845
FXUS63 KLOT 101957
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
257 PM CDT Tue Jun 10 2025

.KEY MESSAGES...

- Wednesday (tomorrow) will likely feature the warmest
  temperatures for the area since May 15th.

- Shower and thunderstorm activity Wednesday night onward may
  initially focus primarily near or north of the IL/WI state line
  and then sag southward Friday-Saturday.

- There is a signal for hot and humid conditions to develop
  across the general region next week with periodic shower/storm
  chances.

&&

.DISCUSSION...
Issued at 257 PM CDT Tue Jun 10 2025

Through Wednesday Night:

Following a downright cool start to the day today (Tuesday), the
warm-up to near/slightly below normal highs this afternoon is
merely the appetizer for the substantial warm-up in store for
Wednesday. Breezy west-southwesterly winds will subside by or
before sunset this evening, with light to moderate southwest winds
the rest of the night. Overnight low temperatures will be propped
up by warm advection and the modest southwest winds to upper
50s/around 60F, except low-mid 60s in and near Chicago.

On Wednesday, it appears that the general haziness from lingering
wildfire smoke should be less pronounced than today per HRRR
vertically integrated smoke forecasts. 925 mb temps in the upper
teens Celsius this afternoon will jump about 5C by this time
tomorrow. With plenty of sun and better moisture return flow still
blocked off by high pressure to our southeast translating to dew
points only in the upper 50s-low 60s in the afternoon, the stage
will be set for mid-summer like warmth. Breezy west-southwest
winds gusting to 25-30 mph and deepening mixing will further
contribute to highs in the upper 80s for most of the area (except
far southeast) and a smattering of highs up in the 90-91F range.
This will particularly be the case for the heart of the Chicago
urban heat island. If Chicago (ORD) and Rockford reach 90+F, it
will be the first 90F temps there since May 15th (94F at both
sites that day).

Turning attention to convective potential Wednesday night into
early Thursday, there was a distinct "less convectively active"
flavor within the CWA in the 12z 6/10 cycle versus previous
cycles. This was even as a previously quasi-stationary boundary is
still progged to make slow southward headway down toward the I-80
corridor by pre-dawn Thursday (a bit faster southward progression
of the boundary near the lake, as is common). A positively tilted
short-wave impulse or two (one in the early evening and another
overnight possibly) should move east-southeast from the upper MS
Valley to Lake Michigan and potentially interact with the slowly
sagging frontal boundary.

The change in the guidance today to lesser shower/t-storm chances
Wednesday night looks to be tied to lower RH in roughly the
850-700 mb layer serving to keep 1500-2500 J/kg of potential
MUCAPE largely capped. At the risk of subsequent cycles trending
back towards more of a convective footprint across parts of
northern IL, opted to take an incremental step of nudging PoPs
downward with this issuance. Chance (30-40%) PoPs are now confined
to the IL/WI state line counties, and tapering quickly southward
from there (<15% in the central and southern Chicago metro).

Fairly steep mid-level lapse rates and up to 30-35 kt of
effective deep layer bulk shear do introduce the possibility for
isolated organized convection, if any storms indeed form.
Conditional upon MUCIN being eroded sufficiently for a few
thunderstorms to develop into far northern Illinois, there is a
level 1 of 5 (marginal risk) threat for a severe storm capable of
producing localized damaging hail and/or damaging downburst winds.
Aside from the lower-end/uncertain/lower confidence shower and
thunderstorm chances for far northern Illinois, it will be a
seasonably warm overnight with lows in the 60s, except upper 50s
near the IL North Shore.

Castro

Thursday through Tuesday:

The extended forecast is looking a bit more soggy with
temperatures in the 80s, and 70s along the shoreline (on day
when the winds are out of the east-northeast). It will not be a
total wash out with some dry periods, but there is lower
confidence on the exact timing of showers and thunderstorms
given so much model disagreement.

For Thursday, models have trended farther north with that
stalled front and slower with any southward progression. The GFS
tries to bring it in during the day, whereas the Euro holds off
until later. As a result, PoPs were pulled back slightly in the
northwest portion of the CWA for the lack of certainty in
coverage. That being said, there should be less cloud cover away
from the boundary (around and south of I-88), with south-
southwesterly flow providing warm air advection and isentropic
upglide, decent CAPE, and around 20 knots of shear leading to
the chance for some showers and storms, especially in the
afternoon and early evening.

There are discrepancies on specific timing, but most ensemble
members are coming into agreement on finally moving the frontal
boundary and its surface low over the area Thursday night
through Saturday. It would not be surprising if Thursday night
ended up being slightly drier given the poor time of day and
lack of confidence initial start of frontal movement. But from
Friday through Saturday, there should be better forcing with the
front as it passes through to allow for better coverage of
showers and thunderstorms. Felt no need to change what the NBM
provided with highest PoPs around 60 to 70 percent around the
Rockford Metro Friday afternoon, and southeast of Interstate 55
on Saturday afternoon. There is about moderate confidence in
these trends for now, but whether or not the guidance is on the
right track with this slower progression of the front remains
to be seen and timing will no doubt be adjusted in future
forecast updates.

A weak mid-level wave is forecast to move over the area on
Sunday. While models are in poor agreement with both timing and
strength of the wave moves over, there is some agreement in a
slightly drier air mass in the mid levels. Sunday may be drier,
but with residual moisture and diurnal heating, there is the
slight chance for additional showers and an isolated storm in
the area.

Heading into next week, the Euro depicts a broad upper ridge
building across the central CONUS on Monday which persists
through a good portion of the week. If this pattern plays out,
it would favor a period of above average and humid conditions
with highs in the 80s and possibly 90s at times. Being on the
eastern periphery of the ridge, it would lead toward additional
showers and storms. However, the GFS is now resolving an upper
level trough that drops down from Saskatchewan moving toward
Wisconsin with a reflected surface low and better forcing for
more vigorous convection with it. The Canadian is sort of an
average of the two, with a weak trough that passes to the north.
Given all the uncertainty, the NBM chance PoPs were left in for
now leaving quite a murky yet wet at times extended forecast.

DK

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1254 PM CDT Tue Jun 10 2025

Main Items of Note:

- Westerly winds continue this afternoon, with gusts in excess
  of 20-22kts possible.

- Hazy skies remain overhead through 00-02z due to wildfire
  smoke aloft.

- Gusty sw winds, 22-24kts, return Wednesday morning as LLJ
  mixing occurs.

Surface high pressure has begun to shift into the Ohio Valley,
allowing for westerly winds to begin a slow shift to wsw through
the afternoon into this evening. Gusty winds, in excess of
20-22kts, will continue over the next few hours before gusts
diminish near 00z. Wildfire smoke will continue to usher hazy
skies overhead this afternoon, however no impacts are expected
at the local terminals with SCT coverage possible in the 5kft
and 20kft levels. Heading through the overnight period, skies
are forecast to clear out, however mid-level moisture is modeled
to move back into the region Wednesday morning. Southwesterly
winds are anticipated to increase again Wednesday morning, after
14z, as low-level mixing occurs.

Baker/Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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