FXUS63 KARX 180535

Area Forecast Discussion
National Weather Service La Crosse WI
1135 PM CST Sun Feb 17 2019

Issued at 1131 PM CST Sun Feb 17 2019

The light snow is really diminishing quickly with very few, if
any, visibility reductions still being reported. Expecting that
the remaining snow will be gone or just flurries in a couple of
hours and plan to let the advisory expire as scheduled at
midnight. Roads remain snow covered and slippery but conditions
should not deteriorate with the snow ending and light winds.


.SHORT TERM...(Tonight and Monday)
Issued at 227 PM CST Sun Feb 17 2019

Upper level shortwave trough driving northeast toward the eastern
great lakes at mid afternoon while its associated sfc low hangs well
to the southeast - closer to the Oh River Valley. However, inverted
sfc trough hangs northwest of the low and into the local area. A
ribbon of higher theta-e air will continue to feed this area and
just north, along with weak qg convergence. Snow will continue as a
result, although be on a diminishing trend in areal extent/intensity
as the evening wears on. Lay out of the forcing and radar trends
continues to favor the higher amounts for parts of northeast
IA/southwest WI - with another 1-2" possible. Some locations will
top +6" with ratios as high as 25:1. However, per coordination with
surrounding offices, opting to hold with the Winter Weather Adv
(mainly along and south of I-90) for now thru midnight. Continue to
give warnings strong consideration, but how widespread the higher
totals will be, and the prolonged period of the snow lean toward
keeping the advisory.

.LONG TERM...(Monday night through Sunday)
Issued at 227 PM CST Sun Feb 17 2019

For Monday night thru Wednesday night: main forecast concerns this
period are the next system and snow amounts Tues night/Wed and
potential impacts.

17.12z model runs in good agreement for rising hgts over the region
Mon night, followed by shortwave ridging aloft ahead of the next
strong trough/shortwave into ejecting into the central/southern
plains for Tue. Tightening consensus on this trough/shortwave energy
as it lifts northeast toward/across the upper MS valley Tue
night/Wed, lifting quickly NE of the region Wed night. Fcst
confidence is average to good this period.

Rather strong and seasonable cold Can high pressure sits across the
region Mon night/Tue, even as hgts rise and shortwave ridging aloft
builds across the region. If skies Mon night end up more clear than
partly cloudy with light winds over the plentiful snow cover, the
blend of guidance lows near/below 0F for Tue morning may well be
as much as 10F two too warm in some of the normally colder low
laying valleys. Tue temps remain below normal as clouds ahead of
the next system increase in the afternoon.

Increasing signal for the next strong trough/shortwave to spread a
strong round of 500-300mb PV advection and ageostrophic flow/
divergence between at least partly coupled 250-300mb jet maxes. This
with mdt lower level thermo-dynamic forcing/lift and PW values in
the 0.25 to 0.5 inch range in the airmass flowing north ahead of the
approaching sfc-mid level low/trough. Consensus of 16.12z models
produced about 1/4 inch of QPF across the area. Consensus among
17.12z models is now on the order of 1/4 to 1/2 inch of QPF across
the area, Tue night/Wed. Dendritic growth zone progged more in the
climatological range of 50-100mb deep, with snow-water ratios more
around 15:1 (vs. the 20-30 : 1 today). Confidence increasing for
this system to produce at least advisory (3 to 6 inches) snowfalls
across the area, mainly between midnight and noon Wed, with impacts
to the Wed morning commute for most if not all of the area. Still
some signal for loss of ice nucleation in the cloud tops across the
SE end of the fcst area during the later morning/afternoon hours.
Confidence not the highest on this, but will continue with a mention
of some -FZDZ in the grids across the SE end of the fcst area on

With an advisory in effect for parts of the fcst area thru midnight,
will not issue any headlines for Tue night/Wed system at this
time. If the model QPF consensus continues to increase in later
runs, tonight's mid crew tonight or Monday's day crew may need to
issue a winter storm watch for parts or all of the fcst area for
Tue night/Wed.

For Thursday thru Sunday (days 4 to 7): main fcst concern this
period is another potential winter system Fri night Thru Sat night.

Medium range model runs of 17.00z/17.12z in decent agreement for
broad SW flow aloft over the region Thu/Fri ahead of the next
trough/shortwave energy coming thru the central/southern Rockies.
Solutions then diverge on the timing/strength/track of the
trough/shortwave energy as it ejects into the plains late Fri/Fri
night. These differences at their maximum Sat/Sat night as system
approaches/crosses the mid/upper MS valley. Fcst confidence average
Thu/Fri then below average for the weekend.

Thu/Fri dominated by sfc high pressure under the SW flow aloft.
Although not much for sfc-925mb gradients for mixing, the airmass
slowly modifies in place thanks to what should be some late Feb
sunshine. Blend of guidance high near normal by Fri continues to
look good. Plenty of between model and run-to-run detail
differences on the trough to lift thru the mid/upper MS valley Fri
night thru Sat night. Given confidence on the low side this
period, can do little more than stay with a model/ensemble
consensus in the grid for now. One trend since the 16.12z runs is
toward colder solutions over the area, with any precip Fri night
thru Sat night looking to be snow vs. a wintry mix. Better
consensus on a drier/cooler airmass to build into the region Sun,
to end the weekend on a dry note.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1131 PM CST Sun Feb 17 2019

Local radar indicates the light snow is dissipating pretty quickly
as the upper level low is almost past the area. This trend will
continue into the overnight with some just flurries expected for
KLSE for a couple of hours. Ceilings have already come up to VFR
and expect them to remain that way until the clouds scatter out
Monday morning as high pressure builds in from the north.





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