835
FXUS63 KLOT 262326
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
626 PM CDT Thu Sep 26 2024

.KEY MESSAGES...

- Windy conditions expected Friday afternoon and evening,
  especially near the lake and south of I-80. Wind Advisories
  issued for areas south of I-80.

- Chances for a brief shower or two over the weekend, with best
  chances southeast of I-55

&&

.DISCUSSION...
Issued at 245 PM CDT Thu Sep 26 2024

Through Friday:

Our attention will be turning squarely towards Hurricane Helene
and its extratropical transition. While some rain/showers will
work their way up here, the primary weather impacts we`ll
experience up here will be tied to increasingly strong/gusty
winds. The wind gust potential continues to inch up, and as a
result, Wind Advisories have been issued with this shift for
locales roughly south of the Kankakee River late Friday
afternoon and evening for gusts in excess of 45 mph.

Helene is forecast to continue intensifying today leading up to
eventual landfall in the Florida Big Bend later this evening.
After landfall, a complex interaction with a sprawling cutoff
upper low that`s been churning off to our south the last few
days will occur. As noted by the overnight shift, this will
result in a much slower-than-usual weakening of the wind field
than would otherwise be seen with a landfalling hurricane.
Guidance today remains relatively unchanged, sans a few to-be-
expected wobbles, regarding the morphology of the primary
surface low which is expected to slingshot north and westward
across the Lower Ohio River Valley on Friday. Record or near-
record low pressures are advertised across the guidance suite,
with many ensemble members (EPS in particular) showing low/mid
980s mb. While the surface high to our north across the Dakota`s
isn`t particularly deep, this exceptionally tight pressure
gradient will lead to an intensifying low-level wind field as we
head through the day on Friday.

While some uncertainty regarding the coverage of precipitation
and lower cloud cover remains (with potential impacts on the
degree of boundary layer mixing), a very dry antecedent airmass
will initially be in place. While some showers look to back into
areas east of about I-57 through Friday afternoon, forecast
soundings generally depict steep lapse rates in the lowest 1 km
or so. With 925 mb winds progged to increase into the 40-50 knot
range late in the day and into the evening, momentum transfer
is expected to result in gusts pushing near 45 mph for locales
roughly near/south of the Kankakee River. Additionally, given
the rapid westward movement of the surface low, an isallobaric
component will exists even though surface pressures will be
gradually increasing, which may ultimately help boost the gust
potential a bit. Have noted perhaps a slightly concerning trend
towards even higher winds immediately off the surface (12z GFS
showing nearly 60 knots in our far south and east by early
evening Friday), and while the full force of these winds will
not be realized at the surface locally, we can`t rule out a
potential for locally damaging gusts to briefly push past 60 mph
south and east of perhaps a Pontiac to Kouts line. Farther to
the north of I-55, generally looking at gusts in the 25 to 35
mph range.

With all of this in mind, elected to hoist a Wind Advisory for
Friday afternoon and evening for locales near/south of the
Kankakee/Illinois rivers within the core of the strongest
925-850 mb flow. As stated above, there`s perhaps a play where
we may need to consider a targeted High Wind Warning upgrade
depending on model and observation trends. Winds will gradually
ease through Friday evening and overnight.

With the strong winds, large/battering waves will develop on
the lake. The wind field is not expected to be large enough to
result in any notable surge effects from Lake Huron, but the
large wave action may nonetheless result in some minor
lakeshore/beach inundation.

Regarding precipitation chances: pretty decent agreement that
an initial batch of showers will meander towards our I-57
locales through Friday afternoon, although suspect the NW fringe
will get eaten away pretty readily as it encounters a wall of
very dry mid-level air (700 mb dewpoints near -30 C near the
WI/IL state line). Additional bands of showers will then develop
Friday night, potentially encroaching even on the Rockford
area, although precip intensity/amounts are not expected to be
significant, generally a few tenths of an inch or less.

Carlaw


Friday Night through Thursday:

The upper level low from Helene will slowly fill in and weaken
over the Ohio River Valley. It will continue to spin and send
weaker rain bands over the area on Saturday and Sunday. While
the better chances will be to the southeast of I-55, there are
some deterministic runs that still have a band passing over the
city of Chicago (and points west). A chance (25 to 40 percent)
for light rain was maintained through the weekend for most of
the forecast area.

