250
FXUS63 KARX 201738
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1238 PM CDT Mon May 20 2024

.KEY MESSAGES...

- Through Tonight-Storms push east through early afternoon then
  re-develop tonight - Severe risk level 1-2 of 5

- Tuesday-Multiple rounds of storms-Severe risk remains a level
  3 out of 5 for much of the area/Heavy rain potential:

&&

.DISCUSSION...
Issued at 345 AM CDT Mon May 20 2024

Overview:

A bit more active across the Plains earlier tonight. Water vapor
satellite imagery, 500mb heights, and lightning showed broad
troughing across much of the western and central U.S. with closed
low pressure over Alberta and Saskatchewan.  A couple of embedded
shortwaves in the flow were noted over the Dakotas and also over the
mid-Missouri River Vally toward Oklahoma.  Numerous clusters of
storms were noted with bowing reflectivity/lightning signature
highlighting the cold pool/outflow structures. The 20.00Z MPX
sounding showed 0.97" of precipitable water (PWAT) with drier
lower levels below 850mb. The latest WSR88D radar mosaic shows
areas of showers and thunderstorms across much of the forecast
area with the shortwave and increasing low level jet. Hourly
rainfall rates vary from a trace to a quarter of an inch
locally, however farther south over parts of southern IA, hourly
rates have been as high as .5 to 1" per hour. Gusty winds 20 to
45 mph and small hail have been associated with the stronger
storms.

Through Tonight-Storms push east through early afternoon then re-
develop tonight - Severe risk level 2 of 5:

Showers and thunderstorms will be ongoing this morning with the
shortwave trough lifting across the forecast area, continued
moisture transport, and support from isentropic lift, warm air
advection, and the low level jet.  The convection is holding the
surface warm front farther south initially, but is forecast to have
a northern semblance, but re-develop across Iowa tonight ahead
of the next shortwave. PWAT values increase to 175% of normal by
12Z, then this moisture axis shifts east...lowering to 125% of
normal by 00Z. As the moisture transport vectors veer with time,
the precipitation will shift eastward through the day. Western
areas may not see much as a result with showers and
thunderstorms lingering across parts of southwest WI into the
early afternoon. SBCAPE is limited, however there is some
elevated CAPE. Deep layer shear is variable 25 to 55kts. Due to
the inversion in the lowest 1km, the shear in not all being
realized. MLCAPE shifts eastward this afternoon along with the
forcing, thus look for decreasing coverage by early afternoon
with a break for many.

Perturbations in the southwest flow aloft continue tonight with the
low level jet once again strengthening across Nebraska and Iowa.
Meanwhile, SBCAPE increases 250-1250 J/kg with strengthening
moisture transport, especially to our west.  The CAMS re-develop
scattered thunderstorms especially along the inverted trough
across the northwest (RST/CCY westward) during the evening
hours and additional showers and thunderstorms within the warm
air advection regime overnight. Deep layer shear increases from
the west overnight. With storms being more surface based and
tapping more instability, strong to severe storms will again be
possible.

Tuesday-Multiple rounds of storms-Severe risk remains a level 3 out
of 5 for much of the area/Heavy rain potential:

An area of 500mb low pressure over the Plains is forecast to lift
northeast and become negatively tilted as it rotates through
Minnesota and Wisconsin.  Initially, there is an inverted trough to
the northwest over central MN and a stationary front through parts
of Iowa.  During the day Tuesday, the surface warm front lifts north
with the forecast area within the warm sector.  During the early
morning hours, moisture transport increases across Iowa with a
strengthening 40kt low level jet. We are starting to get some of the
CAMs going out into the Tuesday period and continue to indicate
multiple rounds of storms.  The CAMs are hinting at potentially
severe storms/a bowing MCS that rides along the thermal
gradient with the warm front lifting north, Additional severe
storms are possible in the afternoon along the front and in the
warm sector due to building instability and favorable
large/curved hodographs, and in the evening with the cold front.
With the surface low to our northwest and occluding, there are
still uncertainties in how far north the tornado threat will be.
The current severe weather outlook risk level 3/5 has been
nudged northward and covers most of the forecast area for
wind/hail threats with current tornado threat generally I90
southward. Higher instability and stronger shear profiles favor
the potential for stronger tornadoes/longer tracks. The
overnight and early morning convection Tues. could affect these
details; potentially hindering airmass recovery or affecting
frontal locations.

Through Tuesday, we have rainfall of 1 to 2" of rainfall
forecast. Forecast soundings deepen the warm cloud depth making
storms more efficient at producing heavy rainfall. Generally
rates remain below 1"/hour, however there are some pockets of
20-40% probabilities of 1"/hr. The HREF 24hr ensemble max shows
the potential for 2 to 5 inch amounts. Overnight, most amounts
have been a trace to .5" with localized higher amounts. The
widespread higher amounts fell just to the south. Through
Tuesday, with multiple rounds of storms and flash flood guidance
of 2.5 to 3", there will be the potential for locally heavy
rain and flash flooding.

Continue to monitor this forecast for updates.

Some spotty rain chance continue Wednesday with Thursday
generally dry. Another front moves in Friday and potentially
lingers into Saturday.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1238 PM CDT Mon May 20 2024

Early convection is clearing the area with a mix of VFR and MVFR
ceilings across the area. Short period of VFR conditions expected
between systems this afternoon into the early evening, but as next
short wave approaches, and instability builds to the west, expecting
the next round of convection to start impacts areas later this
evening.

Speaking of which, convection will drive the aviation forecasts
through the overnight and into much of Tuesday. Hard to narrow down
specifics quite yet, but the probabilities of more widespread storm
activity increase dramatically this evening and peak overnight into
Tuesday morning. This period could see wave after wave of impact at
TAF sites likely leading to periodic IFR with both ceilings and
visibilities. Could see some breaks between convection on Tuesday as
well but hard to work out those specifics this far out.

&&

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

&&

$$

DISCUSSION...Zapotocny
AVIATION...Shea