Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 04/09/18
Area Forecast Discussion
National Weather Service Charleston SC
954 PM EDT Sun Apr 8 2018
.SYNOPSIS...
High pressure will push offshore tonight as a frontal system
approaches from the south to impact the area Monday into
Tuesday. Dry high pressure will then prevail into Saturday. A
cold front could approach on Sunday.
&&
.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
Short term guidance has come into better agreeement in placing
the strongest isentropic assent along the Savannah River into
the Beaufort area overnight. Adjusted pops slightly to reflect
this, but 70-80% pops still look on track. KJAX radar already
showing isolated showers beginning to develop just north of the
Florida/Georgia border as expected. GOES-E imagery shows clouds
are rapidly expanding north and skies should go cloudy all
areas soon. Overnight lows have likely occurred or will soon
occur for most areas. Adjusted hourly temperatures to match
going trends.
Isentropic lift is progged to steadily intensify overnight as a
coastal trough sharpens offshore and a 40 kt LLJ strengthens
across the Southeast States. As is typical in these return
flow patterns, it is proving difficult to determine the spatial
and temporal distribution of rainfall that is likely to
breakout across the forecast area with the initial onset of
290-300K isentropic ascent. It does appear that many areas
could see measurable rainfall prior to daybreak, albeit quite
light, so 70-80 percent pops look reasonable for now. The
initial burst of light rain could be quite spotty at first and
possibly even fall as virga, but do anticipate measurable rains
to occur by 6 AM for many area as the Troposphere steadily
moistens. Made some minor adjustments to hourly pops based on
the latest RAP and H3R, but the general flavor of the rain
forecast was maintained.
Overnight lows will be highly dependent on how quickly clouds
thicken from south to north. Latest high resolution guidance
suggests temperatures may end up being a few degrees warmer than
what the current forecast depicts. Will trend overnight lows up
a few degrees, but not quite as warm as some of the guidance
would suggest. It is very possible temperatures may fall this
evening then hold steady or even slowly rise as the cloud
canopy thickens and lowers. Further adjustments may be needed
with the late evening forecast cycle.
&&
.SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/...
Overview: A very unsettled and cooler than normal period
expected through Tuesday. Models continue to show a series of
low pressure systems moving well south of the area Monday and
Tuesday with widespread light rain, then clearing and more mild
by Tuesday night and Wednesday.
Monday and Monday Night: The day will start off with widespread
light rain across the area as a surface low moves just
southeast of the region over the Atlantic waters. Isentropic
lift/overrunning will produce likely to categorical chances for
light rain through early afternoon, especially for the eastern
half of the area. Rain chances will gradually decrease later in
the day. Expect a short break in rain chances during the
evening, then increasing again later at night as the next
surface low moves south of the region. Have gone slight chance
to chance pops after midnight. High temperatures will be very
cool again, around 60 north, to the mid to upper 60s extreme
south with low level northeast winds of 10-15 mph on the
back/west side of the low pressure center. Lows Monday night in
the upper 40s to lower 50s, coolest north.
Tuesday: Models differ on placement of low center, deepest
moisture and rain chances. NAM and Euro are drier, keeping the
best moisture confined to the southeast and Atlantic waters,
while the GFS holds better moisture/rain chances back further
west over much of the region. Have nudged up pops higher to give
some credit to the GFS forecast, with high end/marginal likely
pops over the southeast, and then generally chance further
inland and north. Depending on exact strength and track of low
center, highs could once again be quite cool, in the low to mid
60s, as the area will be on the west side of the low with north-
northeast low level winds.
Tuesday night and Wednesday: The surface low center will
continue to move east-northeast away from the region, with
clearing and drying conditions. Expect low temperatures Tuesday
night in the mid to upper 40s under clearing skies. Mostly
sunny/sunny on Wednesday as high pressure builds over the area
with light low level winds and high temperatures rebounding into
the lower to mid 70s, closer to normal.
Rainfall: Although light rain will be quite widespread on
Monday, overall rainfall totals are expected to be generally
between 0.25 to 0.5 inches Monday through Tuesday.
&&
.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
Deep layered ridge will prevail Wednesday night into Saturday
before a highly amplified upper trough approaches late in the
weekend. A strong cold front is currently forecast to move into
the area next Sunday. Above normal temps expected most days.
