Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 02/19/21
Area Forecast Discussion
National Weather Service Binghamton NY
951 PM EST Thu Feb 18 2021
A disorganized series of upper level disturbances will move
northeast from the Gulf of Mexico and up the eastern seaboard
through Friday with light snow.Lake effect snow will start late
Friday and continue into Saturday, and will be focused on areas
north of the Finger Lakes and west of Syracuse with scattered
snow showers elsewhere.
.NEAR TERM /THROUGH FRIDAY/...
With the evening updates the main forecast update focused on
adding patchy freezing drizzle to the forecast. Both the HRRR
and NAM nest continue to indicate a lack of ice crystals in the
clouds till precipitation ramps up overnight. 18Z model
guidance along with the 00Z NAM and HRRR were also incorporated
which did shift the main axis of accumulating snow slightly
further east with each update lowering totals a touch as well.
The forecast in Chemung county was revised downward enough to
warrant the cancellation of the winter weather advisory given
expected snowfall of only an inch or two.
225 PM update...
Quick update for on-going trends. Main snow band stayed just
south of the area, now we`re in a bit of a lull due to mid-level
dry slotting between weak upper level disturbances.
Frontogenesis band has been trying to get going straddling the
NY/PA border and into the Catskills, but it has been battling
the drier air aloft even as surface dewpoint depressions have
narrowed. Outside of this band, some patchy freezing drizzle
has also been reported, due to the lack of ice crystals in the
dry slot. With the dry slot expected to diminish early this
evening ahead of the next disturbance rippling up from the
southwest, expect the risk of freezing drizzle to diminish a few
hours from now.
As mentioned yesterday, if the Wed Night/Thu AM snow failed to
come through in the Southern Tier, we`d be hanging our hats on
additional snow banding expected to develop tonight into Friday.
Confidence is increasing for that scenario, even though the
total snow amounts of 2 to 4 (locally 5) inches are a bit
underwhelming for Advisory criteria. The impacts aren`t that
much different one side of criteria or the other, and dropping
the advisory before anticipated snowfall develops would be
unwise. So, while the lack of snow today is far from ideal,
we`ll let it ride.
.SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/...
355 PM update...
By Friday evening, the synoptically-forced snow will have
shifted east of the area and into the Hudson Valley and New
England, with dry slotting in its wake quickly being replaced by
scattered lake-effect snow bands and showers as northwest low
level flow develops. The most persistent lake effect band will
be focused along a trailing surface trough, which will enhance
low-level convergence over eastern Lake Ontario, trailing into
Onondaga County. This band may shift a little to the south and
hug the southern shore of the lake heading into Saturday
morning, with a fetch stretching back up to Lake Huron, further
enhancing snowfall potential. Steep lapse rates from low level
CAA and an approaching shortwave trough also favor the higher
snowfall amounts. These factors should all help balance out the
one limiting factor -- lake temperatures continue to cool. A
Winter Storm Watch has been issued, focusing on northern Seneca,
Cayuga, and western Onondaga Counties. If the CMCregional`s
depiction pans out (and it`s not unlikely given the low level
wind fetch), the band may end up extending well into Cortland
and Chenango Counties for a time on Saturday afternoon. But, its
residence time in those areas may be too short to really rack
up the higher accumulations.
Elsewhere, expect fairly widespread snow showers and snow
squalls as a strong northern stream shortwave trough moves
across the Niagara Peninsula Saturday morning, and into New
England by Saturday evening. We`ll stay in cyclonic flow aloft
through Saturday night, but negative vorticity advection in the
wake of the shortwave trough should disrupt the lake effect
bands to some degree Saturday evening. But, they may try to
redevelop again before dawn Sunday, when low level winds should
shift on the back side of an approaching ridge axis.
Also, Saturday will be fairly blustery, with some gusts in the
20-25 mph range (even stronger gusts possible in snow squalls
due to momentum transfer). Wind chill values will be in the
.LONG TERM /SUNDAY THROUGH THURSDAY/...
355 PM Update...
Lake effect snow showers will quickly diminish Sunday morning
as high pressure approaches from the west. Otherwise, partly to
mostly cloudy skies are expected with lows mainly in the lower
to mid teens. Some single digit readings will be possible across
northern Oneida County and the higher elevations of the
High pressure will briefly be in control on Sunday, which will
bring partly to mostly sunny skies to the region. High
temperatures will likely be in the upper 20s to lower 30s.
Clouds then increase from west to east Sunday night ahead of
the next system.
A cold front will move through the area on Monday with one low
pressure system tracking through the Great Lakes and another
forming southeast of our region. This will bring light snow to
the region, with rain likely mixing in for the valley locations
as temperatures warm to above freezing. Highs are expected to be
in the lower to upper 30s. Behind this front, some lake effect
snow showers will be possible along the NY Thruway corridor
Monday night. Then another weak system looks to bring snow
showers, mixing with rain in the valley locations again on
Tuesday. Highs Tuesday will likely be in the mid 30s to lower
40s. Lake effect snow showers then look to follow again for the
NY Thruway corridor Tuesday night.
High pressure will likely bring dry conditions on Wednesday
before another system may bring a mix of rain and snow showers
for Thursday. Temperatures look to be trending above normal with
highs each day currently expected to be in the upper 30s to mid
.AVIATION /03Z FRIDAY THROUGH TUESDAY/...
Snow will increase during the overnight hours and continue
into Friday. As snow moves across the terminals, expect
visibilities to fall to between 1SM and 3SM with ceilings
between 900 and 2000 feet.
Some improvement in flight conditions is expected between 15z
and 20z with IFR restrictions transitioning to MVFR.
Winds will be easterly at 4 to 8 knots.