As an upper level trough begins to develop on Monday morning
over Western Canada, models are suggesting the remnant low from
Helene will continue to weaken and gradually drift east.
Probabilities for rain diminish on Monday/Monday night with much
of the area having a dry forecast. However, areas closer to the
lakeshore as well as inland areas around and east of I-57
closer to the low still have a slight chance for showers
lingering around. Of course that will largely depend on how
much/quickly the low moves away from the area and weakens.

The previously mentioned upper level trough in Canada will dig
down into Northern Minnesota/Wisconsin through the early part of
next week with a reflected surface front arriving around
Tuesday. Current models project keeping the higher moisture
content farther to the north as well as an upper low to the
south restricting any southerly moisture return. So in all
likelihood it will be a dry front that arrives with winds
switching over to the northwest and ushering in a cooler air
mass with afternoon temperatures potentially in the 60s/low 70s.

Surface high pressure is expected to grow behind the front.
Better height rises and a drier air mass should provide quieter
conditions for the middle to end of the week. Models are
projecting another upper level trough moving over the northern
Plains late in the week. This would switch mid level flow back
to the southwest and advect warmer 850 mb temperatures
northeastward. While the 20C temps should remain down in the
Southern Plains (though the GFS is being more aggressive with
the northeast push), some upper teen C temps will build through
the middle to late part of the week. This should translate into
another warming trend that should help bring surface
temperatures back into mid to upper 70s (maybe 80?) by Thursday.


DK

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 626 PM CDT Thu Sep 26 2024

Forecast concern for the terminals...

- Gusty northeast winds of 30 to 35 kt range Friday, occasional
  gusts to 40 kts possible

A broad surface high overhead this evening will maintain
generally quiet weather conditions for tonight with light
northeast winds around 5 to 10 kts. Though gradually increasing
VFR cirrus is expected as Hurricane Helene makes landfall and
races inland along the Gulf Coast and southeast US tonight.

Winds will quickly begin to increase Friday morning as the
remnants of Helene move into the eastern Ohio River Valley.
Initially, northeasterly gusts in the 20 to 25 kt range are
expected early in the day but higher gusts in the 30 to 35 kt
range are expected to prevail Friday afternoon and evening.
There is also the potential for some occasionally higher gusts
near 40 kts, mainly at the Chicago terminals, depending on how
deeply the atmosphere mixes Friday afternoon. Since the
frequency of the 40 kt gusts are lower confidence (around 20%
chance) have decided to forego a formal mention at this time but
will closely watch trends for future TAF issuances.

While gusts are expected to gradually ease Friday evening, the
30 to 35 kts are not forecast to subside until after the
conclusion of this TAF period. Therefore, have maintained the
gustier winds through the end of the period. Aside from the
winds, expect VFR conditions to prevail through Friday evening
with SCT to BKN 20000 to 25000 ft cirrus overhead.

Yack

&&

.MARINE...
Issued at 245 PM CDT Thu Sep 26 2024

A period of northeast gales is appearing increasingly likely
over the extreme southern tip of southwestern Lake Michigan
Friday afternoon/evening.

What is forecast to be a record strong low pressure system for
the lower Ohio Valley Friday will result in a strong pressure
gradient across southern Lake Michigan. While the air mass over
the lake isn`t progged to be particularly cool, the water temps
are still warm and forecast bufkit soundings suggest a 6-9 hour
wind with the potential for mixing down gale force winds near 40
kt. With confidence increasing in gales, have upgraded the Gale
Watch to a Gale Warning for the Wilmette Harbor to Michigan City
nearshore zones. A tight gradient in winds is expected east of
Michigan City, with winds diminishing quickly with eastward
extent.

Izzi/Carlaw

&&

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

     Wind Advisory from 1 PM Friday to midnight CDT Friday night
     for ILZ023-ILZ032-ILZ033-ILZ039.

IN...Beach Hazards Statement from Friday morning through Sunday
     morning for INZ001.

     Wind Advisory from 1 PM CDT /2 PM EDT/ Friday to midnight CDT
     /1 AM EDT/ Friday night for INZ010-INZ011-INZ019.

     Beach Hazards Statement from Friday morning through Saturday
     evening for INZ002.

LM...Small Craft Advisory from 7 AM Friday to 4 AM CDT Sunday for
     Winthrop Harbor to Wilmette Harbor IL.

     Small Craft Advisory from 7 AM to 1 PM CDT Friday for Wilmette
     Harbor IL to Michigan City IN.

     Gale Warning from 1 PM Friday to 3 AM CDT Saturday for
     Wilmette Harbor IL to Michigan City IN.

&&

$$

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