805
FXUS63 KARX 250010
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
610 PM CST Sun Nov 24 2024

.KEY MESSAGES...

- Drizzle for many areas tonight, transitioning to snow towards
  morning from northwest to southeast before ending. Most
  locales should see minimal snow accumulation, but cannot rule
  out a quick 1 to 2 inches north of Highway 29 (20-30% chance).

- Temperatures steadily cool through the week. Highs by Friday
  will struggle to reach the low to mid-20s. Overnight lows next
  weekend fall to the single digits with wind chills below zero.

- Confidence remains low in the snow forecast for mid-week and
  beyond given that subtleties in the weather pattern will play
  significant roles in snow amounts and timing.

&&

.DISCUSSION...
Issued at 230 PM CST Sun Nov 24 2024

Tonight - Monday: Drizzle, Transitioning to Snow

An elongated 1002-mb surface low was positioned over eastern
Kansas early this afternoon with an attendant warm front arcing
NE to E across central Iowa into north central Illinois. The
surface warm front advances slowly northward this afternoon and
tonight, but stays south and east of the forecast area. A moist
easterly fetch under the 800-700-mb warm front will saturate the
lowest 6-7 kft of the troposphere across the region tonight,
with modest 280-290K isentropic ascent through this cloud
shield--a classic drizzle setup. Given the light QPF and
resultant lower PoPs in the NBM, did manually increase PoPs to
the 50-80% range, values more reasonable for the sensible
weather impacts. The drizzle looks to set in around 00-03Z and
last through 09-12Z before the low passes to the south and cold
air advection ensues.

The main question in the forecast revolves around snow. The
broad and rather disorganized nature of the cyclone does not
lend us any favors in pinning down the rain/snow transition
times and snow amounts. The thicker clouds today kept
temperatures a few degrees cooler than earlier thought across
central Wisconsin, Given the shallowness of the above freezing
near-surface layer, snow is expected to begin 6 hours sooner
north of Highway 29 and amounts over Taylor County have been
increased to 1-2 inches. There is some concern that we may tap
into the secondary dendritic growth zone with a 200-300-mb
isothermal layer in the 0 to -5C region, so also increased snow
ratios to factor in this possibility. Roughly 20% of the HREF
members have 3-4 inches of snow in northern Taylor County.

Attention then turns to the backside of the system Monday
morning as sub-freezing air arrives. While the vast majority of
the short-range guidance have minimal precipitation/snow
amounts, the RAP has been the rouge outlier in strengthening the
low to mid-level frontogenesis band and bringing a quick period
of accumulating snow to the region, mainly north of I-90. Given
the shorter duration of the band, amounts should be limited.
Other convective models are warmer and weaker with the forcing,
but the steadfastness of the RAP does give one pause and
warrants further scrutiny tonight.


Monday Night - Next Weekend: Colder, Low Potential for Snow

The first surge of colder air drives southeastward during the
day on Monday with lows Monday night falling well into the
teens. Did trend cloud cover higher for Monday night given the
cyclonic flow pattern in place and early signal from the
RAP/HRRR long range runs that stratus may linger well into
Tuesday. Tuesday will be quiet and cool as a low-amplitude
shortwave ridge slides through. The main question will be cloud
cover, which is very difficult to ascertain at this range. The
LREF members still differ substantially on how far south the
upstream shortwave pivots with the GEFS and EPS still aligned in
their respective camps, but it is worth noting that the
operational 24.12Z EC has come into agreement with the 24.12Z
GFS in taking the surface cyclone over Missouri and Arkansas,
leaving the local forecast area dry. Will have to assess how
other ensemble members sort themselves out over the next 24
hours, but the forecast for Wednesday is trending drier.

This midweek wave ushers a reinforcing round of polar air
southward as highs slide further into the teens and 20s for the
end of the week. The operational NBM lows fall into the single
digits above zero for the weekend, with a 20% chance of lows
below zero, primarily west of the Mississippi River. The
northwesterly flow pattern for late week and the weekend makes
it difficult to nail down any snowfall potential at this range,
but there is a hint that we could see a clipper wave on
Saturday. Roughly 50% of the LREF members have this snow band,
but placements differ by several hundred miles. Have at least
brought PoPs to mentionable values in the forecast, but much can
change over the next six days.

Looking farther ahead, there is some hope for a rebound in
temperatures after next weekend, but there still exists a 10-20
degree spread in the guidance and most members keep temperatures
below climatological normals for this time of year.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 553 PM CST Sun Nov 24 2024

VFR/MVFR ceilings blanketed the area and will continue to see
more widespread MVFR ceilings develop this evening as a
shortwave trough moves into the area. Conditions will continue
to deteriorate with forecast soundings showing low level
saturation with IFR/Patchy LIFR conditions with drizzle and
light rain overnight into Monday morning. The forecast soundings
vary on the extent that the rain will mix with/change over to
snow with the seeding of snow from aloft,the low level -3 to
minus 5 deg C thermal layer, steep lapse rates, strength of
frontogenesis and how much dry air there is in the mid levels.
For now did mention a mix at KRST and could see a changeover for
parts of southeast MN into WI. Some minor accumulations are
currently forecast for Taylor Co., where surface temperatures
will be colder. Will need to monitor this though. Light east and
northeast winds shift around to the northwest after 07Z and
increasing 15 to 20kts with gusts 25 to 30kts.

&&

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

&&

$$

DISCUSSION...Skow
AVIATION...Zapotocny