Friday night through Saturday night...Restrictions in CNY due
to lake effect snow showers. Mainly VFR at AVP.
Sunday through Sunday evening...Mainly VFR.
Sunday overnight through Tuesday...A couple of systems will
likely bring light snow and associated restrictions.
PA...Winter Weather Advisory until 6 PM EST Friday for PAZ040-048-
Winter Weather Advisory until 1 PM EST Friday for PAZ038-039-
NY...Winter Weather Advisory until 6 PM EST Friday for NYZ057-062.
Winter Weather Advisory until 1 PM EST Friday for NYZ055-056.
Winter Storm Watch from Friday evening through Saturday
evening for NYZ016>018.
Area Forecast Discussion
National Weather Service Bismarck ND
920 PM CST Thu Feb 18 2021
Issued at 911 PM CST Thu Feb 18 2021
Thus far fog has not developed across the northwest, but the
latest iterations of the HRRR and RAP13 continue to show
development in the next couple of hours. However, it is worth
noting that observations still show rather large dewpoint
depressions at the surface across many sites across the northwest
and into Saskatchewan.
Further east, into the north central a new patch of stratus has
developed from central Renville county, south southeast through
Minot, and into far northwestern Burleigh county. It looks as if
this area of clouds should keep expanding while slowly drifting
east. However, it is hard to say how long this batch of stratus
will last and how widespread it may become as most guidance is
handling the development poor. For this update, updated the sky
forecast to better account for the latest observations and trends.
Also tried to bump up low temperatures a bit where thicker cloud
cover looks to be most likely.
UPDATE Issued at 601 PM CST Thu Feb 18 2021
No major changes were needed for this update. The biggest
challenge to the forecast tonight will be whether or not fog
develops across the northwest. CAMs continue to have very
different ideas about coverage and whether or not fog will even form.
.SHORT TERM...(This evening through Friday)
Issued at 253 PM CST Thu Feb 18 2021
A possibility of fog in the west and relatively warmer
temperatures tomorrow highlight the short term forecast.
Tonight behind the exiting trough, anticyclonic flow will take
over with a building shortwave ridge in the west. Stratus and
scattered flurries may linger in the east under the periphery of
the exiting cyclonic rotation, with mostly clear skies in the
With the diffuse pressure pattern and the current snowpack, fog
may develop in areas of the west tonight and early Friday morning.
High-res guidance is inconsistent in placement, with the 18Z RAP
being the most enthusiastic about fog potential in the northwest.
Fog was kept out of the forecast for now but trends will need
Warmer temperatures expected Friday with low level warm air
advection gradually taking place. Highs in the 30s southwest and
teens east expected.
.LONG TERM...(Friday night through Thursday)
Issued at 253 PM CST Thu Feb 18 2021
A chance of light snow Friday night followed by warmer
temperatures and a possibly windy Monday highlight the long term
Upper level flow becomes quasi-zonal through the weekend with
gradually warming temperatures at the surface. A weak wave should
push through the flow Friday night through Saturday morning which
should bring a chance of light snow across much of the west and
central. Otherwise the story remains consistent with highs
forecast to reach the 30s and low 40s by Sunday. On Monday a mild
Pacific air mass arrives along with the nose of an upper level
jet. Warm and possibly windy conditions are should accompany this
arrival, before the jet dives south by mid-week. NBM/MEX/ECM
guidance consistently advertises highs on Monday to reach around
50 in the southwest and 40s elsewhere, which looks reasonable. 12Z
guidance along with NBM probabilities are in good agreement in a
relative cool down mid-week with highs in the 20s and 30s.
.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 601 PM CST Thu Feb 18 2021
Stratus has nearly eroded across the forecast area and VFR
conditions area expected to prevail through he period. The one
exception could be if fog forms across the northwest tonight which
would mainly impact KXWA. Considerable uncertainty remains about
whether fog will even form so have not included it in the TAF
forecast at this time but the potential is there.
Area Forecast Discussion
National Weather Service State College PA
1037 PM EST Thu Feb 18 2021
Light snow and some mixed precipitation Friday morning before
ending in most places by Friday afternoon. Snow showers will
linger over the western Alleghenies into the weekend, while
other locations see drying/improving conditions into Sunday. A
fast moving system will likely bring a period of rain and snow
from late Sunday night through Monday. Beyond the early-week
system, the weather pattern through the end of February looks
less stormy with warmer temperatures trending near to above
.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
Modest warm advection ahead of approaching shortwave and weak
surface low over Ohio will result in occasional light snow
across much of central Pa overnight. Some freezing drizzle
has been noted across the Lower Susq Valley this evening.
However, IR loop and RAP soundings indicate the arrival of
deeper moisture and associated colder cloud tops will result in
any fzdz changing back to light snow down there by around 03Z.
Latest NBM qpf supports additional accumulations between 00Z-12Z
of an inch or less.
Temperatures should remain steady in the 20s overnight under
cloud cover and weak warm advection.
.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
Light warm advection snow should taper off from west to east
Friday morning, as mid level shortwave and associated occluded
front push through. Behind this feature, focus will shift to
light, lake enhanced, orographic snow showers across the
Alleghenies, as a seasonably cold air mass crosses the Grt Lks.
Model soundings indicate cloud top temperatures are likely to be
just above -10C during late morning and early afternoon over
the Alleghenies, so will mention the possibility of spotty
freezing drizzle in that area. Surface ridging and associated
low inversion heights will result in only minor snow accums of
less than an inch over the Alleghenies Friday and Friday night.
Elsewhere, downsloping west-northwest flow behind occluded
front should result in breaking clouds east of the Appalachians
by Friday afternoon. Model RH profiles suggest skies will become
mostly clear across the Susq Valley Friday night.
