284
FXUS63 KLOT 070739
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
239 AM CDT Sat Sep 7 2024

.KEY MESSAGES...

- Scattered lake effect showers to impact portions of northwest
  IN this morning.

- Seasonably cool conditions bring an early sampling of fall
  today and tonight.

&&

.DISCUSSION...
Issued at 239 AM CDT Sat Sep 7 2024

Through Sunday:

A narrow upper-level trough continues to propagate eastward
over Lower Michigan and the eastern Great Lakes this morning.
Along the western periphery of the trough, a mid-level shortwave
disturbance is pivoting down the spine of Lake Michigan where a
narrow convergence axis is organizing due to interaction of two
land breezes (from northeast IL and western Lower MI). As the
shortwave and convergence axis continue to move into the
southern portion of the lake they will begin to interact with an
area of robust lake induced instability (equilibrium levels
around 12000 to 16000 ft) which should allow the developing lake
effect showers become more organized and move inland over the
next few hours. Given the current trajectory, the greatest
coverage of lake effect showers is expected to be mainly into
Porter and LaPorte Counties in northwest IN but Lake County, IN
could see a few of these showers towards daybreak before the
axis shifts east. While a stray rumble or two of thunder cannot
be ruled out, the modest mid-level lapse rates should keep the
thunder threat low.

In addition to the showers, the aforementioned instability in
combination with warm lake temperatures and cool mid-level air
(850 mb temps around 4 to 5 celcius) will also generate
favorable conditions for waterspouts. Any waterspouts that do
materialize are expected to remain near the main convergence
axis and remain over the open waters of the lake, thus no inland
threat is expected. Though, anyone with boating plans this
morning should remain vigilant and heed any marine warnings as
the features can cause locally higher winds and waves capable of
damaging smaller vessels.

As the aforementioned trough and associated shortwave continue
to pivot east this morning, the combination of drier mid-level
air and subsidence behind the shortwave should gradually taper
the lake effect showers and waterspout threat by mid to late
morning (between 10 AM and noon). However, the continued cold
advection behind the trough will make for a seasonably cool
afternoon with temperatures only in the lower to mid-60s.
Furthermore, the cold advection will also allow the atmosphere
to mix into some breezier winds aloft and generate gusts in the
15 to 25 mph range. While wind gusts will subside this evening
as the atmosphere decouples, the dry air mass and clear skies
will allow temperatures to fall into the low to mid-40s (upper
40s in Chicago) overnight. Though, temperatures are forecast to
rebound on Sunday as winds become more west-southwesterly
generating highs in the lower 70s under mostly sunny skies.

Yack


Sunday Night through Friday:

Broad mid-level ridging will develop over the Great Lakes into
early next week, and will remain in place through the work week.
At the surface, an expansive quasi-stationary high pressure
will extend from the Great Lakes to off the East Coast. This
will block off any moisture return and result in large diurnal
temperature spreads through the period.

A weak wave will pass to our north Sunday night but, aside from
throwing some briefly increased mid-level cloud cover our way,
won`t do much to our sensible weather. Following another cool
start to Monday morning, temperatures will rebound into the
lower 80s with strong mixing holding dewpoints in the very
comfortable range. As 925-850 mb temperatures warm and mixing
deepens, a gradual warming trend will establish through the end
of the week, with some 90 degree highs possible Wednesday-Friday
away from the lake. Abnormally dry conditions of late should
result in some dry ground feedback given continued sunny daytime
conditions and dew points mixing out. The next chance for rain
will probably not arrive until sometime next weekend, so short-
term precipitation deficits will continue to grow.

Carlaw

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1213 AM CDT Sat Sep 7 2024

The main aviation weather concerns are:

- Lake effect showers from near GYY and points east

- Timing a weak lake breeze/NNE wind shift at the Chicago-area
  sites this afternoon

All sites except GYY: Winds will remain northwesterly tonight,
with some intermittent gusts around 20 kts at ORD/MDW. Winds
will trend northerly through this morning, and eventually,
expecting an 010-020 change at ORD/MDW towards midday/early
afternoon as a weak lake breeze shuttles inland. No timing
changes have been made to the inherited TAFs at this time.
Winds will then become light and variable for a time Saturday
evening before acquiring a W or WSW direction overnight.

At GYY: Breezy northwest to northeast winds will prevail through
much of the TAF period with predominantly VFR cloud cover. Lake
effect shower activity looks to set up largely just east of the
terminal, although a few passing -shra can`t be ruled out,
particularly towards daybreak and through the morning.

Carlaw

&&

.MARINE...
Issued at 239 AM CDT Sat Sep 7 2024

Waves have continued to build into the 6 to 10 ft range
overnight and should remain as such through this afternoon.
Northerly winds will continue to gust into the 25 to 30 knot
range today, particularly within the land breeze convergence
zone and with more robust lake effect showers. Lake parameters
will become more favorable for waterspouts this morning, with
this threat maximizing near the lake convergence zone which is
expected to be more or less right down the center of Lake
Michigan into the northwest Indiana nearshore zones.

Yack/Carlaw/Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement through late tonight for ILZ006-ILZ103-
     ILZ104.

IN...Beach Hazards Statement through late tonight for INZ001-INZ002.

LM...Small Craft Advisory until 10 PM CDT this evening for the IL
     nearshore waters.

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

&&

$$

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