639
FXUS63 KLOT 260145
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
845 PM CDT Sat May 25 2024

.KEY MESSAGES...

- Periods of showers and thunderstorms on Sunday, with pockets
  of gusty winds and isolated large hail in the morning and a
  highly conditional risk for storms with large hail, damaging
  winds, and tornadoes late afternoon into the evening.

- Widely scattered afternoon and early evening showers (and
  possibly a few non-severe storms) Memorial Day and again
  Tuesday

- Gradual warming and drying conditions for the end of the week

&&

.UPDATE...
Issued at 845 PM CDT Sat May 25 2024

Early-evening water vapor imagery depicts a low-amplitude upper-
level shortwave and implied jet streak moving across the Southern
Plains, ahead of a secondary, more wound-up, upper-level shortwave
centered over Salt Lake City, Utah. The lead shortwave and
associated jet streak have been responsible for lee cyclogenesis of
a surface low in far southwestern Kansas, which is facilitating the
rapid northward expansion of a broad warm sector into the central
Plains (as well as several areas of severe thunderstorms). Closer to
home, a surface high pressure system continues to pass by, allowing
for a truly spectacular late May evening.

Tonight, the surface low will move into the central Plains as
ongoing convection across the central and southern Plains grows
upscale across two primary zones. The first will be across
eastern Kansas into western Missouri along the stalling warm
front and surface-instability axis. The second area will be
further the north across eastern Nebraska and Iowa along the
nose of an intensifying/tightening 925-850mb low-level jet and
elevated instability axis. While the (severe) convection along
the surface-based instability axis will no doubt miss our area
to the southwest, the convection along the elevated instability
axis will be poised to move into northern Illinois after
daybreak and eventually northwestern Indiana by early tomorrow
afternoon. Prospects for meaningful surface-based instability
ahead of the incoming complex of storms continues to look
pretty low, though steep mid-level lapse rates along the edge of
the elevated instability axis may nevertheless support a few
cores capable of producing small to marginally severe hail. With
time, however, the complex should begin to outrun the
instability axis, leading to a gradual decaying process
(especially as it moves into northwestern Indiana). As a result,
much of our area may experience a lull in shower and
thunderstorm activity early tomorrow afternoon as the surface
low approaches from the southwest and a secondary upper-level
shortwave moves toward central Illinois.

Now, the forecast for tomorrow afternoon remains somewhat
unclear. With copious amounts of instability available to all
thunderstorms tonight (MUCAPE in excess of 3000 J/kg overnight),
the environment will be ripe for the development of several
mesoscale convective vortices (MCVs). Anytime an MCV develops in
close proximity to a synoptic-scale low, there is a threat that
they merge together leading to a compact but potent low
pressure system. This leads to two apparent forecast scenarios
for tomorrow afternoon:

Forecast Scenario 1: No MCV merges with the synoptic-scale low.
In this scenario, the secondary upper-level shortwave will
initiate thunderstorms along the warm front well to our south
across central and southern Missouri and Illinois tomorrow
afternoon. As a result, any additional showers and storms in
our area would depend on any clearing/destabilization that can
take place before anvil shading from storms to our south spreads
overhead by early evening. Isolated non-severe thunderstorms
would result near the center of the surface low passing
overhead, with many areas potentially remaining dry. At this
point, CAM-based deterministic and ensemble guidance (including
incoming 00Z guidance) favors this scenario, so will nudge our
official forecast in this direction.

Forecast Scenario 2: There is phasing between the synoptic-
scale low and an MCV, leading to the passage of a strong low
pressure system over northern Illinois tomorrow afternoon. In
this scenario, locally augmented low-level wind fields and
0-3km-centric instability would support a threat for low-topped
supercells with all severe hazards (potentially soon after the
first round of showers and thunderstorms). The deterministic
ECMWF is pretty much the only model showing such a scenario, and
accordingly, is being treated as an outlier.

At this point, our message is to expect a round of showers and
thunderstorms in the morning, and to keep an eye (or ear) out for an
isolated thunderstorm or two in the evening.

Updated products will be sent soon.

Borchardt

&&

.DISCUSSION...
Issued at 310 PM CDT Sat May 25 2024

Through Sunday Night:

High pressure crossing the area this afternoon as led to a dry
and sunny day with temperatures mostly in the 70s away from Lake
Michigan. Thickening upper-level cloud will shift across the
area tonight as the low and mid-levels remain quite dry.

