Chicago, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

863
FXUS63 KLOT 201958
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
258 PM CDT Sun Apr 20 2025

.KEY MESSAGES...

- Widespread showers with embedded thunderstorms tonight. A few
  stronger storms with locally gusty winds possible, especially
  south of I-80.

- Windy conditions on Monday with west-southwest gusts in the
  35-45 mph range.

- Well above normal temperatures expected inland of the lake
  for the rest of the upcoming week after a cool Monday.

&&

.DISCUSSION...
Issued at 258 PM CDT Sun Apr 20 2025

Through Monday Night:

A swath of showers is moving across northern IL early this
afternoon. These showers are blooming north of a warm front as
it progresses northward into our CWA. As of around 2 PM, the
boundary resides near the Kankakee River and conditions are dry
immediately along and behind it. A few embedded thunderstorms
and heavier showers have been clinging onto a MUCAPE gradient
immediately north of the front. The showers and embedded thunder
will continue to be pushed along by the front until they get
shunted north of the CWA late this afternoon and the area should
be largely dry then until later in the evening.

The warm front is expected to get hung up over our CWA, roughly
near the I-88/90 corridors later this afternoon before surging
northward again this evening with the approach of the low
center and cold front. A pretty sharp temperature gradient is
setting up across our area on either side of the front,
especially now that our south has dried out and is beginning to
see some sun. Temperatures should be rather steady this evening
while this front remains stationary. After the front begins its
trek northward again, much of the area north of I-80 will likely
even warm a couple of degrees tonight.

The storm`s cold front will propagate across the CWA late this
evening into tonight, bringing with it an additional push of
rain and probably some embedded thunderstorms. Continued low
level warm advection this evening will generate a pool of
elevated instability sufficient for thunderstorm development
just ahead of the cold front. While severe weather is not really
anticipated, we could see some stronger winds associated with
these storms. Several pieces of guidance are resolving a
corridor of gusty winds coincident with a narrow opportunity for
the low levels to become only weakly stable near the front. An
impressive low level jet will track just off the deck and it`s
very possible that some of these stronger winds could punch
through to the surface, especially with the heavy rain and
storms to help out. This is especially true closer to central
and western IL where they could maintain a little bit of mixed-
layer instability into the evening. An SPC marginal risk abuts
our southwest CWA for this wind potential.

The storms will be off to our east prior to daybreak tomorrow. A
trailing secondary cold front will move across the area during
the morning hours. In fact, morning low temperatures tomorrow
will likely occur well after daybreak behind this boundary. With
appreciable low level moisture hanging on behind the initial
front, some drizzle may materialize during the morning before
the secondary front passes. Conditions should also be windy
tomorrow, especially during the morning, but confidence is shaky
in what sort of winds we`ll be looking at. The lack of
confidence boils down to two primary discrepancies among models:
the magnitude of the low level flow and the quality of low level
moisture. The Euro and UKMET bring in much drier near- surface
air behind the initial cold front than other camps leading to
slightly deeper and overall more efficient mixing in the
morning. These are two of the more aggressive solutions for
winds tomorrow. 925mb winds during the morning are also being
modeled at anywhere between 35 and 45 kt. The HRRR for instance
is stronger with the low level flow so, despite being Monday
Night saturated in the low levels with lesser mixing, it`s
still wanting to resolve gusts to around 40 mph. So it`s tough
to say exactly what sort of flow we will have upstairs and how
efficiently we can mix that down to the surface. But a period of
at least 30 to 35 mph gusts looks likely tomorrow morning,
although 40 to 45 mph certainly looks attainable.

Winds could remain strong into the early afternoon but should
start stepping down during the afternoon as the low level flow
relaxes. After the morning drizzle potential moves away, no
rain is expected for the rest of the day. A feed of dry air will
chip away at the cloudy skies and some good sunshine should
fall on the area later in the day while afternoon temperatures
look to be in the 50s.

Doom


Tuesday through Sunday:

The upper level pattern across the CONUS will transition to more
of a quasi-zonal flow (west-to-east oriented) pattern for the
middle part of the week, before becoming more amplified again
later this week into next weekend. This is expected to curtail the
threat of any stronger (and persistent precipitation generating)
storm systems from impacting the eastern half of the country
Tuesday through at least Thursday. In spite of this, ensemble and
deterministic guidance are in agreement in tracking some smaller
scale perturbations eastward across our region during this
period. Accordingly, while there is likely to be mainly
precipitation free hours, there will be some on-and-off shots of
showers and thunderstorms through much of the week.

