436
FXUS63 KLOT 032342
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
642 PM CDT Thu Apr 3 2025

.KEY MESSAGES...

- Periods of rain, possibly heavy, are expected Friday afternoon
  into Saturday, with localized flooding possible, mainly south
  and east of Interstate 55.

&&

.DISCUSSION...
Issued at 316 PM CDT Thu Apr 3 2025

Through Saturday:

Winds remain out of the west today as the surface cold front
continues to sink down over south-central Illinois and southern
Indiana, becoming pseudo-stationary. Wind gusts remain around
20 mph, but will generally diminish into today. Clouds have
increased as a slug of mid level moisture is moving northward
along the southwesterly steering flow aloft. The better chances
for rain will remain where the better forcing is closer to the
boundary. Any showers that do move up here should remain south
of Interstate 80, though rainfall amounts should be minimal.

As a surface high moves over the region through the night,
winds will switch around to the northwest and eventually to the
northeast by Friday morning. There is some uncertainty with
exact minimum temperatures on Friday morning. Models have
started to key in on the better cold air advection overnight
with northwest winds which helped allow the official forecast
to knock temperatures down. However, considering there should be
enough cloud cover overnight, that should help mitigate any
temperatures to touch freezing. But lows tomorrow morning could
range from mid to upper 30s closer to the Wisconsin state line
to the low 40s south of Interstate 80. High temperatures on
Friday are expected in the upper 40s and low 50s. With the
stationary front to the south, similar high temperatures are
expected on Saturday as well.

A weak surface low will move southwest to northeast through
Central Illinois and Indiana starting Friday afternoon through
Saturday morning. This system will have anomalously high
precipitable water amounts (1.5 to 2 inches) at its core where
downpours and locally torrential rainfall could occur. However,
overnight model runs began to shift the main track slightly to
the southeast and this morning 12Z runs look to echo that
sentiment. It looks to set up a heavy rain event that could net
out a strong gradient from higher precip totals in the
southeastern portions of our forecast area, to lower totals in
to the northwest. Given that GEFS, Canadian, and Euro ensembles
came into better agreement, it looks like the gradient could be
drawn over, or just southeast of, Interstate 55. Higher totals
could be 1 to 2 inches of rain, with a 20 to 30 percent chance
for a locally higher amount (favored areas for this around US-
24). Despite the fact that ensembles are suggesting a general
area of precipitation to the southeast, there are still
individual members of the ensembles as well as some
deterministic models (12Z Euro, HRRR) that have a band of higher
precipitation reaching just north of Interstate 80, including
the Chicago Metro, which allows for some uncertainty still yet
to be ironed out on the specific placement of the north edge of
the precip axis. The southern portions of our forecast area
remain in the WPC`s Slight Risk (level 2 out of 4) for
excessive rainfall and the risk for flooding. Current models are
suggesting the higher rainfall amounts begin to start moving to
the east of the area Saturday morning, though the risk for
lingering light rain is possible through the afternoon.

DK

Saturday Night through Thursday:

Rain associated with the Friday storm system (see short-term
discussion above) will exit the area late Saturday night into
early Sunday morning as a cold front pivots through the area.
Behind the front a surface high will begin to build across the
central Plains and nose itself into northern IL and northwest IN
by Sunday afternoon. Therefore, expect a rain-free conclusion
to our weekend but with seasonably cool temperatures in the
mid-40s to around 50 Sunday afternoon.

The weather turns active again on Monday as a potent shortwave
trough pivots across the Upper Midwest and through the Great
Lakes. At the same time, a cold front will be pushed southward
through northern IL and northwest IN bringing with it a period
of precipitation. Guidance is in very good agreement that the
front should move through during the morning and early afternoon
hours which means that temperatures on Monday will likely
struggle to get much above 40 degrees especially for areas north
of I-80. That said, precipitation should briefly begin as rain
but quickly transition over to wet snow as temperatures cool in
the wake of the front. Given the progressive nature of the
front/precipitation (<6 hours at any one location) and the
higher April sun angle, snow accumulations will likely be
limited and mainly confined to grassy surfaces.

While the snow is expected to taper Monday evening, there is a
modest signal for a few lake effect snow showers to linger into
Tuesday morning as the shortwave swings east over lower MI.
Forecast soundings do show lake ELs getting into the 8000-9000
ft range, but the weaker surface convergence will likely limit
the coverage and intensity of said showers. Regardless, the most
favored areas for lake effect will be mostly northwest IN with
all showers forecast to conclude by daybreak Tuesday morning.
Outside of the lake effect, temperatures Monday night are
expected to dip into the low to mid-20s before moderating into
the low to mid-40s Tuesday afternoon.

Temperatures are forecast to warm back towards more typical
April readings towards the later half of next week. With the
warm up there is a signal for periods of showers to return as
well, but confidence on where and when are low at this time.

Yack

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 642 PM CDT Thu Apr 3 2025

VFR conditions are expected tonight through the day Friday.
Northwest winds this evening will become northeasterly tonight
and continue from the east or northeast for the remainder of the
TAF cycle. Light winds tonight will freshen up a bit Friday
morning and continue around 10kt through Friday evening with
some sporadic gusts in the 15-20kt range possible, especially
late Friday morning. Showers are expected to develop Friday
evening with a gradual deterioration to MVFR conditions
expected.

- Izzi

&&

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

&&

$$

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