829
FXUS63 KLOT 231917
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
217 PM CDT Sun Mar 23 2025

.KEY MESSAGES...

- Rain will sweep across the area this afternoon.

- Westerly winds will increase quickly along and behind a cold
  front this evening with gusts of 40 to 50 mph.

- Chance (30-40%) for showers Tuesday into Tuesday night. A
  little slushy/wet snow may mix in, but no impacts are
  expected.

- Trending toward warmer through the end of the week with
  increasing chances for showers and perhaps some thunderstorms
  towards next weekend.

&&

.DISCUSSION...
Issued at 218 PM CDT Sun Mar 23 2025

Through Monday:

A recent surface analysis revealed a 1000mb surface low
centered along the Minnesota and Wisconsin borders, north of a
secondary area of low pressure developing over far northeastern
Iowa. Both lows are sitting atop a cold front, which stretches
through Central Iowa and north-central Missouri. A solid band of
rain is currently crossing the Mississippi River ahead of the
cold front and along the nose of strong "slabwise" DCVA
associated with the parent upper-level trough approaching the
Lower Great Lakes.

Through the afternoon, the solid band of rain ahead of the cold
front will sweep across our area leading to a 2 to 4 hour period
of steady rain areawide. Right behind the back edge of the slug
of rain, a narrow ribbon of 100-150 J/kg of 0-3km CAPE ahead of
the cold front will encourage a similarly-shaped narrow band of
low-topped showers. With 925mb flow poised to rapidly increase
right behind the front, it won`t take much for a shower to
efficiently mix a 45 to locally 55 mph wind gust particularly
south of I-80 where low-level lapse rates will be modestly
steeper.

This evening, westerly wind gusts will rapidly increase as cold-
air advection (which encourages sinking air) works in tandem
with the tightening low-level pressure gradient (along the
backside of the secondary low lifting toward Lake Michigan).
With RAP 925mb (850mb) flow poised to rocket toward 45kt (50kt)
after 00Z/7 PM across our entire area, as well as several
observations of 45 mph wind gusts across southern Iowa behind
the front (further south and earlier than previously
forecasted), felt the course of least regret was to expand the
inherited Wind Advisory initially just along and north of I-80
to cover our entire area. The peak in the wind gusts of 45 to
locally 55 mph looks to be from 7 PM to 11 PM, after which gusts
should only gradually ease into the 35 to 45 mph range through
the remainder of the overnight hours (the surface low will stall
in northern Lower Michigan overnight). Where wind gusts are
strongest, a few downed tree limbs and even a localized power
outage may result.

Tomorrow, the surface low will slowly meander into southern
Ontario. With our area remaining within the packed low-level
pressure gradient behind the low, west to northwest winds will
remain blustery tomorrow with frequent gusts of 35 to 40 mph.
When combined with morning temperatures near the freezing mark,
wind chills at daybreak will be in the upper teens to lower 20s
(brrr!). Gradually scattering stratus should allow for an
opportunity to see the sun before the day is over and help boost
afternoon highs into the mid 40s (lakeside) to around 50
(central Illinois).

Borchardt


Monday Night through Sunday:

A low to mid-level baroclinic zone will be oriented from across
central Iowa through downstate/central Illinois in the wake of
Monday`s departing storm system. This thermal gradient looks to
remain relatively quasistationary into Tuesday as a series of
sheared and lower-amplitude disturbances shuttle southeast along
it. While some modest north/south placement differences exist
in the medium range guidance, some signal exists for attendant
f-gen forcing to squeeze out a narrow axis of precipitation on
the cold side of the baroclinic zone.