&&
.AVIATION /02Z MONDAY THROUGH FRIDAY/...
VFR conditions will steadily deteriorate overnight as strong
isentropic lift develops across the area. Cigs will steadily
lower from south to north with prevailing IFR conditions
expected after 10z at both KCHS and KSAV. Spotty light rain will
begin to breakout along the Georgia coast by late evening, then
blossom over the terminals Monday morning. Widespread vsbys of
3-5SM are likely in mainly light rain. Vsbys should improve by
late morning as the bulk of the light rain pushes offshore as
low pressure pulls away, but low clouds will linger through the
day. Opted to show improving conditions to low-end MVFR by
early-mid afternoon, but is very possible that IFR cigs could
linger through the remainder of the 00z TAF period.
Extended Aviation Outlook: Monday and into Tuesday: Models
continue to indicate the potential for MVFR/IFR conditions as a
series of low pressure centers move southeast of both sites,
producing periods of low ceilings and widespread light rain. The
best rain chances will be Monday, then again late Monday night
and the first part of Tuesday. Tuesday night through Friday:
High pressure will dominate with generally VFR conditions.
&&
.MARINE...
Tonight: Northeast winds will prevail in the 10-15 knot range
through the evening. The flow could turn a bit more easterly for
a few hours, before the coastal trough sharpens late and the
flow closer to shore becomes more northeasterly. Closer to
sunrise, localized pinching of the pressure gradient will help
push wind speeds into the 15-20 knot range in the Charleston
County waters and perhaps further south. Seas should average 2-4
feet.
Monday and Tuesday: A series of weak low pressure systems will
track just south and east of the waters, producing moderate
northeast winds and periods of light rain/showers. Current model
strengths of the low centers look too weak to produce any
highlights, but winds could still be up to 15 to 20 knots at
times, with seas 3 to 5 feet, highest offshore.
Tuesday Night through Wednesday Night: High pressure builds
back over the area with northeast winds 15 knots or less and
seas 3 to 4 feet.
Thursday and Friday: Still no highlights as high pressure
pushes northeast of the waters, with winds veering to southeast
15 knots or less.
&&
.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.
&&
$$
Area Forecast Discussion
National Weather Service Quad Cities IA IL
725 PM CDT Sun Apr 8 2018
...AVIATION UPDATE...
.UPDATE...
Issued at 633 PM CDT Sun Apr 8 2018
Radar, reports, and HRRR data all support the mesoscale band of
moderate to heavy snow persisting another few hours in southeast
Iowa and west central Illinois. Under that band, 1 to 2 inches has
already fallen in the past 2 hours, and likely another 1 to 3 will
fall. I have extended the winter weather advisory through west
central Illinois to cover this band of higher accumulation.
Temperatures have already crashed to the upper 20s to lower 30s,
so slushy roads are already being reported in that area.
Otherwise, though not cancelling during snow, the amounts in the
northeastern portion of the advisory appear too high, and that may
result in only 1 to 2 inches in Cedar/Jones/Delaware counties for
this event.
&&
.SYNOPSIS...
Issued at 330 PM CDT Sun Apr 8 2018
Shortwave is shifting across the Northern Plains this afternoon.
Strengthening warm advection/isentropic ascent has resulted in a
broad band of snow extending from around Independence through Iowa
City to near Burlington moving east. Various webcams show
accumulation is occurring on grassy and elevated surfaces within
this band, mainly along and north of I-80 where intensity has been
moderate and decent dendritic flakes. Roads appear to be just wet as
pavement temps are running well above freezing this afternoon. The
aforementioned shortwave will traverse east-southeast over the next
24 hours and continue to bring likelihood for some snow, and
potentially drizzle/freezing drizzle Monday as the column dries out
aloft.
&&
.SHORT TERM...(This Evening through Monday)
ISSUED AT 330 PM CDT Sun Apr 8 2018
Tonight...Assessing several of latest model runs and comparing to
what`s occurring upstream, seems incoming warm air advection wing of
snow(with some rain mix on the southwest flank) may be mainly east
of the area by 03z-04z. Incoming convergent axis of LLVL jet and
enhanced upward omega strip, suggests an enhanced band of snow
accums of 1-2" possible by mid evening from just north of Sigourney
IA, southeastward to Burlington and Macomb IL. Most other areas to
get a half inch to and inch through midnight mainly on elevated and
grassy surfaces, except northwestern IL which is being impacted by
dry east-southeast boundary layer fetch any may not even get any
snow until 8-9 PM...sharp gradient drop off in precip to the
northeast. A lull of flurries or drizzle acrs much of the area
generally from 9-10 PM through at least Midnight, before weakening
upper wave/trof progresses toward the CWA from the west.
