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Area forecast discussion
National Weather Service La Crosse WI
518 am CST Sat Dec 7 2019

Short term...(today through monday)
issued at 257 am CST Sat Dec 7 2019

Fair warning - get out and enjoy the milder weather this weekend
while we have it, as big changes remain lurking into much of next
week. More on that below, but in the shorter term, a pretty quiet
forecast overall into Sunday, with the primary concerns being cloud
cover at times and temperatures. Early morning analysis places low
level ridging centered across the mid Mississippi River valley,
while a fast-moving shortwave well to the north and broader warm
advection aloft have contributed to a few flurries north of I-94,
and plenty of clouds across the remainder of the area as of 06z.

Said ridge axis will depart the region today, with broad southerly
return flow ramping up through the afternoon as the gradient really
tightens across the region ahead of a cold front sliding through the
northern plains. That setup should favor a rather mild day today,
with snow-free areas likely well into the 40s, perhaps held back
just a little by additional high clouds working overhead through the
afternoon. The mild weather continues tonight with a breezy night
ahead of the approaching cold front, and in fact many areas are
likely to hold near or even above freezing through the night - not
too often we see that in early December. Sunday continues to look
mild as well, though may deal with increasing stratus as moisture
increases over the snow pack ahead of the front. Tough call on just
how warm things get, but westerly flow ahead of the boundary would
favor some very mild readings in snow-free areas, pending cloud
trends of course.

The real action begins late Sunday night into Monday, as northern
stream energy digs from the northern rockies down through our area,
with a quick uptick in deeper layer positive vorticity advection/warm advection ascent
favoring rapid top-down saturation and light snow development. Still
some small risk we could squeeze out a little freezing drizzle if
lower stratus manifests itself in the pre-frontal environment
Sunday/Sunday night before the arrival of in-cloud ice, but
confidence remains rather low in that regard. Otherwise, snow should
quickly develop into Monday morning, ending from west to east into
the afternoon as the wave departs. As for amounts, still looking
like near-advisory amounts north of I-94, tapering to an inch or
less far south, though we'll have to closely watch how much the
northern stream energy digs as any more southerly solution (gfs,
gem) would favor additional snowfall closer to the I-90 corridor, in
line with very consistent gefs plumes the past few days showing an
average of 0.25" of liquid at lse/rst. Suffice it to say, even with
lighter amounts, travel impacts are likely as temperatures will be
in "free-fall" Mode by afternoon, with any slushy snow early quickly
re-freezing.

Long term...(monday night through friday)
issued at 257 am CST Sat Dec 7 2019

Main forecast concern in the longer term is extreme cold for mid
week. Cold advection kicks in Monday night in earnest, with
temperatures dropping into the single digits for lows Tuesday
morning and most areas staying in the single digits through the day
Tuesday. Ridge axis nears the service area Tuesday night, resulting
in the coldest temperatures of the season so far. Current forecast
lows are below zero areawide. Model certainty tool shows current
forecast lows are slightly on the warmer side of guidance,
suggesting we may not be cold enough on lows in this time frame.
This would especially be true for areas with fresh snow cover.

Have kept flurries in the forecast for Monday night and Tuesday as
stratus is still expected to linger and flurries would be likely
given the extremely cold air mass.

Gusty northwest winds will create wind chill concerns Tuesday night.
Areawide, sub-zero wind chills appear likely through the period,
with parts of the area pushing 25 below Tuesday night. Temperatures
rebound quickly Thursday, moderating for the end of the week.

&&

Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 518 am CST Sat Dec 7 2019

VFR conditions will remain in place the next 24 hours, as winds
increase from a southerly direction today and especially into
tonight. Even with surface wind gusts of 20-30 knots this evening
and tonight, low level wind shear will become a concern for both
lse and rst, with very strong flow of 50-60 knots just off the
ground. That shear will subside quickly toward 12z Sunday, with
bouts of thicker mid and high clouds expected to cross the region
the next 24 hours.

&&

Arx watches/warnings/advisories...
WI...none.
Minnesota...none.
Iowa...none.
&&

$$

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