Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/21/20
Area Forecast Discussion
National Weather Service La Crosse WI
521 PM CST Sun Dec 20 2020
.SHORT TERM...(This evening through Monday)
Issued at 215 PM CST Sun Dec 20 2020
The main forecast problem for this period is accumulating snow
tonight and stronger winds on Monday. A significant but fast-moving
shortwave trough and unstable air mass will shift across the area
tonight. GOES Water vapor imagery shows the Pacific wave-train from
yesterday now extending from Bismark, ND west-northwest into British
Columbia. This energy aloft is combining with a developing N-S warm
front in the Dakotas to cause enhancing radar echoes and rain/snow
observed at the surface. This is the system shifting through the
area overnight with 7.5/8 C/km lapse rates in the 500-700mb layer,
about 50 J/Kg of elevated CAPE, moderate QG forcing through the
depth of the troposphere, and strong but narrow warm advection in
the 925-850 mb layer per latest RAP forecasts. With the main
shortwave and surface low north of the area, it appears duration
will be a limiting factor in snow accumulations /lack of a
deformation area/. The CAMS agree well the N-S band of
precipitation moving swiftly through, possibly 2 hours in
duration, between 10pm and 3 am. That said, it will likely be a
quick, convective 0.10" of liquid equivalent, less south of I-90
where saturation may be an issue in the low-levels as dynamics
drop off. It appears all snow in WI and a mix of rain/snow and
possibly a bit of sleet south of I-90 depending on temperatures as
they warm overnight. Believe the slippery roads in 1-1.5" of snow
would mainly be I-94 corridor and north. This is similar to
previous forecasts.
As Monday progresses, the boundary layer deepens and steep lapse
rates will cause enhanced mixing under low-level cold advection. Low-
level wind speeds ramp up areawide, but look to possibly bring more
impact west and south of La Crosse. The 20.12Z HREF and 20.00Z NAEFS
850mb mean wind is 45-50 kts along and west of the Miss river and
the RAP seems to suggest the same. The 20.00Z ECMWF Extreme
Forecast Index is saying this is an unusual event in about 75-85%
of the ensemble members and targeting the same area. We have increased
winds there to approach Wind Advisory levels (30 mph, gusts to 45
mph). A Wind Advisory may be needed for Monday in southeast MN
and northeast IA. With the steeper lapse rates, have added
isolated snow/rain showers to the forecast for WI and sern MN
where the deepest PBL saturation and instability exists. The snow
shower intensity should not be to the level to cause impacts.
Will increase the wind messaging for Monday, but also try to
target a ~2-3 hour burst of snow overnight versus a ho-hum 1".
.LONG TERM...(Monday night through Sunday)
Issued at 215 PM CST Sun Dec 20 2020
Still not sure we have made a measurable increase in the
predictability for the middle of the week...mainly Wed-Thu. The
ensemble and deterministic models still have considerable spread.
However, the big picture remains: deepening low pressure in the
region and significant shortwave energy moving through, a
pronounced drop in temperatures after a warm up Tue/Wed, and
periods of breezy to windy conditions. The 20.00Z ECMWF ensemble
shows a majority of members with light snow much like the 20.12Z
deterministic run - for Wednesday and Wednesday night. Overall,
with such spread among many model platforms, will focus our
message on the aforementioned bigger picture items until the
precipitation type and amounts, if any, come into better
predictability. It is still in the "buyer beware" phase.
Friday through Sunday. A quiet period with a rapid rebound to
near normal temperatures as zonal flow takes over across the
CONUS. Seems there may be some indications of weather by late
weekend.
