780
FXUS63 KLOT 161218
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
718 AM CDT Wed Jul 16 2025

.KEY MESSAGES...

- Scattered to numerous thunderstorms are expected this
  afternoon into the early evening. There is a level 2/5 severe
  weather risk for most of the area with damaging winds as the
  primary severe weather hazard.

- Hot and humid conditions are expected today with peak heat
  indices of around 100 degrees.

- MUCH cooler and less humid Thursday with gusty northerly winds
  and dangerous swimming conditions at Lake Michigan beaches.

- Periods of warmth, humidity, and thunderstorms expected this
  weekend into early next week.

&&

.DISCUSSION...
Issued at 343 AM CDT Wed Jul 16 2025

Through Tonight:

Today`s main focus is on the likelihood of thunderstorms this
afternoon into this evening as a convectively-augmented
shortwave trough/MCV tracks through the region. At press time,
this feature was located in western Iowa and was powering an
expansive thunderstorm complex extending from central Iowa
southwestward through northeastern Kansas. The expectation is
for this thunderstorm complex to continue gradually losing steam
with time through the remainder of the night while exhausting
an outflow boundary, though (perhaps unsurprisingly in this
moist, unstable mid-summer air mass) it appears that this
complex will persist longer than what CAM guidance is showing,
which does throw some additional uncertainty into the forecast
for today.

Assuming that this complex does eventually largely die off
prior to reaching northern Illinois, additional thunderstorms
are likely to redevelop near the Mississippi River during the
late morning/early afternoon either along the complex`s remnant
outflow boundary or a cold front lagging slightly behind this
outflow boundary. The greatest coverage of thunderstorms is
expected to be found close to the center of the MCV, which CAM
guidance unanimously depicts tracking through southern/central
Wisconsin. Between the MCV/shortwave and convergence along
surface boundaries, there should still be enough forcing to
support at scattered thunderstorms in our forecast area, and
most of the 00Z CAM suite actually depicts a broken or solid
line of thunderstorms marching across our CWA later today. The
orientation of the surface boundaries and the influence of any
morning convection/cloud debris will likely play key roles in
just how widespread convection ultimately ends up being this
afternoon.

While mid-level lapse rates will be lackluster, progged
thermodynamic profiles support a threat for damaging winds from
water-loaded downbursts with the strongest thunderstorms that
develop today. That would especially be the case within any
bowing segments that materialize. While damaging winds by and
large appear to be the primary severe weather hazard today,
deep-layer shear profiles closer to the core of the MCV may be
favorable enough to support at least transient supercellular
structures, which could carry a slightly greater threat to
produce isolated large hail or weak tornadoes in an environment
that is otherwise not overly favorable for either of those
hazards. A lower-end threat for brief, weak QLCS tornadoes also
could not be completely discounted with any northeastward-surging
bowing segments that develop north of I-88 given the expected
presence of 25-30 kts of 0-3 km shear there.

It appears that convection on the MCV`s southern flank should
be progressive enough to preclude an appreciable flash flooding
threat from materializing this afternoon and evening, though it
should still be said that precipitable water values pushing 2"
and 12000-15000 ft warm cloud layer depths will support very
efficient rainfall rates, so couldn`t completely rule out some
hydrologic concerns materializing in some of our usual trouble
spots even if there isn`t much in terms of training convection.
One area to watch for a potentially higher threat for flash
flooding would be our far southern CWA tonight as the outflow-
augmented front may stall out there as it takes on a more zonal
orientation. A strengthening low-level jet overriding this
boundary could foster the development of training convection
just north of this boundary, though this is not a sure thing
yet, and if this does occur, it`s possible that this ends up
occurring south of our CWA.

Lastly, it will be hot and humid prior to the arrival of the
thunderstorms later today. Assuming that storms don`t arrive
earlier than expected, high temperatures are forecasted to
reach the upper 80s to low-mid 90s this afternoon, which, when
combined with low-mid 70s dew points, would result in peak heat
indices near 100F.