Low level chill should be scoured out with arrival of occluded
front Friday, resulting in temperatures rising into the mid and
upper 30s by afternoon over the eastern half of the forecast
area. A return to below normal temperatures is expected Friday
night, as cold advection kicks in. NBM mins range from the teens
over the Allegheny Plateau, to the low 20s over the Lower Susq
.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
12 noon update... In the big picture, the upper-level pattern
is expected to relax across much of the CONUS through next week.
This takes place as the deep central Canadian vortex retreats
and heights rise for most areas. Specifically for the
Commonwealth, this should lead to a less stormy pattern, with
As for the daily sensible weather, we`ll still have lingering
cold and moisture into Saturday behind the storm system
currently affecting the region. This scenario should produce
some residual lake enhanced and upslope snow showers/flurries
over the northern tier and along the Allegheny Front.
Dry weather is expected Sunday as a weak high pressure ridge
builds across the area.
A fast moving wave of low pressure and associated frontal
complex are expected to cross the Commonwealth late Sunday night
and Monday, associated with the likelihood of light snow north
of I-80 and a rain/snow mix farther south. Any light lake
enhanced and upslope rain/snow showers behind this system look
inconsequential from this early vantage point, as the incoming
air mass for the middle of next week doesn`t appear to be
.AVIATION /04Z FRIDAY THROUGH TUESDAY/...
Restrictions will be widespread overnight owing to ongoing snow
in the central and western areas with freezing drizzle at MDT
and LNS. As of 00Z, a band of snow showers extends from JST/AOO
to IPT via UNV. This band of showers is expected to persist for
the next several hours. Additionally, current radar trends
indicate a developing band of snow that will affect BFD through
02 or 03Z.
Ceiling restrictions will persist through daybreak tomorrow,
though snow will lift out from SW to NE after midnight with
visibility improvements following soon after. As winds turn to
come out of the NW during the day Friday, downsloping will help
to improve ceilings and eventually clear skies out in the
central and southeastern airfields.
Fri...Lingering snow showers north and east through midday. Improving
Sat...Mainly dry w/ VFR conditions.
Sun...Prevailing VFR. Increasing clouds. Chance of snow west
Mon...Widespread reductions in rain/snow.
Tues...Lake effect snow showers to bring reduced cigs/vis to
BFD & possibly IPT/JST
Winter Weather Advisory until 10 AM EST Friday for PAZ035-036-
Area Forecast Discussion
National Weather Service Grand Rapids MI
700 PM EST Thu Feb 18 2021
Issued at 320 PM EST Thu Feb 18 2021
- Lake enhanced snow showers Friday with some accumulation
- System snow Sunday night into early Monday
- Another weak front early Tuesday
- Warmer most of next week with possible system Thursday
.DISCUSSION...(This evening through next Thursday)
Issued at 320 PM EST Thu Feb 18 2021
- Lake enhanced snow showers Friday with some accumulation -
The light snow from the system passing south of us this evening
should end by evening as the deep moisture pulls out. I do not see
any significant accumulation from the snow this afternoon into
The snow event on Friday during the day will very likely bring
some locally heavy snow showers to our area. There are now two
parts to this event. The first part is the low level winds being
pushed from easterly to westerly around sunrise Friday. The
second part as a shortwave associated with the coldest air, that
moves through Friday evening. Warm advection at mid levels starts
after midnight so that will slowly end the snow showers during
Meanwhile the DGZ is below 6000 ft Friday but there is strong lift
in it for both events (mid morning and late afternoon into the
evening). We will still be in the deep cold air during this entire
event too so it should be no problem to generate lake enhanced
snow showers. The HRRR has 12 hour localized precipitation amounts
over 0.7 inches near MKG by evening, given a 20:1 ratio that
would be over a foot of snow if it happened. Also, it is worth
noting that most of that falls with the second shortwave during
the evening. Also remember the higher amounts will be very
localized. This may be a slow evening commute along US-31 from
Ludington to Grand Haven. Even so, most areas would get less than
2 inches Friday.
Also, due to the wind being mostly westerly the snow showers will
get all the way across the CWA during the day on Friday. Cloud
level winds will be mostly less than 20 knots, normally that would
not bring the snow bands too far inland. However this
synoptically forced as it is really a weak cold front crossing
The evening event will mostly stay near and west of US-131. This
is when the heavier snow showers are expected. We could see
localized snowfall rates over over 1 inch per hour in the better
- System snow Sunday night into early Monday -
The next system is the last (so we think) significant snow
producing system in the colder weather pattern we have been in. It
is this system that boots the cold air out of here. The Sunday system
was the trailing shortwave that dug into west side of the central
CONUS upper trough. We are in the jet exit region on this system.
That should help us stay cold enough for all snow. The 1000/850
moisture transport vectors are aimed at I-94 Sunday evening with a
flow coming from the Gulf. This will likely be a 6 hour snow
event with a 3 hour period of moderate to heavy snow.
This system will bring a quick 2 to 4 inches across the area.
Maybe areas north of I-96 could see 3 to 5 inches due to the
moisture transport being so well aimed at us while we are in the
jet exit region. Seems the surface low will largely stay near or
south our CWA so it will be all snow. It should be too warm for
lake effect snow behind this system though. Once this system
moves out of this area, we will have a more zonal flow through
- Another weak front early Tuesday -
The lead shortwave moves on large polar jet core from the Pacific,
across the area early Tuesday. That may result in some light snow
showers but nothing of consequence. By then it will be just cold
enough for snow but no cold enough to create lake effect. I will
admit this may end but being more than just flurries since we have
the exit region of a 150 knot jet causing it. This will be
another feature to watch out for.
