NWS Chicago, IL wrote its last area forecast discussion before the first event at Jul 27, 1:30 PM CST, 1 hour 45 minutes earlier. Its key messages:
Area forecast discussion, Jul 27, 1:30 PM CST, 1 hour 45 minutes before
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
230 PM CDT Sun Jul 27 2025
.KEY MESSAGES...
- Possible line of decaying thunderstorms Monday morning with
strong/severe storms possible.
- Dangerous heat and humidity Monday with a heat advisory in
effect.
- Periods of thunderstorms return Monday through Wednesday, some
possibly strong to severe with torrential rain which could
lead to flash flooding.
- Dangerous swimming conditions at Lake Michigan beaches
Wednesday into Thursday, possibly into Friday night.
&&
.DISCUSSION...
Issued at 230 PM CDT Sun Jul 27 2025
Through Monday:
Two primary forecast concerns which are thunderstorm chances
Monday morning and heat indices Monday afternoon.
Isolated showers continue to fester across part of the cwa this
afternoon with a weak frontal boundary near I-80 along with weak
outflows from these showers. Also a lake breeze boundary is
slowly moving in from the lake. Isolated showers will continue
through sunset, mainly focused away from the lake and through
the center part of the cwa. A few showers may persist into the
evening, mainly south of I-80. So far, there hasn't been any
detected lightning, but an isolated thunderstorm will remain
possible. While very localized and brief, heavy rain will also
be possible.
Attention then turns to thunderstorm development already
occurring across west central MN, which is also a bit earlier
than current guidance was suggesting for development.
Additional activity is expected later this evening with a
consensus for a turn to the southeast into southwest WI/
northeast IA in the predawn hours and then into northwest IL
around daybreak. The HRRR remains the most consistent and also
the most bullish for this activity remaining together as it
moves across at least the western third/half of the cwa just
after daybreak. There is plenty of instability even at this
early hour with the limiting factor perhaps being shear, though
sometimes these complexes become well organized and persist
longer and stronger then the models show. Given the potential,
have increased pops into the 40 percent range, generally west of
the Fox Valley and then along I-55. These will need to be
adjusted as trends emerge later this evening and tonight. The
more organized the storms are, the better the chance for strong/
severe storms with damaging winds the main threat. Ahead of
this potential line, the airmass is so moist and unstable, there
may be isolated showers or thunderstorms around daybreak across
the local area. Have added slight chance pops starting at 10z,
mainly for the IL counties for this potential. Regardless of
coverage, the storms are expected to quickly dissipate by mid
morning, with the rest of Monday looking dry.
If the area remains dry Monday, temps should quickly recover by
early afternoon with highs into the lower 90s for most areas,
possibly into the mid 90s for some areas including the Chicago
metro. With no change to the airmass, dewpoints in the mid to
upper 70s will push peak heat indices into the 105-109 range for
most of the area and have issued a heat advisory from 17z to
01z. The wind field may become quite light, dependent on storm
coverage in the morning and this may allow for a lake breeze
near the Lake Michigan shore. Quite a bit of uncertainty for how
far this may move inland and even more uncertainty for how
temps may be impacted. Current temps (Sunday afternoon) are
still in the mid 80s not far from the shore. If anything it may
limit peak heat indices right near the shore.
Finally, fog is possible overnight into early Monday morning,
though confidence is low with the expectation for increasing
cloud cover and then possible storms into the northwest cwa by
daybreak. For now, have no fog mention in the forecast. cms
Monday Night through Sunday:
Remnant outflow from Monday's potential MCS would likely drift
back north across Iowa through early Monday evening. While
overall large scale forcing looks meager, a plume of extremely
steep mid-level lapse rates will continue to advect eastward
atop seasonably moist low-levels. Even a glancing blow from a
modest remnant MCV/vort max or other shortwave(s) embedded
rotating around the northern periphery of the subtropical high
could serve as a focus to reignite storms off to our north and
west. Extended CAM guidance seems to generally support the idea
of another cluster of thunderstorms organizing somewhere
upstream of us and subsequently evolving into a forward-
propagating MCS Monday night, although specifics (as usual at
this range) remain unclear. Specifically, how far east and north
any such complex initially develops will play a large role in
whether storms manage to push back into our region through the
late overnight and into the morning hours on Tuesday.
Similar to tonight/Monday's setup, forward-propagating Corfidi
Vectors seem to be oriented a bit more easterly as opposed to
south or southeast, and 850-300 mb thickness lines also appear
to be laid out in a WNW-ESE orientation, both of which suggest
MCS motion that could end up staying north of the local area,
particularly if initial develop takes place across the
Minneapolis area. However, the instability/MUCAPE axis is
forecast to take on more of a NW-SE orientation and build more
solidly into our region, and with the propensity for mature MCSs
to favor these gradients, that does portend some potential for
activity to attempt to build into our region. Whether any
complex manages to sustain an appreciable severe weather threat
into our area also remains uncertain, given the unfavorable time
of night with increased nocturnal convective inhibition and
generally easterly-directed deep layer shear (0-7, 0- 8 km
layers). The latter would support outflow attempting to develop
ahead of any NW-SE moving complex. Still, the presence of
potentially 2500-3500 J/kg of MUCAPE, at least modest high level
flow, and significant upstream DCAPE (1000-1500 J/kg), the
threat for an MCS with some strong-severe storm potential exists
late Monday night into early Tuesday morning.
