813
FXUS63 KLOT 051121
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
621 AM CDT Sat Jul 5 2025

.KEY MESSAGES...

- Today will be hot with the combination of heat and humidity
  making it feel like the upper 90s.

- Shower and thunderstorm chances return late today/this evening
  especially across northwest IL. Waves of showers and storms
  are expected in the general region overnight into Sunday.
  Torrential downpours will provide localized relief to ongoing
  drought conditions, and some areas will stay completely dry.

- A period of dry and comfortable weather conditions is on track
  to start the workweek.

&&

.DISCUSSION...
Issued at 302 AM CDT Sat Jul 5 2025

Through Sunday night:

Evening upper air analysis places a 5920 meter H5 high pressure
center across downstate IL, with a ridge axis extending
northward in the the western Great Lakes region. The ridge axis
is progged to gradually shift east of the forecast area by this
evening, as a mid-level short wave trough over the Plains
advances slowly eastward. The influence of the ridge, coupled
with low-level warm advection downstream of the approaching
short wave, will make for a fairly hot day across the area. Low-
level thermal progs (925 mb temps around +25C) should support
afternoon highs in the low-mid 90s this afternoon. Humidity
levels are expected to be lower than with some of our recent
more oppressive episodes however, as surface dew points mix out
into the mid-upper 60s in many spots (even low-60s across the
Chicago metro) resulting in peak heat indices in the 95-100F
range. Breezy southwest winds gusting 20-25 mph this afternoon
(perhaps 30 mph across far north/northwest IL) will also keep
things from feeling too oppressive.

Warm mid-level temps (and poor mid-level lapse rates) depicted
in forecast soundings will keep things capped with respect to
diurnal shower/thunderstorm potential across the eastern half of
so of the forecast area through this afternoon, though isolated
to widely-scattered thunderstorms ahead of a cold front across
MN/IA may spread into northwest IL later this afternoon and
evening as ascent ahead of the approaching short wave helps to
weaken/erode the cap. Greatest chances (40-60% along with some
isolated gusty wind potential) appear to be west of about a
Peru-Harvard IL line per various CAM solutions, with decreasing
coverage/probability farther east later in the evening/overnight
with the loss of diurnal instability although isolated showers
and storms would remain possible overnight.

Shower/storm trends look a little messy Sunday morning, as both
the mid-level short wave trough and the surface cold front push
slowly across the forecast area. Can`t rule out a few
showers/isolated thunderstorms during the morning, though
greatest chances (50-70%) look to be during the midday/afternoon
hours ahead of the front mainly along/southeast of the I-55
corridor. These storms will have the potential to produce some
torrential downpours, as forecast soundings depict weak west-
southwesterly flow through the column (slow cell movements) with
p-wats approaching 2". We`ll have to keep an eye on any training
of cells along/ahead of the front, for localized flooding
potential. While the best focus for greater storm coverage
appears to be mainly south of Chicago proper, there are
indications of a "backdoor" push of north- northeast low- level
flow off Lake Michigan behind the front in the afternoon, which
may locally increase low-level convergence and shower/storm
potential for a time across the city as well, as the 00Z HRRR
and 03Z RAP suggest.

Scattered showers/thunderstorms and increased cloud cover will
keep temperatures lower on Sunday than over the past few days,
with highs expected to range from the lower 80s northwest behind
the front to the mid-upper 80s southeast. Temperatures may drop
into the 70s along the IL Lake Michigan shore as winds shift
northeast later in the afternoon. It will feel muggier however
along/ahead of the front with dew points pooled in the low-70s
and heat indices 90-95F despite lower air temps. Lingering
showers and storms will settle south of I-80 Sunday evening,
gradually ending from north to south as the front continues to
sag south overnight.

Ratzer


Monday through Friday:

Monday into Tuesday, the frontal boundary responsible for waves
of showers and storms should sag east-southeastward especially
as northeasterly onshore flow becomes entrenched across northern
Illinois and northeastern Indiana. Suppose we can`t rule out a
shower or storm Monday afternoon southeast of I-57 if the
progression of the frontal boundary is slower than expected,
though for now, do favor a dry forecast for both days. Mostly
sunny to partly cloudy skies each day will help facilitate warm
afternoon temperatures into the mid to upper 80s, except near
the lakeshore where highs will be limited to the upper 70s.

Forecast confidence lowers Tuesday night onward owing to the
complex interaction of a building ridge across the southwestern
US, a cut- off low pressure system off the coast of California,
and a northern stream trough digging into southwestern Canada.
Generally speaking, the net result should be a broad region of
weak upper-level northwest flow from the central Plains to Great
Lakes, opening the door for slow-moving convectively-augmented
waves with associated episodes of showers and storms. Now,
whether or not any impact our area to provide much needed
rainfall will be something to pin down as we get closer and can
better resolve/track individual shortwaves. For now, will have
to shoulder nearly continuous low chances (20- 40%) for showers
and storms offered by the NBM until we can provide meaningful
adjustments. Regardless of shower and thunderstorm chances,
ensemble model guidance supports a continuation of near to above
normal highs and low temperatures from the middle of next week
and beyond (save for cooler readings along the Lake Michigan
shore).

Borchardt

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 621 AM CDT Sat Jul 5 2025

Regional radar imagery and surface observations depict an axis
of showers and thunderstorms extending from eastern Kansas
northeastward toward western Lake Superior. The showers and
storms are located ahead of a weak frontal boundary and surface
low pressure system in the Plains. The Great Lakes are located
well ahead of both features at press time.

Over the next few hours, southwesterly winds will gradually
increase in magnitude and gust in the 20 to 25 kt range by early
afternoon. By late afternoon, the aforementioned axis of showers
and storms will cross the Mississippi River. Confidence
continues to increase that thunderstorms will reach RFD by early
evening, allowing for the inherited PROB30 group to be
converted into a TEMPO group focusing on the 22 to 02Z
timeframe. Meanwhile, dry air ahead of the front should tend to
limit any development ahead of the main axis of showers and
storms through sunset.

After sunset, the showers and storms to the west should begin to
wane in coverage and intensity. Nevertheless, the axis should
continue sliding eastward through the overnight hours as the
parent upper-level trough moves directly overhead. Will carry PROB30
groups for thunder at DPA/ORD/MDW/GYY during the early morning
hours of Sunday with relatively limited coverage being the main
setback to introducing higher probabilities (e.g., TEMPO
groups).

Behind the axis of showers and storms, a modest westerly wind
shift characterizing the aforementioned weak frontal boundary
will move through the area. Gridded guidance ubiquitously
advertises IFR to MVFR cigs spreading eastward across the
terminals behind the front. Given the same guidance is handling
the ongoing stratus across Minnesota, felt confident enough to
introduce low cigs tomorrow morning at the end of the 30-hour
TAF windows for ORD/MDW.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 1 PM this afternoon to 8 PM CDT this
     evening for the IL nearshore waters.

&&

$$

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