471
FXUS63 KARX 061754
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1154 AM CST Sat Dec 6 2025

.KEY MESSAGES...

- Winter storm impacts the local area this afternoon through
  tonight with the threat for several inches of snow becoming
  more likely along and south of I-90. A Winter Storm Warning is
  in effect for portions of northeast IA where 5 to 7" of snow
  is likely. A Winter Weather Advisory is in effect for counties
  along and south of I-90 for snow accumulations of 2 to 6".

- Below normal temperatures persist throughout the next week
  with Tuesday being the "warmest" day with highs rising into
  the 30s.

- Additional snow chances continue into next work week. Rain
  will likely mix in with snow late Tuesday for areas south of
  I-94.

&&

.DISCUSSION...
Issued at 335 AM CST Sat Dec 6 2025

Today - Tonight: Winter Storm Moves in this Afternoon, Highest
Amounts in Northeast Iowa

Our winter storm that remains the main focus of this forecast cycle
remains a challenging forecast as spatial disagreements between the
global ensembles and Hi-res guidance persists with their 06z runs
early this morning. Overall the conceptual model for this event is
fairly well understood, quasi-zonal to northwesterly flow will pivot
a 500mb shortwave, that is currently located over central MT,
through the Great Plains throughout the morning and afternoon and
eventually into the Upper Midwest. As this occurs, a surface low
pressure center will deepen and progress eastward through Iowa
during the evening hours. The million dollar question even just 12-
18 hours away from the snow moving into the region is where exactly
this deepening surface low tracks and positions itself. Currently,
the GEFS/EC group of solutions keep the surface low near the
MO/NE/KS triple point with much of the heaviest snowfall just south
of the local area. This is contrasted by much of the CAMs which, as
shown in the 06.03z RAP, has the surface low positions in
southeastern IA, roughly 50-100 miles further north. Additionally in
this scenario, the hi-res guidance has a much more dynamic system
with fairly strong 700-900mb frontogenesis across northeast IA where
saturation in the dendritic growth zone primarily takes place.
Consequently, the 06.00z HREF has a fairly decent footprint for
1"/hr snowfall rate probabilities in northeast IA over the course of
the evening. Timing with this system is higher confidence with snow
generally entering the local area from west to east during the late
afternoon and exiting during the pre-dawn hours on Sunday.

Snowfall amounts will be dictated heavily by which scenario
ultimately plays out in this GFS/EC vs hi-res CAMs showdown. In the
first scenario, this would keep the heaviest totals south of the
region with amounts likely in the 1-4" range near and south of I-90
and lower likelihood for 1"/hr rates with the strongest forcing
further south of the area. In the hi-res CAMs solution, this would
favor likely amounts of 6"+ across most of northeast IA and
southwest WI where this strong frontogenetic footprint would be
present. In either case, snow to liquid ratios will be on the
slightly drier side, perhaps around 15:1 with somewhat limited DGZ
residence shown in the RAP/HRRR in northeast IA. However, this is
with the caveat that strong frontogenesis in the DGZ in the hi-res
scenario could allow for increased ratios.

In any case, have opted to take a somewhat middle of the road
approach in this forecast cycle. Have upgraded portions of northeast
Iowa to a Winter Storm Warning for amounts of 5-7", where the
06.00z HREF has the highest probabilities for snowfall amounts
of 6 inches or greater. Additionally confidence is higher that
this is where amounts of at least 3-6" would be likely in either
scenario. Elsewhere in northeast IA and southwest WI, have kept
with a Winter Weather Advisory for amounts of 3-6" at this
juncture. However, it needs to be understood that if the more
aggressive hi-res solutions end up gaining higher confidence,
this will likely warrant an upgrade to a Winter Storm Warning
for other portions of northeast IA and southwest WI. The exact
northern extent of this system also remains a key question due
to the uncertainties on the position of the surface low.
Currently, have opted to carry a Winter Weather Advisory up to
along Interstate 90 with high probabilities for amounts in
excess of 2" (70-100%) in the 06.00z HREF. Regardless, what is
important to note as well is much of the snow will fall within a
fairly short timeframe as the bulk of the accumulating snow
will fall between 6pm to 3am across the local area. As a result,
if you have any plans to travel during this timeframe, be extra
cautious as snowfall rates will reduce visibility significantly
and roadways will quickly become snow-covered and slippery.

