428
FXUS63 KLOT 281759
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1259 PM CDT Tue May 28 2024

.KEY MESSAGES...

- Scattered thunderstorms expected this afternoon into early this
  evening, the strongest of which may produce locally gusty winds
  to 50 mph and hail to the size of a penny.

- Cool, but dry, weather expected Wednesday and Thursday,
  followed by moderating temperatures back into the lower 80s by
  early next week.

&&

.UPDATE...
Issued at 1026 AM CDT Tue May 28 2024

As yesterday`s upper level wave has moved to the southeast,
clearer skies have developed over northeastern Illinois
prompting a downward adjustment to the sky cover forecast for
the short term. But all eyes are on satellite and radar as the
next wave is clearly visibly moving southeast out of Minnesota
and western Wisconsin. For now, there has not been any
indication of lightning with this broken line of showers.
However, as they move down, they will be able to interact with
just over 1000 J/kg of MUCAPE that recent mesoanalysis displays
over northern Illinois. Time of arrival still looks good for
around 3 PM around the Wisconsin/Illinois border, with numerous
showers expected and the chance for thunderstorms through this
evening with the potential for stronger wind gusts and maybe
some penny size hail.

DK

&&

.DISCUSSION...
Issued at 307 AM CDT Tue May 28 2024

Through Wednesday:

Water vapor imagery early this morning depicts our mid-level
impulse responsible for yesterday afternoons scattered
thunderstorms currently sliding east-southeastward right across
southeastern WI/ and far northeastern IL. The lingering showers
associated with this impulse will continue to hug the
southeastern WI shores of Lake Michigan through sunrise. Most of
our area remains dry this morning, but cannot rule out a few of
the ongoing showers in southeastern WI briefly shifting into
parts of Lake county IL during the predawn hours, before the
focus quickly moves offshore.

Following a quiet and dry morning, the focus for this afternoon
and evenings weather quickly shifts to the next impulse currently
noted in the water vapor imagery across the eastern Dakotas. This
feature will be the driver for yet another round of afternoon and
early evening thunderstorms as it digs southeastward into our
area. Similar to yesterday, low-level moisture is expected to
remain marginal through afternoon, as dewpoints remain in the
lower 50s. Accordingly, MLCAPE values will remain lackluster, with
values only peaking in the 500-800 j/kg range. Deep layer shear
is also not expected to be overly favorable for organized severe
storms as a result of the stronger mid-level flow remaining
displaced to the west and southwest of the area. Nevertheless,
unseasonably cold temperatures aloft (e.g. 500 mb temperatures
around -20C) are expected to again foster rather steep low to mid-
level lapse rates as surface temperatures warm into the low 70s
this afternoon. This may thus act to compensate for the meager
MLCAPE values and poor shear to at least support some low threat
for a few isolated pulse-type storms and loosely organized multi-
cell clusters that could become capable of producing a few
instances of locally gusty winds to 50 mph and possibly some small
hail. The threat of storms will wane shortly afternoon sunset,
though some lingering showers could persist through the evening.

The weather will quiet down for Wednesday, but temperatures will
be below average. Expect high temperatures Wednesday to only be
in the 60s to low 70s. The coolest temperatures will be near the
lake due to the persistent breezy onshore flow. Dangerous swimming
conditions are also expected along Lake Michigan beaches on
Wednesday due to enhanced wave action from the breezy northerly
flow down the lake.

KJB


Wednesday Night through Monday:

The windward side of a departing upper trough will spread
surface high pressure across the Great Lakes late Wednesday and
remain in the region into early Friday. This should keep
conditions quiet and clear Wednesday night and Thursday. We can
expect seasonably mild temperatures on Thursday with highs
forecast to reach the lower and middle 70s for most of us. Light
onshore flow will keep areas near the lake several degrees
cooler.

The high will begin moving away to the east early Friday and the
mild return flow will help pull highs into the middle and upper
70s to close out the work week. Clouds are anticipated to build
during the day as moisture is advected in aloft ahead of an
approaching shortwave disturbance. This wave is expected to
bring a push of showers across the region late Friday and
Saturday. There is spread on the timing and magnitude of the
wave with the GFS being the most aggressive with forcing. In any
case, while embedded thunderstorms look very achievable, really
no guidance is hinting at an overly supportive thunderstorm
environment. At this point, the greatest precip potential for us
here looks to be late Friday night through Saturday morning
meaning we`ll hopefully spend the bulk of Saturday rain-free.

There is rather poor agreement among medium-range guidance on
the regional synoptic pattern beginning early next week. It
does look to be a moderately unsettled one with various
disturbances floating around causing models to spit out sporadic
rain and storm chances Sunday through Tuesday. The GFS and Euro
over several runs have been bringing a cold front through the
region sometime between late Sunday and early Tuesday with an
appreciable thunderstorm environment building ahead of it. We`ll
continue to monitor this potential as model confidence for this
period builds. There is considerable agreement among
deterministic and ensemble guidance in a warmup to more early
summer-like conditions early next week just in time for the
dawn of meteorological summer!

Doom

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1259 PM CDT Tue May 28 2024

Forecast concerns for the terminals...

- Lake breeze may impact the Chicago terminals early this
  afternoon

- Scattered showers and thunderstorms this afternoon and evening

- Period of low-end VFR to MVFR ceilings late tonight into
  Wednesday morning

Scattered to broken VFR clouds will continue to develop this
afternoon with otherwise light northwest winds around 6 to 8
kts. However, a lake breeze is trying to surge inland across
northern IL and northwest IN this afternoon which may result in
an easterly wind shift at ORD and MDW. Given that winds with the
lake breeze are equal to the prevailing winds, confidence is
low as to whether or not the boundary will actually surge
through ORD and MDW or stall just east of the airports.
Therefore, have decided to forgo a formal mention of an easterly
wind shift at ORD, but since the lake breeze is almost to MDW
did add a wind shift at 1830z. Regardless, winds at the Chicago
terminals should turn back northwest as showers arrive later
this afternoon.

Scattered showers and thunderstorms are expected to develop
across northeast IL and northwest IN this afternoon as an upper-
level disturbance pivots through the area. While instability
continues to look modest, shower and storm coverage is forecast
to be more robust than yesterday thus confidence on storms is
higher today especially in the 22z to 00z timeframe.
Additionally, there is the potential that the strongest storms
may produce locally gusty winds and small hail but widespread
severe weather is not expected.

Showers and storms will come to an end this evening (likely by
02z at the latest) with dry conditions expected for the rest of
the period. However, guidance continues to hint that some 2000
to 3000 ft ceilings may try to develop overnight and linger into
Wednesday morning. While most guidance is in good agreement on
the ceilings, ensemble probabilities are only around 30% for
MVFR conditions tonight so have decided to maintain the SCT020
mention for now. Any MVFR ceilings that develop tonight will
erode by midday with otherwise northwest winds becoming
northeasterly Wednesday afternoon.

Yack

&&

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

&&

$$

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