&&
.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 521 PM CST Sun Dec 20 2020
Precipitation mainly in the form of snow to a wintry mix will
quickly move into and through the area tonight. Brief reductions to
IFR conditions are possible around 06z at KRST and between 06 and
09z at KLSE. Snowfall accumulations are expected to be light and
minimal, but the quick burst of snowfall could drop visibilities to
near 2 SM and ceilings in the 600 to 900 ft AGL range. LLWS threat
is possible between 10z and 13z, but did not include in the TAFs at
this time as surface winds would be strong enough by early tomorrow
morning to mitigate this potential impact. Winds then increase from
the northwest with gusts from 25 to 35 knots then lasting through
much of the day. MVFR ceilings are expected to persist over the
region through the afternoon, with a few possible snow showers. They
will be cellular in nature and scattered across the region, so was
not confident enough to include in the TAFs at this time. Any
passing snow shower could lead to brief visibility reductions.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&
$$
SHORT TERM...Baumgardt/Wetenkamp
LONG TERM...Baumgardt
AVIATION...Peters
Area Forecast Discussion
National Weather Service Green Bay WI
549 PM CST Sun Dec 20 2020
Updated aviation portion for 00Z TAF issuance
.DISCUSSION...Tonight Through Tuesday Night
Issued at 255 PM CST Sun Dec 20 2020
--Key Messages--
1.) Monday: Two rounds of widespread light precip expected that may
lead to minor travel impacts across the area.
2.) Wed-Thu: Potential for a mixed precip wintry weather system
that may bring strong winds to Wisconsin.
3.) Thu-Fri: Colder temperatures expected across Wisconsin.
--General Discussion--
Monday: Confidence remains high for a wintry weather system to
impact Wisconsin. The first round of snow will likely arrive
overnight, beginning around 2 am or so in Wood County, and moving
west throughout the morning hours, arriving in the Fox Valley
around 5 am or so. Snow amounts with this first band should be
near to under 1 inch, and impacts will mostly be visibility
reduction to 1-3 miles.
The second round of snow will come as the core of this system pushes
across wisconsin from NW to SE throughout the daytime. Total
snowfall amounts will be light- around 1 inch or less- and impacts
will, again, be reduced visibility to 1 to 3 miles during periods
of snow/flurries. During the afternoon, precipitation type may
become mixed.
Wed-Thu: Forecast confidence remains generally low across the board
on the Wed-Thu system. Like the Monday system, we may see widespread
light, potentially mixed wintry precip. One notable change from
previous days is the potential for increased winds behind this
system Wednesday afternoon/overnight. Will watch this closely, as
any light snow/flurries that are accompanied with these strong winds
will lead to reduced visibility.
Thu-Fri: Confidence is increasing in a fairly short-lasting slug
of cold air immediately behind this system. The aforementioned
potentially strong winds may play a factor in wind chill values
Thursday and Friday, especially during the morning hours.
Temperatures may make it down to the single digits Thursday and
especially Friday morning. If wind speeds remain elevated, wind
chill values Friday morning may approach the -10 to -20 degree
range across northern/north-central Wisconsin, which would
certainly be the coldest air of the season thus far. By the
weekend, however, it looks like temperatures will rebound back
into the 20s, at least. So this will not be a prolonged cold spell
of below normal temperatures.
--Technical Discussion--
Mon: GOES-R imagery starting clearly picking up on the shortwave
mentioned throughout the weekend being located on the Canadian side
of North Dakota. The HRRR/ESRL-HRRR seem to have been handling this
system the best throughout the Saturday morning hours as this
shortwave has been journeying across the Rockies... so have leaned
towards its evolution. As it nears Wisconsin, models continue to
show decently good agreement with an organized line of 300-500mb Q-
vector convergence oriented form NW-SE propagating across our CWA
from 00z Mon-04z Mon, with precip trailing shortly behind...
suggesting a banded orientation. Of note, hi-res models converging
on a pocket of stronger low-level frontogenetical forcing over
central to south-central WI that precedes a pocket of low-level
moisture flux convergence. This, paired with slightly steeper
lapse rates, suggests this band is worth keeping an eye on as it
makes its way across the state. The HRRR family has been trending
this potentially stronger pocket of snow along the band further
south and keeping most of it in the ARX/MKX CWA. But there is
certainly a chance that it may clip Wood to Waushara counties.
In a change from previous forecasts, have boosted wind speeds for
Monday in the western portions of our area into the 15 to 25 kt
range as the pressure gradient has tightened up to 3 mb/100km.