After Midnight...expect an uptick in snows again as the upper wave
moves in from the west, like mentioned above. This wave may produce
up to another inch or two generally along and north of the I80
corridor, and west of the MS RVR by 12z Monday...areas east of the
river 0.5 to 1 inch by sunrise. Loss of ice crystals, lower liquid-
to-snow ratios may foster more rounds of drizzle/freezing drizzle
mixing in with less intensity wet snow or flurries acrs the southern
third of the CWA late tonight, making for lower snow accums of a
half inch or less in these areas. But additional shifts will have to
watch out for another southward pull of the upper low and it`s
associated main lift/forcing axis...which could actually place a
higher snow amount band during the 06z-12z window from southwest of
Iowa City, to east of Burlington again. The latest available run of
the ESRL HRRR suggests this. Lows tonight mainly in the upper 20s
acrs most of the CWA...around 30 in portions of the south.
Monday...Additional light snow accums generally of a tenth or two
acrs the northern 2/3`s of the CWA through mid morning...before low
to mid layer drying may support more of a drizzle or freezing
drizzle regime where lift persists. Expect mainly marginal sfc temps
diurnally for much of any freezing drizzle impacts on traveling
surfaces Monday morning. Any lingering light precip should become
mainly regular drizzle or spotty light rain in steepening low level
lapse rates and sfc temps increasing into the upper 30s to low 40s
by afternoon. All in all, with dry air to overcome/much of initially
lift going into saturation, and marginal sfc temps...storm total
snow amounts acrs most of the CWA to range from 1-3 inches, lower in
northwest IL. Will expand the ongoing advisory a tier of counties
south, with the southward trend in forcing/lift regime of both the
WAA wing this afternoon/evening, and the overnight upper wave. Will
continue to advertise the Advisory areas getting a total range of 2-
4 inches, but lowered confidence in these locations getting toward
the higher end of that range. ..12..
.LONG TERM...(Monday Night through next Sunday)
ISSUED AT 330 PM CDT Sun Apr 8 2018
The main story or emphasis is the transition to a warmer pattern
with at least a couple days of potential to be above to much above
normal.
Monday night-Tuesday will remain seasonably cool and dry with high
pressure in control.
Tuesday night into Wednesday... This is the first transition toward
much warmer conditions, as high pressure shifts east and southerly
flow strengthens. Temps will likely level off in the evening and
then rise overnight right on through the afternoon Wednesday. Will
have to watch the Wednesday afternoon time for fire weather concerns
with the gusty south winds and lowering RH. An upper level
disturbance is shown to move across the area mainly to our north
Wednesday night and will bring an attendant chance of showers and
possibly a few storms with strengthening nocturnal low level jet and
moisture transport.
Resultant cold front from the upper level disturbance will settle
in/near the area on Thursday, and from here leads to considerable
uncertainty and challenges with high temps both Thursday/Friday
dependent on the location/movement of the boundary. Potential for
well above normal warmth with 70s to near 80 in the warm surge south
of boundary. Meanwhile, north of this boundary will be much cooler
in the 40s/50s. Location of the front will hinge some on whether any
easterly fetch and lake enhancement off Lake MI occurs. This time
period of late week also offers potential for strong convection
in/near the area with right now seemingly more of a consensus on
this being to our south/east. Nonetheless will be a period to watch.
Then next weekend it appears to be cooler and more seasonal in wake
of departing low and cold front.
