614
FXUS63 KLOT 240845
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
345 AM CDT Tue Jun 24 2025

.KEY MESSAGES...

- Afternoon heat indices over 100 are expected south of I-80
  again today where a Heat Advisory remains in effect.

- Scattered showers and thunderstorms are anticipated later
  today into Wednesday. A low threat for localized damaging wind
  gusts exists this afternoon and evening, with a localized
  flooding threat persisting overnight into Wednesday.

- Peak afternoon heat indices will still be in the upper 90s to
  near 100 through much of the week.

- Periodic shower and storm chances exist through early next
  week. The strongest storms may be capable of localized
  damaging winds and flash flooding.

&&

.DISCUSSION...
Issued at 345 AM CDT Tue Jun 24 2025

Through Wednesday:

Early this morning, we find a decaying line of showers
extending from eastern IA up into central WI. This line is
churning along a stationary front to our northwest and had
recently featured several embedded thunderstorms, but diurnal
cooling had done away with those. A couple of outflow boundaries
originating from this line made their way across northern
portions of the CWA late last evening bringing brief periods of
gusty winds and leaving sites slightly cooler in their wake.

This line should remain focused to our northwest through the
morning, although some scattered showers or storms could very
well make it into our northwest during the morning, especially
after instability builds southeast of the front as conditions
warm. But it`s unlikely that we`ll see any activity get into the
Chicago metro prior to noon as profiles dry quickly as you move
away from the front. However, during the afternoon and evening,
the front will begin to pivot south into southern WI and deep
layer moisture will pool over northern IL increasing shower and
storm chances around the area for the latter half of the day.
Despite this deep moist advection, coverage looks largely
isolated to scattered. In fact, precip coverage for today has
been on a consistent downward trend among guidance.

The upper high in the lower Midwest responsible for the recent
heatwave will keep the better shear and forcing mechanisms
shunted well to our north. This should help inhibit any
organized severe convection later today. However, upwards of a
few thousand J/kg of MLCAPE could build over the area by this
afternoon with ELs on forecast soundings as high as 15 km AGL.
Accordingly, we may see a few taller storms quickly go up this
afternoon, but with the lack of shear, hopefully they`ll be just
as quick to drown themselves out. Any deeper storms may produce
gusty to marginally damaging winds, especially earlier in the
afternoon while there`s still a good amount of dry layer air for
entrainment. Thunder chances should wane after sunset as
diurnal cooling eats away at instability, although enough will
remain where a few storms or heavier downpours could develop
during the night.

We`ve been closely watching for a potential for localized
flooding today. With a unidirectional deep shear profile
oriented roughly parallel to the front today, there are concerns
that training convection could ensue, mainly later today and
tonight after the deep moisture fills in. Models aren`t really
showing this signal explicitly, but a majority of them continue
to paint scattered splotches of 1-2" of QPF around the CWA
through Wednesday morning. The seemingly greatest flooding
potential exists across our north and northwest and where they
will spend more time in the ample environment near the front,
but the flooding threat in general appears rather low and
localized at this time.

The low levels will be nearly saturated tonight into Wednesday,
which will promote low-hanging stratus and possibly some minor
fog through Wednesday morning. Scattered showers are looking
pretty likely Wednesday morning, but the activity should wane
through the day as the surface front and deep moisture retreat
back to our north.

On the topic of temperature, it should be another hot summer
day out there today, although not quite as oppressive as we`ve
seen in recent days, especially across our north. There is a bit
more spread in temperature guidance for today than we`d like to
see, mainly in our north, but that`s because it`s being
influenced by the potential scattered convection. Our north and
northwest CWA look to be the "coolest" today with highs forecast
in the middle and upper 80s while lower 90s are expected near
and south of I-80. Afternoon heat indices look to range from the
lower 90s in our north to around 100 near and south of I-80. No
changes were made to the Heat Advisory issued yesterday for
counties south of I-80 for the cumulative effects of now four
consecutive days of heat indices near or above 100 degrees.
Highs on Wednesday look slightly cooler in the 80s to around 90
with peak heat indices in the upper 90s and approaching 100 in
our south. Onshore flow around the lake during the day will keep
conditions there several degrees cooler with lower and middle
70s expected along the shoreline during the afternoon.

Doom


Wednesday Night through Monday:

The upper level ridge is progged to flatten through mid-week,
and eventually shift eastward this weekend. While this occurs,
a series of mainly low-amplitude mid-level short waves are
expected to track across the upper Midwest and Great Lakes
region, with a quasi-stationary, west-east oriented frontal
boundary settles across the area. This will set the stage for
periodic shower and thunderstorms to affect the forecast area
through the week, with greatest chances focused during the
afternoon/evening peaks of diurnal heating and instability.
Models often don`t do well with timing of these shorter
amplitude waves (or MCVs generated by upstream convection)
farther out in time, so confidence is somewhat low in timing
periods of higher pops.

As indicated in the previous discussion, the air mass along and
south of the stationary frontal boundary will remain warm,
moist and conditionally unstable through the period. The column
remains relatively warm aloft, maintaining somewhat weak mid-
level lapse rates, while the core of the mid-upper level jet
stream stays largely north of the forecast area. The modest mid-
level flow and lapse rates, coupled with PWATs approaching 2"
at times would support diurnally strong thunderstorms capable of
producing wet downbursts, in addition to the threat of
localized flooding.

While not as hot and oppressive as the past few days have been,
it will remain seasonably warm and humid through the week, with
daytime highs in the mid-80s to lower 90s, with peak heat
indices around 100 south of the front. There will likely be some
days where onshore winds off of Lake Michigan provide some
cooling near the shore, though that will likely depend on the
day to day position of the frontal zone and outflow boundaries.

Ratzer

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1250 AM CDT Tue Jun 24 2025

Key Messages:

- Gusty northwest to north wind shift moving across the Chicago
  metro at the start of the TAF.

- Lake breeze to turn winds east to northeast this afternoon.

- Isolated to scattered showers/storms possible this afternoon.

A long lived outflow boundary continues to progress across the
Chicago metro early this morning. This resulted in a brief
period of gusts in excess of 20kt out of the northwest at ORD
and suspect this will continue through MDW and potentially GYY.
Winds then ease and turn light and variable the remainder of the
overnight hours. A stray shower can`t be ruled out in the wake
of this outflow boundary, but so far they have struggled to hold
together.

Winds settle back into a west to southwest direction after
daybreak. Convection currently across northwest MO and
southwest IA is forecast to gradually move toward the area
through overnight through the morning hours. This may attempt to
form into a weak MCV which could aid in storm development here
locally by this afternoon as it moves from west to east across
the area. Thus have opted to maintain PROB30 groups for TSRA to
account for this potential. Variable gusty winds may accompany
any of these storms in addition to heavy downpours leading to
reduced visibility.

Meanwhile a lake breeze is also progged to push inland during
the early to mid afternoon timeframe turning winds east to
northeasterly. This boundary could also serve as another
mechanism for lift for additional thunderstorm development if
the aforementioned MCV related storms don`t materialize.

In the wake of the afternoon/early evening showers and storms,
MVFR ceilings may develop tonight into early Wednesday morning.
For now have introduced a lower-end VFR CIG and a scattered MVFR
mention due to lower confidence at this range.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Heat Advisory until 7 PM CDT this evening for ILZ023-ILZ032-
     ILZ033-ILZ039.

IN...Heat Advisory until 7 PM CDT /8 PM EDT/ this evening for
     INZ010-INZ011-INZ019.

LM...None.

&&

$$

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