843
FXUS63 KLOT 252010
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
310 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

&&

.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 /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 112 PM CDT Sat May 25 2024

Forecast concerns for the terminals...

- Lake breeze turning winds easterly at the Chicago terminals
  this afternoon

- Couple rounds of showers and thunderstorms possible Sunday,
  first round in the morning with a second round in the late
  afternoon/evening

High pressure will continue to build into the area this
afternoon which will keep sunny skies and light winds in place.
However, a lake breeze is surging inland across northeast IL and
is expected to reach ORD around 19z and MDW between 18z and
1830z. Behind the lake breeze winds will become easterly with a
slight uptick in wind speeds into the 8 to 10 kt range.
Elsewhere, winds will maintain a north-northwest direction
through this afternoon before turning southeasterly overnight.

Heading into Sunday, a line of showers and thunderstorms is
expected to develop across the central Plains and advance
across northern IL Sunday morning through early afternoon.
While initially these showers and storms should arrive in a
decaying state, instability is forecast to be gradually
increasing through the morning as the showers/storms approach
the Chicago terminals (around 16-17z). Therefore, confidence
remains modestly high that showers and storms will impact the
terminals Sunday morning and have maintained the prevailing SHRA
and VCTS mentions. Though, have left GYY dry as the storms are
not expected to reach the airport until after 18z which beyond
their respective TAF period.

Depending on how quickly this initial line of showers/storms
exits the area Sunday afternoon will determine how much
destabilization occurs and whether or not a second round of
storms develops late Sunday afternoon/evening (similar to
yesterday`s forecast). Guidance is unfortunately showing little
agreement on whether or not the environment will recover and
therefore confidence is lower on any impacts at the terminals.
However, felt there was still a sufficient signal to warrant
the addition of a PROB30 to the 30-hour TAF sites to account for
this potential.

Yack

&&

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

&&

$$

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