758
FXUS63 KLOT 062352
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
652 PM CDT Wed May 6 2026

.KEY MESSAGES...

- Areas of frost tonight away from the urban Chicago core, with
  localized sub-freezing temperatures possible.

- While the majority of the time will be dry, there will be a
  few opportunities for showers Thursday through the upcoming
  weekend.

- Other than a possible brief warm up Saturday, temperatures
  will remain mostly near to below average through early next
  week.

&&

.DISCUSSION...
Issued at 126 PM CDT Wed May 6 2026

The temperature/frost/freeze forecast tonight remains tricky. An
expansive ribbon of upper-level cloud cover continues to stream
northeast out of the Colorado Front Range, and this looks to
pivot across roughly the southeast third of the forecast area
late tonight. Farther to the northwest, cloud cover across Iowa
and Wisconsin should generally diminish this evening with the
loss of heating/mixing. At this time, it looks like this will
open up a window for generally clearer skies this evening/early
overnight before additional cloud cover associated with the next
disturbance approaches around or perhaps just prior to daybreak
Thursday. Any thicker cloud cover that does develop in the
region has the potential to significantly alter overnight
temperature trends.

With how dewpoints have trended today, even if temperatures dip
solidly into the mid 30s, widespread frost development is
uncertain. The main concern would be for temperatures to tank
much below blended guidance in the dry airmass, with a threat
for sub-freezing temperatures to become more widespread than
anticipated. Confidence in widespread sub-freezing temperatures
is too low to justify a Freeze Warning, however. With this
forecast package, elected to hoist Frost Advisories for our
Illinois counties north of about I-80 (and outside of the
Chicago) where prolonged clearing looks more likely, with a
potential for temps to undercut the forecast. Interestingly, raw
model guidance remains several degrees colder than statistical
(MOS) output. Farther to the south and east, the potential for
increased mid cloud cover, and generally warmer guidance
precludes the issuance of any frost/freeze headlines at this
time, but won`t rule out the need for expansions to the
headlines this evening. Within the frost advisory area, have
limited frost wording to "areas" at this time given the current
dry surface layer.

On Thursday, a fast-moving and fairly robust shortwave
(currently shifting out of southern Saskatchewan) will push
across the region through midday and into the afternoon.
Guidance has trended a bit more robust with the associated
forcing with this feature, and 500 mb temperatures even look a
bit colder than they did yesterday, with pockets of -30 C
temperatures forecast to drift across northern Illinois. While
the boundary layer will be quite dry (surface dewpoints could
fall into the mid/upper 20s), attendant steep low and mid-level
lapse rates may make up for the lack of more appreciable
moisture, resulting in slivers of weak instability.
Additionally, 700-500 mb moisture is forecast to increase
markedly near/south of I-80 in the vicinity of enhanced mid-
level f-gen.

Based on this, there may be two "corridors" favored for shower
activity on Thursday: one across far northern/northeast Illinois
tied to colder mid-level temperatures, and a second near/south
of I-80 in the vicinity of increased f-gen forcing. Have
continued to slightly boost PoPs to account for these regimes on
Thursday. The limiting factor for higher PoPs (likelies) right
now is the progged dry boundary layer. Given the cold
temperatures aloft, can`t rule out the potential for a few
lightning strikes, but at this time, this signal is too low to
justify including a mention in the gridded forecast.

Broad warm advection is forecast to continue through Thursday
evening south of about I-80. It remains unclear how much
saturation will remain through this time period, but will
continue to hold onto some precip chances in this area to
account for the persistent isentropic upglide.

Another robust impulse arrives on Friday. Strong low/mid-level
convergence will yield and increase in 850-700 mb frontogenesis
once again, facilitating an increase in shower (and potentially
isolated thunderstorm) development through the late morning and
midday. Noting some north-south spread in the track of the
shortwave trough, as well as overall trough amplitude which
casts some uncertainty in the coverage and location of
precipitation. While the latest NBM output has shifted the
corridor of highest PoPs a bit farther south, given the current
ensemble envelop, elected to continue to hold onto likelies a
bit farther to the north.

Saturday looks like it`ll be warm and breezy. A cold front,
accompanied by another shortwave will bring the next chances for
showers to the region Saturday evening into Sunday. This will be
followed by another period of near/below average temperatures
to start next week.

Carlaw

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 652 PM CDT Wed May 6 2026

Key aviation messages for the 00Z TAFs:

- Breezy west-northwest winds diminish quickly with sunset this
  evening, then redevelop midday Thursday with gusts in the
  20-25 kt range.

- Chance (~30%) for scattered SHRA Thursday afternoon.

West-northwest winds, still gusting 20-25 kt at issuance time,
will diminish quickly with sunset and the loss of daytime
heating. Broken VFR deck in the 7-8 kft range will decrease as
well leading to mostly clear VFR conditions overnight and into
Thursday morning.

West-northwest winds will become breezy with gusts 20-25 kt
again later Thursday morning and can`t rule out a few higher
(25-30 kt) gusts during the afternoon hours. In addition, high-
based VFR deck (6500-7500 ft again) will develop again with the
approach of a mid-level disturbance. Have maintained a PROB30
group for scattered SHRA at all sites during the afternoon
hours. Can`t completely rule out isolated TS potential with
steep mid-level lapse due to unseasonably cold air aloft,
though chances remain too low for TAF inclusion at this time.
Should see an end to SHRA and a decrease in both winds/cloud
cover again with sunset Thursday evening. There is some spread
in model wind direction forecasts Thursday evening, though most
continue to depict a westerly component at this time.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Frost Advisory from 1 AM to 8 AM CDT Thursday for ILZ003-
     ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-
     ILZ019-ILZ020-ILZ021-ILZ103-ILZ105-ILZ106-ILZ107-ILZ108.

IN...None.
LM...None.

&&

$$

Visit us at weather.gov/chicago