Wed-Thu: Long-range models continue to diverge on the handling of
this system. Interestingly, the 12z ECMWF appears to be heading
towards the GFS camp- which has remained fairly consistent with its
output. Specifically, the 12z ECMWF now has more emphasis on
highlighting the northern low in this system, versus having the
southern stream portion "break off" into a closed low near the
Ozarks. At this point, it is not worth digging too much into
mesoscale details, since there is still so much potential for
variability... that, and the fact the synoptic scale is not very
resolved. Certainly noteworthy is that both camps tend to bring a
huge slug of dry air all the way into canada through much of the
day Wednesday- hammering the ice introduction layer. Further,
forecast sounding cloud thicknesses struggle to get much beyond
5kft though much of Wednesday. This would initially point towards
more dz/fzdz through much of Wednesday. Any lingering precip
should --> all snow fairly quickly Wednesday night when the cold
air advances over our area.
The 12z GFS solution in particular points at very strong winds
behind this system as it drops the sfc pressure to ~987 mb, while
pushing a ~1025 mb sfc high along the North Dakota/Canada border
behind the main low. Should this scenario pan out, the potential
for strong winds, up to Wind Adv. criteria will need to be
monitored. With cold air coming in behind this, wind chill values
could easily drop to the -10F range Thursday/Friday. Again,
confidence not high enough to lock onto this evolution yet.
&&
.AVIATION...for 00Z TAF Issuance
Issued at 549 PM CST Sun Dec 20 2020
Variable cloudiness is expected this evening with scattered to
broken clouds between 1000 and 2000 ft. An Alberta Clipper system
will bring IFR conditions and snow to the area towards daybreak or
shortly thereafter. Snow accumulations or an inch or two are
expected in most area, with the snow tapering off by Monday
afternoon. IFR ceilings will likely continue Monday afternoon and
slowly rise to MVFR Monday night as drier air arrives behind a
weak cold front. &&
.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
DISCUSSION.....Kotenberg
AVIATION.......RDM
Area Forecast Discussion
National Weather Service Houston/Galveston TX
532 PM CST Sun Dec 20 2020
.AVIATION [00Z TAF Issuance]...
Not much change made to 18Z TAFS. With light winds and clear sky
skies think conditions right for radiation fog as occurred last
night. HRRR vsby progs suggest may be more southern sites for the
more dense conditions...possibly CXO on south...but in any case
should start be shallow. Not much of a signal in the SREF guidance
but that may be due to shallow nature of fog. Will evaluate the
fog threat further for the 06Z issuance...and update as necessary.
18
&&
.PREV DISCUSSION /Issued 355 PM CST Sun Dec 20 2020/...
.SHORT TERM [Tonight through Monday Afternoon]...
High pressure over Central Texas will drift eastward through the
region setting the stage tonight for shallow fog to form under
favorable radiational cooling conditions by midnight. Then thicken
and may need dense fog advisories for the region after 3 am-ish.
The thick fog should dissipate between 830-930 am Monday. A very
pleasant day on tap Monday with light southwest and south winds
and a few degrees warmer with highs in the upper 60s far west to
lower to mid 60s east under sunny skies in the afternoon.
45
.LONG TERM [Wednesday Through Sunday]...
The warming trend will continue as we approach Christmas Day, though
the passage of a strong cold front on Wednesday will provide SE TX
with cool and generally pleasant conditions for the holiday
(however, we remain concerned about fire weather conditions...more
on that in a bit). With onshore flow continuing to promote warm and
moist advection, we should continue to see highs slightly above
seasonable normals on Tuesday with most locations seeing the upper
60s to low 70s. By Wednesday, persistent increases in moisture will
result in building cloudiness as a deepening surface low over the
Northern Plains/Great Lakes brings an associated surface cold front
through most of the central CONUS. There continues to be
discrepancies amongst the global deterministic models regarding the
timing of this feature`s arrival in SE TX. ECMWF & Canadian continue
to favor a less progressive scenario with the boundary arriving in
the northern counties later on Wednesday, while the GFS continues to
depict the front already clearing the coastal waters by midnight on
Thursday. The NAM solution is a relative middle ground between these
two camps. As such, while we expect showers and storms to develop
along and ahead of the front (though, less widespread and intense
than in the previous fropa due to reduced instability and a
relatively unfavorable position of the upper jet streak), the timing
of the precipitation is less certain. I have slightly hedged my PoPs
between these different scenarios given the continued uncertainty
and maintain marginal chances into the early hours of Thursday.
