Winter Storm Watch 6, NWS La Crosse, WI

Area Forecast Discussion
National Weather Service La Crosse WI
1040 PM CST Sun Dec 27 2020

.SHORT TERM...(Tonight through Monday night)
Issued at 148 PM CST Sun Dec 27 2020

It took awhile, but saturation finally occurred today as short wave
passes through the area. Drier air led to saturation in a cooler
temperature range producing mainly snow, outside of a brief period
of mixed precipitation in far southern areas earlier. Deformation
bands will continue to produce some light accumulations and impacts
on highways mainly in Wisconsin heading into the evening. Not great
timing for travelers as holiday weekend ends. Advisories continue
for parts of central and southwest Wisconsin.

This system should continue to progress east fairly quickly with
snow ending from west to east. Expect some subsidence behind this
wave with peaks at sun on Monday.

Depending on clearing, light winds Monday night could lead to a
chilly but quiet night.

.LONG TERM...(Tuesday through Sunday)
Issued at 148 PM CST Sun Dec 27 2020

Had made assumptions yesterday that the midweek storm might start
looking a little clearer by today. I should know better than that.
While medium range guidance may be coming together on some aspects,
still seeing differences run to run and ensemble members are
becoming more spread which does not help confidence much.

One item that has become more uniform in 27/00z and 27/12z guidance
is keeping approaching short wave out of the southwest more of an
open wave now. This will translate to a faster system. More
influence too from northern short wave trough which also boosts
confidence in faster translation. Also better agreement in timing
because of this, with main impact area moving in late Tuesday.
Influence of northern short wave may push main snow axis a little
further south and east than earlier suggestions, as noted in drift
of 27.12z GFS. Items to keep an eye on.

While there is still ample moisture with this system, progressive
nature in model trends suggest warm layer may be more limited, or at
least not advect as far north. Still could see brief window of 1-3C
layer and brief melting in south, but more likely time for any
freezing precipitation would be with dry slot and loss of saturation
behind main precipitation shield. Certainly main focus will be
snowfall and related impacts though.

Higher snowfall rates with this system expected Tuesday evening and
into the overnight ahead of main wave as upper air dynamics,
including diffluent jet flow, associated lift, moisture advection all
increase and come together. In fact, higher snowfall rates will be
keen at start of this event to reach higher totals. Refinement of
snowfall totals certainly will take place next 48 hours but still
seems like low end warning criteria with general 4 to 8 inch band
across area. Feeling like more conservative approach given trends
might be best so might need to use wider probability ranges
initially. Mean of 50 ECMWF EPS has gone from 8 to 7 to 6 inches
over past 3 runs at KLSE /La Crosse, WI/ so just doesnt support a
more aggressive forecast. Faster overall speed should bring end to
precipitation Wednesday but lingering impacts likely.

Lots of great collaboration between WPC and surrounding forecast
offices today. With previous mentioned reasoning, lower trends, and
ongoing advisory, have opted to hold off on any watches at this
point. But fully expect a watch in later shifts as event draws
closer.

Active flow out of southwest may not end with 2020. Now seeing
suggestions in outer ranges of another potential short wave ejecting
out of Texas late in the week and lifting toward Great Lakes. This
too would have ample moisture to work with, including a more favored
deformation zone compared to midweek system. Too far out for details
and exact track but given some consistencies between ECMWF and GFS,
could see another snow event impact the region to start the New Year.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1040 PM CST Sun Dec 27 2020

Cigs: MVFR could linger into the overnight, but briefly, before
generally SCT/VFR through the overnight. Another upper level
shortwave/sfc front slides through the region and looks to bring
more MVFR cigs after 18z Mon. These should shift southeast of the
TAF sites later Mon evening thanks to a sfc high moving in.

WX/vsby: the shortwave looks to suffer from deeper saturation and is
somewhat weakly forced. Could see some flurries out of its
associated cloud band, but not looking for impactful -sn at this
time.

Winds: northwest becoming a bit more westerly for Mon afternoon.
Mostly holding near 10 kts.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

SHORT TERM...Shea
LONG TERM...Shea
AVIATION....Rieck

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