Area Forecast Discussion
Issued by NWS La Crosse, WI

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
000
FXUS63 KARX 241128
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
628 AM CDT Sun Mar 24 2024

.KEY MESSAGES...

- Impactful winter storm remains on target to reach the region
  by mid to late morning and last through tonight. Have expanded
  the winter weather advisory to encompass a band of quick
  hitting snow with high accumulation rates late this morning
  through early this afternoon.

- Despite the storm being on our doorstep, there remains ample
  uncertainty in snowfall amounts owing to questions on snowfall
  rates and the timing of when the warmer air aloft arrives.

- Monday will be warmer with rain showers lingering into Tuesday
  before cooler air moves in and changes precipitation back to
  light snow.

&&

.DISCUSSION...
Issued at 400 AM CDT Sun Mar 24 2024

Early this Morning

A leading band of isentropic ascent with embedded regions of
frontogenetical forcing continues to transect the region early
this morning ahead of our main storm system later today. Did
increase forecast snow amounts to around 1 to 2 inches based on
upstream webcams and road conditions, with this band lifting to
the northeast through the morning and eventually merging with
our main system coming from the southeast.

Today and Tonight: Winter Storm Details

This is by no means a slam dunk winter storm forecast. While the
synoptic weather pattern has been well resolved for days, the
forecast continues to be adjusted as the mesoscale details of
the pattern emerge. Most recent changes have been to expand the
winter weather advisory southward well into Iowa to account for
a convectively enhanced band of theta-e advection (tied to a
500-mb wave/jet streak) that surges through the region later
this morning.

Explicit model reflectivity progs show values of 40 to 50 dBZ
as this band lifts through northeast Iowa into southeast
Minnesota and western Wisconsin. Snowfall forecasts from the
HRRR/RAP/ARW/NMM are all painting a swath of 3 to 5 inches of
snow with this band by this evening, much of it falling within
2-4 hours. Therefore, based not only on potential amounts but
also the high snowfall rates, have expanded the winter weather
advisory south and west to align with the latest QPF/snow progs.
However, given the fast forward motion of the wave and high
snow rates (plus melting later today), the snowfall amount
forecast within this band is very sensitive to minor changes to
timing or rates.

Attention then turns to north of I-90 where the thermal profiles
will remain coolest the longest and higher snow amounts remain
forecast. The decision to upgrade Trempealeau and Jackson
counties to a warning was not easy, but with the latest QPF
forecasts coming in higher for these counties owing to the
previously mentioned band of snow coming out of Iowa, new storm
total snow progs were a solid 6 to 7 inches over these counties
and elected to play it safe and upgrade to a warning. However,
the snowfall forecast within the winter storm warning from
Wabasha to Jackson counties remains on the lower side of the
confidence spectrum owing to the possibility that snow may end
sooner than forecast. Confidence in warning level amounts is
higher further to the north in Clark and Taylor counties. As the
warm nose approaches these two counties towards sunrise Monday,
there exists a window in which the precipitation could
transition to freezing rain before surface temperatures rise
above freezing by midday. Therefore, have extended the winter
storm warning for these two counties through 15Z to account for
this hazard.

Monday through Tuesday: Weather System Lingers

With an upper level dry slot cutting across the forecast area
ahead of the 700-mb low/trough by Monday morning, any rainfall
during the day Monday will be more showery in nature as alluded
to by the short-range guidance. The forecast PoPs remain quite
high--around 90-100 percent--owing to the fact that nearly all
areas should see showers at some point during the day, but by no
means are we expecting an all day washout. The surface low and
associated warm sector have slowed for Monday afternoon and are
positioned further west than forecast yesterday. This looks to
limit the thunderstorm potential and is reflected well in the
latest SPC Day 2 Outlook that has shifted the Marginal Risk to
the west of the area. Have also trended highs cooler for Monday
as a result.

Showers fill back in with the approaching lower tropospheric
low for Monday night into Tuesday morning, though the coverage
may be more showery in nature once again. The deformation zone
pivots across southeast Minnesota into western Wisconsin Tuesday
afternoon and evening. With the overall moisture fetch well
displaced to the east, snow amounts should remain around an inch
or less though there are a few outliers pushing 2 inches if the
ingredients line up just right. Winds do increase with the
tightening pressure gradient with the snow, so there may be
some minor drifting snow impacts.

Wednesday through Saturday: Moderating Pattern

In the wake of our early week system, surface high pressure
builds in from the west and brings with it a shot of cooler air
for midweek. There are uncertainties with how much snow will
remain on the ground from our current storm, so the temperature
forecast may be slightly too warm if a snow pack exists
(especially overnight lows). Upper level ridging builds in for
the latter part of the week and brings warmer air with it. A
weak impulse rides the crest of this wave for Friday into
Saturday, though only about 20 to 30 percent of the ensemble
guidance have any rain with its passage. The better potential
for rain does not show up in the forecast until early next week,
but timing and placement differences make pinning down any
specifics difficult.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 628 AM CDT Sun Mar 24 2024

IFR and LIFR conditions likely today across the area due to
heavy snow during the afternoon. Once snow transitions to rain
sometime in the late afternoon/early evening, flight categories
are expected to remain around MVFR/IFR due to both lowered CIGS
(between 1000 and 3500ft) and visibilities (between 2 and 4SM)
into Monday. While it is snowing, snowfall rates of 1" per hour
can`t be ruled out. Southeast winds will have gusts between 25
and 30kts during the afternoon and into the evening hours.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory from 10 AM this morning to 7 PM CDT
     this evening for WIZ053.
     Winter Storm Warning from 10 AM this morning to 1 AM CDT
     Monday for WIZ032>034.
     Winter Weather Advisory from 10 AM this morning to 10 PM CDT
     this evening for WIZ041>044.
     Winter Storm Warning from 10 AM this morning to 10 AM CDT
     Monday for WIZ017-029.
MN...Winter Weather Advisory from 10 AM this morning to 7 PM CDT
     this evening for MNZ094>096.
     Winter Storm Warning from 10 AM this morning to 1 AM CDT
     Monday for MNZ079.
     Winter Weather Advisory from 10 AM this morning to 10 PM CDT
     this evening for MNZ086>088.
IA...Winter Weather Advisory from 10 AM this morning to 7 PM CDT
     this evening for IAZ008>011-018-019-029.

&&

$$

DISCUSSION...Skow
AVIATION...Cecava


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.