Given the potential for morning convection and debris cloud cover,
this really muddies the heat index forecast on Tuesday. Without
any storms, things are easily primed to push well into the
105-110 range (perhaps even a threat for some isolated 115
readings), mainly away from the lake with signs of a late-
morning/midday lake breeze push. With a morning MCS, heat
indices could still recover to near advisory levels, but the
areal extent would likely be diminished, and perhaps relegated
to our southwest half/third. Given all of this, we're taking the
stance of handling heat headlines on a day-by- day basis, but
will continue to message the potential for dangerous
heat/humidity. As it stands right now, the greatest chances for
heat indices over 105 F on Tuesday exist south and west of a
Rockford to Rensselaer line.
For Cook County where there is a separate 2-day Extreme Heat
Warning threshold of heat indices over 105 F, there is some
potential we threaten this if storms don't disrupt the
environment as it looks like we'll be close to 105 F on Monday.
At this point, with an afternoon lake breeze looking like a
greater possibility on Tuesday, the extent/duration of these
higher readings seems more limited on Tuesday, lessening the
potential for busting the 2-day criteria.
Finally, if no MCS materializes Tuesday morning, isolated to
widely scattered storms could develop in the afternoon in a
weakly capped, highly unstable environment. Coverage would
likely remain quite limited though given weak overall forcing.
Better signal for storms in the guidance suite exists during the
Tuesday evening into Wednesday timeframe as a cold front pushes
south out of Wisconsin. Any storms or surging complex would
carry with it another threat for severe weather given the highly
unstable environment, and increasing shear through the 0-6 and
0-8 km layers.
Moisture doesn't look like it'll get fully scoured out on
Wednesday behind the front, with low to mid 70s dewpoints
lingering and PWATs still pushing 1.5 to 2 inches. A shortwave
riding atop the frontal zone could yield a continued threat for
showers and storms through the day as a result.
A period of much more tranquil and pleasant conditions is forecast
to develop from about Thursday and through the upcoming weekend
as a sprawling 1020-1030 mb high builds overhead. Strong/gusty
northerly winds down the lake will promote building waves and a
prolonged threat for rip currents/dangerous swimming conditions
Wednesday and Thursday, and potentially even lingering into
Friday and Friday night.
Carlaw
&&
.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1240 PM CDT Sun Jul 27 2025
The primary concerns for the 18Z TAF period include:
* Periodic MVFR cigs possible this afternoon
* Potential for rain and storms Monday morning, greatest at RFD
A densely scattered cloud deck between 2 and 3 kft AGL has been
filling in periodically to bring flight categories down to
MVFR. While predominantly VFR is expected this afternoon,
periods of MVFR will remain possible.
A decaying system of showers and thunderstorms will approach
northern IL from the NW around dawn on Monday. There are
uncertainties in coverage as the wave moves across during the
morning. The greatest potential for morning shower/thunder
coverage will be at RFD where a PROB30 was introduced during
this TAF cycle. The Chicagoland TAFs were kept dry for now as
probs appear too low to include, but trends will be closely
monitored through the night. A period of MVFR would also be
possible early Monday if the rain holds together.
Meanwhile, easterly winds below 10 kt will veer to southerly
early this afternoon and remain as such through the night.
Monday will feature SW winds below 10 kt into the late afternoon
before a lake breeze may approach the terminals turning winds
to NW then possibly to NE toward the end of the 30-hr TAF
period. Additionally, winds may go NE for a time Monday morning
associated with the rain potential. Even if the rain doesn't
quite make it into Chicago, outflow from nearby convection may
also result in a shift to NE.
Doom
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Heat Advisory from noon to 8 PM CDT Monday for ILZ003-ILZ004-
ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-
ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-
ILZ105-ILZ106-ILZ107-ILZ108.
IN...Heat Advisory from noon CDT /1 PM EDT/ to 8 PM CDT /9 PM EDT/
Monday for INZ001-INZ002-INZ010-INZ011-INZ019.
LM...None.
&&
Visit us at weather.gov/chicagoSpecial weather statement, Jul 27, 3:04 PM CST, 11 minutes before
Special Weather Statement
National Weather Service Chicago/Romeoville
404 PM CDT Sun Jul 27 2025
ILZ013-104-272130-
DuPage IL-Central Cook IL-
404 PM CDT Sun Jul 27 2025
...A GUSTY THUNDERSTORM IS IMPACTING EASTERN DUPAGE AND WESTERN COOK
COUNTIES...
At 403 PM CDT, Doppler radar was tracking a gusty thunderstorm over
Elmhurst, Westchester, and La Grange. The storm is nearly
stationary.
HAZARD...Localized wind gusts up to 50 mph.
SOURCE...Radar indicated.
IMPACT...Gusty winds could knock down tree limbs and blow around
unsecured objects.
Locations impacted include...
Lyons, Westchester, Clarendon Hills, Addison, La Grange, Bellwood,
Berkeley, Hillside, River Forest, Oakbrook Terrace, Northlake,
Maywood, York Center, Westmont, Western Springs, Franklin Park, Stone
Park, Elmhurst, McCook, Hinsdale, North Riverside, Hodgkins, Melrose
Park, Oak Brook, and Lombard.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
If outdoors, consider seeking shelter inside a building.
Torrential rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.
&&
LAT...LON 4177 8795 4183 8801 4190 8803 4195 8801
4195 8786 4190 8780 4181 8779 4177 8784
TIME...MOT...LOC 2104Z 270DEG 5KT 4188 8794
MAX HAIL SIZE...0.00 IN
MAX WIND GUST...50 MPH
Borchardt