Oak Park, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

081
FXUS63 KLOT 150822
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
322 AM CDT Fri Aug 15 2025

.KEY MESSAGES...

- Uncomfortable heat and humidity will return this weekend and
  last into early next week. Daily heat indices could top 100
  degrees particularly near and south of I-80 Saturday through
  Monday.

- While plenty of dry time is in store, there will periodic
  episodes of thunderstorms in the general region tonight
  through Tuesday. The strongest storms will be capable of
  producing wind damage and flash flooding.

- A cooler (seasonable temps), less humid, and primarily dry
  stretch appears likely from the mid to late workweek into or
  through next weekend.

&&

.DISCUSSION...
Issued at 322 AM CDT Fri Aug 15 2025

Today through Saturday Night:

An anomalously strong (997 mb surface pressure) low pressure
system in northern Manitoba well north of strong upper-level
ridging centered over the Southern Plains is driving a belt of
westerly flow across the northern United States. Several upper-
level shortwaves are embedded within the westerly flow, with one
currently moving across the Great Lakes while another (or two)
are driving a few clusters of convection in the central Plains
at press time. Over the next 36 to 48 hours, the southern US
upper-level ridge will expand northeastward as the Canadian low
shifts eastward, setting the stage for a quintessential
summertime pattern marked by a battle between building heat and
humidity with episodic clusters of thunderstorms in the Great
Lakes beneath quasi-zonal to northwesterly flow.

Today will be "table-setting" day as the upper-level ridge
begins expanding northeastward. The outward spread of the
thermal ridge (850mb temperatures warming by +3C/24 hours or so)
will allow for temperatures this afternoon to top off in the
upper 80s to lower 90s. While southwesterly advective winds
won`t be particularly strong today (mean wind of around 10 mph
in the mixed layer), evapotranspiration should help dew points
rise about 10 degrees or so from readings yesterday and toward
the upper 60s to lower 70s. The net effect will be heat indices
in the upper 90s to around 100 this afternoon, which will be
quite noticeable after the pleasant mid-week temperatures. The
exception will be along the shoreline of Lake Michigan, as a
lake breeze will hold temperatures in the lower 80s this
afternoon. Daylight hours are currently expected to be dry.

By this evening, the center of the upper-level ridge should be
in the vicinity of the Kansas/Oklahoma/Missouri/Arkansas border
region placing the western Great Lakes along the precarious
northeastern flank where storm clusters often roam. Now, timing
out the cadence and location of storm clusters in this pattern
is often an unforgiving exercise given inherent dependencies on
convectively-invigorated shortwaves and varying position of
convectively-reinforced fronts that forecast models often fail
to correctly place and/or resolve. Accordingly, will offer a
hearty warning to limit consumption of inevitably ever-changing
CAM guidance in favor of watching observational trends for the
forecast for tonight through Saturday night.

Based on the current vigor of convection in Nebraska, a
somewhat favored forecast scenario is for a resulting MCV or two
to track eastward and kick off a new round of storms this
evening somewhere in the southern Minnesota to northern Iowa
vicinity. High PWATS (nearing 2"), high MUCAPE (>3000 J/kg), and
mostly uni-directional westerly shear should support upscale
growth into a forward-propagating line of storms with time.
Assuming this occurs, northwesterly steering flow along the
thickness gradient would guide the line of storms into northern
Illinois after dark presenting a primary threat for gusty to
locally damaging winds. With the low-level jet axis focused
into southern Minnesota, would have to think any threat for
regenerating convection would remain north and west of our area,
and that any line of storms moving into northern Illinois may
outrun the best low-level shear leading to a gradual outflow-
dominant decay. With that said, confidence in the forecast for
tonight is fairly low. Note that other scenarios exist tonight
ranging from convection developing further upstream only to
arrive into our area by daybreak Saturday, to no convection
tonight at all. For now, will maintain the inherited low
(20-30%) PoPs in favor of letting the day shift take another
look.

The forecast for Saturday obviously hinges on what happens
tonight, though there remains a play for hot and humid
conditions to materialize in our region in scenarios with
minimal coverage of thunderstorms. This is especially true since
the center of the thermal ridge will nudge even more-so
northeastward. Accordingly, rising forecast 850mb temperatures
of +22 to +23C and evapotranspiration-enhanced dew points
(perhaps pooled near any residual outflow boundary) will support
high temperatures in the low to mid 90s with afternoon heat
indices in the 100 to 110F range. Saturday will hence be a
candidate day for a Heat Advisory particularly south of I-80
where the threat of convective outflow is perhaps relatively
lower compared to areas further the north.

With such hot and humid conditions, the stage will be set for
another episode of convection in the neighborhood during the
afternoon, evening, and/or overnight period tied to any incoming
shortwaves rounding the crest of the upper-level ridge. The
favored time window will come down to the exact arrival time of
any incoming shortwaves, though west to northwesterly steering
flow would once again put our area in the strike zone for
convection. Somewhat lacking mid-level shear may limit an
otherwise concerning parameter space defined by steep mid-level
lapse rates, high PWATs, and high instability values. A signal
for somewhat unidirectional flow parallel to any residual
frontal boundary does leverage concern for notable flash flood
threat in the general region, as well. For now, we`ll take it
day by day and refine our forecast as we get closer.