Northerly winds and clearing skies behind the front should prove
favorable for a cool and dry (albeit gusty) Christmas Eve and
Christmas Day. However, as echoed in the previous discussion by my
colleague on the midnight shift, we continue to monitor the
potential for elevated fire weather conditions in the wake of the
fropa. The 12Z ECMWF has backed off on the aggressive single-digit
dew points that its 00z counterpart depicted, though both the EC and
GFS still show very dry conditions on Thursday and Friday. We will
continue to monitor this situation closely in the coming days.
Onshore winds will return by early Saturday as surface high pressure
drifts eastward and another period of slightly warmer and more humid
conditions should begin next weekend. With global models placing the
area in the favorable left front quadrant of the dominant 300mb jet
streak during this time, the potential for showers and storms will
return on Saturday and Sunday.
Cady
.MARINE...
Winds relaxing overnight as high pressure drifts through SETX
tonight/Monday and into LA Tuesday. Southerly flow becomes the
dominating issue Tuesday and dewpoints rise with windspeeds on
the increase and Tuesday evening may need SCEC flags for
southeasterly 15-20kt winds and seas building to 4-6 feet going
into Wednesday. Strong cold front will be racing southeast and
should cross the coast early Wednesday evening with strong CAA
likely bringing wind gusts of at least 35 knots to the coastal
waters and may peak closer to 45 knots. May need to hoist a Gale
Watch with the Monday evening marine package. Winds start to let
up during the afternoon Thursday but will still have a strong
offshore flow into Thursday evening. Thunderstorms possible but
iffy over the coastal waters with the frontal passage.
45
.FIRE WEATHER...
Very dry air will stream through the area Wednesday night/Thursday
in the wake of the strong cold front. May reach elevated fire
weather conditions with RH values below 20 percent if current
dewpoints hold...and this is question with much lower dewpoints in
some of the guidance.
&&
.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL) 37 66 42 67 55 / 0 0 0 0 0
Houston (IAH) 44 64 44 66 55 / 0 0 0 0 0
Galveston (GLS) 51 61 55 65 61 / 0 0 0 0 0
&&
.HGX WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...None.
&&
$$
DISCUSSION...Cady
NEAR TERM...45
SHORT TERM...45
LONG TERM...Cady
AVIATION...18
MARINE...45
FIRE WEATHER...Cady
Area Forecast Discussion
National Weather Service Lincoln IL
1005 PM CST Sun Dec 20 2020
.SYNOPSIS...
Issued at 300 PM CST Sun Dec 20 2020
A fast moving disturbance tracking through the Upper Midwest into
the Great Lakes may bring a period of very light rain or light snow
to locations along and north of I-74 during overnight into early
Monday morning. Once the system departs, strong northwesterly
winds will gust 30 to 40 mph through Monday afternoon. Seasonably
mild temperatures will continue across the area through Wednesday.
Lows tonight will be in the lower 30s. Highs Monday will be in the
mid 40s to lower 50s.
&&
.UPDATE...
Issued at 958 PM CST Sun Dec 20 2020
A shortwave will push across Illinois over the next 6 to 9 hours.
Low level dry air will limit the potential of measurable precip,
especially since our area is cold enough for all snow in most
areas. The HRRR shows a fast forward motion of the forcing for
precip, which will also limit the chances of measurable precip.
Have included flurries/sprinkles in the forecast to trend down
with measurable rain/snow. The upstream obs show cloud bases at
7K feet at the lowest, which is on the high side for precip to
reach the ground. Time frame for light precip still looks to be
mainly between midnight and 6 am from west to east. That timing is
handled well in the forecast database already.
Cloud cover will limit diurnal cooling, so temps will likely be
nearly steady the rest of the night.
&&
.SHORT TERM...(This evening through Monday)
ISSUED AT 330 PM CST Sun Dec 20 2020
Mid afternoon surface map shows a weak frontal trof over central
IL/MO and passing thru dry with scattered to broken mid/high
clouds ahead of it in eastern IL and IN. Temps were in the 40s
over much of CWA at 3 pm, with Galesburg the cool spot at 38F.