- Warmer most of next week with possible system Thursday -
That 150 knot jet exit region is from a broad upper jet that
extends across 40 degrees of longitude. The system behind that
digs a trough on the west coast, which in turn lifts that jet
north of us by Wednesday. We will then be on the anticyclonic
side of the polar jet and should be in the warm air. Well at least
sort of anyway.
Here is where it gets glitchy. That large jet streak moves east
and we are in the entrance region of that jet by early Thursday.
Then there is a southern stream jet feature coming out of the
base of the trough, created by that next Pacific system. The
latest ECMWF couples the two jets (exit region incoming southern
stream jet with the entrance region of the departing northern
stream jet). That of creates enough lift for a broad area of
precipitation from central New York State, across lower Michigan,
into Iowa early Thursday. This could be mixed precipitation here
or maybe just snow we will have to see. The GFS does is similar
with the upper air pattern has but no precipitation with the event.
Again this is yet another event to watch.
So there is the bottom line, while it may seem we could have a dry
period of weather next week, it just may turn out not nearly as
dry as we were thinking.
.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 700 PM EST Thu Feb 18 2021
Light snow will continue to produce widespread MVFR vsbys between
3 and 5 miles this evening, primarily south and east of GRR. Some
brief/localized IFR vsbys are also possible. As that snow pulls
away later tonight, we may see several hours of VFR in it`s wake.
However as lake effect snow develops Friday, expect MVFR or lower
cigs/vsbys to redevelop. That will occur first at the lakeshore
(MKG) around 12Z, reaching inland toward LAN/JXN by afternoon.
Some of the snow showers Friday could be locally heavy at times
so brief LIFR vsbys are possible.
Issued at 320 PM EST Thu Feb 18 2021
At this point we do not have enough wind with any of the system
moving through the area into early next week to be very concerned
about issuing marine headlines. The ice build up in the near shore
helps to mitigate some of that anyway.
Area Forecast Discussion
National Weather Service New Orleans LA
836 PM CST Thu Feb 18 2021
.UPDATE...Forecast is running on track for the most part. A few
sites are running a little slower this evening but the forecast
lows still look good for a majority of the area, maybe a few
southshore sites forecast could be a tad to warm but will hold off
on an adjustments at this time. Looking downstream slightly drier
air has started to work southeast once again and dewpoints are
still forecast to drop into the mid 20s to lower/mid 30s across
the area however, the concern for tonight could be winds. Winds
are forecast to not decouple and could remain around 15-20kts at
h925. Other concern is how strong is the CAA? Looking at the
latest HRRR it initialized too cool in the LL. Our 00z sndg at
h925/h85 was 7C and 9.5C respectively (HRRR was 4C and 9C) while
JAN`s 00z sndg measured -1C and 4C (HRRR -2C and 1C). Too offset
the wind expected tonight CAA needs to be strong and not really
seeing a lot to indicate overpowering CAA. That said there is
drier and colder air sliding in and if for some reason winds shut
down and hour or two before sunrise temps could drop pretty good
in a short time and we would hate to make any significant changes
to the forecast or cancel any warnings at this time but it
wouldn`t be a shock if we just don`t reach our potential tonight.
Tomorrow night is a different story and could be a VERY good
radiational cooling set up. /CAB/
.PREV DISCUSSION... /issued 336 PM CST Thu Feb 18 2021/
SHORT TERM (Tonight through Saturday Night)...A longwave trough
axis will sweep through the Gulf South tonight into tomorrow. The
main impact from this trough axis will be a pool of much colder
air advecting into the region, and the continuation of cloud cover
through tomorrow morning. Temperatures will fall below freezing
across the entire forecast area tonight, and a hard freeze with
readings 23 to 26 degrees is expected for areas along and north of
I-12. The thermal trough will remain over the area tomorrow, and
daytime highs will struggle to reach the middle to upper 40s even
with ample solar insolation in the afternoon hours. The coldest
night of the period will occur tomorrow night due to a combination
of light winds and clear skies. Lows will once again fall below
hard freeze criteria for areas along and north of the I-12
corridor with readings in the upper 20s and lower 30s across the
remainder of the area. Another round of freeze and hard freeze
warnings can be expected. Have opted to go slightly colder than
guidance through tomorrow night given the extensive snowpack just
north of the CWA.
Saturday will be a day of transition as the trough axis pulls to
the east and a weak upper level ridge moves in from the west.
Increasing subsidence aloft will clear skies and warmer
temperatures to the area. Highs will remain about 10 to 15 degrees
cooler than normal, but will warm into the middle to upper 50s.
This same trend will continue into Saturday night as temperatures
fall into the lower to middle 30s north of I-10/12 and the lower
40s south of the tidal lakes and along the Louisiana coast.
LONG TERM (Sunday through Thursday)...The weak ridge axis will
shift to the east on Sunday, and an increase in cloud cover is
expected through the day as a shallow shortwave trough and
associated front moves into the Southern Plains. The combination
of increasing lift in advance of this trough and some moderate
return flow around 850mb will be sufficient enough to bring
increasing cloud cover to the area throughout the day. Warmer
temperatures can also be expected with this return flow off the
Gulf, and highs should be closer to average in the mid 60s.
The weak shortwave trough will push through the Gulf South Sunday
into Monday, and surface front will accompany this trough axis.
Increased low level forcing along the frontal boundary will
combine with stronger lift aloft to spark off scattered showers
Sunday night into Monday morning. The POP forecast will still be
relatively low at 20 to 40 percent during this period, and
instability parameters will remain very weak. As a result,
thunderstorm activity is not anticipated with this frontal passage
Sunday night. Temperatures will be near average Sunday night with
lows in the 40s and lower 50s, but some weak cold air advection
into the area on Monday will cool temperatures a few degrees into
the upper 50s and lower 60s. The main heart of the cold pool will
pass over the area Monday night and lows should dip into the upper
30s and lower 40s.
