767
FXUS63 KLOT 270834
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
334 AM CDT Mon Jul 27 2026

.KEY MESSAGES...

- Thunderstorms possible through early this evening, any storms
  that occur could be severe with damaging winds and potentially
  some large hail.

- Heat and absolutely stifling humidity will result in
  dangerous heat indices outside of any storms today.

- Dangerous swimming conditions due to large waves and strong
  currents are expected late tonight through early Wednesday
  morning.

&&

.DISCUSSION...
Issued at 334 AM CDT Mon Jul 27 2026

Dense Fog This Morning:

The exceptionally moist low-level air mass and and southwesterly
warm/moist advection led to the development of (rare for late
July) fairly widespread dense fog. To this point, dense fog
coverage has been highest generally west of the Fox Valley (ie.
western half of the CWA). Opted to issue a Dense Fog Advisory
until 8 AM and assess observations and webcams for any eastward
expansion.

Castro

Thunderstorms and Associated Severe Threat Today:

While Mother Nature may finally be starting to tip her hand with
regards to convective trends, there remains a good deal of
uncertainties today into this evening. Early this morning there
is a band of widely scattered thunderstorms from north of
Milwaukee to arcing northwest to around Duluth and then
northwest to extreme southeast Manitoba. Convective coverage
has been somewhat limited thus far, and even the size of the
more intense supercells over and near western Lake Superior
haven`t been that large. This has prevented any meaningful
clustering and amalgamation of cold pools up to this point. The
12km NAM and RGEM both had and MCV having already developed by
06z, which clearly hasn`t happened yet.

There appears to be a shortwave trough on water vapor imagery
from northeast North Dakota into northwest Minnesota, which
seems to be driving or at least aiding the convective
development across northern Minnesota. Not seeing any guidance
that has much of a reflection of this shortwave seen in water
vapor imagery, though to the east of this feature is where the
RGEM and NAM have already blown up a strong MCV. The million
dollar question is will a MCS organize, and if so, when and
where?

Really not much of a low level jet out there with 20-30kt
westerly flow noted from on DMX and MSP VWPs. The 00z GRB
sounding depicted quite a bit of convective inhibition, even
for elevated parcels, which would match what SPC mesoanalysis
page shows across MN and WI. This is probably why coverage of
convection thus far has been as minimal as it has been. Suppose
it can`t be ruled out that there will be an uptick in convective
coverage yet this morning, given the apparent wave noted in
water vapor imagery. RAP also shows a modest low level jet
developing yet this morning across northern MN aiming into
northwest Wisconsin. That is the area that seems most prone to
potential clustering and MCS development, so we will be watching
that area closely in the coming hours.

Most CAMs do develop an MCS, so if an MCS doesn`t evolve out
this scattered convection, that pretty much renders most CAM
guidance useless for convective trends today. Plan to maintain
some low chance pops this morning, especially northeast
IL/northeast IN in case any of this scattered elevated
convection were to build southward. Do ramp pops upward by early
afternoon, especially eastern CWA, in deference to the model
guidance. Extrapolation would also suggest that the subtle
shortwave noted on water vapor would likely be near southern WI
to western lower Michigan 18-21z this afternoon. So perhaps that
could be an impetus for convective initiation.

More formidable shortwave is noted farther northwest across
south central Manitoba. This is the primary shortwave that will
carve out a deep trough over the Great Lakes region this evening
and lead to sfc cyclogenesis over lower Michigan. If more
widespread convection with this lead shortwave or an MCS from WI
rolls across the area this afternoon, that should greatly limit
if not eliminate the threat of storms this evening. If there
isn`t earlier convection in our area, then that ups the odds a
bit for convection along the cold front and associated with the
more pronounced shortwave this evening. However, the strongest
large scale ascent with this shortwave looks to remain more to
our east and with considerable dry air aloft, it is possible
that storms may struggle to form or remain isolated along the
cold front.

Unfortunately, a volatile air mass will be in place across the
area through this evening with MLCAPE over 4000 J/kg and
unseasonably strong (40-50kt) 0-6km shear. Any mature storms
in this environment would likely become severe with primarily a
damaging wind threat, but weak low level shear and long
hodographs would also support intense supercells which could
also pose a large hail threat. This severe threat all hinges on
there being storms, which isn`t necessarily a lock, given that
there are still substantial uncertainties whether storms will
occur. The chance of storms increases with eastward extent
across the CWA and PoPs reflect that today.

