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000
FXUS63 KARX 141958
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
258 PM CDT Sun Apr 14 2024

.KEY MESSAGES...

- Still dry area-wide and becoming breezy Monday west of the
  Mississippi River with elevated fire weather conditions.

- A potent spring storm to bring widespread rainfall (60-100%
  1"+), strong winds and the potential for severe weather to
  the area Tuesday and Wednesday.

- Chance for precipitation Thursday then a more seasonable to
  end the week into the weekend. Cold lows Saturday morning
  could lead to frost/freeze; damage to vegetation.

&&

.DISCUSSION...
Issued at 250 PM CDT Sun Apr 14 2024

Overview:

Water vapor satellite imagery and heights show a 500mb ridge of high
pressure over the Rockies with a closed area of low pressure over
Nevada/California. Lightning was occurring with the storm
complexes over parts of the northeast U.S. and north of the
closed low over Oregon. Of note was the dry air south of this
closed low and the tropical mid and high level moisture
streaming across Baja and the southern U.S. Strong 110kt upper
level jets extended across southern Canada and from southern
California into Nevada. The 850mb thermal ridge is now shifting
south of the forecast area, but not before allowing most
locations to warm into the 70s and lower 80s today with low
relative humidity values 15 to 25% with good mixing due to the
northwest winds with gusts 20 to 30 mph.

Tonight into Monday:

Surface high pressure will dominate with the mid-level ridge
building in from the west.  A broad area of mid-level warm air
advection increases across the Dakotas into Minnesota with parts of
the area under the influence of the right entrance region of the
jet.  The surface front over the Plains lifts north as a warm front
Monday toward the mid-Mississippi Valley.  During this time,
moisture transport is increasing through the Southern and Central
Plains.  850mb temperatures around 8-14 deg C are expected today and
similar temperatures 8-12 deg. C temperatures are forecast for
Monday.  Forecast soundings show increasing mid and high level
clouds with weaker mixing, thus temperatures still above normal, in
the 60s to lower and mid 70s.  Frontogenesis increases and lapse
rates steepen from the Dakotas into Minnesota with some of the
deterministic/hi-res models developing elevated showers/convection
in this area. The 14.15Z HRRR spills some low probabilities of
showers/convection into Buffalo and Trempealeau Co. by 20Z and
along I94 during the late afternoon and evening. Other hi-res
models hinting at this. Expect ACCAS to develop and could see
some showers, unsure how far southeast they will develop into
the local forecast area. Will include 10-20 pops to cover this.
Should these showers develop due to the forcing, they should
dissipate during the early evening.

A potent spring storm to bring widespread rainfall, strong winds
and the potential for severe weather to the area Tuesday and
Wednesday:

The closed 500mb low pressure over the Rockies moves into the
Western Plains Monday night, through the Mid-Missouri River Valley
Tuesday and eastward to the Mid/Upper Mississippi River Valley.
We still see very good general model agreement on the synoptic
pattern, with continued strength and track differences which
will affect the track of the surface low and the resultant
precipitation and severe weather potential. The upper low
continues to push through Tuesday night into Wednesday with
showers generally east of the area Wednesday.

Broad synoptic lift increases with the approaching low pressure
system.  Upper level support increases with strong warm air and deep
moisture advection. We should see showers and thunderstorms
increasing across southwest Iowa Monday evening along the warm
front. The showers and storms will become more widespread across the
region Tuesday morning with continued strong moisture advection. The
left front exit region of the jet will also help support the showers
and storms as they move north. There is a ribbon of 40 to 50kts of
deep layer shear with the first band that lifts north. Freezing
level forecasts vary by model from 8500 to 10500, so cannot rule out
the potential for severe storms with elevated hail.  Mean
GFS/EC/Canadian ensembles show limited CAPE available with 200-600
J/kg of SBCAPE increasing Tuesday afternoon with increasing deep
layer shear of 45 to 55kts.  The potential for severe weather
continues to be 15 to 30% for parts of northeast Iowa and southern
Wisconsin based on the current storm track, instability, forcing are
expected.  The greater risk for severe weather is still south of our
local area.  With this being a dynamic system, we will continue to
monitor how far north the warm front will make it in the short term
as well at the potential for elevated storms north of the warm
front.

Welcome rains; strong winds:

The mean 14.00Z GFS/EC/Canadian ensembles show a 30 to 60%
probability of 1" or more of rainfall for 24hrs by 7am Wednesday.
Our current forecast is leaning toward the higher end of guidance
with widespread 1 to 2 inches across the forecast area. Precipitable
water values continue to forecast in the 200-275% normal range for
Tuesday. The NAEFS has PWAT 2 to 3 standard deviations above normal.
Forecast soundings become saturated with a warm cloud depth of 3km
and a storm movement possibly increasing to 40kts. With multiple
rounds of showers and convection, this faster movement would affect
rain total.  We`ll continue to monitor the forecast, but the area
could take an inch or two, however short term heavy downpours could
result in minor hydro issues related to ponding on roads and filling
of poor drainage areas.

We continue to see strong east winds develop Tuesday with gusts 30
to 45 mph. As the surface low tracks toward the area Wednesday
morning, the winds decrease, but increase again Wednesday afternoon
from the northwest with 30 to 45 mph.

Precipitation chances for Thursday - Still colder for the weekend:

Low pressure if forecast to track across Canada Thursday with a cold
front pushing through Thursday.  Showers are forecast with this
system, but snow could try to mix in as the system exits the area
Thursday night.  Still more seasonable for the weekend with
temperatures from the upper 20s to the lower 30s Saturday morning.
Due to the mild temperatures being followed by freezing
temperatures, we have the potential for frost/freeze headlines
Saturday morning.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1223 PM CDT Sun Apr 14 2024

Ideal aviation weather continues across the region - VFR from
horizon to horizon. Some gusts with northwest winds behind passing
front but that should subside going into the evening. As we look
ahead towards larger approaching system, look for gradual wind
shifts early Monday and perhaps some mid to high level clouds
approaching from the west as initial signs of lift and higher level
advection approach. Still all VFR though.

&&

.FIRE WEATHER...
Issued at 250 PM CDT Sun Apr 14 2024

We remain under the influence of surface high pressure Monday with
light north and east winds with low RH values area-wide.
However later in the day, easterly winds increase west of the
Mississippi River with gusts 15 to 25 mph. High temperatures
again top out in the 60s to mid 70s with minimum relative
humidity of 20-30%. Due to the stronger winds and the warmer
temperatures with drier air in place, elevated fire weather
conditions may develop over parts of southeast MN and northeast
Iowa during the day. Relative humidities improve Monday evening,
however wind gusts will actually strengthen with the
approaching storm system overnight.

&&

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

&&

$$

DISCUSSION...Zapotocny
AVIATION...JAR
FIRE WEATHER...Zapotocny