197
FXUS63 KLOT 151736
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1236 PM CDT Tue Jul 15 2025

.KEY MESSAGES...

- Isolated to widely scattered showers and thunderstorms
  expected this afternoon near/south of Highway 24.

- Thunderstorms are likely late Wednesday afternoon into
  Wednesday evening, some possibly severe posing a threat of
  damaging winds, as well as torrential downpours that could
  lead to some localized flash flooding.

- Wednesday will be hot and humid with peak heat indices around
  100 degrees.

- After a brief break Friday, humidity and the chance for
  occasional bouts of storms expected to return over the weekend
  and stick through next week.

&&

.DISCUSSION...
Issued at 325 AM CDT Tue Jul 15 2025

Sheared out vort max over Missouri will move across central
Illinois this afternoon and into northern Indiana this evening.
Scattered showers and thunderstorms have been developing in the
vicinity of this stretched out mid-level circulation for the
past couple of days and anticipate the same again this
afternoon. Greatest coverage of storms will probably end up just
to the south of our southern CWA, but still expecting to see
some scattered convection at least as far north as Highway 24
and possibly up to around I-80. Precip chances decrease with
northern extent with drier mid level air mass and likely
stronger inhibition with slightly more mixed out low level
moisture. Can`t rule out a lingering shower or two tonight over
southeastern CWA as this shear axis moves east of the area.

Attention Wednesday will be on the shortwave trough currently
over Wyoming. This wave will likely lead to severe convection
that should evolve into an MCS tonight over central Plains.
Seems likely that an associated MCV will develop and the
shortwave trough with an embedded MCV or two will move eastward
into the mid-Mississippi Valley by later in the day Wednesday.
Ahead of this wave, look for hot and humid conditions across the
area Wednesday with highs around/in the lower 90s and heat
indices around 100 degrees in the afternoon. The hot and humid
conditions should fuel moderate to strong instability with
MLCAPE of 2-3k J/kg during the afternoon.

There remain some modest timing and track differences among the
various model guidance, but the general consensus would support
strong-severe thunderstorms developing near or west of our CWA
during the afternoon Wednesday. This activity is expected to
spread east into the CWA late in the afternoon into the evening
hours. Any embedded MCV would likely favorably augment the
otherwise fairly modest background deep layer shear leading to a
locally enhanced threat for a corridor of damaging thunderstorm
winds. Impossible to pin down precise location at greatest risk
at this distance given the mesoscale features that will likely
drive the risk, but the latest SPC SWODY1`s slight risk area
nicely depicts the area most at risk.

In addition to the threat of locally damaging thunderstorm
winds, very high precipitable water values (near 2"), along with
deep warm clouds should lead to storms being very efficient
heavy rain producers with rainfall rates >=2"/hour in stronger
storms. Anywhere that sees even a bit of training of convection
with those type of rainfall rates could lead to some flash
flooding threat as well.

Anticipate convection will weaken after sunset Wednesday
evening as it attempts to move eastward across the CWA,
encountering increasingly greater convective inhibition after
sunset. Hard to say how far east convection will make it, so
maintain POPs across our CWA later Wed night into Thursday
morning, but do taper them off with eastward extent as
convection could mostly dry up before reaching northwest
Indiana.

Position of the composite outflow boundary and cold front will
play a key role in determining where the greatest chances of
renewed convective development will be Thursday afternoon. GFS
and NAM both would have this boundary far enough south that much
of the convection Thursday afternoon could end up south and east
of our CWA. Didn`t have enough confidence to make any changes
to NBM likely POPs for Thursday afternoon south/east of I-55,
but given the tendency for these type of boundaries to end up
farther south/east than progged, it is possible that precip
chances may need to be lowered considerably Thursday, especially
north of I-80 where it looks likely Thursday will be dry.

Fairly brisk northerly winds are still expected down the full
fetch of Lake Michigan later in the day Thursday through early
Friday morning. The lake sea surface temps are quite warm now,
making it easier for these northerly winds to kick up some
moderately large waves. If guidance remains consistent with
the moderate northerlies then we could be headed for a beach
hazards statement late Thursday into early Friday for Lake
Michigan beaches. Thursday night through Friday night, the
boundary should be safely south of our CWA, leaving the area
with cooler and less humid conditions.

This weekend through much of next week, medium range guidance
remains in good agreement in depicting a strengthening upper
ridge becoming established across the southern half of the
country. There will likely be a largely zonally oriented low
level baroclinic zone along the northern periphery of this upper
ridge separating a hot and humid air mass from a more moderate
air mass to its north. Low amplitude shortwaves riding along
the northern flanks of this upper ridge will likely lead to MCSs
tracking across the Plains into the Midwest and Great Lakes
region. Predicting timing and track of these low amplitude waves
at this distance is largely futile, but all indications are
that our CWA will be within the larger "cone of uncertainty"
with a threat for occasional MCSs with attendant threat of heavy
rainfall and severe weather. If at any point the boundary
shifts north of the area, then we could also be at risk for a
period of very hot and humid conditions.

- Izzi

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1236 PM CDT Tue Jul 15 2025

Main Concern:

- Potential for gusty TS late Wednesday afternoon.

Varying south to south-southwest winds may shift to southeast or
east-southeast for a time this evening at MDW and ORD as the
lake breeze pushes inland and then washes out. Expect light
southerly to VRB/calm winds the rest of the night thereafter.
The light winds and humid conditions may set the stage for minor
VSBY reductions at RFD, DPA, GYY early Wednesday.

On Wednesday, VFR conditions and southwest winds (gusts to ~20
kt PM hours) are expected until the mid-late afternoon, when a
broken cluster of TS may be approaching from the west. There`s
uncertaininty on the exact timing and coverage, but felt the
signal was strong enough for possible TS impacts in the last
couple of hours of the ORD/MDW 30-hour TAFs to add a PROB30.
Downburst/outflow winds with any TS would likely be from a
west/west-northwest direction, with gusts to 30 kt or higher
certainly possible.

Castro

&&

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

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

LM...None.

&&

$$

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