429
FXUS63 KARX 191915
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
215 PM CDT Thu Sep 19 2024

.KEY MESSAGES...

- Showers and thunderstorms are expected to increase for this
  afternoon and evening (70 to 90%). A slight risk (level 2 of
  5) for severe weather continues for much of the area. Main
  hazards are large hail and damaging winds, an isolated
  tornado, lightning and local downpours of heavy rain.

- Above normal temperatures continue through Saturday before cooling
  to more seasonal temperatures lasting into early next week.

- Chances for rain return Saturday and possibly Sunday as next
  storm system moves into the region.

&&

.DISCUSSION...
Issued at 210 PM CDT Thu Sep 19 2024

Overview:

Late morning water vapor satellite imagery, 500mb heights, and
lightning showed an area of closed low pressure over the
Saskatchewan/Manitoba border with a ridge from parts of the Great
Lakes into the Mid-Mississippi Valley to the Southern Plains.  A
second area of closed low pressure was over the Southwest U.S.
Lightning was noted to the east of the upper level trough from parts
of northern Minnesota into western Wisconsin/central Iowa toward
northwest Missouri. An additional isolated storm was noted near the
front in northeast Neb.  Surface dewpoints at 16Z were mostly in the
upper 50s to lower 60s. Dewpoints in the upper 60s to around 70 were
pooled ahead of the surface front in southwest Minnesota. At
17Z...Visible satellite imagery has some clouds locally with a large
clearing area to the west with temperatures warming into the 80s.

An 18Z radiosonde was launched at MPX. Lapse rates were steep
around 8.7 deg C/km with SBCAPE of nearly 3000 J/kg and
downdraft CAPE of near 1000 J/kg.

The latest reflectivity shows storms growing across northern
Iowa with one severe thunderstorm warning issued already. This
is an hour or two ahead of some of the CAMs and these storms are
more elevated.

Strong to severe storm potential with locally heavy rain:

The 500mb trough axis is forecast to swing through the upper
Mississippi Valley this afternoon into this evening dragging a
cold front into the forecast area; gradually exiting the area
Friday morning. The axis of instability over western Minnesota
and northwest Iowa will shift eastward with increasing moisture
transport, an strengthening deep layer shear (pockets of
40-60kts) and 0-1km shear (30-40kts). MLCAPE tends to be
strongest 21-03Z (1000-2500J/kg) in the western portions of the
forecast area, clipping west central WI.

A few of the CAMs (WRFARW/NAMNest) have more convection into early
afternoon for portions of northeast Iowa, thus will continue to
monitor this area along the moisture gradient and gradient of
MUCAPE.  Attention then turns to storms increasing ahead of the cold
front with upscale growth along the front.  Discrete storms will be
capable of producing of severe weather with large hail,
damaging winds, and isolated tornadoes. Some of the forecast
soundings are showing slightly more veered winds versus backed
winds, thus the tornado threat will be conditional on the
ability to tap the backed flow.  The most recent HRRR/RAP show
multiple southwest to northeast broken lines and potential for
embedded bows or surges with some of the line segments. The
HRRR/RAP do try to converge these lines as they move into
western WI by 02 or 03Z, but not as much as the other hi-res
data. Additional storms develop to the west during the evening
and linger until the front pushes through.

A severe weather watch was being coordinated and issued for
parts of southeast MN, wcntrl WI, and parts of northeast Iowa.

Initially, the MPX 19.12Z sounding has a low level inversion and dry
air below 850mb.  Deeper moisture is arriving though with a 15 deg.
C 850mb dewpoint.  Precipitable water values at MPX were nearly 1.5
inches and are forecast to increase across the forecast area 1.5 to
2".  The 2" axis around 00Z is in the western parts of the forecast
area (parts of SE MN/WC WI) exits the area around 06Z with drying
from the west.  Forecast soundings show a warm cloud depth of around
4km.  We have some 10-40% probabilities of 1" per hr. rates on the
19.12Z The 3hr FFG is 2.5 to 3.5".  The storm movement is on the
faster side 25 to 35kts which is less supportive of flash flooding.
However, will want to monitor rain rates and storms moving over the
same area.  HREF PMM for 3hr shows some isolated spots could see
amounts in excess of 2 to 3 inches.

Above normal temperatures through Saturday then cooler for the
weekend into next week:

Surface high pressure builds in for Friday, however 850mb
temperatures do not cool significantly until the next cold front
Saturday night. Highs Friday and Saturday should range from the
mid 70s to the mid 80s. As another 500mb trough moves through
central Canada and the closed low over the Southwest U.S. moves
into the Four Corners region, moisture returns northward and
ahead of the trough Saturday. The forcing appears to be somewhat
split, but there is enough to mention rain chances 40-70%
Saturday night and possibly into Sunday. There is not great
agreement for Sunday on the strength of the push of dry air with
the cold front and the strength of the storm system moving
through Colorado with the deterministic runs. The ensembles do
have 40-70 probabilities of 0.25" or more. We`ll continue to
monitor this though with the deterministic EC being drier for
Sunday. Similarly for Monday, depending on the closed low
tracking into the region, precipitation may affect parts of the
forecast area. We are on the cool side of the front Sunday into
mid-week, thus temperatures remain seasonable in the 60s to
lower 70s.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1236 PM CDT Thu Sep 19 2024

Thunderstorms will be the main aviation concern this evening with
IFR category reductions possible at times. As a result, have
included a prevailing MVFR conditions with a tempo for IFR visbys
where confidence is highest on timing. Winds may shift to westerly
with strong wind gusts being possible as storms move through. With
convective initiation still 4-5 hours away, likely will need to make
forecast adjustments prior to the 00z issuance depending on
convective trends. Storms move eastbound through the region during
the evening and eventually exit towards midnight. As a front passes
through the region, cooling overnight temperatures and recent
rainfall in combination with clearing skies will create a fairly
favorable fog/low-stratus setup into the morning hours on Friday.
However, there remains some questions with how some stronger winds
to around 20 kts in model soundings (recent HRRR/RAP/NAM) just off
the surface will impact any fog/low-stratus formation. Consequently,
held with the 3SM but will likely introduce IFR/LIFR reductions in
subsequent issuances as forecast confidence increases.

&&

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

&&

$$

DISCUSSION...Zapotocny
AVIATION...Naylor