777
FXUS63 KLOT 030534
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1234 AM CDT Fri May 3 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 /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1220 AM CDT Fri May 3 2024

Aviation Key Messages:

- SHRA persist through pre-dawn hours. Embedded TSRA threat
  diminishing but non-zero, especially toward KGYY.

- Period of MVFR ceilings develops pre-dawn and lingers until a
  cold front passes after sunrise.

- Winds shift NNW with cold front after sunrise, then turn NNE-
  NE by mid-morning. NE winds persist through remainder of
  period.

Somewhat diffuse area of low pressure was lifting north-
northeast across eastern WI as of midnight, with a cold front
trailing through far western IL into western MO. This low was
associated with a mid-level disturbance over central IL, and
will continue to lift northeast with the cold front moving
across the terminals by sunrise. SHRA and embedded TSRA will
persist ahead of the front, with TSRA growing more isolated with
time, with greatest coverage of TS likely toward KGYY. MVFR
ceilings along the cold frontal zone will spread across the
terminals pre-dawn, eventually scattering out to VFR by mid-
morning.

Winds, while somewhat disorganized by earlier stronger
convection, will be generally light W-SW ahead of the cold
front, then shift NNW behind the front by sunrise. Winds will
then continue to veer northeast by mid morning behind the front,
and will generally remain northeast through the day.

Winds turn light easterly Friday evening, and there is some
model guidance indication that MVFR stratus may form east of the
area across Lake Michigan and IN, which could drift west into
the terminals later in the night. Confidence is low this far
out, but have included a FEW020 mention in TAFs.

Ratzer

&&

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

&&

$$

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