-- Discussion containing changed information from previous version are highlighted. --
903
FXUS63 KARX 211935
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
235 PM CDT Tue May 21 2024

.KEY MESSAGES...

- A significant severe weather event with a MODERATE RISK
  (Threat level 4 of 5) for severe storms today south of I90 and
  a Threat level 3 of 5 north of I90. Multiple rounds of storms
  continue morning and later this afternoon into evening. The
  highest severe threat appears to begin late afternoon and into
  the evening with damaging wind gusts and tornadoes being the
  primary threats, large hail is a secondary threat.

- With the multiple rounds of storms expected, a flood watch
  has been issued for southeast MN and northeast IA

- Small rain chances Wednesday with more rain chances Friday

&&

.MESOSCALE...
-- Changed Discussion --
Issued at 235 PM CDT Tue May 21 2024

At 19Z, surface analysis showed the main low pressure center
undergoing cyclogenesis in southwestern IA with 5mb/3hr
pressure falls over nwrn WI. A maritime tropical airmass warm
front was located roughly along I-80 with gusting southerly
winds to 25-30kt. Secondary frontal boundary extends as an
inverted trough from the low to about Duluth...another area of
convergence and destabilization occuring south of it.

Mainly monitoring the return flow and destabilization over the
next hours as the wind field is in place. KDMX wind profile has
70+ 0-6km bulk shear, and 37 kts in the 0-1km layer with a
potent 300 m2/s2 of 0-500m SRH. A very serious wind profile for
supercell tornadoes, and severe weather in general.

Using WoFS guidance over the area, values of 5+ surge northward
to about I-94 (remarkably!) but just south of the Twin Cities
by 6 pm, so some serious destabilization with big dynamical lift
coming in. There seems to be a slight preference for southeast
MN into northcentral WI for a higher probability footprint for
0-2km vertical vorticity swaths over nc IA into sern MN, with
more sporatic signals further south, meaning there may be a
slight preference for that area through 6 pm for tornadoes.
Current forecast is on track, with generally a 5pm to 9 pm
window from I-35 to I-39 in WI.

Also, there is signal that an initial line will move in to the
western areas just east of I-35, but storms will develop out
ahead and possibly a discrete phase of storms along the Miss
river in concert. This is verified in the HRRR and WOFS runs
over the past many hours. So, be careful with assuming steady
state motion of the inital storms and no downstream
development.
-- End Changed Discussion --

&&

.DISCUSSION...
Issued at 330 AM CDT Tue May 21 2024

Overview:

Water vapor satellite imagery, 500mb heights, and lightning showed a
shortwave trough to the east with a broad trough over MT/WY
and a closed low over Saskatchewan. Ahead of the broad trough,
several clusters of storms were noted across parts of
CO/NE/SD/IA/MN. At the surface, a 983 surface low was located
over southeast Colorado with a stationary from northeast through
KS/NE/IA into srn MN. A secondary surface low was near
northwest IA with a boundary through central Iowa into southeast
IA and northern IA. Storms were on the boundary and somewhat
loosely organized over parts of IA/MN during the late evening,
with both right movers and left movers. Some areas were lifting
north with the increasing low level jet, while others were
shifting east with the perturbations in the southwest flow
aloft. The SPC mesoanalysis shows the SBCAPE axis from Kansas
extending northeast toward western Iowa into southern MN. VAD
wind profilers show the strong 35 to 50kt winds at 850 at
DMX/EAX/TWX and 30kts locally. The effective wind shear was
greater to the west of the local area at 06Z. The MPX 21.00Z
sounding had 1.19" precipitable water (PWAT) and steep 7.5 C/km
lapse rates.

Storms overnight and Tuesday morning:

Seasonably strong upper level jets are forecast from the
Dakotas into Manitoba/Ontario and also from New Mexico into the
Southern Plains; shifting eastward through 18Z. Portions of the
Central/Northern Plains and Upper Mississippi Valley are
forecast to experience upper level divergence/a coupled jet.
Meanwhile, a 500mb trough in the lee of the Rockies moves into
the Plains with surface low pressure deepening over southwest
MN/western IA. Moisture transport increases across the Plains
and into the local area by 12Z, but really, ramps up through the
Missouri River Valley. This then shifts east across western
Iowa through 18Z. The low level jet across the Plains is
forecast to strengthen 50 to 55kts through 12Z and makes
progress eastward through 18z...but not as strong more in the 30
to 40kt range.

