946
FXUS63 KLOT 201730
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1230 PM CDT Mon Apr 20 2026

.KEY MESSAGES...

- Warming trend through Thursday (except near the lake on
  Wednesday).

- Next chances for thunderstorms as early as Tuesday afternoon,
  but mainly Tuesday evening and overnight near/south of I-80.

- More widespread showers and thunderstorms with another cold
  front towards the end of the week.

&&

.DISCUSSION...
Issued at 118 AM CDT Mon Apr 20 2026

The core of a mid 1020s mb surface high will shift east of the
region through the morning with tranquil conditions to start the
work week. Synoptic flow will promote the development of a lake
breeze later this morning and afternoon which will focus
primarily across Illinois lakeside locales and perhaps the
immediate lakefront in northwest Indiana. Away from the lake
breeze influence, temperatures will warm into the mid to upper
50s. Shaved a few degrees off the blended dewpoints this
afternoon, particularly across our southwest where deeper mixing
should occur into an 850 mb dry layer.

A southwesterly LLJ will develop tonight along with a gradual
increase in occasional 20-25 mph surface gusts, mainly near the
urban Chicago core. This LLJ will also transport the leading
edge of another stout EML plume into the region through the day
on Tuesday. The base of this EML plume is forecast to be quite
warm, with 800 mb temperatures near +10 C, and this will keep
our area decidedly capped (at least to surface-based convection)
through the morning and likely much of the afternoon. Given
the very steep nature of lapse rates within the EML, however,
can`t rule out some elevated/high based convection developing
during the afternoon north of I-80, but this potential presently
looks too low to justify an inclusion in the gridded forecast.
Otherwise, Tuesday will feature breezy southwesterly winds
(strongest south of I-80 where gusts to of 35+ mph will be
possible) and warm conditions with temperatures in the 70s.

A low-amplitude shortwave embedded within broad northwesterly
flow aloft is forecast to push across the region late in the
day as a weak frontal boundary pushes out of Iowa and Wisconsin.
Forecast soundings now generally depict more lifting/cooling
occurring at the base of the EML through late in the day and
into Tuesday evening, with a commensurate increase in the number
of ensemble members convecting in our vicinity during this
time. However, chances look to increase through the evening as a
southwesterly LLJ nocturnally intensifies. Blended PoPs have
increased into the 30-40 percent range near/south of I-80 during
this period which looks appropriate at this point. Instability
looks quite sparse, with generally limited moisture return.
Still, stout northwesterly winds aloft will promote at least
modest deep layer shear within the cloud-bearing layer, with a
low potential for some updraft organization and a small hail
threat Tuesday night. If storms do manage to fire and sustain
(which still remains unclear at this time), we would also need
to keep an eye on a very localized training/heavy rainfall/flash
flood potential given the flow orientation and likely slow
storm motions.

Wednesday generally looks quiet as shortwave ridging will build
in overhead. Onshore flow, along with the potential for marine
fog/stratus will hold immediate lakeside high temperatures in
the 50s. The GFS--which is an outlier at this point--holds onto
fairly extensive boundary layer moisture and stratus on
Wednesday with resultant notably cooler temperatures compared
to the rest of the guidance suite. Given its outlier nature at
this point, made no changes to the blended forecast which
advertises highs in the mid and upper 70s away from the lake.

The next large scale/synoptic trough is forecast to arrive in
the region during the Thursday afternoon into Friday morning
timeframe. Strong large scale forcing will promote the
development of widespread showers and some thunderstorms. At
this time, the progged late arrive of the system`s cold front,
overall dearth of instability, and limited deeper layer shear
suggests a muted severe threat into our region. A faster (or
slower) arrival closer to peak heating Thursday or Friday
afternoons would increase the severe threat. High pressure is
expected to build back into the region through next weekend.

Carlaw

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1230 PM CDT Mon Apr 20 2026

Aviation forecast concerns for the 18Z TAFs:

- Breezy south-southwest winds develop tonight (gusts 20-25 kts
  at urban metro sites) and persist Tuesday.

- LLWS away from the metro sites late evening/overnight with 45
  kt low level jet around 2 kft AGL.

- Low (~20-25%) chance for scattered SHRA/TSRA development late
  Tuesday/Tuesday evening. Greater potential appears to be just
  off to the south of terminals, but will need to monitor
  trends.

Quiet VFR aviation weather across the terminals this afternoon,
with surface winds gradually becoming south-southwest while high
pressure drifts away to the east. Pressure gradient is forecast
to tighten up late this evening/overnight, with southwest winds
increasing and likely becoming at least occasionally gusty in
the 20-25 kt range around/after midnight for ORD/MDW/GYY.
Farther west, where surface winds more likely to decouple, a 45
kt southwesterly low level jet will support LLWS conditions at
DPA/RFD. Surface winds then become breezy southwest at all
sites after sunrise Tuesday.

A surface cold front will approach the area from the northwest
late Tuesday afternoon and evening. Some guidance depicts
scattered convective SHRA/TSRA across parts of the area, with
high-res ensembles favoring development just south of the
terminals after 21-23Z. While a few model runs do show some
potential across the terminals. probability of that currently
appears to be less than 30 percent and thus have not mentioned
in the ORD/MDW 30 hour forecasts. Will need to continue to
monitor trends into Tuesday for any increase in potential across
the terminals.

Ratzer

&&

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

&&

$$

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