Breezy sw winds 10-20 mph was giving wind chills in the low to mid
30s over parts of central IL. Aloft an upper level trof extended
from Ontario through AR with 580 dm 500 mb high near the CA coast.
Latest models take short wave energy over the northern plains near
the Canadian border quickly se into central/northern IL by sunrise
Monday, and shift east of IL Monday evening. Broken to overcast
mid level clouds over central and nw IA to spread se over CWA
during this evening. HRRR model also shows some low clouds
currently just south of IL lifting ne over eastern IL, east of
I-57 during late evening and overnight, and some fog possible by
overnight near the Wabash river valley. A brief period of light
rain/snow showers possible over ne CWA, mainly from I-74 ne from
2-8 am Mon morning but no snow accumulations expected. Lows in the
lower 30s likely reached this evening before clouds move in, then
nearly steady or even rising a bit during overnight in sw flow.
A 1003 mb surface low pressure moves into central MI by Monday
afternoon with a surface trof passing thru central IL during Mon
morning. Strong WNW winds behind this trof later Mon morning and
afternoon could gust 30-40 mph, strongest ne CWA. Highs Mon in the
mid to upper 40s central IL (lower 40s in Knox, Stark and Marshall
counties), with lower 50s in southeast IL. More sunshine will be
possible south of I-74 on Monday.
.LONG TERM...(Monday night through Sunday)
ISSUED AT 330 PM CST Sun Dec 20 2020
Low pressure quickly moves off the NJ coast by sunrise Tue with
surface high pressure ridge over the MS river valley including IL
by dawn Tue. Upper level ridge also shifting east into the MS
river valley by midday Tue. A pretty nice day expected Tue with a
lighter winds and a fair amount of sunshine. Lows Mon night in the
mid to upper 20s, mildest in southeast IL. Highs Tue in the low to
mid 40s, with upper 40s sw of Springfield. Dry/fair weather to
continue thru Tue night across central and southeast IL.
A stronger storm system off the Pacific NW coast to track across
the northern Rockies/Plains and into ne MN by dawn Wed with a cold
front approaching IL Wed morning. Rain showers to likely develop
ahead of this front during Wed over central and se IL. Highs Wed
range from upper 40s west of the IL river, to the mid 50s near the
Wabash river. Models show a deep upper level trof digging into the
Midwest Wed night and Thu, though still diverge with speed of
arctic front passing east thru IL. GFS is quickly with cold front
moving thru IL during Wed morning while slower ECMWF and GEM
models takes cold front east thru CWA late Wed afternoon and Wed
evening. Quicker GFS model also shunts moisture/qpf east of IL by
Wed evening while slower ECMWF and GEM have qpf wed night
especially Wed evening and Ecmwf has deformation light snow Thu
afternoon into Thu night. Will stay close to consensus of models
which has chance pops of rain showers across area Wed afternoon,
then highest pops Wed evening as rain turns to snow from nw to se
during Wed evening and early overnight Wed night. Lingered 20-30%
of light snow in eastern IL Thu morning with flurries possible
thereafter. Early morning highs Thu range from lower 20s west of
the IL river, to the mid 30s near the Wabash river and falling
during the day. Lows Thu night around 10F northern CWA and mid
teens se of I-70. Wind chills to get in the single digits below
zero Thu night into mid Fri morning along and north of I-70 and
zero to 4 above south of I-70. Coldest air of the season arriving
Christmas eve and Christmas Day, with highs Christmas day only in
the mid to upper 20s despite return of sunshine as strong upper
level trof starts shifting east of IL. Lows Fri night in the mid
to upper teens with lighter winds.
Temps quickly modify to near or above normal next weekend with
highs in the mid to upper 30s Sat and upper 30s to lower 40s next
Sunday. GFS has a weather system brings some qpf into IL Sat night
and Sunday while slower Ecmwf has qpf across IL by Sunday
afternoon. GEM is keeping it dry and forecast blend is dry but low
confidence of a dry forecast next Sunday.
&&
.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 559 PM CST Sun Dec 20 2020
VFR conditions will prevail over the next 24 hours, despite a
shortwave passing across our northern counties later tonight.