&&
.AVIATION...(For the 00Z TAFS through 00Z Monday Evening)
ISSUED AT 647 PM CDT Sun Apr 8 2018
Ongoing snow across the area this evening will continue into the
overnight hours. KDBQ was observing MVFR conditions, but
conditions have been deteriorating as the snow has caused visibilities
and ceilings to drop. More snow and IFR to LIFR conditions will
be seen through the evening, especially in areas south and west
of KDBQ, impacting KCID, KMLI, & KBRL. A fairly narrow band of
heavy snow is expected to continue through at least mid evening,
however may not hit any of the terminals. By 04Z, snow should be
becoming light in all areas, but visibilities will stay low with
fog setting in. Precipitation may change to drizzle on Monday
morning, and if temperatures haven`t risen above freezing could
get some patches of light freezing drizzle. Amounts should be very
light though so don`t expect a large impact from this. After 18Z
on Monday visibilities and ceilings will be improving, but likely
to stay at least MVFR with through the majority of this forecast
cycle.
&&
.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Winter Weather Advisory until 10 AM CDT Monday for Benton-
Buchanan-Cedar-Delaware-Des Moines-Henry IA-Iowa-Jefferson-
Johnson-Jones-Keokuk-Linn-Louisa-Muscatine-Washington.
IL...Winter Weather Advisory until 10 AM CDT Monday for Henderson-
Mercer-Warren.
MO...NONE.
&&
$$
UPDATE...Ervin
SYNOPSIS...McClure
SHORT TERM...12
LONG TERM...McClure
AVIATION...Brooks
Area Forecast Discussion
National Weather Service Sioux Falls SD
652 PM CDT Sun Apr 8 2018
.SHORT TERM...(This evening through Monday)
Issued at 407 PM CDT Sun Apr 8 2018
Vigorous storm system continues to move eastward. Surface low
pressure is currently located in central NE, with a strengthening
upper short wave moving into northeast SD. The mid and upper level
QG forcing is rapidly exiting our eastern zones early this evening
in front of this wave. However PV forcing in the 700-600mb layer
remains strong behind the QG forcing across the northeast half of
our forecast area all evening long. This helps to keep deep layer
saturation going for those locations, with much drier mid levels
heading toward the Nebraska border which currently are causing a
light wintry mix. The 850mb low off of the RAP13 is projected to
head toward Yankton SD by 01Z and 02Z this evening, from its current
location near Chamberlain. Therefore a constant low level feed of
moisture is supplied ahead of this low using the 850mb streamlines,
and water vapor imagery also showing a feed in the upper reaches of
the atmosphere from the southwest. The radar mosaic is filling in
nicely across east central and northeast SD as the 850mb low tracks
southeast. Therefore expecting another two to three inches of snow
over the headlined area this evening.
Thought about changing some of the winter weather advisory to a
winter storm warning but there are some concerns. First, the
frontogenetic part and instability of the event is largely gone.
Therefore not sure if rates will be sufficient enough to produce a
widespread 8 inches of snow or so. In the headlined area, we already
have a lot of 5 to 7 inches of snow forecast, so not sure if the
message radically changes between these amounts and 8 inches of
snow, especially noting that half of the snow depth will be gone
by Monday due to melting and compaction. The HRRR and HRRRx continue
to show a small heavier band of snow very late this afternoon and
early this evening in extreme southeast SD and extreme northwest IA.
So there may be some enhanced amounts just south of KFSD if those
solutions pan out. At any rate, that will be the main challenge in
the next few hours is worrying about headline changes. As the next
round of snowfall moves southeastward, the freezing drizzle/light
sleet threat should recede through the early evening hours. Although
the threat for a light wintry mix overall will not totally end in
our southern zones.
The wave will slowly exit our eastern zones overnight. Light snow
and a light wintry mix remains a threat in our eastern zones well
into the overnight and late night hours. RH time sections keep the
area pretty cloudy with stratus. Therefore despite the new snow
cover over many locations, opted to not tank lows too much. Mainly
used raw model consensus values. On Monday, lowered highs a few
degrees over many of the snow covered areas which will produce
another way below normal day.
.LONG TERM...(Monday night through Sunday)
Issued at 407 PM CDT Sun Apr 8 2018
The upper flow becomes more quasi-zonal for the mid week which
ushers in more of a Pacific air mass. On Tuesday, continued to lower
highs a couple/three degrees across the northeast half of the
forecast area. However with a southwest wind, raised highs over
consensus values in our southwest zones to give readings in the 60s
across our south central SD zones. The temperature range could be
very large from southwest to northeast across our CWA on Tuesday.
A quick moving short wave should pass harmlessly by on Wednesday.
