212
FXUS63 KLOT 172331
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
631 PM CDT Thu Jul 17 2025

.KEY MESSAGES...

- High waves and dangerous swimming conditions at Lake Michigan
  beaches through Friday morning.

- Strong/severe thunderstorms possible Saturday afternoon.

- Several rounds of thunderstorms may occur early next week,
  with strong/severe thunderstorms possible along with
  torrential rain and localized flash flooding.

- Dangerous heat and humidity possible next Wednesday and
  Thursday.

&&

.DISCUSSION...
Issued at 258 PM CDT Thu Jul 17 2025

Through Friday:

What a difference 24 hours makes! It feels much more like fall
out there this afternoon with temperatures in the mid-upper 60s
across northeast Illinois and near the lakeshore with 70s
farther inland. Some lower 80s did sneak into far southern areas
this afternoon, mainly south of US 24. Low-level saturation
north of I-80 has been sufficient for patchy drizzle to develop
at times mid morning into the afternoon. Not something we see
too often in the middle of July during the day! Coverage has
been greatest over Chicago and even resulted in visibility
reductions to as low as 1 mile near the lakeshore at times. The
drizzle has been rather persistent and confidence in its
longevity is admittedly low, but suspect it may linger in spots
through the afternoon over parts of the Chicago metro.

Meanwhile an axis of showers is also lifting across the
southern CWA this afternoon associated with a low-amplitide
mid-level wave/vort lobe moving across central Illinois. This
will lead to periodic showers continuing this afternoon into
the evening mainly south of a Pontiac to Kankakee to Rensselaer
line. We are on the edge of the better instability and forcing
so it appears that the thunder threat will remain low. For now
have held off on a formal thunder mention but a few lightning
strikes can`t be fully ruled out across far southern Ford,
Iroquois, and Benton counties late afternoon/early evening.

Expect temperatures to cool overnight into the mid 50s across
interior rural Illinois(!) with low to mid 60s expected
elsewhere. Will have to keep an eye out for additional spotty
shower development south of I-80 late tonight into early Friday
morning but will let the evening shift get another look at
things to determine of additional precip chances are needed.

Warm air begins to lift back into the region during the day on
Friday allowing temperatures to warm back into the low to mid
80s across much of the area in the afternoon. The southeast to
easterly flow will likely keep temperatures cooler along the
Illinois lakeshore, with highs only in the 70s expected, likely
reinforced by a lake breeze. Any shower/storm chances look to
hold off through the daytime hours.

Petr


Friday Night through Thursday:

An active pattern is shaping up through the middle of next week
with the potential for several rounds of thunderstorms.

Convection is expected to develop well west and northwest of the
area Friday evening ahead of a cold front and move southeast,
possibly reaching the local area around daybreak Saturday
morning. Much of the 12z guidance model suite as well as the
ensembles show this activity weakening as it reaches the local
area. Thus, confidence remains low for thunderstorm coverage
Saturday morning. The more activity there is Saturday morning,
the more uncertain trends become for Saturday afternoon/evening.
A cold front is expected to be moving southeast toward the area
Saturday afternoon and then move into the local area Saturday
night. Scattered thunderstorms are possible along/ahead of this
front Saturday afternoon/evening, but coverage, location and
even intensity will be dependent on any possible morning
activity. And it will likely still be a few more model cycles
for trends to become more clear. Strong to severe thunderstorms
will be possible Saturday afternoon with SPC updated Day3 now
showing a level 2 risk for the southern half of the area. Heavy
rain and localized flooding would also be possible though storm
motion based on the soundings would suggest the storms could be
fairly progressive, limiting the time period for heavy rain.

The cold front noted above may then slow or stall across the
southern cwa through Sunday leading to the potential for
additional showers and thunderstorms across the south through
Sunday evening. Have maintained low chance pops across the
north for Sunday, but as trends look now, at least the northern
third of the cwa looks to remain mainly dry. could also be a
bit cooler perhaps upper 70s/lower 80s. North/ northeast winds
may be high enough for moderate conditions for rip currents at
Lake Michigan beaches Sunday. Will have to monitor for a period
of stronger winds then currently indicated by guidance.

There may be a brief break in the active weather sometime Sunday
night into Monday, maybe. Blended pops are still high chance for
much of this time period and made no changes. There is quite a
bit of uncertainty for when the frontal boundary returns north
as a warm front, which current timing looks to be Monday night
into Tuesday. There will likely be one or more thunderstorm
complexes during this time period nearby or over the local area
as the upper ridge also builds into the region. This could be a
time period for the best torrential rain and flooding potential.
This transition and more specifically how fast it happens, will
affect temperatures with highs on Tuesday potentially only
reaching the upper 80s for most areas.

By Wednesday, good agreement the upper ridge will be solidly
across the region with highs likely in the mid 90s. Dewpoints
have been progged by the models to be in the upper 70s/near 80
for this time period for several runs. With mature crops now,
dewpoints in the lower 80s are certainly possible outside of
urban areas. These temps/dewpoints would push heat index values
into the 110-115 range. Similar conditions are possible on
Thursday. It does appear there will be a cold front moving
south across the western Great Lakes next Thursday into Thursday
night. Should this arrive sooner, that would impact high temps.
cms

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 631 PM CDT Thu Jul 17 2025

Key aviation messages for the 00Z TAFs:

- MVFR continues to slowly lift to VFR this evening.

- Northeast winds become more ENE Friday (SE inland at RFD) and
  SE Friday evening.

Early evening surface analysis depicts high pressure spreading
southeast across the upper Midwest and the northern Great Lakes
region. Counter-clockwise (anti-cyclonic) low-level flow will
maintain generally northeast winds across the terminals through
tonight, though a weak land-breeze may turn ORD/MDW briefly
light NNW late tonight. Flow will turn more east-northeast on
Friday as the surface high continues to move east across the
Great Lakes, then eventually turn light southeast Friday evening
(earlier at RFD).

Visible satellite imagery continues to show extensive stratus
and stratocu from far southern WI/LM/MI into IL/IN, though
advection of drier air from the northeast has allowed cloud
bases to slowly rise. While RFD has just improved to VFR, a
combination of satellite trends and high-res guidance suggests
bases will likely continue to slowly lift early this evening
across the Chicago terminals, becoming VFR roughly 02-04Z with
GYY likely latest. There is some indication in guidance that a
brief ~2500 foot MVFR deck may develop after sunrise Friday
morning, though it should be short-lived as daytime warming
helps lift bases VFR.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement through Friday morning for ILZ006-
     ILZ103-ILZ104.

IN...Beach Hazards Statement through Friday morning for INZ001-
     INZ002.

LM...Small Craft Advisory until 4 AM CDT Friday for the IL and IN
     nearshore waters.

&&

$$

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