581
FXUS63 KLOT 010136
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
836 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.

&&

.UPDATE...
Issued at 836 PM CDT Tue Apr 30 2024

Recent radar trends across southern Iowa show that a cluster of
generally discrete supercells proceeded by disorganized
convection toward the Des Moines area has congealed into a more
organized MCV near Ottumwa. Unlike as was depicted in earlier
CAM guidance, this organization does not display a cohesive MCS
signal. Expectations are for the current MCV to track just south
of east into the relatively more favorable convective
environment. This would bring the track along or just south of
the far southern CWA. Concerns of a potential small wake low
with 50mph+ winds remain valid, but the smaller footprint of
organized convection and weaker cold pool/trailing meso-high
suggest the overall threat is waning.

Farther north, residual mid-level forcing may be able to
maintain some lingering isolated showers and storms across most
of northern Illinois through the overnight hours. Given the
existing low-level dry airmass and only gradual nocturnal
surface decoupling in the warm sector, any (decaying)
shower/storm would still be capable of producing gusty winds at
the surface.

Kluber

&&

.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 /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 617 PM CDT Tue Apr 30 2024

Aviation Key Messages:

- 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

- Diminishing winds turn northwest late Wednesday and eventually
  light northeast later Wednesday evening

Surface winds will continue to back to the southeast early this
evening, in response to low pressure currently moving east
across the mid-Missouri River Valley.

Area of strong/severe TSRA across IA-NW MO-KS are expected to
weaken quickly as they move east this evening and outrun better
instability to our west. Decaying showers will likely make it
into northern IL, though may be only for a brief period
especially farther east into the Chicago metro terminals. Of
greater interest, has been multiple successive runs of the HRRR
CAM, which had shown a consistent signal for very strong west-
southwest winds of 40-50+ kts associated with the decaying
showers. Latest (22Z) HRRR run has backed off these higher wind
magnitudes, though along with other guidance continues to
suggest a period of gusty winds near 30 kt overnight. Will
continue to monitor trends with convection to our west and any
associated enhanced winds.

Forecast soundings indicate a period of MVFR ceilings are
possible behind a cold front which will move through the area
pre-dawn, with a better signal farther west/northwest for KRFD
than our other terminals. These should erode after sunrise
Wednesday, with breezy west winds expected. Winds are expected
to diminish and veer northwest late in the day as surface high
pressure spreads into the area, and then shift northeast later
Wednesday evening.

Ratzer

&&

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

     Small Craft Advisory until 1 PM CDT Wednesday for the IN
     nearshore waters.

&&

$$

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