But this wave will help to produce very mild conditions south of it
across our southern zones. With 850mb temperatures projected
to be +10C to +12C on the ECMWF in our far south, raised maximum
temperatures on Wednesday to 65 to 70 degrees in our zones from
south central SD to near Highway 20 in northwest IA.
All eyes are then trained to the very strong system exiting into the
plains late this week in the Thursday night through Friday night/
Saturday time frame. Obvious timing differences exist in the
deterministic solutions with the GFS being much quicker then the
ECMWF. But latitude placement wise, the models continue to trend on
the cool side overall in this forecast area. The one exception may
be late Thursday night and early Friday in our eastern zones who are
temporarily in the warm sector - but are also largely dry slotted to
the south of an occlusion at that point, at least on the ECMWF.
Taken literally, another snow event is the offing on the backside of
the low but will continue to monitor. It appears next weekend will
be very much below normal again in terms of temperatures.
&&
.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 651 PM CDT Sun Apr 8 2018
Snow will continue north and east of KFSD during this evening,
with an additional inch or two of accumulation. The best forcing
has passed KHON, so they are likely done with accumulating snow.
But KFSD and KSUX will likely both have several more hours of
freezing drizzle mixed with light snow. An additional light glaze
should be expected for these terminals. By 09/0400z, any drizzle
looks to become very light, perhaps more of a mist. By the early
morning, fog may develop during the morning hours as high pressure
builds in behind this system. Expect areas of fog through the
morning, perhaps even into the afternoon as moist, relatively stagnant
air remains over the region.
&&
.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory until 1 AM CDT Monday for SDZ038>040-
053>056-061-062-066-067-071.
MN...Winter Weather Advisory until 7 AM CDT Monday for MNZ071-072-080-
081-089-090-097-098.
IA...Winter Weather Advisory until 7 AM CDT Monday for IAZ001>003-
012>014-020>022.
NE...None.
&&
$$
SHORT TERM...MJ
LONG TERM...MJ
AVIATION...VandenBoogart
Area Forecast Discussion
National Weather Service Wilmington NC
1103 PM EDT Sun Apr 8 2018
.SYNOPSIS...
Low pressure will develop along the Gulf Coast along a stalled
front tonight, then will move near the Carolina coast Monday.
Widespread rain on Monday could linger into Tuesday, along with
continued chilly temperatures. The low will move out to sea
Tuesday night and Wednesday as high pressure builds in from the
west. This high will move offshore Thursday, with much warmer
temperatures expected through the weekend. Wet weather may
return Sunday ahead of the next cold front.
&&
.NEAR TERM /THROUGH MONDAY/...
As of 1030 PM Sunday...Minor changes to account for temperature
curve and cloud cover, and also to slow precip onset overnight.
Zonal flow aloft is south of a subtropical jet moving through
the Mid-Atlantic. Beneath this jet, high level moisture from the
Pacific is streaming overhead as high level cloud cover, and
this is "saving" the area from frost/freeze conditions tonight.
Despite rapid cooling earlier, temperature curves are leveling
off as the clouds thermally insulate the ground from longwave
cooling. This will keep mins from falling much below 40 except
in the coldest sheltered locations of Bladen and Pender
counties, with low 40s expected across most of the area.
The other concern tonight is onset of precip as well as areal
coverage before daybreak. Weak impulses within primarily zonal
flow will spawn a low pressure off the GA coast late tonight
which will then lift NE into Monday. Isentropic lift atop the
surface high combined with forcing in the vicinity of the low
and onshore flow will cause the column to saturate, and showers
will develop from south to north towards dawn. Latest RAP
soundings and HRRR output suggest a much slower development of
precip than previously forecast, so have slowed the timing, but
still expect widespread light rain beginning well after
midnight, and increasing into Monday. Relevant portion of
previous discussion below:
As of 300 PM Sunday...A quick moving shortwave moving through
the Tennessee VAlley will crank up isentropic lift around the
290k surface early Monday morning. This will first increase
clouds late tonight with light rain shortly thereafter. We are
advertising categorical pops through most of the day Monday but
overall rainfall amounts should be light. Highs Monday will be
in the middle 50s.
&&
.SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/...