Guidance remains in good agreement with tracking an initial low
amplitude disturbance eastward across the Midwest into the western
Great Lakes through Tuesday night. Weak height falls associated
with this waves should encourage the development of a modest
southerly low-level jet and strengthen theta-a advection Tuesday
night. This will in turn support our first decent potential
(50-60% chance) for showers and storms late Tuesday into
Tuesday night. At this time the best chances for this reside
along and north of I-80. Another, similar quick moving impulse
may shift into the area and result in another period of (~30%
chance) of rain late Wednesday into Wednesday night. Otherwise,
our next good shot (50-60% chance) of showers and storms comes
late in the week (Thursday night into Friday) as a stronger cold
front moves across the area.

Temperatures are expected to be on a warming trend to solidly
above average temperatures through the middle of the week,
particularly for inland sections of northern IL and IN. As is
often the case this time of year, lake breezes each afternoon will
keep temperatures notably cooler along and near the Lake Michigan
shore. Temperatures are then likely to cool back closer to
seasonable levels next weekend following a cold front passage late
this week.

KJB

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 101 PM CDT Sun Apr 20 2025

Key Messages:

- Widely scattered VFR -SHRA this afternoon, greatest coverage
  west of the Chicago terminals.

- Period of showers (and possibly a few thunderstorms) mid to
  late this evening through ~3am Monday morning.

- Easterly winds turn south-southeasterly this evening, then
  increase from the south-southwest overnight. Gusty west-
  southwest winds expected Monday, with MVFR CIGs.

A robust storm system will track northward from eastern KS
into WI by early Monday morning. As it does so, a period of
widely scattered -SHRA will move across parts of northern IL
this afternoon, with the highest coverage likely remaining near
and west of the main Chicago terminals. A few embedded
thunderstorms will also accompany this activity.

Following a brief break in the showers late this afternoon and
early evening, another line of showers (and possibly some
embedded thunderstorms) will develop and shift eastward across
the terminals in advance of an approaching cold front later this
evening. The favored timing for these showers remains on track
to be in the 04-05Z through 08Z timeframe tonight at the main
Chicago terminals, and 03Z to 06Z timeframe at RFD.
Accordingly, I made no changes to the going tempo for SHRA
during this period. The only thing we will have to keep an eye
out for is any showers and/or storms that attempt to develop in
advance of the main line. If this occurs, some showers and
storms could impact the Chicago terminals as early as 02-03Z.
However, confidence on the coverage and occurrence of this
potential earlier activity remains too low for much more than a
VCSH mention at this point.

Winds will shift southerly and become increasingly gusty
overnight, before ultimately becoming west-southwesterly during
the daylight hours of Monday. Expect frequent gusts of 30-34 kt,
particularly Monday morning. A deck of MVFR stratus also looks
to shift into the area following the cold frontal passage late
tonight. This deck of stratus may hang on across the area for
much of the day, but bases are likely to undergo a gradual
diurnal improvement through the afternoon.

KJB

&&

.MARINE...
Issued at 358 AM CDT Sun Apr 20 2025

A Small Craft Advisory is in effect today. A Gale Warning goes
into effect tonight and lasts into tomorrow evening.

Low pressure at or just under 1000 mb will track from
Iowa tonight to northern Lower Michigan Monday afternoon and
across Lake Huron Monday evening. Breezy east-southeast winds
tonight will result in hazardous waves for small craft along
the Illinois and Indiana nearshores. A period of gales is also
possible late tonight coincident with a push of showers and
thunderstorms. Then in the wake of the system on Monday, winds
will quickly shift to southwest and then west while rapidly
increasing. While some uncertainty remains, felt confidence was
high enough to upgrade the watch to a Gale Warning for tomorrow.
Also made the decision to have the warning go into effect at 06Z
for the brief gale potential tonight.

Doom/Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 7 PM this evening to 1 AM CDT Monday
     for the IL and IN nearshore waters.

     Gale Warning from 1 AM to 7 PM CDT Monday for the IL and IN
     nearshore waters.

&&

$$

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