Due to the uptick in forcing overnight, progress of the
perturbations and the shortwave trough, deepening of the surface
low, and the favorable moisture and instability, continue to look
for showers and thunderstorms to increase in coverage overnight. The
CAMs are somewhat similar in showers and thunderstorms ongoing
across much of the forecast area south of I94 through 12Z.  While
the instability is there, the deep layer wind shear and effective
shear profiles remain more prohibitive for organized severe weather
early on. We will continue to see strong to potentially severe
storms this with damaging winds and some hail with the
instability and forcing with heavy rain still a concern,
especially along the boundaries and where storms repeat over the
same area. A cluster of storms progress during the morning
hours, these could tap into better shear with some severe
storms. Some of the CAMS hint at bowing structures, while others
do not.

Storms this afternoon and evening:

This afternoon, the 500mb trough over the Missouri River Valley will
rotate northeast into the Upper Mississippi Valley.  Surface low
pressure is forecast to lift northeast along the boundary with the
warm front lifting north and dragging a cold front across the
forecast area during the afternoon and early evening.  Surface based
instability is forecast to increase with area 3000+J/kg.  A ribbon
of strong mid-level winds are forecast to increase across
Nebraska into Iowa this afternoon and into Wisconsin this
evening, resulting in lengthening hodographs and
increasing/large 0-1/0-3 storm relative helicity values. The
most recent CAMS indicate a lull in activity possibly 17-21Z. If
this occurs, it would allow for additional destabilization/very
steep lapse rate 8 deg C/+ ahead of the approaching cold front.
A line of broken storms is forecast to grow upscale across Iowa
into western Wisconsin. It will be important to note any outflow
boundaries and the location of the warm front, where supercells
may develop. Short term mesoanalysis will be critical early on.
Damaging winds, large hail, tornadoes and heavy rain continue
to be the main hazards. Due to the large 0-1/0-3 SRH values a
few strong tornadoes could occur. Damaging winds of 75 to 90 mph
and QLCS tornadoes will be possible with the line of storms.
The highest threat appears to be 20/21Z until 01-02Z with the
storms exiting the forecast area by mid-afternoon south of I90.
Due to the above factors coming together this afternoon and
early evening, the severe weather risk has been increased to a
level 4 of 5.

Heavy Rain Potential:

Dodge and Mower County did see localized heavy rain, potentially 3
to 5 inches based on radar, but there could be some hail
contamination in those values.  PWAT values increase to 150-200% of
normal by 12Z and continue through Today.  Forecast soundings show
warm cloud depths increasing through the day becoming more efficient
rainfall producers. The 21.00 HREF mean values of for 24hr rainfall
show 1.5 to 2" through 22.00Z with max values of 3 to 7".  Multiple
rounds of storms are expected. Highest confidence is highest
over parts of southeast MN where heavy rain occurred Monday and
along the surface boundary. A broad look as the radar shows the
increased convection along the boundary from eastern Nebraska
across Iowa. The RAP has this boundary gradually lifting north
during the day. Morning storms may affect how north it makes it.
The main negative will be the progressive nature of the storms;
cloud level storm motions increase 20kts to 35kts to 45kts
through the day. That being said, the multiple rounds should
make up for it. For now will include southeast MN and northeast
Iowa in a flood watch for flash flooding until 00Z for the
rounds of morning and afternoon convection.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1242 PM CDT Tue May 21 2024

Main taf concerns are storm chances at both RST/LSE taf sites
this afternoon into this evening. Then...MVFR/IFR conditions at
both RST/LSE taf sites tonight into Wednesday morning. A cold
front will move across the region this afternoon and evening.
Showers/storms will develop along the front and impact the taf
sites late this afternoon and evening. There is potential for
severe wind gusts of 50 knots or greater. For now have capped
wind gusts to 45 knots and see how storms develop along front to
introduce higher gusts in tempo group. Behind the front
ceilings will lower into MVFR and continue into early Wednesday
morning at both taf sites. Conditions will improve to VFR around
15z Wednesday at both taf sites.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...Flood Watch until 9 PM CDT this evening for MNZ079-086>088-
     094>096.
IA...Flood Watch until 9 PM CDT this evening for IAZ008>011-018-019-
     029-030.

&&

$$

MESOSCALE...Baumgardt
DISCUSSION...Zapotocny
AVIATION...DTJ/Zapotocny