Increasing clouds in the mid and high levels will be followed by a
brief period of light snow showers for our I-74 terminals between
09z and 13z. Have VCSH to cover the potential of light snow, but
very little impact is expected to aviation, if at all. The HRRR
and RAP output keep MVFR ceilings north of our CWA, but the I-74
terminals could see a brief dip into MVFR ceilings later tonight.
The other concern will be LLWS conditions late tonight into Monday
morning, as 45KT SW low level winds develop ahead of the trough.
Surface winds behind the shortwave/trough will increase from the
W-NW into the 15-20KT range gusting to 25-30KT at times.
&&
.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
UPDATE...Shimon
SYNOPSIS...07
SHORT TERM...07
LONG TERM...07
AVIATION...Shimon
Area Forecast Discussion
National Weather Service Louisville KY
950 PM EST Sun Dec 20 2020
.Forecast Update...
Issued at 950 PM EST Sun Dec 20 2020
Regional webcams and satellite imagery are showing patchy dense fog
developing in our east and southeast CWA. Soundings show a low level
inversion overhead helping to trap low level moisture. In addition
to present moisture, clear skies and lights winds are contributing
to fog development. Guidance has been struggling with visibilities
for the coming hours, but think we should be able to see a few hours
of patchy fog develop across the entire region as clear skies
prevail. Still expecting to see a low stratus deck move in during
the overnight, which should help limit fog potential. Have updated
the wx grids to include patchy dense fog and collaborated with
neighboring WFOs. For now, have issued an SPS for the fog for our
eastern half of the CWA. If fog continues to develop region-wide, a
Dense Fog Advisory may be needed. Will continue to monitor for now.
The rest of the forecast remains on track, but did blend in some
newer hi-res data for now until tomorrow afternoon. Updated products
will be published with this update.
&&
.Short Term...(This evening through Monday)
Issued at 255 PM EST Sun Dec 20 2020
A pair of fast moving shortwaves will be flowing through the
longwave pattern as upper level troughing extends across the OH and
MS Valleys. Latest surface analysis shows this morning`s cold front
now washed out with the weak boundary now located roughly over a NNE-
SSW oriented line from CVG-SDF-BWG. Current satellite shows the low
clouds persisting east of the I-65 corridor with clearing taking
place west of there. Radar continues to show light returns
associated with very light drizzle. RAP soundings suggest this
1500ft layer of stratus will degrade over the next couple of hours
and with it the occurrence of any light precipitation, therefore
weather grids will show chance of sprinkles for area east until
roughly 21z.
Clearing will be brief tonight as low level stratus is expected to
redevelopment. This thin blanket of clouds should keep our Monday
morning min temps from bottoming out too much, with expected lows in
the mid 30s for most. Low clouds will begin to break up by late
morning, coincident with the arrival of a clipper system approaching
from our NW. There`s a strong ~120kt jet core aloft associated with
this system along with a 40-50kt LLJ at 850mb. Therefore, expect to
see breezy conditions Monday late morning through the afternoon as a
tight pressure gradient results from high pressure over the Central
Plains and the aforementioned clipper to our north. Winds will be
out of the WSW with gusts up 30mph possible. Most of the moisture
associated with this system will be well off to our NE, but we could
still see a chance of light sprinkles in the Bluegrass Region
sometime Monday afternoon.
.Long Term...(Monday night through Sunday)
Issued at 312 PM EST Sun Dec 20 2020
Tuesday, surface high pressure and upper ridging bring clear skies
and near normal temperatures to the region, but as quick as this
pattern arrived, it`s gone.
The real focus in the long-term is the potential for snow on
Christmas Eve. A strong cold front is expected to move through the
CWA somewhere between 0z to 12z Thursday morning. The latest run of
the Euro is around 6 hours slower than the GFS. The timing of the
system will continue to get adjusted as the system nears.
Ahead of the front, southerly winds around 15 mph with gusts between
25 to 30 mph advect warm moist air northward from the Gulf of
Mexico. This lifts high temperatures across the region into the mid
to upper 50s on Wednesday and the 40s on Thursday, and with these
warm temperatures in place, rain will arrive with the front.
