711
FXUS63 KLOT 042009
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
309 PM CDT Fri Oct 4 2024

.KEY MESSAGES...

- Gusty winds developing Saturday. A few gusts to 35 mph late in
  the afternoon.

- Low chance (around 20%) for showers and possible thunderstorms
  late Saturday night into early Sunday morning (mainly N. of
  I-80).

- Combination of gusty northwest winds and dry conditions looks
  to create elevated fire threat on Sunday

- Dry and above normal conditions expected through at least the
  end of next week

&&

.DISCUSSION...
Issued at 307 PM CDT Fri Oct 4 2024

Through Saturday:

Moisture channel loops reveal the shortwave--in part responsible
for this morning`s showers--shearing and gradually diminishing
across northern Missouri. Lingering modest warm advection
associated with this feature will drift eastward into our forecast
area this evening and overnight. As this occurs, a reservoir of
mid-level moisture currently situated across central Illinois will
slosh back across the region resulting in some increase in cloud
cover, particularly south of I-80. At this point, it`s a bit
unclear just how widespread/expansive cloud cover will be, but
have increased sky cover grids a touch. Suppose it`s not totally
out of the question a few sprinkles materialize late tonight into
Saturday morning if cloud cover remains deep enough, but chances
for this look much too low to include any mention in the gridded
forecast.

Tomorrow will feature increasingly gusty south to southeasterly
winds through the day along with notably warmer temperatures as a
robust surface low takes shape across Manitoba. Expecting gusts to
peak late in the afternoon (3 to 5 pm or so) near 30-35 mph as
mixing deepens PBL heights to around 4-5 kft. Any morning cloud
cover should scatter through the early afternoon, and with 850 mb
temperatures warming through about +15 C, high temperatures will
push into the low to mid 80s, with cooler conditions along the
Illinois lakeshore.

On Saturday night, the aforementioned surface low will push
rapidly into Ontario as it occludes. With the core of the
associated upper low displaced well to our north, we`ll be on the
southern flanks of the more notable large scale ascent. This
forcing won`t be totally negligible, however, with 60 to 80 m/12
hour 500 mb height falls forecast, along with at least a glancing
blow of DCVA scraping the Illinois/Wisconsin state line. While
essentially every piece of global guidance (and respective
ensembles) are precipitation-free in our area, a plume of very
steep mid-level lapse rates will advance ahead of the incoming
cold front. Just a slight misdiagnosis of moisture in the 800-650
mb layer will be the difference between a totally dry outcome and
one featuring unforecast storms.

For this forecast iteration, it does look like guidance has
generally dried this layer out a bit, as well as warmed the base
of the lapse rate plume, resulting in more formidable capping to
elevated parcels. While the latest 12z ECMWF isn`t producing
precipitation over our area, forecast soundings near the state
line really don`t look all that far away from supporting
convection, however. As a result, don`t want to entirely rule out
the potential for a few storms, however, so have maintained some
low-end PoPs across the northeastern half of the CWA on Saturday
night. If storms do materialize, effective inflow bases look like
they`d be elevated (above 925-850 mb or so), resulting in more
muted effective deep layer shear compared to surface-based
parcels. This should limit the severe threat, but a few stronger
storms can`t be ruled out.

Carlaw


Saturday Night through Friday:

Following the passage of a cold front late Saturday night, the
forecast in the long term looks awfully quiet for us here. We pick
on Sunday morning where we should find dewpoints circling the drain
behind the departing front with midday Tds forecast in the middle
and upper 30s. Highs in the lower and middle 70s yield midday RH
values in the 25 to 30% range. Breezy northwest winds will spread
over the area during the day with gusts getting up to around 25 to
30 mph. The breezy winds and low RH in conjunction with dry fuels
brings about fire weather concerns for Sunday. The going forecast
keeps us below red flag criteria for now, but it doesn`t miss by
much.

The windward side of the departing upper trough will drop a broad
area of surface high pressure across the Plains on Sunday. The
trough will be very slow to move out of the NE US with a prominent
upper ridge building in behind it, which will then spill over the
Midwest later next week. Consequently, this should lead to several
days of dry, uneventful weather while surface high pressure meanders
about the region for the better part of next week. Sunny and
seasonable to slightly warmer-than-normal conditions are expected
through late week.

Doom

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1201 PM CDT Fri Oct 4 2024

The only aviation weather concern is a period of strong/gusty
south/southeasterly winds developing Saturday.

VFR fair-weather cumulus will develop at the terminals through
this afternoon with northeasterly breezes, along with some
intermittent gusts near 20 knots. Winds will ease this evening
and trend easterly. Through mid-morning Saturday, winds will
increase as they veer southeasterly. Gusts should become more
frequent, and increase into the mid-upper 20 knot range through
the afternoon. A few gusts could inch near 30 knots late in the
day (roughly 21-23z) as wind directions trend towards 180
degrees.

Just beyond the ORD/MDW extended TAF windows: a very strong
low-level jet will develop Saturday evening and overnight. While
intermittent gusts are probable through the overnight, a period
of LLWS appears likely as winds will near 55-60 knots at 2 kft.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 1 AM Saturday to 4 AM CDT Sunday for
     the IL nearshore waters.

     Small Craft Advisory until 4 AM CDT Saturday for the IN
     nearshore waters.

&&

$$

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