NWS-La Crosse Forecast Discussion

 

						FXUS63 KARX 220400
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1100 PM CDT Sun Apr 21 2024

.KEY MESSAGES...

- Patchy frost possible tonight with higher chances for
  frost/freeze Tuesday night.

- Low RH and breezy winds may lead to elevated fire weather conditions
  Monday afternoon, especially across Wisconsin.

- Generally light rain amounts possible Monday into Tuesday,
  with a wetter, stormier period increasingly likely late this
  week.

&&

.DISCUSSION...
Issued at 149 PM CDT Sun Apr 21 2024

Tonight-Tuesday:

GOES visible imagery early this afternoon revealed clear skies
across much of the region, helping temps rebound back to near
seasonal values. While it will be another chilly night tonight, weak
return flow/warm advection should keep temps a touch warmer than
last night with the higher chances for subfreezing temps confined
north and and east of I-94 in the typically cooler locales. With
light return flow developing, especially west of the Mississippi
River, and dry air, frost may be more patchy and confined to
typically cooler or low-lying areas. As a result, will hold off on
any frost advisories for now and let the next shift re-evaluate.

A breezy, warmer day is ahead for Monday as southwest low-level flow
increases ahead of an approaching front. Gusty south winds will help
temps climb into the 60s. The highest gusts (30-40 mph) will be
favored just ahead of the front across southeast MN/northeast
IA within the low-level jet axis. Deep mixing ahead of the front
should keep RH low, with elevated fire weather possible,
especially across WI. See the fire weather discussion below for
details.

Warm advection and convergence on the nose of the low-level jet
should aid in scattered shower development during the
afternoon/evening ahead of the front, although showers may battle
dry low-level air. Overall, expect some increase in showers through
the evening (40-70% chance), especially into Wisconsin, but with any
rainfall on the light side. CAPE looks minimal, so thunder chances
are quite low. Scattered additional showers/storms are possible (30-
70% chance, highest for WI) Tuesday beneath the mid-level cold pool
with steeper lapse rates/some instability as the main trough swings
through. Rain amounts continue to trend lower, with chances less
than 50% for 0.10" west of the Mississippi River and chances for
0.25" only 20-30% northeast of I-94.

Wednesday-Sunday:

A pattern change will take place later this week leading to a
warmer, stormier period. However, for mid-week, Canadian high
pressure will maintain dry, seasonable air across the Upper Midwest.
Wednesday morning is expected to be chilly with 50-80% chances for
subfreezing temps south of I-94 and >90% chances north of I-94. As
return flow kicks in later Wednesday and Wednesday night, the risk
for subfreezing temps is highest across central Wisconsin.

Deeper moisture will begin to spread back north by Thursday as the
mid-level ridge axis reaches the Upper MS Valley and flow aloft then
transitions southwesterly heading into the weekend. Although the
details of this pattern evolution remain unclear, it appears a few
shortwave troughs will eject from the Rockies northeastward Friday
through early next week, tapping into the moisture/instability axis
extending from the plains through the Mississippi Valley. There is
still quite a lot of spread in the model suite regarding the
placement/timing of these waves, but at least one wave looks to
impact the area by Friday with higher rain chances (70-90%).
Predictability then decreases through the weekend.

The track of the shortwaves will influence temps and the northward
extent of instability during this time. It's worth noting the CSU
GEFS-based machine learning severe weather probabilities show 5-15%
chances for severe storms extending into the area Friday-Monday with
the higher chances residing to the south where chances for greater
destabilization exist. With the increasing probabilities for
precipitable water over 1" in the deep southerly flow, this pattern
would support some healthy rain amounts, dependent on placement of
convection.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1057 PM CDT Sun Apr 21 2024

VFR conditions will prevail through this TAF period. Look for
light winds and clear skies through the overnight period;
however, as a wave shifts west to east across the area on
Monday, southerly winds will increase with gusts in the 25-30kt
range across much of the area. As this wave traverses the area,
a mid-level (5000-6000ft) VFR ceiling will move in. While dry
air will limit precipitation potential Monday, there is a 20-30%
chance for rain showers through Monday afternoon.

&&

.FIRE WEATHER...
Issued at 149 PM CDT Sun Apr 21 2024

Elevated fire weather conditions may develop on Monday afternoon
with warmer temps into the 60s and increasingly gusty southerly
winds. Occasional higher gusts (40-45 mph) are possible (30-60%
chance) for open areas of Iowa/Minnesota west towards I-35, but
gusts of 20 to 35 mph are expected area-wide. Some increase in
mainly mid/high clouds will occur with shower chances (30-50%)
increasing by afternoon, especially north of I-90. However, with
deep mixing to near 850 mb, RH should fall to 20-30% (locally
lower values into the upper teens), lowest across Wisconsin,
with modest improvement through the afternoon possible in
southeast Minnesota and northeast Iowa as moisture begins to
increase. Rain amounts look light through Tuesday with potential
for 0.10" or a bit higher mainly across parts of Wisconsin.

&&

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

&&

$$

DISCUSSION...JM
AVIATION...JAW
FIRE WEATHER...JM