Focus then turns to potential severe weather in and around our
region for Sunday. A lot needs to unfold with regards to
expected convection from southern Nebraska into northern
Oklahoma late this afternoon into this evening. Per recent
observational trends and morning CAM guidance, clusters of
convection ahead of a lead wave ejecting from New Mexico into
eastern Colorado will likely enhance the wave with a couple
embedded MCVs while tracking toward the Mississippi River Valley
late tonight. Ongoing elevated linear convection should near
the western CWA in the 6 or 9am window. The overall evolution
beyond that point becomes less clear as sufficient low-level
moisture may not advect northward early enough to take advantage
of impressive 8+C/km mid-level lapse rates. Given recent
guidance, the convection should remain mostly sub-severe through
much of the CWA, with the primary hazards being locally large
hail and perhaps some pockets of strong gusty winds. However, if
this decaying storm complex slows by as little as a couple
hours, the low-level environment will become increasingly
favorable for surface- based convection due to additional
heating and increasing moisture with a warm front lifting
northward. In this scenario, all hazards are possible roughly
east of I-57 early in the afternoon.

After the low-level environment attempts to recover in the wave
of the morning convection, a conditional severe weather risk
with all hazards exists early to mid-evening. Given sufficient
heating, isolated to scattered discrete convection could develop
along the cold front as deep-layer shear increases ahead of a
stronger mid-level wave. Current trends suggest this will be a
low probability (<30%) chance as airmass recovery will occur too
late for sustaining stronger updraft cores before the loss of
daytime heating. If the initial wave in the morning trends even
faster than expected, severe storm chances would be notably
higher across much of the area through at least mid-evening.

Kluber


Monday through Saturday:

As the front moves east, drier conditions are expected on
Monday, even if momentarily. Another short wave trough descends
down the longer wave aloft on Monday providing another chance
for showers in the late afternoon and early evening. Moisture is
limited, lapse rates aloft are not particularly robust and wind
shear through the column is fairly weak. However, there is a
little instability aloft that could make some rumbles of
thunder. Better chances are for areas closer to the Wisconsin
state line, but kept at least minimum slight chances for the
entire forecast.

As an almost repeat event on Tuesday, the final short wave
trough descends down out of Wisconsin for the chance for showers
and storms Tuesday afternoon. Precipitable water amounts are
projected to be even less than Monday afternoon with weaker
instability. Current projections have the wave hugging the lake
as it descends, so there are better probabilities for showers
closer to the lake.

Beyond Tuesday, an upper level ridge begins to grow over the
Plains and drift east over Illinois. With heights rising and mid
level subsidence developing, a gradual warming trend with drier
conditions are expected for the end of the week, with a chance
for temperatures away from the lake to start creeping back into
the 80s by Friday and Saturday.

DK

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 646 PM CDT Sat May 25 2024

Forecast concerns include...

Chance of thunderstorms Sunday morning.
Chance of thunderstorms Sunday afternoon/evening.
Gusty southeast winds Sunday.
Mvfr cigs Sunday night.

Convection developing over the central Plains this evening is
expected to continue through the night and reach northwest IL
around daybreak Sunday morning, likely in a slowly weakening
phase but continue across all of the terminals by early Sunday
afternoon. Adjusted timing a little earlier, starting 16z for
the Chicago terminals, but further refinement will be needed
once trends emerge overnight. After this activity moves east,
there may be a lull for a few hours in the mid/late afternoon
and then there will be a chance for at least isolated
thunderstorms late Sunday afternoon into early Sunday evening.
There is still uncertainty for timing and coverage but prob
mention seems reasonable at this time.

Southeast winds under 10kts will continue tonight and then
increase to 10-15 kt Sunday morning with gusts into the 20kt
range developing by late morning. Winds will likely turn more
southerly mid/late Sunday afternoon and then southwesterly
Sunday evening, but there remains quite a bit of uncertainty
for specific wind directions during this time. Gusts should
diminish with sunset Sunday evening.

While mvfr cigs will be possible with any thunderstorms,
prevailing low mvfr cigs may develop Sunday evening or early
Monday morning as cooler air spreads into the area behind a cold
front. Low confidence for mvfr cigs but scattered mention seems
reasonable from this distance. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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