396
FXUS63 KLOT 271921
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
221 PM CDT Fri Sep 27 2024

.KEY MESSAGES...

- Winds will increase in magnitude this evening with gusts of
  30 to 40 mph across much of the area, and 45 to 55 mph
  along/east of I-55 and downwind of Lake Michigan.

- A narrow band of showers and embedded thunderstorms will
  develop overnight across northeastern IL and northwestern
  Indiana and last through Saturday morning.

- Drier conditions and moderating seasonable temperatures are
  expected next week.

&&

.DISCUSSION...
Issued at 207 PM CDT Fri Sep 27 2024

Through Saturday:

An incredibly large storm system, which is the result of the merging
of an upper-level low and the remnants of Hurricane Helene,
encompasses the eastern half of the United States. The center of the
sprawling storm system is currently in central Kentucky, and will
retrograde westward toward Paducah this evening. An expansive area
of rain extends from northeastern Arkansas and roughly along the
Ohio River Valley back into southern Ohio, with the northern terminus
currently just south of our forecast area.

This evening, surface winds will steadily increase (both sustained
speeds and gusts) as the low-level jet intensifies ahead of the
approaching inverted surface pressure trough. While mixing heights
will be encouraged to collapse after sunset, the tightness of the
pressure gradient (20-25mb down the length of Illinois) should be
enough to support widespread gusts of 30 to 40 mph across the area.
Locally stronger gusts, perhaps tagging 45 to 55 mph, are poised to
develop within a 2-4 hour window (6 to 10PM or so) immediately
downwind of Lake Michigan as well as along and southeast of I-55.
For this reason, kept the inherited Wind Advisory but opted to add
Will, central/southern Cook, and Lake (IN) through 11 PM tonight.

Tonight into Saturday morning, the expansive reservoir of moisture
to our southeast will contract into a narrow plume of surface-based
instability and PWATs near 2" in northeastern IL and northwestern
IN. At the same time, a narrow region of frontogenesis is
expected to develop along the northern periphery of the stalling
storm system to our south. Taken together, the stage appears to
be set for a narrow band of showers and embedded thunderstorms
to develop and train overnight through Saturday morning.
Conceptually, these kinds of scenarios can lead to surprisingly
efficient rain rates, so rainfall totals in a narrow area may
exceed 1" through tomorrow morning. CAM guidance appears to have
a reasonable handle on the mesoscale forcing and depicts such
narrow banding developing along an axis from Pontiac, IL to
Gary, IN, though in reality, anywhere along and south of a line
from Mendota to Waukegan, IL will be "fair game" for rain
overnight.

Tomorrow, the storm system will effectively stall in western
Kentucky, but begin to "wind down" as it will remain cut-off from
the mean baroclinic upper-level flow. As a result, tomorrow will
feature breezy northeast winds and mostly cloudy skies, as well as a
few lingering showers immediately downwind of Lake Michigan.

Borchardt

Saturday Night through Friday:

A stray shower cannot be ruled out for much of the forecast area
Sunday and into Monday, though models are continuously trending
drier making the rain chances fairly hit-or-miss. The main focus for
showery activity will be for locations east/southeast of I-57;
however, the exact confidence in the shower chances/location will be
closely tied to how the remnants of the Helene fall apart over the
Ohio River Valley and drift east (as it is projected to do through
Monday). Much of the area will be dry with temperatures in the 70s
under mostly cloudy skies.

A long wave trough over southwestern Canada is projected to dig into
the Upper Midwest early next week. It will have a surface cold front
associated with it that is expected to pass over the region on
Tuesday. Winds will flip around to the northwest ushering in a
cooler (near 3C at 850 mb) and much drier airmass (dewpoints
potentially in the 30s and 40s) in the wake of this front. Overnight
temperatures Tuesday night into Wednesday morning look to become
downright chilly, with readings potentially bottoming out in the low
to mid 40s outside Chicago and inland areas away from the lake.

The front should be rather progressive as the surface high builds in
its wake. Winds should switch to a south-southwesterly direction
again Wednesday night into Thursday following the eastward passage
of the surface high. With better warm air advection from the south,
850 mb temperatures will increase to the upper teens. This should
result in surface temperatures on Thursday potentially reaching well
into the 70s (maybe even 80) ahead of what looks to be another
approaching cold front late in the week. The extent and duration of
this late week warm-up will be largely dependent upon the timing of
the next cold frontal passage, which at this time remains somewhat
unclear. Precipitation chances beyond Monday will remain very low at
<10%.

DK

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 108 PM CDT Fri Sep 27 2024

The aviation weather concerns through the 18Z TAF period
include:

* Strong NE winds through the period

* Showers in the area and MVFR cigs late tonight through the day
  on Saturday

NE winds will continue to gradually build into mid-afternoon
with frequent gusts to near 35 kt expected at Chicagoland
sites with less frequent gusts in excess of 35 kt likely. Gusts
will come down some later in the evening but look to persist
above 20 kt tonight and through the day tomorrow. Direction all
the while looks to hold steady at 030-050.

Rain is expected to move into Chicagoland from the south after
midnight. There is a notable amount of uncertainty regarding
rain coverage tonight through tomorrow. Model guidance is in
good agreement that a big majority of the rain in the area will
fall out of a narrow band of rainfall with only light, isolated
pockets of rain expected outside of this feature. Models are in
far lesser agreement regarding the position of this band and
rain coverage within it. The gist: best guess is that all
Chicagoland sites will see at least intermittent showers through
tomorrow with the best chances seemingly late tonight and
tomorrow morning. Moderate rainfall rates will be possible at
times which could potentially pull vsbys down into MVFR or high-
end IFR territory.

Additionally, MVFR cigs are expected to move overhead late
tonight and stick around through the end of the TAF period.
It`s possible that cigs could get pulled down to high-end IFR at
times, particularly beneath any rainfall.

ATTN RFD:

Winds at RFD this afternoon will be marginally lighter than
over Chicagoland at 25 to near 30 kt. The rain looks to remain
south of RFD although an isolated sprinkle or very light shower
will be possible anytime tonight through tomorrow. Additionally,
MVFR cigs look to move over RFD closer to 12Z tomorrow and
should last through the period.

Doom

&&

.MARINE...
Issued at 617 AM CDT Thu Sep 27 2024

An extremely large storm system will continue moving along the
Ohio River and stall this evening. The strong pressure gradient
between a surface high pressure system in southern Ontario and
the storm system over the Ohio River Valley will continue to
support a period of 35 to 40 kt northeasterly gale force winds,
with the peak generally from evening through early Saturday
morning. During the period of peak winds, waves will also build
to 8 to 12 feet with occasional waves as high as 16 feet. While
gale force winds are expected to cease overnight, winds and
waves will be slow to taper through the weekend.

A Gale Warning continues from Winthrop Harbor to Michigan City
through 8Z / 3 AM Friday. A Small Craft Advisory will be issued
when the Gale Warning expires.

Borchardt

&&

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

     Wind Advisory until midnight CDT tonight for ILZ104-ILZ105-
     ILZ106-ILZ107-ILZ108.

     Wind Advisory until midnight CDT tonight for ILZ023-ILZ032-
     ILZ033-ILZ039.

IN...Beach Hazards Statement through Monday morning for INZ001.

     Wind Advisory until midnight CDT tonight for INZ001.

     Wind Advisory until midnight CDT /1 AM EDT/ tonight for INZ010-
     INZ011-INZ019.

     Beach Hazards Statement through late Sunday night for INZ002.

LM...Gale Warning until 3 AM CDT Saturday for the IL and IN
     nearshore waters.

&&

$$

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