Sunday: Colder with Clearing Skies

As we head into Sunday, subsidence on the backside of the upper-
level trough that guided the aforementioned winter storm will aid in
the development of surface high pressure into the region. This along
with increased cold air advection will help clear our skies some and
allow our temperatures to drop fairly respectably with median high
temperatures in the NBM likely staying in the upper single digits to
teens for much of the region. Overnight lows will likely fall into
the single digits below zero across much of the region with minimum
wind chills of around -5F to -15F. Certainly, a reminder that we
have entered meteorological winter!

Next Week: Below Normal Temperatures with Additional Winter Systems
Passing Through the Region

As we head into next week, the synoptic flow pattern really does not
change at all with northwesterly to quasi-zonal upper-level flow in
place with periodic shortwave perturbations sneaking into the
region. Which will essentially result in a forecast consisting of
below normal temperatures will periodic bouts of precipitation, in
most instances snow. The only exception to this will be Tuesday
night where additional precipitation types may be in play.

Monday evening and into the overnight features the first of these
shortwaves which pushes north of the region bringing likely some
light snow near and north of I-90. Overall amounts with this first
wave appear on the minimal side with the grand ensemble
(GEFS/EC/Canadian ensemble) only having medium probabilities (30-60%)
for an inch or greater of accumulations north of I-94 with this. As
we head into Tuesday, southerly surface flow ahead of an incoming
secondary clipper system will allow us to "warm up" with the inter-
quartile range in the NBM for highs on Tuesday ranging from the
lower to upper 30s south of I-94, much of these warmer temperatures
persisting into the evening and overnight ahead of the cold frontal
passage associated with the surface low passage. As a result, seeing
some indication that rain may be the predominate precipitation type
at times south of I-94 as a fairly robust warm nose is present in
the recent GFS soundings. If wetbulbing near the surface can be
efficient, cannot rule out some freezing rain potential however the
grand ensemble (GEFS/EC/Canadian ensemble) is not overly enthusiastic
about this scenario with low probabilities (0-20%) for measurable
ice accumulations. Regardless, this wave Tuesday evening and into
early Wednesday appears to be the most messy but likely has the most
dynamics going for it early in the week.

By the second half of the work week, the pattern remains steady
state with additional shortwave perturbations across the global
members that try to sneak their way in. With cold advection
reestablishing well below normal temperatures for the local area, any
of these shortwave perturbations would likely feature snow as the
favored precipitation type. However, details regarding the strength
or exact track of these clipper systems remains low at this time.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1131 AM CST Sat Dec 6 2025

Incoming snow seen on radar imagery traversing the Missouri
River Valley from eastern South Dakota through western Iowa
early this afternoon. Accompanying impacts likely drop into IFR
visibilities per local climatology. IFR to MVFR ceilings linger
post snowfall overnight with low level cold air advection
saturating the lowest levels.

Drier air within this colder airmass suggests VFR potential
Sunday morning as cold air advection weakens. Low confidence
situation given model bias to remove impacts at longer forecast
hour as well as select model biases in sustaining low level
saturation.

Subsequent aviation impacts frequent the longer term forecast:
accompanying small show chances Sunday night through Monday
morning and again Monday through Thursday.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory from 6 PM this evening to 6 AM CST
     Sunday for WIZ041>043-053>055-061.
MN...Winter Weather Advisory from 3 PM this afternoon to 6 AM CST
     Sunday for MNZ086-087-094-095.
     Winter Weather Advisory from 6 PM this evening to 6 AM CST
     Sunday for MNZ088-096.
IA...Winter Storm Warning from 3 PM this afternoon to 6 AM CST
     Sunday for IAZ008-018-019-029.
     Winter Weather Advisory from 3 PM this afternoon to 6 AM CST
     Sunday for IAZ009-010.
     Winter Weather Advisory from 6 PM this evening to 6 AM CST
     Sunday for IAZ011-030.

&&

$$

DISCUSSION...Naylor
AVIATION...JAR