As of 300 PM Sunday...Low pressure off the coast not to make much
progress through most of the period. As such it will be hard to get
rain completely out of the forecast for much of the time, though
expected amounts will be negligible. The final wave of energy aloft
finally swings through Tuesday night and the surface low finally
accelerates off to the northeast. This will mark the beginning of
dry weather. Temperature-wise both nights will be seasonably cool
thanks to cloud cover whereas these same clouds will keep Tuesday
highs below climatology.
&&
.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
As of 300 PM Saturday...The upper trough responsible for the
chilly weather the first half of this week should finally push
offshore late Wednesday. Moisture trapped beneath a growing
subsidence inversion between 8-10kft should keep skies at least
partly cloudy Wednesday, maybe even mostly cloudy at times as
shown on the GFS. Highs should only make it into the mid-upper
60s.
Surface high pressure across North Carolina on Wednesday should
push offshore Thursday. The 12z GFS is the fastest of all model
guidance with the departure of the high, and I`m delaying the
onset of southerly return flow until Thursday, in line with the
bulk of model guidance. With lingering marine influence along
the coast I`m also trending toward the cooler side of high
temperature guidance on Thursday along the coast with upper
60s/lower 70s contrasting with upper 70s farther inland. A flat
upper ridge will build across the Southeast Thursday and
Friday, building more strongly as it pushes offshore Saturday
and Sunday. Southwesterly winds should bring much warmer air
into the Carolinas late in the week with inland highs expected
to reach the lower 80s Friday and Saturday. Healthy seabreeze
circulations should develop each afternoon Thursday through
Saturday with onshore winds increasing by 5-8 mph above the
background synoptic wind.
The GFS is again the fastest of all available guidance with the
eastward progression of a strong trough over the lower
Mississippi Valley region Saturday night into Sunday. A deep
plume of tropical moisture lifted northward ahead of this
feature will move across the Southeast states, probably reaching
the Carolinas on Sunday. I`ll put Sunday`s PoPs at 50 percent
for now. Model ensembles show this could be a significant rain-
maker for the Carolinas early next week with continued slow
movement of the upper level system beyond our extended forecast
period.
&&
.AVIATION /03Z MONDAY THROUGH FRIDAY/...
As of 00Z...A low pressure area will move northeast along a
frontal boundary located offshore this TAF valid period. VFR through
12Z with lowering cigs and light winds becoming northeast by
daybreak. Light rain will gradually overspread the terminals as
cigs lower to MVFR shortly after daybreak. IFR cigs appear most
likely around 15Z KCRE/KMYR, and around 17Z at the remaining
terminals. Vsbys should remain VFR as rain should remain light.
Rain tapers off west to east by the end of the TAF valid period,
but cigs remain IFR.
Extended outlook...Mon night-Tue, IFR/rain. Wed-Fri VFR.
&&
.MARINE...
NEAR TERM /THROUGH MONDAY/...
As of 1030 PM Sunday...No significant changes with the evening
updates other than to drop the SCEC early. Very light winds will
persist across the waters overnight, with direction slowly
becoming easterly and then increasing to 10-15 kts towards
morning as weak low pressure develops. Seas of 1-2 ft will build
late to 2-3 ft, and then further on Monday. Relevant portion of
previous discussion below:
As of 300 PM Sunday...Northeast winds veer to east overnight
and increase as low pressure develops to the west and moves
rapidly east. Speeds increase to 15-20 back from the northeast
Monday afternoon. Seas will increase possibly into small craft
criteria late Monday.
SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/...
As of 300 PM Sunday...A nearly stalled area of low pressure will be
off the coast Monday night into Tuesday. The gradient may become
sufficiently pinched to necessitate Small Craft Advisory. If so then
these flags may be lowered Tuesday as the low weakens and/or starts
moving to the NE, though a period of SCEC is tough to rule out.
Conditions continue to abate Tuesday night as high pressure builds
in from the NW.
LONG TERM /WEDNESDAY THROUGH FRIDAY/...
As of 300 PM Sunday...High pressure across North Carolina on
Wednesday will slowly move off the coast late Wednesday night
and Thursday. The GFS model is the fastest of our available
model guidance with the movement of the high; virtually all
other models are 6-12 hours slower. Northeast winds should veer
around to the south during the day Thursday, and veer to south-
southwest on Friday as the high moves farther away from the
coast. Also by Friday look for the reappearance of our typical
afternoon seabreeze, with nearshore wind speeds becoming 5-8
knots higher than 10-20 miles out.