A return to weak upper level ridging and increased subsidence
aloft along with southerly flow in the low levels will lead to
clearing skies and warmer temperatures for the middle of next
week. Highs should climb back to more normal readings in the upper
60s and lower 70s by Wednesday and overnight lows will warm from
the 40s Tuesday night into the 50s by Wednesday night. Cloud cover
should also increase Wednesday night due to a combination of the
warmer air advecting in from the south over the cooler nearshore
waters, and the approach of another low pressure system. This
system should impact the area at some point late next week, but
timing differences between model solutions are in place. Have
opted to stick with a blended solution and indicate low end chance
POP for Thursday. Temperatures will remain near average with highs
in the upper 60s and lower 70s expected.
.AVIATION (00Z TAF DISCUSSION)...Low stratus will linger over the
terminals until around 15z in the morning. IFR ceilings are
expected at all of the terminals during this period. Winds will
also remain gusty from the north at more than 12 knots at KNEW and
KMSY as boundary layer winds mix down to the surface after passing
over the warmer lake waters. After 15z, skies are expected to
clear rapidly as drier air aloft mixes down, and prevailing VFR
conditions can be expected through the end of the forecast period.
.MARINE...Strong gradient flow associated with a colder airmass
moving into the area will keep winds and seas elevated into small
craft advisory range through tomorrow evening over the open Gulf
waters. In the lakes, the winds should begin to decrease below
advisory levels later tonight. The surface high associated with
this colder airmass will shift toward the eastern Gulf over the
weekend, a onshore flow of around 15 knots is expected to take
hold of the coastal waters. This onshore flow will continue
through Sunday night before another cold front moves through.
Winds will shift back to the north at around 15 knots by Monday
and remain from the north through the first half of next week.
.PRELIMINARY POINT TEMPS/POPS...
MCB 24 44 20 57 / 0 0 0 0
BTR 26 45 23 55 / 0 0 0 0
ASD 28 49 24 59 / 0 0 0 0
MSY 32 48 31 56 / 0 0 0 0
GPT 31 50 27 57 / 0 0 0 0
PQL 31 50 25 58 / 0 0 0 0
LA...Freeze Warning from midnight tonight to 9 AM CST Friday for
Hard Freeze Warning until 9 AM CST Friday for LAZ034>037-039-040-
GM...Small Craft Advisory until midnight CST Friday night for GMZ532-
Small Craft Advisory until 3 AM CST Friday for GMZ530-534.
MS...Hard Freeze Warning until 9 AM CST Friday for MSZ068>071-077.
GM...Small Craft Advisory until midnight CST Friday night for GMZ536-
Small Craft Advisory until 3 AM CST Friday for GMZ534.
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
857 PM CST Thu Feb 18 2021
857 PM CST
Forecast in generally good shape this evening. Other than keeping
a mention of flurries beyond midnight for northern/northeast IL
and adjusting some hourly temp trends for effects of variable
cloud cover, see little that needs significant changes.
Radar trends this evening have indicated a gradual reduction in
snow shower intensity in inland areas with the loss of diurnally
enhanced low level instability after sunset. Backing of the low
level wind field from northeast to north-northwest over southern
Lake Michigan has also allowed for a decrease in lake-effect snow
showers, though less organized lake-induced light snow does
persist from along the Cook county shore into northwest Indiana.
This should continue to shift southeastward out of Chicago over
the next couple of hours as winds continue to back more west-
northwesterly, with light snow showers persisting and gradually
shifting east across northwest IN overnight. Across north central
and northeast IL, continue to see a bloom of weak radar returns
(in sensitive VCP-31 mode) associated with flurries. This appears
to be in association with forcing tied to the approach of a
compact mid-level short wave over eastern IA, which will translate
eastward across northern IL through 06-09Z. 00Z DVN sounding and
RAP initialized forecast soundings suggest moisture is fairly
shallow, but right in the -10C to -20C range perfect for
squeezing out a few flakes with the assist from the approaching
wave. The short wave trough/vort axis looks to move through the
Chicago area by 3-4 am, which should allow any lingering flurries
Otherwise, GOES IR depicts areas of stratus/stratocu which will
bring partly to mostly cloudy skies across a good portion of the
area into the pre-dawn hours. However, where clearing or breaks in
the cloud cover occur, sharp temp drops are likely (KARR dropped
to 10 as clouds scattered out, but has risen back to 16 as clouds
moved back in). Wouldn`t be surprised to see a few spots dip below
zero by morning, but for now think going forecast has a decent
handle on things with lows approaching zero across a good portion
of northern IL and higher single digits into the heart of the
metro and northwest IN.
205 PM CST
Through Friday night...
Our earlier cohesive area of steady lake-enhanced snow has
become notably more cellular in nature. With surface temperatures
having risen into the low to mid 20s, steepened low-level lapse
rates have allowed a sliver of surface-based instability to
develop. Where this has co-located with enhanced near-surface
convergence, we`re seeing a decent coverage of snow showers
rotating south and westward across parts of northeast Illinois.