It`s still difficult to say if that will occur over our forecast
area or off to the south, but the ensemble signal continues to
justify chance PoPs across much of the forecast area on Tuesday.
Thermal profiles currently look like they`d support
predominantly a cold rain, although the presence of stronger
ascent and associated dynamic cooling could end up supporting a
changeover/mix to a wet and slushy snow in spots. Additionally,
skies could remain fairly clear Monday evening, allowing
temperatures to drop swiftly with dewpoint still in the mid 20s.
For now, have just added a slight chance of a little snow for
now. Even if heavier snow rates were to materialize, surface
temperatures still look like they`d relegate any slushy
accumulations to grassy and elevated surfaces.

Precipitation chances (showers) will continue through Tuesday
afternoon and into the evening as another fast-moving wave
arrives. High pressure then looks set to briefly build overhead
on Wednesday yielding near seasonable temperatures with just
some northwesterly breezes and diurnally-building cumulus.

Intensifying low-level south to southwesterly flow ahead of a
lower amplitude (but robust) shortwave traversing the Trans-
Pecos Region will encourage the next bout of warm advection
across the central CONUS on Thursday. At the same time, broad
surface troughing will develop across the High Plains,
facilitating sharpening boundaries to our south and west. The
aforementioned burgeoning warm advection may foster the
development of scattered showers and even some elevated
thunderstorms Thursday evening into Friday morning as the
eastern fringes of an expansive EML begin to surge eastward.

A notable, negatively-tilting shortwave is forecast to eject out
across the plains during the Friday - Saturday timeframe,
although sizable timing, intensity, and placement differences
exist across the guidance suite. Regardless, the signal for
another storm system developing in the region is high into the
upcoming weekend, along with a commensurate increase in
precipitation chances.

Carlaw

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1255 PM CDT Sun Mar 23 2025

VFR conditions at the Chicago terminals will deteriorate over
the next 2 to 3 hours, with IFR ceilings and low-end MVFR
visibility expected with a band of rain shifting across the area
by around 20Z. SSE winds gusting to around 25 knots through
mid-afternoon will slowly veer SSW as the band of rain moves
overhead. The main band of rain will exit to the east by 22-23Z,
but another corridor of low- topped convection (without TS) is
expected across the Chicago metro late this afternoon into early
evening. While the line of SHRA is expected to be scattered to
broken, the SHRA could produce brief gusts over 30 knots.

Shortly after the SHRA early this evening, west winds will gust
in excess of 40 knots for several hours before settling in the
30 to 35 knot range overnight into Monday morning. SCT to BKN
MVFR stratocumulus is expected during this time. Ceilings will
scatter and lift into VFR levels by Monday afternoon as WNW
gusts diminish to under 30 knots.

Kluber

&&

.MARINE...
Issued at 218 PM CDT Sun Mar 23 2025

A deepening low pressure system will move through the Upper
Great Lakes this evening and stall near the Mackinac Straits
tonight into Monday morning. The cold front of the system will
sweep across the region this evening, ushering in westerly gale
force winds to 45 kt. A storm force gust to 50 kt cannot be
ruled out, particularly on high platform observing platforms.

Gale force speeds/gusts will ease gradually tonight toward 35 kt
but persist through Monday afternoon. West-northwesterly winds
will then steadily diminish through Monday evening and may even
turn light northerly prior to sunset along portions of the
Indiana nearshore.

A Gale Warning is in effect for the nearshore waters from Winthrop
Harbor IL to Michigan City IN from 7 PM CDT this evening through
3 PM CDT on Monday.

NWS Chicago

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Wind Advisory until midnight CDT tonight for ILZ003-ILZ004-
     ILZ008-ILZ010-ILZ011-ILZ019.

     Wind Advisory from 6 PM this evening to midnight CDT tonight
     for ILZ005-ILZ006-ILZ012-ILZ013-ILZ020-ILZ021-ILZ023-
     ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Wind Advisory from 6 PM this evening to midnight CDT tonight
     for INZ001-INZ002.

LM...Small Craft Advisory until 7 PM CDT this evening for the IL
     and IN nearshore waters.

     Gale Warning from 7 PM this evening to 3 PM CDT Monday for the
     IL and IN nearshore waters.

&&

$$

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