&&
.ILM WATCHES/WARNINGS/ADVISORIES...
SC...None.
NC...None.
MARINE...None.
&&
$$
SYNOPSIS...TRA
NEAR TERM...JDW/SHK
SHORT TERM...MBB
LONG TERM...TRA
AVIATION...MRR
MARINE...
Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
732 PM CDT Sun Apr 8 2018
.UPDATE...
730 PM CDT
No major changes are planned to the forecast tonight. We are
monitoring moderate to even heavy snow across eastern Iowa across
western Illinois, but a gradual weakening with eastward movement
is expected with this. While some of these rates could clip
Dixon to Pontiac to Watseka before weakening, the potential
accumulations of 1" to possibly 2" in that area from 9pm-4am
still are on track and look to have minimal impacts. Less snow
remains forecast further north and east.
Water vapor imagery early this evening indicates the primary
closed wave across central/southern MN with a couple attendant
waves to its south. One of these is enhancing the 850-700mb warm
advection and ascent across eastern IA and western IL and is
responsible for the modest snow. Multiple locations have gone down
to 1/2SM visibility within the first hour of snow, including
Galesburg, Peoria, Lincoln, and Springfield. While strong mid-
level frontogenesis is moving over the forecast area, lower very
dry air with temperature/dew point depressions of 15 to 30 degrees
and lack of a low-level jet per the 00Z DVN sounding are keeping
this only virga over the CWA as of 730 pm.
Snow will gradually inch its way eastward with warm advection
aloft, reaching near the I-39 corridor by 9 pm and to the western
and southern Chicago suburbs by midnight. Again while rates may be
occasionally moderate, especially in the far western and
southwestern CWA, much of the snow is expected to be light with
any accumulation mainly on grassy or cooler surfaces.
The primary wave presently across MN will begin to dampen as it
loses jet support and reaching eastern Wisconsin by daybreak.
Light snow gradually diminishing in coverage is anticipated
between 6-10 am in the morning. Impacts to the morning commute,
especially in Chicago, are expected to be minimal with mainly
occasionally reduced visibility and wet possibly some minor
slushy side roads.
MTF
&&
.SHORT TERM...
259 PM CDT
Through Monday...
Main concern for the short term forecast period will remain the
unseasonably low temperatures and the snow amounts for tonight into
tomorrow.
While the models are in relatively decent agreement on the larger
scale pattern and potential for snow tonight and into tomorrow,
there remains a bit of uncertainty as to the timing and strength of
individual shortwaves, which could, ultimately, impact the timing of
the periods of more impactful snowfall. The first complicating
factor is the very dry air mass across the CWA. Sfc dewpoints range
from single digits to middle teens this afternoon as temperatures
top out in the upper 30s to lower 40s. A lake breeze that pushed
inland earlier this afternoon has kept the immediate lakefront a bit
cooler. Also, the day started out with ample sunshine which would
allow road surface to warm up some, though increasing cloud cover
through the afternoon will limit the full potential for road surface
warming. So, combining these factors, expect that the onset time of
precipitation will be a little later than previously anticipated to
give time for top-down saturation to occur and for pcpn to reach the
ground. Temperatures should still be well above freezing at the
time that pcpn starts, so a short period of rain is likely before
changing over to a rain/snow mix and then a change to all snow for
the late night and early morning hours. The latest RAP
moisture/temp/UVV profiles indicate that strongest forcing will be
in the the -10 to -15C range, on the lower end of the most favored
DGZ, so snowfall rates may not be quite as high as could be expected
with a more favorable DGZ. Also, low level thermal profiles will be
somewhat marginal as well. So, in general, expect a period of wet
snow, with highest amounts generally over the swrn portions of the
CWA, including the Greater Pontiac Metro area, which could see an
inch or so of wet snow. The northeastern portions of the CWA,
including the Chicago Metro Area should see around a half inch.
Given the relatively warm road surfaces, much of the accumulation
will likely be on raised or grassy surfaces. Temperatures tomorrow
will remain much below normal, with highs only in the upper 30s
along the lake, where flow will remain onshore, to the lower 40s
inland.
&&
.LONG TERM...