Anticipate that these will begin to wane with the loss of heating
through the rest of the afternoon and into the evening hours, but
have maintained a pocket of likely PoPs to account for this the
rest of the afternoon. Locally sharply varying visibilities to
under 1-2 miles are likely with this activity, although what seems
to be fairly large flake sizes will help hold the visibilities up
a bit more than we`d otherwise expect given the radar
Am somewhat uncertain how additional lake effect may evolve this
evening and overnight. With the low-level flow becoming more west
to northwesterly with time, the expectation is for any lingering
LES to begin to slosh eastward more into NW Indiana. This idea
seems to be supported reasonably well with this afternoon`s
guidance and have limited things to just a mention of flurries
later this evening across parts of northeast Illinois as a result,
transitioning to somewhat better snow chances into Lake and
Porter counties. Increasing low-level winds with time should hold
the residence time of any lake band down, but given high ratios,
couldn`t rule out a quick inch or so towards and just after
Our next disturbance, currently pinwheeling across the Dakotas, is
forecast to push into our region later on Friday afternoon and
evening. While the ECMWF and EPS remains precipitation-free, the
GFS/GEFS, and CMC/GEPS continue to squeeze out some light
precipitation amounts south of I-80 where enhanced low-level
moisture coincides with better near-surface lapse rates. Given how
sharp this feature appears on upstream moisture channel loops, did
elect to throw in some low-grade snow chances into Friday evening,
although note that this remains a somewhat lower confidence
forecast. Depending on how fast this system scoots east of the
region, slackening winds and potentially clearing skies as a
surface ridge builds overhead could allow patchy (freezing) fog
development, but this remains too conditional to warrant a mention
in the gridded forecast at this time.
248 PM CST
Saturday through Thursday...
Generally quiet conditions are expected for the start of the
weekend as the persistent upper-low and broader loosely blocked
pattern continues to break down. Weak low-level WAA will lift
northeast across the CWA midday into the evening under a passing
mid-level speed max. Have kept the forecast dry for now, but if
there is any semblance of thicker stratus over the area, a light
wintry mix of fine snow and freezing drizzle cannot be ruled out.
Focus then turns to the potential for accumulating snow area-wide
Sunday and Sunday night. A slow-moving mid-level trough over the
central Rockies Saturday night will slightly phase with another
trough tracking ESE though south-central Canada. While the
resulting low-level surface and 850 hPa lows will not have the
typical favorable track for heavy snow across northern Illinois,
tracking from southern Iowa into southeast Wisconsin in this case,
the combination of a slightly negative tilt to the mid-level
trough and corresponding steep lapse rates aloft indicate that
this system has the potential to produce a period of moderate to
briefly heavy snow. Thermo profiles showing limited DGZ depth amid
continued low-level WAA favor SLR values in the 10-13:1 range.
While there may be enough WAA to allow for some rain to mix with
the snow across the far southeast CWA, wetbulb profiles and
dynamic cooling favor all snow as the precip type for much of the
Timing from ensemble and deterministic guidance continues to
become more refined, with snow spreading west to east across the
area late Sunday morning west of I-39 to early/mid afternoon in
the Chicago metro and northwest Indiana. Steady light to moderate
snow with some pockets of briefly heavy snow will likely continue
through the evening before tapering from west to east overnight.
While confidence in accumulating snow is quite high, confidence in
amounts is much lower. Timing will be critical in order to see
higher amounts as low-level moisture transport ahead of the system
will be commencing right as it is progged to move across the CWA.
Conservative estimates yield 1.5-3" for much of the area, with a
higher-end potential of a narrow swath of 3-5". Given the decent
dynamics and lapse rates in play, high snowfall rates will be
possible with most of the snow falling in 9 hours or less.
Beyond Sunday, the cold pattern that has been locked in place
across the area will finally break, giving way to more seasonable
temperatures well into the 30s for much of the upcoming week. Even
with the warmer conditions, the amount of snow melt is less clear
with nighttime temps falling back below freezing, the necessity
to warm/prime the existing snowpack, and some potentially breezy
periods amid drier conditions. And while sunny days will help melt
snow at the top of the snowpack, the snowpack is likely deep
enough to absorb and hold onto much of this melt water. With all
that said, any substantial days of melting will increase flooding
For the 18Z TAFs...
A somewhat lower confidence forecast through the next 6-9 hours or
so with persistent lake-enhanced snowfall pivoting across northern
Illinois. Prevailing MVFR/IFR -SN appears to be gradually pushing
inland at this time, with light east to northeasterly winds noted
now near the lake. Based on the TORD TDWR, it looks like there may
be one last push of IFR -SN to push across the ORD/MDW airfield
through 20z or so with perhaps an additional one half inch of
Based on visible satellite imagery and some recently shared photos
from the Chicago area, bonafide cumulus is developing with a bit
of daytime heating. Guidance suggesting a semi-broken line of snow
showers seems plausible as a result along the enhanced area of
surface convergence. Given a hint of instability at the surface,
any snow showers could lay down some decent rates (for brief
periods), and have introduced a TEMPO for 2 sm -SN at DPA as a
At this time, the thinking is this SHSN type activity
will develop west of the ORD/MDW terminals but may impact DPA into
the mid-afternoon hours. RFD looks to be far enough west to
maintain just occasional flurries into this evening. GYY offers up
a conundrum, as it`s conceivable additional lake effect snow
develops and pushes south off the lake this evening. This signal
isn`t terribly strong at this time, but sufficient to justify a
PROB30 for -SN overnight.
Otherwise, activity should diminish this evening with winds
trending northwest and eventually westerly. Could develop some
occasional gustiness Friday afternoon as well. Another wave will
pass overhead, and it`s possible this also squeezes out some
additional snow showers. Latest indications are this activity may
not develop until closer to Friday evening.
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AREA FORECAST DISCUSSION
National Weather Service Morristown TN
1008 PM EST Thu Feb 18 2021
The forecast remains on track for the overnight. Just inputed
recent observations of temps, dew points, and winds to adjust the
hourly curve. Latest RAP analysis places the cold front just E of
the S Appalachians, but the colder air has been slow to cross the
Cumberland Plateau into our region. Current temps W of the
Plateau are in the upper 20`s, with mostly upper 30`s/low 40`s in
our region. This cold air W of the Plateau will spill in over the
next few hours. Lift from a 160 kt jet overhead has worked with
abundant low-level moisture to continue areas of light rain and
drizzle through the evening, and this will continue through about
06Z before changing to light snow showers and flurries from W to E
as the colder air finally works into the region. At the same time
however, forecast soundings show the low-level moisture beginning
to erode after 06Z, so expect any light accumulations to be
confined to the higher terrain of the Plateau and E TN/SW VA
mountains late tonight. Slightly higher amounts of up to 1 inch
are still expected in the E TN mountains as weak NW upslope flow
00Z TAF DISCUSSION.
