108
FXUS63 KARX 132331
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
530 PM CST Mon Jan 13 2025

.KEY MESSAGES...

- Cold again tonight with wind chills of 20 to 25 below, though
  a quick warm-up is in the offing afterwards. Look for a 30
  degree swing in temperatures between today and Friday.

- Another cold snap looms for the weekend and early next week.
  There is a 20-30% chance that temperatures (not wind chills)
  fall below -20 degrees early next week.

- Minimal risk for accumulating snow through the next seven
  days, though a few bouts of flurries remain possible.

&&

.DISCUSSION...
Issued at 250 PM CST Mon Jan 13 2025

Tonight through Tuesday Night: Cold Weather Continues

Cyclonic flow is in place this afternoon across the north-
central CONUS with an elongated trough stretching from central
Ontario to the Four Corners region. Water vapor imagery easily
depicts the multiple perturbations propagating southward out of
Manitoba and into Minnesota within this regime. Ambient moisture
is limited and thus the flurries are confined closer to the
circulation centers of these vort lobes. Diurnal cumulus
developed off the snowpack over west-central Minnesota midday
and are working into south-central Minnesota this afternoon.
These clouds move through later this evening.

Low temperatures tonight hinge on the degree of cloud cover and
decoupling that takes place. The NBM 10th percentile
temperature forecast depicts lows in the -10 to -15 degree range
along and north of the I-90 corridor, though these will only be
realized in sheltered and lower elevations given the modest
pressure gradient in place. This is a conundrum with a blended
forecast. Either we do not decouple and temperatures stay 5 to
10 degrees warmer, or temperatures bottom out with calm winds.
Confidence in either of these scenarios exclusively happening is
low and by blending in colder temperatures with slightly lower
(but not calm) wind speeds, wind chills are likely too cold in
the gridded forecast. Adding cloud cover to the mix, the wind
chill forecast for tonight is, needless to say, on the low side.
Therefore, have withheld going with a cold weather advisory to
see how the cloud, wind, and temperature elements of the
forecast unfold.

Tuesday looks quiet weather-wise with shortwave ridging sliding
through, though with synoptic cyclonic flow remaining in place,
any pockets of lower tropospheric moisture could possibly spit
out flurries. Surface ridging moves through Tuesday night and
also presents an opportunity for temperatures to bottom out
lower than the current NBM guidance. It is interesting to note
that the probabilistic temperature guidance for Tuesday night is
quite not as cold as tonight west of the Mississippi River,
which is likely a function of the lower tropospheric thermal
trough moving eastward during the day on Tuesday and winds
possibly increasing towards sunrise on Wednesday. That being
said, with Tuesday`s highs not much warmer than today, lows have
the potential to quickly bottom out below the current forecast
and have trended lows colder based on this possibility.

Wednesday - Friday: Fast Warm Up, Mostly Dry

The pattern takes an abrupt shift with the passage of the ridge
on Tuesday night. A ribbon of Pacific, downslope-modified air
works in Wednesday afternoon, helping to start the warming trend
for the afternoon and overnight on Wednesday. The flow turns
southerly late Thursday and brings +3 to +6 C 925-700-mb
temperatures into the area for Friday. Surface highs respond for
Friday, likely reaching the low 40s for many locales, though
the snowpack in central Wisconsin may modulate temperatures to
some degree. A cold front races southward for Friday night and
heralds the arrival of much colder air for next week.

The precipitation forecast for this period is mostly dry. There
is a window of opportunity for light snow towards central
Wisconsin Wednesday night as a jet streak/wave drives out of
northern Minnesota. The NBM forecast came in with near zero
PoPs, so did boost these closer to the LREF probabilities of
20-30 percent and at least mentioned flurries in the grids.

Early Next Week: Another Cold Snap

Guidance is in strong agreement that arctic air plunges out of
Alaska and into the central CONUS as the polar vortex weakens
and elongates, reforming further south over Hudson Bay by early
next week. Lower tropospheric temperatures plunge below -30C
Monday morning with the longer range guidance in unanimous
agreement that we will see an anomalous cold snap. The only
question is how cold will temperatures fall. If we had a pre-
existing snow pack, we could be looking at temperatures plunging
well below -20F; however, the lack of a snowpack may keep
temperatures from falling off a cliff. No matter how you dice
the forecast, cold weather headlines will likely be needed as we
get closer to the event.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Updated at 530 PM CST Mon Jan 13 2025

CIGS: thin stratus deck working across the TAF sites early this
evening, tracking southeast. Trends favor clearing KRST by mid
evening, but possibly not until after 06z at KLSE. A period of SKC
would then occur, but mid level clouds dropping south out of the MN
arrowhead could be working across the local area before 12z. Whether
just SCT or possibly a few hour of BKN is uncertain.

WX/vsby: no impacts currently expected.

WINDS: northwest winds through the period, mostly near or just below
10 kts.

&&

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

&&

$$

DISCUSSION...Skow
AVIATION.....Rieck