After the passage of the front, temperatures will quickly fall as
brisk winds veer towards the northwest, but precipitation will
likely remain as rain through most of the event until temperature
drop enough to change things over to snow on the back side of the
precipitation. If what the models are showing holds up, there is no
question that this will happen, and the models have came into better
agreement with around an inch across the area.
Behind the system, temperatures are going to be the main story.
Christmas morning lows are in the teens everywhere except
Louisville. They stay in the low 20s. Highs on Christmas stay below
freezing, but temperatures slowly warm through the weekend.
&&
.Aviation...(00Z TAF Issuance)
Issued at 625 PM EST Sun Dec 20 2020
Regional webcams and satellite imagery shows clearing skies this
evening. However, these clear skies and VFR conditions are not
expected to last long. Redevelopment of low stratus is expected
during the overnight hours and persist through Monday morning.
Guidance is struggling to deal with this stratus feature, and mixed
solutions make for a lower confidence forecast. Light winds and low
level moisture will linger under a shallow inversion, but the
question remains if fog will develop. Currently think winds in the
morning may be too strong and mixy for fog development. For now,
will continue to highlight the borderline LIFR/IFR ceiling that will
be overhead for several hours Monday morning.
Winds pick up late tomorrow morning and help clear out clouds. Winds
of 15-20 kts and gusts of 25-30 kts are possible. Guidance does
suggest some low level wind shear at the very end of this forecast
period, but decided to leave out of TAFs at this time to allow the
focus to remain on the stratus layer.
&&
.LMK WATCHES/WARNINGS/ADVISORIES...
IN...None.
KY...None.
&&
$$
Update...CJP
Short Term...CG
Long Term...KDW
Aviation...CJP
Area Forecast Discussion
National Weather Service Memphis TN
850 PM CST Sun Dec 20 2020
.UPDATE...Evening Update.
&&
.DISCUSSION...
Low stratus was eroding over West TN earlier this evening but now
seems to be expanding to the East once again. There is also a
little bit of erosion over East Arkansas, but for the most part
all of the Midsouth is beneath a very persistent deck of low
clouds. There is also a light Southwest wind across the region.
Both of factors should help limit the development of fog
overnight, but we still may need a dense fog advisory towards
sunrise, especially if winds relax. Visibility across most of the
region is in the 3-5 mile range, but areas may dip lower for brief
periods, especially sheltered valleys. Will continue to monitor
conditions for the need of a dense fog advisory. Overnight lows
should cool into the mid 30s across most of the area. We should
see significant clearing tomorrow with high temperatures
approaching 60 degrees.
30
&&
.PREV DISCUSSION... /issued 601 PM CST Sun Dec 20 2020/
As we head into the new work week dry and mild conditions are
expected Monday and Tuesday. Temperatures into the mid to upper
50`s with some southern spots getting to 60 are possible. It`s
going to be a bright, bright sun-shiny day or rather two days.
As our weather is quiet Monday and Tuesday, a trough is building
over the Pacific Northwest. This trough will bring our next round
of weather. Rain and wintry precipitation late Wednesday and into
Thursday is expected. The potential for wintry precipitation is
minimal and dependent on how fast the trough exits the area. There
is model disagreement on timing and strength. While the Euro is
slower and stronger, the GFS is quicker and weaker. This will
need to be watched as the system develops and progresses. This
system will also usher in cold air. High temperatures will occur
very early Thursday morning and fall throughout the day. This cold
air sticks around through the holiday into Saturday morning.
Sunday and into early next week milder temperatures and the sun
return to the area.
SMW
&&
.AVIATION...
Low clouds and some patchy fog linger across the Mid-South this
evening. The western edge of the cloud deck is slowly advancing
and guidance continues to point to clearing from west to east
overnight which makes sense given the decent westerly flow at
925mb. Concern is whether fog/more low clouds will develop as
skies clear out. Latest HRRR guidance indicates that patchy dense
fog will develop mainly east of the MS River after 21/06z with
some low stratus as well. VFR conds should return areawide
21/12-21/15z and continue through the end of the period. Light
winds overnight will become west at 10-12 kts with higher gusts by
late morning on Monday.
SJM
&&
.MEG WATCHES/WARNINGS/ADVISORIES...
AR...None.
MO...None.
MS...None.
TN...None.
&&
$$