Flash Flood Warning 5, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
247 PM CDT Thu Jul 10 2025

.KEY MESSAGES...

- Several waves of showers and storms are possible from this
  afternoon through Saturday across the area. Some storms may be
  severe with damaging winds and flash flooding.

- Provided low coverage of thunderstorms, Friday may be hot and
  humid with heat indices near 100 degrees.

&&

.DISCUSSION...
Issued at 247 PM CDT Thu Jul 10 2025

Through Friday Night:

Several rounds of showers and thunderstorms are expected to impact
at least portions of the area late this afternoon through Friday
evening. With these storms, the primary concerns will be the
threat for strong to potential severe storms, along with very
heavy rainfall and possible flash flooding.

An MCV continues to be noted in satellite imagery shifting over
southeastern MN, with an associated warm air advection wing noted
extending southward along the Mississippi into northwestern IL at
the time of this writing. To the east, a very warm and moist low-
level airmass (low 70s surface dewpoints) continues to
destabilize across northern IL early this afternoon, and has
recently supported the development of a few isolated showers and
storms across southern Livingston and Ford counties in IL. We
expect thunderstorm coverage to be on the increase through the
remainder of the afternoon, with at least scattered coverage
(40-60%) anticipated over northern IL into early this evening as
the warm air advection wing near the Mississippi continues to
slide eastward into northern IL.

Low to mid-level flow is increasing this afternoon (~30 kt) across
the Corn Belt along the southern periphery of the southeastern MN
MCV and overtop of backed southeasterly near surface flow. This is
resulting in ample shear that will be supportive of organized
storms late this afternoon into this evening, possibly including
some supercells. As such, a low tornado and damaging wind threat
exists (level 1 of 5) with these storms into this evening.
Outside of the severe threat, a high Theta E airmass (with
precipitable water values of 1.75"+) will also be supportive of
torrential downpours with these storms.

Additional thunderstorms are likely later this evening and
tonight as the parent MCV shifts into southern WI. This is likely
to promote waves of backbuilding and training east-southeastward
storms over the same areas of southern WI and/or northern IL
overnight. Given the presence of deep rich moisture (precipitable
water values 1.8"+), there is increasing concern that excessive
rainfall amounts (2-3"+) could fall across parts of northern IL
tonight and potentially result in flash flooding. Flash flooding
would particularly be of concern if the heaviest rainfall is
concentrated over the Chicago and/or Rockford metro areas. We
gave strong consideration to issuing a flood watch for portions
of northern IL (north of I-80) given this potential, but with
some uncertainty in which areas are most at risk, we opted to
hold off at this time to allow the evening shift to get a better
handle of observational trends going into the evening. For now,
we plan to highlight this threat in our forecast graphics and
in an ESF.

While some showers and storms could linger into Friday morning, it
does look like there will be a several hour break in the activity
in the morning and early afternoon. However, additional showers
thunderstorms appear likely to redevelop Friday afternoon and
evening along the remnant outflow boundary from tonight's
activity. This activity will once again be developing within a
well sheared and unstable environment which will support
organized and potentially severe storms. Additional instances of
heavy rainfall and possible flash flooding will also be of concern
with this activity into Friday evening.

KJB

Saturday through Thursday:

Another round of strong to severe thunderstorms (and localized
flash flooding) appears quite possible Saturday afternoon,
particularly for areas east of I-39. There may be a fairly early
end to the storms, with an increasing potential for the
footprint of ongoing storms towards/around sunset to be
primarily southeast of I-55 and perhaps even farther southeast.

There are a few noteworthy mesoscale wildcards/unknowns that will
undoubtedly modulate the PM thunderstorm forecast and associated
strong to severe t-storm threat. The first of these, as is very
common, is the extent of convection lingering into Saturday
morning. Deterministic guidance is extremely mixed on this aspect,
with a wide range of plausible outcomes.

On one end of the spectrum, an extensive robust convective
footprint (with associated heavy rain/flash flood/localized
severe threats) could plausibly linger through early to mid
morning Saturday. The stabilizing cold pool from this may then
serve to limit afternoon thunderstorm coverage and intensity,
especially with northwestward extent. There's likely even a
plausible scenario in which the afternoon "zone of concern"
shifts well southeast of I-55 or so. Then on the other end of
the spectrum, if shower/storm coverage is at a minimum through
most of Saturday morning, that would increase the likelihood of
more widespread afternoon storms with a damaging wind threat for
a larger chunk of the CWA (more on this a few paragraphs down).

Synoptically speaking on Saturday, mid-level short-waves embedded
in a trough axis extending back to northwest Ontario will be
accompanied at the surface by a weak surface low and cold front
progressing across the region through Saturday evening. The front
will encounter a very warm (highs ~84-90F) and moist (exactly how
unstable TBD) air mass with dew points well into the 70s. A final
piece of the puzzle is the timing of the cold frontal approach and
passage, which will also in all likelihood be modulated by the
morning unknowns. Operational guidance has overall trended a bit
faster with the incoming cold front, which could veer winds
earlier, reduce boundary layer convergence, and limit storm
coverage, particularly from the I-39 corridor and west. While the
ensemble signal for ~60-70% PoPs/coverage is strong enough to
maintain those PoPs area-wide in the afternoon, robust convective
initiation may favor the eastern or southeastern half of the CWA.
Thunderstorm chances into the early evening, likely focused
primarily southeast of I-55 (and possibly only over our far
southeast CWA) will then quickly come to an end behind the front.

Regarding the severe threat, deep layer shear is forecast to be
somewhat muted (20-30 kt), which would limit the chance for
widespread intense/organized storms. That said, 20+ kt of shear
plus steep low-level lapse rates/moderate to high DCAPE and precip
loading in downdrafts are ingredients that may yield at least
localized corridors of wind damage, especially if small forward
propagating segments can develop. Given the overall uncertainty in
the setup and storm intensity limiting factor, SPC's threat level
1 of 5 (marginal risk) in the day 3 outlook update appears quite
reasonable.

Saturday evening's cold front should push far enough southeast
into Sunday, along with very dry mid-level air and neutral to
positive mid-level height tendencies, to result in a mostly (if
not entirely) dry and less humid period at least through Tuesday
morning. The 12z NAM was regarded to be an unrealistic outlier
with additional showers and storms later Saturday night into
Sunday. Thunderstorm chances may then again uptick midweek with
the next short-wave trough and accompanying cold front. Temps
will continue to average solidly above normal with moderately
humid to muggy conditions common to mid July, and then possibly
cool to below normal by the end of the work week in the wake of
the cold front passage.

Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1239 PM CDT Thu Jul 10 2025

Key messages for the 18Z TAF period include:

* Scattered showers and thunderstorms this evening and
  overnight, some of which could feature very heavy rainfall.

* Additional showers and storms expected on Friday.

A system of rain and thunderstorms is anticipated to move in
from the west later today, mid-late afternoon near RFD and early
evening in Chicagoland. The highest coverage is anticipated
overnight. Periods of very heavy rain will be possible beneath
these showers. The latest TAFs limit vis reductions to MVFR, but
IFR is certainly plausible. Rain will move away to the east
during the early morning. Additional showers and storms are
expected Friday afternoon and evening.

Meanwhile, easterly winds largely below 10 kt will go variable
tonight with the showers around and may get locally gusty at
times. Expect SW winds near or below 10 kt during the day on
Friday.

Doom

&&

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

&&

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