316
FXUS63 KLOT 010608
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
108 AM CDT Wed May 1 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 /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 108 AM CDT Wed May 1 2024

Key Messages:

- Showers at/near the Chicago metro terminals during the early
  overnight hours.

- Period of southerly wind gusts to around 30 kts possible
  through daybreak, then westerly winds gusting to around 25
  kts this morning and afternoon.

A batch of decaying showers is moving through northern Illinois
at press time. These showers should end no later than about 07Z
at DPA, 08Z at ORD and MDW, and 09Z at GYY, and are not expected
to produce any meaningful visibility reductions. While some
cloud bases on the backside of this activity are near or below
3000 ft AGL at this time, their coverage appears to be too
sparse to warrant advertising MVFR ceilings in the TAFs, but
will continue to monitor observational trends.

The threat of strong (40+ kt) wind gusts tonight has diminished,
but upstream observations still suggest that southerly wind
gusts to around 30 kts will remain possible over the next few
hours. Winds will eventually turn westerly this morning and gust
to around 25 kts or so through this afternoon before subsiding
and turning northeasterly this evening. Another round of showers
may be approaching RFD, ORD, and MDW right towards the end of
their respective TAF periods, but have withheld a mention of
that in this set of TAFs and will let this be addressed in the
12Z TAF issuance.

Ogorek

&&

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

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

&&

$$

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