FXUS63 KARX 162022

Area Forecast Discussion
National Weather Service La Crosse WI
222 PM CST Sun Dec 16 2018

..SHORT TERM...(Tonight thru Monday night)
Issued at 222 PM CST Sun Dec 16 2018

The main short-term fcst concern remains temperatures.

Data analysis had a weak mid level shortwave spreading mainly cirrus
clouds across the SE half of the region as of 18z. A weak cold front
also continued to push SE across the region with more W to NW lower
level flow behind it. Even with this, mid-day temps across the Upper
Midwest were mostly in the mid 30s to mid 40s, some 10 to 15 degrees
above the mid Dec normals.

16.12z model runs initialized well and offer quite similar solutions
as hgts rise and shortwave ridging builds over the Upper Midwest
tonight thru Mon night. Short term fcst confidence is good.

In the short term, stronger low level cold advection behind front
spreads across the area late this afternoon and tonight with 925mb
temps below 0C by 12z Mon. Low level warm advection then returns by
later Mon and Mon night with 925mb temps mostly back above 0C by 12z
Tue. Model soundings/time-hgt X-sections showing limited moisture
over the area tonight thru at least Mon evening, with potential
for some thickening cirrus over the area later Mon night. Mostly
clear skies tonight, with Monday shaping up to be another mostly
sunny day. Some W-NW winds 5-10kt tonight for BL mixing to keep
lows from cooling too far. Blend of guidance lows in the mid teens-
low 20s reasonable. If winds decouple some lows of 5-10F above
likely in the normally colder low laying areas along/NE of I-94.
NW sfc- 925mb flow continues most of Mon, with model soundings
showing mixing to only 750-1000 ft. Blend of guidance highs
generally in the 30s looks good. Sfc ridge axis drifts across
later Mon afternoon/evening, for a period of light/calm winds and
clear/mostly clear skies, lows Mon night looking to be around
midnight, then steady/slowly rise overnight into Tue morning as
the gradient tightens and S/SE sfc winds increase.

.LONG TERM...(Tuesday through Sunday)
Issued at 222 PM CST Sun Dec 16 2018

For Tuesday thru Wednesday night: main fcst concerns this period
are warm highs Tue and precip chances/types Wed/Wed night.

Model runs of 16.12z in good agreement on the shortwave ridging
quickly moving east of the area Tue, then for a northern/southern
stream shortwave combo to carve out a rather deep longwave trough
over the lower thru upper MS valley by Wed night/Thu morning. Some
detail differences impacting precip chances by Wed/Wed night, but
this more-or-less expected. Short-term fcst confidence good Tue,
then more medium by Wed/Wed night.

Mdt S-SW sfc-850mb flow spreads across the area Tue with the low
level ridge axis shifting east. Low level warming continues with
mixed 925mb temps by afternoon supporting highs well into the 40s
most locations. Tue could be another rather warm day but there is a
signal for thickening cirrus clouds thru the day with model
soundings showing mixing only to around 1K ft. For now consensus
highs mostly 35 to 45 on Tue are reasonable but upside potential for
highs in the 40-50 range if skies stay mostly sunny thru the day.

Warming Tue sets the stage for a warm low level airmass to be in
place as hgts fall and the trough/shortwave energy dig in for
Wed/Wed night. 925-800mb temps progged above 0C thru at least 06z
Thu, with any precip thru then falling as liquid and ultimately
controlled by the BL temp (FZ or non FZ liquid types). Nam progs
increasing sfc-925mb moisture into the area already late Tue night
along with some lift in this layer. This moisture progged to only be
about 2k ft thick, shy of the minimum of about 1KM needed for
DZ/FZDZ production. Will leave late Tue night dry and stay with the
consensus of precip chances increasing and spreading across the area
Wed/Wed night. Perhaps a bit of an icy mix Wed morning before
warming BL temps would have precip mainly as -RA/-DZ Wed afternoon/
evening. Lower level cooling to spread in later Wed night behind
passage of the sfc-850mb trough axis, enough so for precip becoming
a wintry mix overnight into Thu morning across the NW 1/2 to 3/4 of
the fcst area. Blend of guidance lows looks too cool Wed night with
thick clouds/precip, otherwise will stay near the model/ensemble
consensus for precip chances/types until the details come into
better focus.

For Thursday thru Sunday (days 4 to 7): main fcst concerns this
period are exiting precip chances/types Thu and a small -SN chance
north on Sat.

Medium range model runs of 16.00z/16.12z in good agreement for the
highly amplified pattern at 12z Thu to slowly progress Thu/Fri, with
shortwave ridging building over the Upper Midwest for Fri/Fri night.
Timing differences by day 6 as expected, but some consensus for a NW
flow shortwave to drop into the northern plains Sat, swing across
the north end of the region Sat night followed by more NW flow aloft
for Sun. Day 4-7 fcst confidence is medium.

Models differing on how quickly 850-700mb drying behind the trough
axis would spread SE across the area Thu. GFS fastest, and would
have precip all but out of the area by Thu morning. Other models
slower and linger what would be a -RA/-SN mix or -SN (column cools
with ice still present in the cloud tops) across mainly the SE half
of the fcst area mainly in the morning. Will stay with the model
consensus for now. Any -SN amounts looking to be light. Drier/colder
airmass to settle in for Thu night thru Fri night, with temps only
looking to cool to near the normals. Wave approaching/passing north
of the area Sat/Sat night drags a weak sfc-700mb trough/front and
some moisture across the fcst area Sat. Some small -SN chances
mainly across the north 1/3 of the fcst area with a colder column/
more saturation OK for now. High pressure with seasonable
temperatures to finish out the pre-Christmas weekend.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1128 AM CST Sun Dec 16 2018

VFR conditions expected through the TAF period with only some cirrus
moving through occasionally. Winds will remain out of the west/
northwest, with some gusts of 18 kts at RST this afternoon. Low
level wind shear could be an issue again this evening, especially at
LSE, so introduced a mention from 00Z to 06Z. Looks like the
strongest winds aloft will stay north and east of RST, but will be
something to monitor for later TAF issuances.




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