Borchardt


Sunday through Monday:

Into the early to mid morning hours of Sunday, we`ll need to
watch for lingering convection pushing south overnight from
Wisconsin and far northern IL. The best chance (~30%) for storms
in the 7-10am timeframe looks to be from near/south of I-90 in the
northern Chicago suburbs south-southeastward to portions of east
central IL and northwest Indiana. This potential early-mid
morning convection should be associated with a lake enhanced
composite outflow/backdoor front getting an extra "shove" south by
1020+ mb high pressure building into the northern Great Lakes. To
whatever extent showers and storms persist past sunrise Sunday,
would anticipate diurnal weakening/decaying by the mid-late
morning.

For the rest of the day on Sunday, this fairly healthy composite
outflow/backdoor front will likely be the main impetus for
convection as it interacts with moderate to strong instability
inland of the lake shadow. With fairly dry mid-levels (potential
for dry air entrainment into updrafts) and minimal large scale
forcing (neutral height tendencies), shower and thunderstorm
coverage spurred by the front should generally be isolated to
widely scattered, primarily inland of the lakeshore. With this
forecast issuance, our highest PoPs of about 30% generally reside
from the Kankakee Valley of Illinois to interior northern Illinois
where frontal convergence theoretically would be maximized.

Sunday is another day with fairly large temperature and peak heat
index "bust potential", though with areas near/south of I-80 less
likely to see the oppressive heat and humidity combination
disrupted by convection related effects. These areas appear to
have the best chance of 105-110F heat indices (due to highs in
the low-mid 90s and dew points well into the 70s) and possibly
needing a Heat Advisory (perhaps a continuation from Saturday).
Farther north, it will certainly be hot and uncomfortably humid
inland of lake cooling, but shy of advisory criteria. The
immediate lakeshore should be limited to the lower 80s before a
bit more pronounced cooling later in the day. We`ll also need to
watch for building waves and strengthening currents at Lake
Michigan beaches from 10-15 mph or higher sustained onshore winds.

The Sunday night-Monday period has similar uncertainty to Saturday
night-Sunday. Impulses shifting east-southeast atop the strong 500
mb ridging centered over the lower Ohio Valley are favored to move
through the central and northern Great Lakes, with the large
MUCAPE reservoir supporting a fairly extensive convective
footprint. This would probably entail outflow effects leading to a
general bleeding southward of showers and storms, potentially into
northern Illinois (30% PoPs Monday morning north of I-80). Then
into Monday afternoon, we could be looking at another period with
widely scattered thunderstorm coverage primarily inland of Lake
Michigan (and to the north of I-80). Once again, locales near and
south of I-80 appear more likely to reach or exceed the 105+F peak
heat index threshold for heat advisory criteria.

Deep layer wind shear through this period should be marginal for
widespread organized convection, so similar to much of this
summer, the main threat aside from lightning would be localized
strong to damaging downburst winds. In addition, slow storm
motions, high PWATs, and large MUCAPE will also yield a localized
flooding threat.

Castro

Monday Night through Thursday:

A synoptic cold front will eventually push through the entire
region, but this may not take place until Tuesday night into
Wednesday, which will keep off-and-on thunderstorm chances going
in the region. Tuesday may feature the highest daytime thunderstorm
coverage (30-40% PoPs area-wide) of our latest hot and humid
stretch, followed by any isolated storms on Wednesday focusing
south of I-80. The main thunderstorm hazards will likely again be
lightning strikes, localized downburst wind gusts, and localized
flooding.

In the wake of the cold front passage, there`s a continued strong
signal in the guidance suite for cooler and less humid conditions
in the region during the middle and end of next week. In addition,
primarily dry conditions are currently favored midweek into or
through next weekend.

Castro/KJB

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1224 AM CDT Fri Aug 15 2025

Main Concerns:

- Lake breeze wind shift likely at MDW and GYY in the mid to
  late afternoon, reaching ORD in the ~00z-01z timeframe.

- Low chance of TSRA moving across the terminals late
  tonight/early Saturday.

With fairly light south-southwesterly winds aloft during the
daytime hours today, expect south-southwest surface winds to
remain generally below 10 kt. Also due to the lighter winds
aloft, a lake breeze should push slowly but steadily inland this
afternoon. Introduced a wind shift at MDW (east-southeast) and
GYY (east-northeast) at 21z. Confidence in the boundary reaching
ORD during the afternoon is low, with a better chance it occurs
around or after 00z this evening.

There`s a 20% or less (too low for inclusion in the TAFs)
chance for TSRA to develop upstream and push across the area
toward and after 06z Saturday. Aside from this lower end but
certainly worth monitoring potential, VFR conditions will
prevail through the current TAF period. Cumulus clouds with
bases at 5-6kft AGL are expected to develop inland of the lake
breeze late this morning into the afternoon. Any 5-6kft clouds
above the Illinois shore at the start of the air show practice
today should clear out once the lake breeze pushes inland.

Castro

&&

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

&&

$$

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