816
FXUS63 KLOT 120538
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1238 AM CDT Tue May 12 2026

.KEY MESSAGES...

- Tuesday will be warm and start dry with showers and perhaps a
  thunderstorm during the afternoon and evening hours.

- Cooler conditions are expected on Wednesday before a longer-
  lasting warm up arrives to close the week.

- The May 16 through 19 timeframe may feature several rounds of
  showers and storms in the general region.

&&

.DISCUSSION...
Issued at 138 PM CDT Mon May 11 2026

A large surface high pressure system continues to serve as the
main factor of influence today, and is leading to sunny skies,
an east to northeasterly breeze, and cool temperatures in the
upper 50s to lower 60s.

Tonight, the surface high will shift eastward causing winds to
gradually veer southeasterly. A gradient in overnight lows is
expected with partly cloudy skies holding values in the upper
30s to lower 40s across Illinois and largely clear skies
allowing for temperatures to drop into the mid 30s across
northwestern Indiana. Do expect at least patchy fog to develop
across northwestern Indiana overnight, but will hold off on a
Frost Advisory given the expectation for temperatures to remain
above freezing.

Tomorrow, an upper-level shortwave (currently propagating along
the Montana, North Dakota, and Saskatchewan borders) will dive
southeastward into the Great Lakes. Ahead of the wave,
southwesterly flow will increase markedly leading to breezy
surface winds through the afternoon hours. With a very dry start
to the day (dew points in the upper 20s), continue to lean on
the more "mix-y" guidance which advertises gusts peaking near 40
mph by early afternoon. Consequently, a plume of warm
temperatures (850mb values near +12C) will advect into the
region via the strong southwesterly flow, which with the dry
profile will support highs in the upper 70s to lower 80s
areawide.

During the afternoon hours, gradually increasing DCVA ahead of
the approaching trough will support an area of southeastward-
moving showers ahead of a southeastward-moving cold front. With
the residual dry airmass, it may take time for top-down
saturation to allow for raindrops to survive to the ground
through the afternoon hours. In fact, local enhancements to the
wind field may occur as evaporation leads to more efficient
downward mixing. With that said, suspect that saturation will
finally be archived with southeastward extent, supporting a
gradual increase and broadening of PoPs through the afternoon
hours with time. With effective equilibrium levels expected to
terminate right around minus 20C, chances for thunder continue
to look on the lower end of the spectrum and around 15 to 20% at
any given point.

With respect to the threat for severe weather along the cold
front tomorrow evening, am not impressed with the exceedingly
marginal thermodynamics (MUCAPE of 100-200 J/kg) and apparent
lineage to preceding rainfall (to moisten the low-level moisture
profile) to take advantage of what would otherwise be a
supportive forcing/kinematic regime. In fact, suspect little to
no convection will be sustained along the front, favoring a dry
evening and overnight period. So, will withhold any mention of
severe weather in any outgoing products.

Quickly rising surface pressure in the wake of the cold front
will facilitate the efficient advection of cool air in the wake
of the front Tuesday night into Wednesday. A trailing shortwave
arriving in the morning hours may actually bolster cold air
advection through the day. Falling 850mb temperatures toward 0C
will set the stage for steep low-level lapse rates leading to an
expansive stratocumulus deck by mid-morning. In fact, would not
be surprised in the least to see sprinkles or a few showers
throughout the day (have manually introduced "silent" 10 PoPs
for now). When taken altogether, Wednesday is shaping up to feel
a bit more like fall than spring.

Toward the end of the week, the upper-level pattern will
undergo a transition from predominant toughing to quasi-zonal
flow along the US-Canadian border. As a result, the much-
anticipated transition toward warmer weather remains on track to
arrive by this weekend. Will also note an ensemble signal for
rounds of showers and storms in the general region in the May
16-19 timeframe.

Borchardt

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1238 AM CDT Tue May 12 2026

E/ESE winds under 10 knots at TAF issuance will steadily
increase and veer this morning. While there may be a period of
SSE gusts to 20 knots around and after sunrise, winds are
expected to shift just W of S by 14-15Z at ORD/MDW. Gusts will
continue to increase through the afternoon as deep diurnal
mixing develops. SW winds will frequently gust over 30 knots
through the afternoon, including a period of the highest gusts
around 40 knots mid afternoon. Winds will slightly diminish
early this evening before shifting NW with gusts around 25 knots
behind a cold front late evening and overnight.

While conditions will remain VFR through the period, there
could be a few exceptions. Substantial virga from high-based
SHRA this afternoon may become heavy enough to reach the ground
and produce brief bouts of MVFR visibility and additional strong
gusts. There is also a 10 to 20 percent chance of TSRA with the
cold front mid- evening. Maintained a dry forecast as moisture
availability appears too low for coverage to warrant a mention
in the TAF at this time. Finally, patches of high-end MVFR
ceilings may brush the area to the northeast overnight.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 9 AM this morning to 7 PM CDT
     Wednesday for the IL nearshore waters.

     Small Craft Advisory from 9 AM this morning to 1 AM CDT
     Thursday for the IN nearshore waters.

&&

$$

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