452
FXUS63 KLOT 201124
AFDLOT

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

.KEY MESSAGES...

- Breezy and cool today with showers at times.

- 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 358 AM CDT Sun Apr 20 2025

Through Monday:

A pronounced trough continues to eject into the southern Plains
this morning and will begin to lift into the mid-Mississippi
Valley later today becoming more negatively tilted as it does
so. At the same time, the baggy surface low over central TX will
gradually deepen as it too lifts into the Mississippi Valley
forcing the stalled frontal boundary across the Ohio River
Valley to propagate northward as a warm front into our CWA.
Recent radar trends continue to show showers festering along the
frontal boundary which will pivot into northern IL and
northwest IN towards the mid-morning hours (between 8 AM and 10
AM CDT). However, the dry low-level airmass overhead and
advancing dry slot this afternoon are expected to work in tandem
to generate a more scattered coverage of showers across our
area especially during the afternoon hours. So expect periods of
dry weather today especially with eastward extent.

Aside from the showers, the advancing warm front will also
generate a gradient in temperatures across the area today. Those
to the north of the boundary (roughly those north of I-88) will
likely remain in the lower 50s today as stout east winds advect
cooler air off of Lake Michigan, meanwhile those to the south
of the boundary will likely see highs in the upper 50s to around
60 (possibly mid-60s near US-24) as winds will favor a more
south of east direction. Regardless, the tightening pressure
gradient and modest mixing will cause winds to become quite
breezy with gusts in the 25-30 mph range.

Heading into tonight, the aforementioned surface low and
associated trough will be pivoting across IA as they track
towards WI and eventually northern MI. This will force the
system`s cold front to swing through the area overnight bringing
with it a more widespread coverage of showers and embedded
thunderstorms. Given that the front and associated
showers/storms are not expected to arrive until well after
sunset (after 11 PM for most), instability will be in the
process of waning and mainly centered above 850mb. Thus the
thunderstorms should be in a decaying state as the front moves
through, however, the strong forcing and stout kinematic field
(effective shear around 40-50 kts) may still allow a few
stronger storms to linger into our CWA particularly south of
I-80. Any stronger storms that do materialize will have a threat
for locally gusty winds up to 50-60 mph.

The showers and storms will taper Monday morning as the front
exits to the east and the low pivots more into northern MI.
Therefore, dry conditions and gradually clearing skies are
expected for Monday. However, the cold advection behind the
system will keep temperatures on the cool side the highs only
peaking in the mid to upper 50s. Additionally, the cold
advection in combination with the tight pressure gradient behind
the low will also generate rather windy conditions. That said,
gusts on Monday will likely peak in the 35-40 mph range out of
the west-southwest but there is the potential for higher gusts
up to 45 mph if deeper mixing is achieved.

Yack

Monday Night through Saturday:

In the wake of today/Monday`s system, pattern across the CONUS
will transition to mostly zonal flow for the remainder of the
upcoming work week. A stronger than usual Bermuda High and lee
troughing will result in persistent and broad southerly flow off
the Gulf, pulling moist air north across much of the central
and eastern portions of the country.

Guidance is in surprisingly good agreement bringing a low
amplitude wave rippling through the zonal flow across the
Midwest and western Great Lakes later Tuesday and Tuesday night.
The height falls with this waves will encourage a modest
southerly low level jet to develop and strengthen theta-a
advection Tuesday night. The should result in a period of
showers and thunderstorms Tuesday night, especially north of
I-80 as it looks now.

Another, more pronounced northern stream shortwave trough will
pass mostly north of the area later in the week. Cold front
trailing from the accompanying sfc low should move across the
area late Thursday/early Friday time frame, bringing with it
another opportunity for some showers and storms.

After seasonably cool temps Monday, look for a warming trend to
solidly above average temperatures Tuesday through Thursday. As
is so often the case this time of year, lake breezes each
afternoon should keep temps notably cooler near the lake. A
return to more seasonable temperatures is expected just in time
for the weekend in the wake of the late week cold front.

- Izzi

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 624 AM CDT Sun Apr 20 2025

- WDLY SCTD -SHRA expected from late this morning through early
  evening, greatest coverage expected at RFD

- Period of heavier showers expected late this evening into
  overnight hours tonight into Monday morning

VFR conditions are expected through most of daytime hours today,
though some WDLY SCTD -SHRA are expected from mid morning
through the early evening hours. Greatest coverage of showers
will be west of the Chicago area, closer to RFD, with some
virga and sprinkles/very light showers more likely at the
immediate Chicago terminals. A more formidable band of showers
will sweep across the region late this evening into the pre-dawn
hours Monday.

East winds will gradually increase today with at least
occasional gusts developing this morning. Easterly winds will
strengthen further later this afternoon into this evening with
OCNL gusts to around 30 kt possible. Winds will shift to south
for a brief time overnight, before shifting to southwest with
gusts to 30kt developing. Brief period with stronger gusts,
possibly up to 40kt possible during the predawn hours Monday as
cold front moves through. Strong/gusty southwest winds will
gradually trend to westerly after sunrise Monday morning.

- Izzi

&&

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

Small Craft Advisory issued for period of 25-30 kt winds
tonight with a Gale Watch maintained for Monday.

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. Then in the wake of the
system on Monday, winds will quickly shift to southwest and then
west while rapidly increasing due to a tight pressure gradient.
Given that some uncertainty remains in the exact magnitude of
wind gusts on Monday, the Gale Watch was maintained with this
forecast package. Therefore, the marine forecast will have west-
southwest gales to 35 kt mentioned but there remains the
potential for higher gusts near 40 kts if deeper mixing is
achieved.

Yack

&&

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

     Gale Watch from Monday morning through Monday evening for the
     IL and IN nearshore waters.

&&

$$

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