155
FXUS63 KARX 190933
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
433 AM CDT Sun May 19 2024

.KEY MESSAGES...

- Showers and thunderstorms becoming more widespread tonight and
  continuing into Monday. Some risk for severe storms overnight
  and Monday morning (2-5%).

- Increasing confidence for severe thunderstorm potential with
  the Enhanced Risk (a 3 on the 0 to 5 SPC risk scale) now for
  much of the region Tuesday.

&&

.DISCUSSION...
Issued at 342 AM CDT Sun May 19 2024

Overview:

The cold front came through during peak heating.  Temperatures maxed
out in the 80s to around 90 degrees.  Surface dewpoints were
generally in the 50s to lower 60s as expected and the HRRR with
stronger shear ended up verifying better.  With steeper lapse rates,
heating being maximized, and the 12 deg C. 850mb dewpoints pooled
ahead of the front, the storms were perkier with warnings being
issued and large hail reported across parts of west central WI
(Trempealeau, Jackson, Taylor, Clark, and Richland counties)
especially from 2030Z to 2330Z.

Water vapor satellite imagery, 500mb heights, and lightning showed a
closed low over Ontario westward toward Alberta with troughing
extending southward into the Plains and southwest U.S.  Lightning
was noted ahead of the cold front over the Great Lakes with another
convective complex moving out of Colorado into the Plains.  The MPX
19.00 sounding had dried out with 0.54" of precipitable water. The
latest subjective surface analysis had the cold front over eastern
WI with surface high pressure building in.  Dewpoints were in the
30s and 40s with temperatures in the 50s.  Skies were generally
clear locally, however clouds were mushrooming over parts of
Nebraska and Kansas with the convective complex there. By 09Z
the complex had made it to GRI/HSI.

Through Monday Night:

Within the southwest flow aloft, we`ll see a shortwave trough track
across the Plains and into the Upper Mississippi Valley by 03Z
tonight.  On its heals, a couple other shortwaves in the flow will
continue lifting northeast across the same region overnight into
Monday.  A seasonably strong 105kt upper level jets across the
northern U.S. with provide upper level support with a strengthening
low level jet over the Plains working east overnight (35 to 45kts)
into Monday morning.  Precipitable water values initially around
0.2" (50% of normal) today increase to 1.5 to 1.75" (150-200% of
normal)by 09Z through 18Z Monday before decreasing.

Through the day today, forecast soundings/timesections show top down
moistening with areas of shear increasing 40 to 60kts within a low
CAPE environment. 850-700mb warm advection increases 21Z to 00Z and
continues into Monday.  Moisture transport vectors ramp up 03-06 and
gradually shifting in a southern IA to Lake MI areas through the day
Monday.

The latest CAMs show the convection over the western High Plains
lifting northeast to the Missouri River Valley by 12Z, but differ on
the coverage as it moves toward the local area, generally weakening
as it lifts northeast into the drier air, but some showers working
into the forecast area by mid afternoon and into west central WI by
late afternoon or early evening. Instability remains limited
farther north with some 250J/kg creeping into the south by 03Z
increasing to 1250J/kg for parts of northeast IA by 06Z. As a
result, mainly showers with thunder potential increasing with
the increasing instability and low level jet. Isolated storms
per the CAMs/HREF show some potential for gusty winds and hail.
At this time, there is some risk due to the instability (MLCAPE
500-1000J/kg in northeast IA) and high shear during the
overnight hours. The HREF shows some low probabilities for 1"
per hour rainfall rates (10-20%) Monday morning for parts of
Taylor Co. and Grant Co. 13-14Z with the warm front lifting
north. The EC/GFS/Canadian ensemble probabilities are 30-60% for
0.5" or more with the higher probabilities of 70% just north of
the forecast area. The latest flash flood guidance for 6hr is
2-3". The HREF Max 6hr shows the potential for localized 1.5-4"
amounts possible both in NE IA and just to the northwest of the
forecast area, so will want to monitor if storms repeat over the
same area.

Severe weather/heavy rain potential Tuesday:

There continues to be decent model agreement that a stronger
trough over the Plains Tuesday becomes negatively tilted and
lifts northeast with deepening surface low pressure over the
Upper Mississippi River Vally Tuesday. Moderate instability of
2500 to 3500 J/kg is progged (EFI-extreme forecast index-values
with 60 to 80% higher than the climate max) with deep layer
shear of 55kts+. The current Colorado State University machine
learning probabilities/CIPS analogs show the ingredients for
severe weather coming together across eastern a large area of
the Plains into the Mississippi River Valley. A large portion of
our forecast area is included in an enhanced risk (3 of 5) for
severe weather Tuesday. There remain difference in the frontal
details along with timing and strength differences and the model
precipitation forecasts vary greatly. The EC/GFS/Canadian
ensemble probabilities are 50-70% for 0.5" or more with the
higher probabilities of 90% just northwest of the forecast area.
We will continue to monitor the severe weather potential and
the potential for heavy rain. Continue to stay tuned for
forecast updates.

Not as warm today through Tuesday with highs generally in the
70s to low 80s. More seasonable temperatures for the rest of the
week.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1025 PM CDT Sat May 18 2024

CIGS: mostly SKC conditions through Sunday morning, then an increase
in high/mid level clouds for the afternoon and early evening.
Short/medium range guidance shows a dip into MVFR overnight Sunday
(as an upper level shortwave spins across the region).

WX/vsby: no impacts expected through Sunday afternoon. Rain starts
to look likely for late evening through the overnight hours Sunday
as a shortwave trough spins across the region. Enough instability to
support at least scattered storms. A few of the short term models
suggest showers are possible Sunday afternoon - but soundings only
point to mid level saturation with a very dry sub cloud layer. Virga
would be seem to be more favored in this scenario, unless more
vigorous forcing can be realized.

WINDS: west/northwest winds this evening with speeds dropping off
into the overnight - becoming mostly light and variable. Weak high
pressure meanders across the region on Sunday, suggesting low end
speeds (under 10kts) with mostly a southeasterly direction. Winds
will be back on the uptick moving into Sunday night as the sfc
pressure gradient tightens with that shortwave.

&&

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

&&

$$

DISCUSSION...Zapotocny
AVIATION...Rieck