Poor flight conditions with drizzle, mist, and IFR will continue
through the evening at CHA and for most of the night at TYS and
TRI. Used TEMPO groups for brief LIFR at TYS and TRI this evening.
Drier air building in will allow CHA to scatter out to VFR by mid
morning Friday, but it will take until late afternoon for TYS and
TRI to return to VFR. Winds will generally be N to NW at 3 to 8
kts through the period, except NE at times at TYS.
.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN 29 43 22 47 27 / 20 0 0 0 0
Knoxville McGhee Tyson Airport, TN 29 39 20 42 25 / 30 10 0 0 0
Oak Ridge, TN 28 40 18 42 25 / 30 0 0 0 0
Tri Cities Airport, TN 31 38 18 38 23 / 40 10 0 0 0
...Updated Aviation Forecast Discussion...
Issued at 250 PM CST Thu Feb 18 2021
The primary forecast concerns are chances for light snow or light
freezing drizzle Saturday night into Sunday. We could also see a
little light snow tonight.
Large scale mid tropospheric pattern features a trough extending
from MN down into west TX, with a ridge from British Columbia to
off the coast of CA. There does appear to be a weak shortwave
trough moving southeast across the central US now. 2 pm surface
analysis showed high pressure dominating much of the region.
Tonight, it does appear that there will be some potential for
light snow or snow flurries as an area of moisture and lift move
through the area. Cloud temperatures have been favorable for ice
crystal growth the past 24 hours or so and that appears that it
will continue tonight. Recent RAP model runs continue to depict
potential for an area of light snow moving from west to east
through the area overnight, in association with a another trough
in the northwest flow. Right now it appears amounts of an tenth
of an inch or two are possible.
Temperatures have warmed up into the upper teens and lower 20s at
most spots with dewpoints in the single digits and teens. Lows
tonight are tricky with the variable cloud cover, snow cover and
fairly light winds. Expect most lows in the single digits, but
localized radiational cooling could result in some lows below
We will see a gradual warming trend the next few days. Areas near
the SD and KS borders could approach freezing tomorrow, with many
spots in the 30s Saturday. The next disturbance of concern will
start to approach from the west Saturday night. Large scale lift
will increase, but there is some question as to moisture depth and
amount of ice aloft. Some soundings show the moisture too shallow
to generate ice and dry layers aloft. For now we will keep a
mention of both light snow and freezing drizzle, with details
likely becoming more clear the next few model runs. There appears
to be potential for around 1 inch of snow and a light glaze of
ice with this system. Chances will mostly be in the decrease
Sunday afternoon and we look for highs in the lower or mid 30s.
The big story next week is warmer temperatures. We should see
highs in the 40s Monday, with 40s to lower 50s Tuesday. The
warmest highs should be where snow cover is the lowest. It looks
like a cooler airmass will move in Tuesday night or Wednesday. As
that happens we could see some light snow, but timing for snow
leads to low overall confidence on nailing down precipitation in 6
hour increments. We will leave precipitation chances at less than
20 percent for now from next week. Highs on Wednesday and
Thursday look to be in the mid 30s to lower 40s.
.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 512 PM CST Thu Feb 18 2021
VFR conditions through the period. There is a small chance of
mainly flurries at KOFK 06-09z, and KOMA 08-13z. Believe the snow
may stay just north of the KLNK site. Maybe only about 0.1
Area Forecast Discussion
National Weather Service Mount Holly NJ
959 PM EST Thu Feb 18 2021
A couple of low pressure systems will track northeastward along
a frontal boundary located offshore of the east coast tonight
through Friday. High pressure approaches the area from the
southwest Friday night through Saturday, before building over
the Mid Atlantic region Saturday night into Sunday. This high
will build offshore Sunday night in advance of our next storm
system expected Monday into Monday night. High pressure builds
across the southern states with a couple of disturbances
possibly affecting the Mid Atlantic region.
.NEAR TERM /THROUGH FRIDAY/...
Mid level dry air has moved into the region especially in the
700-500mb layer which caused a loss of cloud ice this evening
which is confirmed by the 00z soundings at IAD and OKX. With the
loss of mid level RH, steady precip has come to an end however,
PW in the low levels is still rather high, and based on RAP low
level data there does appear to be at least enough lift and warm
air advection to allow for patchy drizzle across the region.
With this evenings 00z soundings from both IAD and OKX
indicating quite a bit of low level moisture and little in the
way of ice nucleation, patchy freezing drizzle has been
consistent through the evening and I anticipate that will
continue through periods of the overnight hours with
accumulations of up to another tenth possible.
Previous discussion...Impressive sloped ascent from this
morning has translated offshore this afternoon, leaving freezing
rain/ rain/ sleet/ and snow in its wake. The threat for snow
across DE/ southern NJ and southern PA looks pretty low as mid-
level dry air comes in from the southwest. This can be seen
pretty clearly on WV. Low level 925 MB flow continues to veer
from the southeast this evening. This upslope flow has allowed
for reflectivity to again blossom across the Lehigh Valley and
Poconos. Most of the precipitation overnight from the upslope
flow will be very light in nature, but this light frozen
precipitation will make for slippery road conditions. Given
this, we have converted the Winter Storm Warning over to a
Winter Weather Advisory for all zones.