156 PM CDT
Monday night through Sunday...
The main weather story during the period will be the likely shift
towards at least a couple days of above normal temperatures mid
through late week.
Surface high pressure is forecast to shift over the area for
Tuesday, which should result in a dry but continued cool day
across the area. However, things begin to change Tuesday night
into Wednesday as a mild spring airmass begins to shift into the
area. This will occur as the lower-level flow turns southwest on
the back side of the surface high and as upper level heights
begin to rise in response to a western U.S. upper ridge shifting
eastward over the central CONUS. Breezy southwest flow over the
area on Wednesday looks to result in a quick response to
temperatures, with 925 mb temperatures expected to warm to around
+10C, which local climo for this time of year suggests would
support highs around 60.
A fast moving upper level disturbance is progged to shift eastward
late Wednesday and Wednesday night atop the broad upper ridge
over the central CONUS. This is expected to induce an uptick in
the low-level jet over the region Wednesday night. As this occurs,
it may induce a period of showers and perhaps some thunderstorms
across northern portions of the area Wednesday night. Otherwise,
gusty southwest winds will continue Wednesday night, so overnight
lows are likely to be quit mild.
The real warmth for the area looks to be late in the week. During
this period a large upper trough will shift eastward into the
plains, essentially inducing the development of a potent surface
low over the plains and into the Upper Midwest by Friday. This
surface low is likely to send a warm front northward over the area
late Thursday. A good surge of warmer air in the wake of this
front may support temperatures well into the 70s (~80 ???),
especially on Friday. Thursday also looks to be a mild day,
however, there are some questions as to if the winds will try to
shift easterly off the lake across northern Illinois in advance of
the northward moving warm front. If this occurs, areas near the
lake will likely be significantly cooler than areas farther
inland.
Will also be some low end chances for showers and storms within
the vicinity of the warm front later in the week, but at this
time it appears the main threat of showers and storms may set up
north of the area by Friday. Depending on the actual timing of the
storm system entering the Upper Midwest on Friday, it will shift
a cold front across the area sometime Friday night into Saturday
morning. This would likely be or best chance to get a few showers
and storms before colder air begins to return to the area in the
wake of this frontal passage next weekend.
KJB
&&
.AVIATION...
For the 00Z TAFs...
A band of snow has crossed the Mississippi River, but in between
the snow and terminals is a lot of dry air. Dew points are slowly
climbing indicating moistening air, but the arrival of snow will
be slow thanks to the dry air. In addition, models are showing
that the snow will be weakening as it moves over the region, so I
have low confidence in how long snow will occur.
East winds diminish this evening and light snow should start
falling at the terminals between late this evening and midnight.
MVFR cigs should accompany the snow, and I have low confidence in
IFR or lower cigs. Models show a few stronger snow showers early
Monday morning, so I left 2SM visibilities in the window of
stronger snow showers. Steady snow itself may come to an end
quicker than currently forecast, but we may have snow showers
through Monday morning.
Some models indicate additional light rain showers may form Monday
afternoon, but I have very low confidence in occurrence and
coverage. Therefore, I put a VCSH in the TAFs to account for the
possibility of additional precipitation. Cigs should become VFR
Monday afternoon, and light northeast winds will back to north and
perhaps north-northwest late Monday evening.
JEE
&&
.MARINE...
203 PM CDT
Surface high pressure is currently in place across the Great Lakes
Region, and this is producing fairly light and variable winds
across the lake. Overall, expect fairly quiet conditions on the
lake for the next couple of days before the winds begin to
increase out of the southwest late Tuesday and Wednesday. A quick
moving area of low pressure is forecast to shift eastward over the
lakes region Wednesday night into early Thursday, so a period of
southerly winds of 15 to 25 kt looks likely for Wednesday,
followed by at least a short period of winds approaching 30 KT
Wednesday night. This will likely result in the need for a small
craft advisory for the IL and IN near shore waters.
A stronger low will take shape in lee of the Rockies late in the
week (Thursday night time frame) and then slowly move east as
strong high pressure holds across central Canada. This will allow
a sharp frontal zone to setup across the lake with ENE winds north
and southerly winds south. Depending on the strength and path of
the low, gales are a possibility even though the low should be
weakening as it shifts eastward.
KJB/KMD
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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