195
FXUS63 KLOT 021953
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
253 PM CDT Thu May 2 2024

.KEY MESSAGES...

- Scattered showers and thunderstorms will increase in coverage
  this afternoon, and continue through the overnight hours.

- A few storms may be severe this afternoon with hail up to 1
  inch in diameter, 60 mph winds, and soaking downpours. The
  severe threat should wane after sunset.

- Periodic chances for showers and thunderstorms will continue
  from this weekend onward, with some potential for severe
  weather on Tuesday.

&&

.DISCUSSION...
Issued at 253 PM CDT Thu May 2 2024

Through Friday night:

Showers and thunderstorms are off to an early start along the I-55
corridor, seemingly due to the interaction with an eastward-moving
mid-level gravity wave and the northward-moving warm front.
Additional showers and storms are expected to develop throughout
the afternoon and especially after sunset as the right entrance
region of an upper-level jet streak moves over the Great Lakes.
As we already saw with the first batch of showers and
thunderstorms, a few may pulse to severe levels with 1" hail and
60 mph "splat" downbursts through about midnight thanks to
MLCAPE 1000-1250 J/kg and meager deep layer shear. With that
said, widespread severe weather is not expected today, and the
threat should wane with the loss of heating after sunset (though
a few non-severe thunderstorms may continue through the
overnight hours).

Showers will likely continue through daybreak Friday as a cold front
sweeps across the area. Chances for rain will then end from west to
east Friday morning behind the front as drier air works into the
region. Clearing skies and highs in the lower 70s are expected
tomorrow afternoon, though stout northerly winds off Lake Michigan
will keep lakeshore locations some 10 to 15 degrees colder,
particularly in northwestern Indiana.

As a surface high pressure system slides overhead tomorrow
night, slacking winds and clear skies will allow for efficient
radiational cooling. Overnight lows should dip into the upper
40s with locally cooler readings in typical cold spots. If drier
guidance verifies (allowing for even more efficient radiational
cooling), some lower 40 degree readings cannot be ruled out.

Borchardt

Saturday through Thursday:

Primary forecast concerns are the potential for severe
thunderstorms Tuesday and possibly isolated strong/severe
thunderstorms Saturday afternoon/evening.

The models are now showing a compact system moving across the
local area Saturday afternoon/evening with a chance for
thunderstorms. The ensembles are in reasonable agreement for
this potential and there is some instability. There will be the
potential for a few strong/severe storms during peak heating in
the afternoon/early evening. Blended pops are generally in the
low chance range but these may need to be increased if these
trends continue.

Sunday looks mainly dry and there is quite a bit of uncertainty
for precip chances Sunday night into Monday morning with the
Canadian/ECMWF fairly dry with only the GFS showing
precipitation. After the wave on Saturday afternoon/evening and
as the pattern shifts into early next week, this time period may
end up being dry but made no changes to the blended pops which
are likely too high, in the chance range, for Sunday night into
Monday.

By Monday afternoon into Monday night, convection is expected
to develop well west of the area and possibly reach northwest IL
by daybreak Tuesday and if it does, most likely in a decaying
phase. With temps likely to reach 80 and dewpoints in the 60s on
Tuesday, this will provide plenty of instability for additional
thunderstorms to form Tuesday afternoon and evening and some of
these thunderstorms may become severe. Precipitable water
values look to reach into the 1.5 to 1.75 inch range, which will
allow for heavy rain.

The pattern stays unsettled into the end of next week with
periodic chances for showers and thunderstorms, though there
will also be several dry periods. cms

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1237 PM CDT Thu May 2 2024

Key Messages:

- Southerly wind shift expected at the terminals by 20z this
  afternoon.

- Isolated to widely scattered late afternoon and early evening
  thunderstorms around the terminals after 21z.

- A period of more widespread showers (and a few storms)
  expected mid to late this evening through early Friday
  morning.

- West-southwesterly winds shift northerly by daybreak Friday,
  then northeastward through the day.


A couple periods of active weather are expected later today and
tonight as a storm system tracks north-northeastward from IA
into the Upper Great Lakes. In the near term, a southerly wind
shift is expected in the 19 to 21z timeframe as a surface warm
front lifts northward across the terminals. Once this occurs, a
warm and moist airmass will allow for the development of some
isolated to widely scattered thunderstorms later this afternoon.
While it remains a bit unclear if these storms will move over a
given terminal late this afternoon, there is moderate
confidence that a few will be around the area. For this reason,
we continue to advertise this potential in a tempo group in the
22 to 24z timeframe.

More widespread showers (with some embedded storms) are expected
to develop/shift eastward across the terminals tonight as a cold
front approaches. The best timing for the Chicago terminals
looks to be in the 04 to 09z timeframe, and more in the 03 to
08z period at KRFD. A period of MVFR CIGs and VSBYs may also
accompany some of the rain overnight. MVFR CIGs may linger for a
few hours following the end of the rain late tonight, but
conditions should improve back to VFR through the morning
following the cold front.

Winds will shift north-northwesterly late tonight following the
cold frontal passage. Directions will then settle into a
northeasterly direction during the day Friday.

KJB

&&

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

&&

$$

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