643
FXUS63 KLOT 101107
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
607 AM CDT Thu Oct 10 2024

.KEY MESSAGES...

- Dry, breezy and unseasonably warm on Friday with potential
  elevated brush fire risk Friday afternoon.

- Notable cooldown early next week with the potential for
  nightly frost and localized freeze conditions.

&&

.DISCUSSION...
Issued at 336 AM CDT Thu Oct 10 2024

Through Friday:

Continued dry and warmer weather conditions are expected for
the end of the work week, particularly on Friday as temperatures
likely top out in excess of 15 degrees above average and no
lake cooling. The breezy and warm weather Friday afternoon,
coupled with our ongoing drought conditions may support an
elevated risk of brush fires.

Surface high pressure will build southeast across the Great
Lakes region today, in the wake of a deep upper trough and
embedded closed upper low moving east across the Canadian
Maritimes and Atlantic Coast. Increasing subsidence and weak
low-level warm advection beneath generally cloudless skies will
allow for moderating surface temperatures, with afternoon highs
ranging from the low-mid 70s east to the upper 70s/near 80 west.
Light east-southeast surface winds will again provide some
lake-cooling to locations along the Lake Michigan shores, though
to a much lesser extent than yesterday with weaker winds. An
upper level ridge will build into the region tonight, while
farther to the northwest a mid-level short wave tracks along the
U.S./Canadian border region. Surface low pressure associated
with this wave will pass well north of the region and should be
approaching James Bay Ontario by morning, with a cold front
trailing across the upper Midwest and into the central Plains.

The short wave passing to our north will flatten/suppress the
upper ridge axis south of the area on Friday, placing us beneath
a west- southwest 25 kt low level jet ahead of the approaching
cold front. This will maintain warm advection across the cwa
under mostly sunny skies (some increasing high clouds possible
in the afternoon), while making for breezy southwest winds with
gusts 20-25 mph (strongest across the northwest half of the
cwa). Progged 925 mb temps in the +19C to +21C range support
afternoon highs in the low to mid 80s across the area (warmest
N/NW), some 15 to nearly 20 degrees above average. The breezy
southwest winds will bring this warmth right up to the Lake
Michigan shores, with no lake cooling expected. These
unseasonably warm, breezy and dry conditions will also support
an elevated threat for brush fires Friday afternoon, as
afternoon relative humidity levels drop below 30 percent and
especially where winds are a little stronger across the
northwest half of the forecast area. At this time, model
guidance keeps the cold front just north of the forecast area
through sunset.

Ratzer


Friday Night through Wednesday:

A shortwave trough is expected to be pivoting across the
northern Great Lakes Friday night into the day on Saturday with
a couple of subtle impulses traversing the base of the trough.
At the surface a cold front is progged to be moving across the
upper Midwest and should move through northern IL and portions
of northwest IN late Friday night and Saturday morning. However,
the front is not expected to fully clear the CWA as it looks to
stall in central IL and IN. Therefore, a gradient in
temperatures remains likely Saturday afternoon with locales
south of the front (namely the Kankakee River Valley and points
south) seeing highs in the upper 70s to around 80 while areas to
the north see readings in the upper 60s to lower 70s.

In terms of rainfall potential, guidance continues to hint at
some spotty showers developing along the front late Friday night
and then redeveloping along the boundary Saturday afternoon as
the aforementioned impulses pivot overhead. However, forecast
soundings continue to show a large quantity of dry mid-level air
in place which I suspect will severely hamper the coverage of
any showers. That said, I did trim back the NBM POPs on Saturday
to slight (15- 20%) chances through early evening just in case
a shower does manifest. Note, there is a higher chance mention
(around 30-40%) Saturday evening mainly in northwest IN where
the front should be as a second shortwave begins to move into
the Great Lakes. While I am still not sold on the coverage of
showers for the same reason described above, a better coverage
of showers is forecast for northern and central IN so to
maintain tolerance with eastern neighbors have left these higher
POPs in for now.

A stronger shortwave trough is expected to move into the Great
Lakes Saturday night into Sunday which will force another cold
front through northern IL and northwest IN. This time the front
will have the force to clear the CWA by Sunday evening which
means that notably cooler temperatures are in store areawide for
Sunday and into early next week. Similarly to the last front,
rain chances continue to look minimal due to limited moisture,
but the much stronger forcing in combination with lake induced
instability may generate some scattered showers near Lake
Michigan in IL and northwest IN Sunday through Tuesday.
Additionally, winds behind the front on Sunday will also become
gusty as the cold advection allows the atmosphere to mix into
the stout mid-level winds aloft. Therefore, gusts in the 25 to
35 mph (25 to 30 kt) range seem likely Sunday afternoon and
evening especially near and over the open waters of Lake
Michigan.

While winds are expected to diminish Sunday night through the
day on Monday, the aforementioned shortwave trough is forecast
to broaden and remain over the Great Lakes through the middle of
next week. As a result a notable cooldown (at least to what
we`ve had) is in store for Sunday and the early part of next
week with daily highs in the 50s to around 60 and nightly lows
in the 30s to lower 40s. In fact, there is even a decent signal
for some areas to see near freezing temperatures at times,
particularly on Monday and Tuesday nights, pending cloud cover.
Not only would these temperatures be hazardous to sensitive
plants, but they will also have the potential to develop frost
which could further harm some plants. Despite the higher
confidence, this period is still 5 to 6 days out so some
uncertainty in the coverage of the coldest temperatures and
frost does remain so have maintained the patchy frost mention
for now.

Looking beyond the next 7 days, guidance is hinting at the idea
for a blocking pattern to develop as a building upper ridge
gets sandwiched between the aforementioned trough, that should
shift to our east, and a new trough forecast to move into the
western CONUS. Depending on how far east the west coast trough
gets before it stalls, our area could be within the southwest
flow downwind of the trough which is a favorable spot for more
frequent precipitation chances. Obviously there is still a great
deal of uncertainty as to where our area will be within this
pattern to have much confidence on what to expect locally, but
it will be a period worth watching especially given our lack of
rainfall as of recent.

Yack

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 607 AM CDT Thu Oct 10 2024

Tranquil weather to continue at the terminals through the
forecast period as high pressure stays over the Great Lakes.
Therefore, expect easterly winds today to gradually become
south-southeasterly overnight and then southwesterly Friday
morning. While speeds will generally remain light through the
period with values in the 5 to 7 kt range, winds will begin to
increase Friday morning with gusts in excess of 20 kts possible
Friday afternoon. Otherwise, expect skies to remain clear
through tonight before some cirrus drifts overhead Friday
morning.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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