034
FXUS63 KLOT 302008
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
308 PM CDT Tue Apr 30 2024

.KEY MESSAGES...

- Wind gusts 50 mph or higher possible around/just after
  midnight as a line of decaying showers/isolated thunderstorms
  moves across the area.

- Gusty westerly winds to 30 mph Wednesday.

- A couple periods of showers and thunderstorms mid to late
  week, most notably Wednesday night into early Thursday
  morning, then again late Thursday night into Friday morning.

&&

.DISCUSSION...
Issued at 308 PM CDT Tue Apr 30 2024

Through Wednesday Night:

The primary forecast focus and concern is regarding what the
HRRR and several other models are showing late this evening into
the early overnight hours, which is the potential for wind gusts
50 mph or higher. As showers and a few thunderstorms move into
the western area late this evening, they will be in a weakening
phase and as they dissipate across the local area, the models
are showing the potential for strong winds mixing to the
surface. Of note with the HRRR is that it has been doing this
consistently, run after run today. There is a narrow time window
of strong winds via a low level jet and whether the models are
mixing into this low level jet or trying to develop a wake low
is not clear. Confidence is low for how this will evolve and
trends with convection across IA will need to be monitored into
this evening. Have bumped up winds tonight, with gusts into the
30-35 mph range but if what the HRRR and some of the models are
showing does continue in later runs this evening, its possible
a short duration wind headline may be needed. The models show
these stronger winds diminishing in the predawn hours.

Winds turn westerly Wednesday morning and now appear to be a bit
stronger than previously expected, especially along and north of
I-80 where wind gusts to 30 mph are possible. Winds slowly turn
northwesterly Wednesday afternoon with speeds/gusts slowly
diminishing in the mid/late afternoon. This may allow a lake
breeze to develop into northwest IN by late afternoon but
confidence is low for a lake breeze into northeast IL late
Wednesday afternoon.

The next system will bring a chance of showers and possibly a
few thunderstorms to the area late Wednesday night into early
Thursday morning. Confidence is fairly low for coverage and its
possible that much of the area may remain dry, with the best
chances across the northwest cwa. cms

Thursday through Tuesday:

Warm air advection driven showers (and some embedded
thunderstorms) may be ongoing across portions of
northern/northwestern IL at the start of the period Thursday
morning as a surface warm front lifts northward into the area.
However, the focus for this early morning warm air advection
driven convection is expected to shift north of the area through
midday as the surface warm front continues its northward shift
towards the WI state line. Thereafter, a primarily precipitation
free afternoon is anticipated, as we await our next good
potential for showers and storms with an approaching cold front
later Thursday night into Friday morning.

While most areas may end up dry Thursday afternoon, temperatures
could end up varying considerably across far northeastern IL
(northern Chicago suburbs). This is due to fact that Lake
Michigan may slow the northward progression of the warm front
into WI near the lake, thus resulting in continued persistent
cool east-southeasterly winds through the day. In such a scenario,
temperatures Thursday afternoon would be held down in the lower
60s near the lake in far northeastern IL, while areas to the south
of the warm front experience summer-like temperatures (in the low
to mid-80s) and breezy south winds.

Mild weather will continue into Thursday night as our area remains
locked in the warm sector of the surface low tracking northward
across the upper Midwest. This low is forecast to occlude over
northern MN and far southwestern Ontario into early Friday
morning. As it does so, the attendant surface cold front will
slide eastward into our area very late Thursday night into
Friday. A period of showers, and a few embedded thunderstorms, is
likely to accompany this front, with the highest chances (60-80%)
coming early Friday morning. We will need to continue to monitor
the potential for any hydrologic concerns during this time frame,
especially given the recent heavy rainfall the area has
experienced. However, signs continue to point at the heaviest
rainfall amounts potentially remaining west of the NWS Chicago
forecast area in closer proximity to the track of the surface
low.

The rain threat is expected to end from west-to-east on Friday
following the cold frontal passage. Thereafter, temperatures are
favored to return closer to normal going into the weekend. Another
approaching disturbance may result in another period of
precipitation late Saturday into Saturday night, but the signal
for this is not all that strong at this point. Otherwise, slightly
warmer temperatures are favored into early next week with the
overall weather pattern continuing to look rather active across
the central CONUS.

KJB

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 105 PM CDT Tue Apr 30 2024

Main Concerns:

- Wind shift to southeast early this evening

- Decaying SHRA moving across the area late tonight

- A period of strong (>40 kt) south-southwest wind gusts
  possibly accompanying the decaying SHRA, with at least 25-30
  kt range gusts appearing probable

- Potential for a period of MVFR CIGs early Wednesday, primarily
  at RFD

West-southwest winds around 10 kt with occasional gusts will
shift to southeasterly early this evening. A true lake breeze
shift to easterly appears unlikely.

Tonight, expected severe TS west of the MS River will weaken
quickly as they move east-southeast across the region, with
TEMPO mention in the TAFs. The HRRR model has shown a fairly
consistent run to run signal for very strong south-southwest
winds gusting to 40-50+ knots associated with the rapid decay of
the SHRA. Change with this TAF issuance was to bump up prevailing
south-southwest winds to ~15-20 kt with ~25-30 kt gusts, which
appears probable, even if the HRRR scenario doesn`t pan out.

Forecast soundings suggest MVFR CIGs are briefly possible behind
an early morning cold front passage. Included TEMPO mention in
the RFD TAF where confidence was a bit higher. Winds will shift
to westerly with 20-30 kt gusts behind the front, strongest
Wednesday afternoon.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 11 PM this evening to 7 PM CDT
     Wednesday for the IL nearshore waters.

     Small Craft Advisory from 11 PM this evening to 1 PM CDT
     Wednesday for the IN nearshore waters.

&&

$$

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