749
FXUS63 KLOT 260803
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
303 AM CDT Sat Apr 26 2025

.KEY MESSAGES...

- Seasonably cool and dry conditions today, with the possibility
  for patchy frost outside of the Chicago urban heat island
  tonight.

- Turning very warm on Monday, with highs in the lower 80s.

- A chance for strong to potentially severe thunderstorms as a
  cold front moves across the region Monday night into Tuesday
  next week.

&&

.DISCUSSION...
Issued at 303 AM CDT Sat Apr 26 2025

Through Sunday:

Breezy northerly winds, in advance of high pressure shifting
into the western Great Lakes, will set the stage for a
seasonably cool day across the area today. This in spite of
early morning low-level cloud cover giving way to mostly sunny
skies this afternoon. The coolest locations will be near the
Lake Michigan shore, where temperatures this afternoon will
struggle to get out of the 40s. Farther inland, some recovery is
expected this afternoon, but only to around the 60 degree mark.

Winds will abate from west to east this afternoon and evening
as the center of the surface high begins to move over Lake
Michigan. Light winds under this surface high will be supportive
of efficient radiational cooling after sunset this evening. Low
cloud cover just after sunset will also aid in radiational
cooling this evening, but there are some indications that some
mid and high-level cloud cover may begin to stream into the area
overnight. While it remains somewhat unclear how much of impact
this cloud cover will have on overnight low temperatures
tonight, a rather chilly night is expected, with mainly
locations likely to bottom out in the 30s into early Sunday
morning. Accordingly, at least some patchy areas of frost appear
plausible in areas outside the Chicago urban heat island,
particularly in and around the Fox and Kankakee River Valleys.

Winds will turn southeasterly on Sunday as the surface high
shifts towards the eastern Great Lakes. This will allow for a
decent rebound in temperatures Sunday afternoon following a
chilly start. The warmest locations inland from the lake should
top out well into the 60s to around 70 degrees. However, the
southeasterly wind component will favor a northeastern IL lake
breeze with more persistent onshore flow, and thus notably
cooler conditions near the IL lakeshore areas Sunday afternoon.

KJB

Sunday Night through Friday:

An upper-level wave will exit the Rockies on Sunday and trigger
surface cyclogenesis over the northern Plains Sunday night.
Models are showing fairly good agreement with the track of the
surface low moving east from western Nebraska to Lake Superior
by Monday evening. Cloud cover will increase as the low
approaches Wisconsin and a tightening pressure gradient should
help develop a fairly breezy day with southwesterly surface wind
gusts potentially in excess of 35 mph. These southwesterly winds
will also drive strong warm air advection across Illinois during
the day, likely fostering high temperatures into the upper 70s
and low 80s. However, as the low moves through Wisconsin toward
Lake Superior, a cold front will pass over northern Illinois and
with it the threat for strong and potentially severe storms.

As it move east late Monday, a jet streak will strengthen
providing additional synoptic lift. As the front moves into the
area, it has some of the quintessential characteristics of a
potent shower and thunderstorm threat: steep mid level lapse
rates, 1500 to 2000 J/kg MLCAPE, and 30-40 knots of 0-3 km
shear. But the track and exact timing are two things that lower
confidence in overall impacts in our area form this system.
Models have ever so slightly nudged the track of the low to the
north. With it, the better forcing and wind shear remains to the
west and north of the forecast area. Additionally, models have
consistently had the frontal passage through the overnight and
thus less than favorable time of day. With all the ingredients
in place, it is likely that there will be showers and
thunderstorms Monday night into Tuesday morning.

As for the severe risk and overall timing, trends will have to
continually be monitored. The threat will likely be higher
around the Rockford metro and points west-northwest into the
Upper Midwest. Even with the uncertainty of how well the system
can maintain its strength overnight, it is justifiable that the
Storm Prediction Center has put most of the forecast area in a
Slight Risk (level 2 out of 5) for severe weather, though parts
of Lee, Ogle, Winnebago, and Boone counties are in an Enhanced
Risk (level 3 out of 5).

The cold front should continue to move east away from the area
on Tuesday. How quickly it exits will determine the storm and
severe threat on Tuesday. Recent models have sped up the
front`s movement keeping the (20-40 percent) chance for showers
and storms south and east of Interstate 55 before lunchtime.
Winds will turn to the west behind the front and eventually the
northwest in the afternoon on Tuesday ushering in a cooler air
mass, capping high temps in the 70s.

Wednesday could provide a brief respite from the rain, but the
next upper level wave is projected to eject out of the southern
Plains and move toward Lake Michigan. Showers and storms will
likely move in from the south along a warm front Wednesday night
into Thursday, before a surface low moves east through the
area. Models have come into a little better agreement with the
surface low track compared to just 24 hours ago, but with it
being 5 to 6 days away, there is still time to iron out timing
and impacts.

DK

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1239 AM CDT Sat Apr 26 2025

Key Aviation Messages for the current TAF period:

- MVFR cigs expected through Saturday morning

- Gusty winds to 25 knots overnight will gradually diminish into
  Saturday afternoon

No precipitation is expected through the current TAF period as
high pressure gradually increases through the weekend. Stratus
clouds have blanketed the area resulting in widespread lower
MVFR cigs. there are isolated pockets of terminals observing IFR
cigs, but they are mostly on the southern end of Lake Michigan
and widespread IFR is not forecast. Clearer skies are gradually
sinking southward out of Wisconsin and Minnesota. VFR cigs will
eventually return to the terminals on Saturday. There is
moderate confidence on the timing in the TAF, but it would not
being surprising if around daybreak it flopped a little going
VFR then back to MVFR as breaks in the clouds happen.

The last impact to terminals in the current TAF window is from
the gustier wind conditions in the short term. While most of the
winds are out of a "northerly" direction, it varies at
specific locations (020 at KORD and points west, but 350 at
GYY and areas south and east). Gusts are expected to be between
25 to 30 knots through daybreak and then gradually diminish to
around 20 knots in the early afternoon. Winds should slacken by
00Z and become more widespread northeasterly.

DK

&&

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

&&

$$

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