Ogorek


Thursday through Tuesday:

Fairly strong high pressure (~1020mb) for mid-summer combined
with push from earlier convection should send the composite
outflow boundary/cold front south of our area Thursday. In the
wake of the front, look for much cooler and less humid air to
filter into the region. Morning stratus should break up into a
SCT V BKN stratocumulus deck during the afternoon, with this
cloud likely aiding in keeping temps cooler. Thursday afternoon
temps could be around 20F in the Chicago area with readings in
the low to mid 70s with a brisk wind off the lake. That brisk
wind off the lake will also help build up larger waves and lead
to hazardous swim conditions at unprotected Lake Michigan
beaches Thursday into Thursday night.

Friday will be warmer, but with still comfortable humidity
levels. Boundary that will have pushed south of our area
Thursday should stall near the Ohio River before heading back
north as a warm front Friday night into Saturday. There`s been a
pretty consistent signal in bringing a convectively enhanced
shortwave across the region later Friday night into Saturday.
Still several days out, so exact timing of this feature and
where exactly it tracks may need to be refined in coming days,
but the general idea of an MCS moving across the mid-Mississippi
Valley in this time frame, possibly affecting portions or all
of our area, continues.

Medium range guidance has trended toward slightly more
amplified northern stream troughing over the Great Lakes into
New England later in the weekend. This would allow for a
transient sfc high to move across the Great Lakes Sat night into
Sunday. This, combined with the prior MCS, could push the
effective boundary and rain/storm chances south of our area.
Confidence is too low to reduce or remove NBM POPs for Sunday,
but there is an increasingly plausible scenario where the
second half of the weekend could end up dry and a bit cooler
and less humid.

Looking ahead to the first half of next week, there continues to
be a consistent signal in medium range guidance that a strong
upper level ridge and associated dome of heat will build over
the central and eastern portions of the nation. In this time
frame, we`d expect that the front that pushes southward some
Sunday will lift back north bringing the potential zone of MCS
activity with it back into our area for a time early next week.

By the middle of the week, the upper ridge is progged to become
anomalously strong and centered over the lower and middle
Mississippi Valley. Assuming MCS activity doesn`t significantly
impede the northward movement of the boundary, our area could
get into a bout of some potentially dangerous heat during the
middle and end of next week. Operational runs of the GFS and
ECMWF both are explicitly forecasting 80F+ dewpoints
accompanying the heat, which given we`ll be in the heart of
evapotranspiration season, may not be unreasonable. Still plenty
of uncertainty regarding if convection can temporarily impede
the northward building heat, but concern is growing that
potentially dangerous heat wave could develop next week in or
near our area.

- Izzi

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 718 AM CDT Wed Jul 16 2025

Key Messages:

- Thunderstorms expected this afternoon into the early evening.

- MVFR ceilings possible tonight into tomorrow morning.


Scattered to numerous thunderstorms are expected to spread into
the area from the west this afternoon. The precise timing of
thunderstorms affecting our TAF sites still remains an item of
somewhat low confidence, though the 3-4 hour long TEMPO groups
for TSRA continue to give a good ballpark estimate for when
storms are most likely to occur. Will need to keep a close eye
on the storms in eastern Iowa as, if they persist, then the onset
time of any showers or lightning may be several hours earlier
than presently advertised in the TAFs. The low-level jet that`s
driving the Iowa convection is expected to continue weakening,
and the overall trends on both radar and satellite with those
storms have been down, so it appears more likely than not that
these storms should dissipate prior to reaching any of our TAF
sites, but confidence in that outcome is not overly high. Any
storms that do affect our affect the terminals today would be
capable producing strong winds and torrential rainfall that
could reduce visibilities to IFR levels or lower.

After storms clear the area and a westerly wind shift occurs
either with the storms or with an incoming cold front, MVFR
ceilings are likely to move into the area tonight and persist in
some capacity into tomorrow morning. A period of IFR ceilings
isn`t out of the question either, but didn`t have high enough
confidence in this possible outcome to advertise it in the TAFs.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Air Quality Alert until midnight CDT tonight for ILZ005-ILZ006-
     ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-
     ILZ107-ILZ108.

IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...None.

&&

$$

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