- Izzi

Heat today then beach hazards and smoke:

Outside of storms, the other big headline today will be the
oppressive humidity and resultant dangerous heat indices.
Widespread 80F+ dewpoints were observed Sunday afternoon within
this incredibly moist air mass that is seeping eastward across our
CWA. Widespread upper 70 degree dewpoints at this time of day is
very impressive and likely portends fairly widespread low to
locally middle 80 degrees dewpoints today once evapotranspiration
kicks in diurnally. One the fog and low clouds lift and burn off,
highs in the lower 90s with dewpoints in the low to mid 80s should
support heat indices of 110 to perhaps locally near 120 degrees
this afternoon over our western CWA. In collaboration with DVN and
ILX, we have issued an Extreme Heat Warning for these areas,
despite the 48 hour criteria not quite being met.

Farther east, kept forecast high temps a few degrees lower, "only"
in the upper 80s to near 90 degrees. These areas should see heat
indices 105-110 degrees. If storms don`t materialize until very
late in the afternoon or early evening, then heat indices up to
around 110F can`t be ruled out farther east into the Chicago metro
area south down the I-57 corridor. Confidence is lower in these
areas, so opted to go with a heat advisory here. Ran all heat
headlines until 02z, but where/if storms occur, then portions of
advisory/warning can be canceled early.

Behind the cold front Monday night, look for a quick ramp up in
northerly winds behind the front. The strong northerly winds down
the full fetch of Lake Michigan will result in large waves and
dangerous currents. A beach hazard statement has been issued for
all southern Lake Michigan beaches through early Wednesday
morning.

Only other item of note is the potential for wildfire smoke
getting back into the area. Satellite imagery Sunday showed what
appeared to be fairly thick smoke across Montana and western North
Dakota. The northwesterly flow aloft should advect this smoke into
our area later tonight into Tuesday. I`m bumped up sky cover to
account for this smoke through Tuesday, but will likely need to be
extended out farther. At this point, it doesn`t look like there
will be high near surface concentrations of smoke, so have not
added it into the weather grids. Hopefully the smoke stays mostly
above the surface this go around.

- Izzi

Wednesday onward...

High pressure will largely remain in control of our sensible
weather through much of the work week yielding dry and more
comfortable conditions, particularly Wednesday and Thursday. The
moist and instability axis will start to build back towards the
region Thursday afternoon into Friday, and there`s a signal for
one or several disturbances embedded within the northwest flow
aloft to pivot across the Great Lakes region. This may yield the
next chances for showers and thunderstorms, which may linger
into Saturday before another cold front arrives during the
Saturday night/Sunday timeframe.

Carlaw

&&

.AVIATION /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1257 AM CDT Mon Jul 27 2026

Main Concerns:

- Lower flight categories early this morning

- TS with gusty to damaging wind potential late this morning
  through mid afternoon and a low chance for additional TS this
  evening

- Wind variability in the wake of TS today and then a wind shift
  to northerly tonight behind a cold front

The very moist airmass over the region has supported rapid
expansion of low CIGs and reduced VSBY in fog/BR over interior
northern Illinois. Outlying terminals will likely see the lowest
flight categories (particularly RFD), but a period of LIFR CIGs
is a distinct possibility even at ORD and MDW.

Turning to the TS potential, confidence is still on the lower
side with specifics, but starting to have a better handle given
upstream trends across the far northwestern Great Lakes region.
This entails a later TS timing, anytime from the late morning
until mid afternoon or so. Opted to carry a PROB30 with this
issuance, but will (hopefully) be able to refine a TEMPO window
with the 09z AMD. If a well organized line of TSRA surges south
across the area, 40-50+ kt gusts from the north to northeast
would probably accompany the leading edge of the complex. Gusty
east to southeast winds may then follow, briefly returning to
southwest before veering more westerly with the approach of the
cold front this evening. TSRA coverage with the cold front
itself may be quite sparse, so no additional mention in the TAFs.

Finally, winds will shift to north-northeasterly (~15-20 kt
gusts) in the wake of the cold front passage, around/shortly
after 06z Tuesday.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Extreme Heat Warning until 9 PM CDT this evening for ILZ003-
     ILZ008-ILZ010-ILZ011-ILZ019-ILZ021-ILZ032-ILZ039.

     Heat Advisory until 9 PM CDT this evening for ILZ004-ILZ005-
     ILZ006-ILZ012-ILZ013-ILZ020-ILZ023-ILZ033-ILZ103-ILZ104-
     ILZ105-ILZ106-ILZ107-ILZ108.

IN...Heat Advisory until 9 PM CDT /10 PM EDT/ this evening for
     INZ001-INZ010-INZ011-INZ019.

LM...None.

&&

$$

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