Overnight tonight, temperatures will fall only slightly with
widespread cloud cover and southeasterly low level flow. The
main story will be an area of low pressure in the low levels
that looks to try and organize. At 250 MB the upper level jet
streak is actually forecast to strengthen late tonight/ early
Friday morning. In the mid-levels, impressive height falls are
forecast as the main mid-level wave approaches from the west.
Guidance is split here on the location of the development of
precipitation in response to the weak surface cyclogenesis.
Confidence here on the location of the precipitation is low as
guidance is all over the place. Some models are showing the
precipitation spreading up near the I-95 corridor, while other
guidance is showing precipitation spreading northeast offshore.
Given the weak lift described above, have gone ahead and
extended the Winter Weather Advisory out to 21z and 00z Friday/
Saturday respectively. Precipitation type with this next system
will again be a mix, with snow across the northwestern zones
and a snow/sleet/ rain mix across the coastal plain of NJ.
Highs Friday afternoon will be in the low to mid 30s with skies
.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
Quieter conditions are expected over the weekend, although it
will not have a completely dry forecast through Saturday. As an
area of low pressure continues to move to the northeast Friday
night along a frontal boundary located offshore of the east
coast, there could remain some lingering snow showers across
eastern Pennsylvania and New Jersey, although the chances are
pretty small. Most everyone will be precipitation free however.
On Saturday, most areas will be precipitation free, but a
steady northwest flow will be in place across the region as low
pressure will be located to our northeast and high pressure
builds to our southwest. Wind gusts of 20-25 mph can be expected
through the day across the area. There will also be a chance of
snow showers or flurries for portions of northeast Pennsylvania
and northern New Jersey as low-mid level lapse rates increase
along with an increase in moisture as well. As a short
wave/vorticity impulse moves across the area later in the day,
snow showers/flurries could develop for portions of the area.
High pressure continues to build into the region Saturday
night, and centers itself across the Mid Atlantic region on
Sunday. Dry but cool conditions will prevail Saturday night
.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
The first half of Sunday night will remain dry, however a warm
front may lift northward across the area late in the night which
could combine with a short wave/vorticity impulse lifting
across the area to create some light snow across the area. A
frontal boundary is expected to move across the area on Monday,
while a triple point low could also develop along the front and
move across the area. Temperatures will be cold enough for some
area to see snow, but this will be dependent on the track of the
developing low. While many areas may begin as snow, a lot of
places will likely change over to rain before ending Monday
For the Tuesday through Thursday period, high pressure is
forecast to build across the southern states. Meanwhile, several
disturbances could move across the area Mid Atlantic region
each day. Tuesday, and especially Thursday when a more well
defined frontal boundary and increase in moisture, have the best
chances of precipitation.
.AVIATION /03Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Tonight... Now that much of the steady and heavy precipitation has
ended, a period of lighter precip and low clouds will prevail across
the area tonight. Much of the time, CIGS will be IFR or LIFR, but
some periods of MVFR are possible for the NW sites. Light drizzle or
freezing drizzle are possible from time to time. NE winds will be
around 10 knots overnight.
Friday... Low clouds will continue for many areas, some breaks are
possible N/W. It`s a rather low confid fcst with low pressure and
precip moving northward along the coast. For the most part IFR/MVFR
will prevail across the area. Some improvement possible late. N
winds 5 to 10 knots.
Friday night...MVFR or IFR conditions are likely to start off
the evening, before improving to VFR overnight. West to
northwest winds 5-10 knots.
Saturday...Mostly VFR, ceilings may briefly lower to MVFR
during the day. Northwest winds 10-15 knots, with gusts 20-25
Saturday night-Sunday...VFR conditions expected. West to
northwest winds 5-10 knots, shifting to southwest and
diminishing later Sunday.
Sunday night...VFR conditions are expected to start the night,
before potentially lowering MVFR overnight. South winds around
Monday...MVFR or IFR conditions likely with a period of rain or
snow. South to southwest winds 5-10 knots, with gusts of 15-20
Monday night...MVFR or IFR conditions likely to begin the
evening, before improving to VFR overnight.
Tuesday...Mostly VFR, ceilings may briefly lower to MVFR during
the day. West to southwest winds 5-10 knots, with gusts 15-20
Low pressure will translate northeast this afternoon with northeast
wind 20 to 25 knots, with gusts of 25 to 30 knots on our
coastal waters through tonight. The wind is forecast to
diminish slowly on Friday as it backs to the north behind
another area of low pressure.
A Small Craft Advisory remains in effect for our ocean
waters until 6:00 PM Friday, and for Delaware Bay until 6:00 AM
Friday. The Small Craft Advisory for the Atlantic Ocean waters
continues through Friday evening.
Friday night...Small Craft Advisory has been extended through
Saturday-Saturday night...Winds and waves likely to remain
above advisory levels through Saturday night before lowering
Sunday-Sunday night...Conditions likely to remain below
advisory levels through Sunday night, although winds may gust
around 20 knots at times.
Monday-Tuesday...Conditions may remain near advisory levels
with seas near 5 feet and winds gusting near 25 knots at times.
PA...Winter Weather Advisory until 7 PM EST Friday for PAZ055-062-
Winter Weather Advisory until 4 PM EST Friday for PAZ054-060-
NJ...Winter Weather Advisory until 7 PM EST Friday for NJZ001-
DE...Winter Weather Advisory until 7 PM EST Friday for DEZ001>003.
MD...Winter Weather Advisory until 7 PM EST Friday for MDZ012-015-
MARINE...Small Craft Advisory until 6 AM EST Saturday for ANZ450>455.
Small Craft Advisory until 6 AM EST Friday for ANZ430-431.