516
FXUS63 KLOT 261722 CCA
AFDLOT

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
1222 PM CDT Thu Sep 26 2024

.KEY MESSAGES...

- Windy conditions expected Friday afternoon and evening,
  especially near the lake and south of I-80

- Showers likely Friday night, mainly southeast of a Valparaiso
  to Pontiac line

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

&&

.DISCUSSION...
Issued at 318 AM CDT Thu Sep 26 2024

Today:

Mostly sunny and rather warm conditions expected. High temps Wed
afternoon ended up on the higher end of the guidance spectrum
and with 925mb temps progged to be about 1-1.5C warmer this
afternoon, would anticipate high temps today will be about 2-3F
warmer than Wednesday. That would place most areas around 80F
with the exception being near Lake Michigan, where highs should
top out in the lower 70s.

Friday and Friday night:

Our weather will become increasingly influenced by Helene with
strengthening winds and probably some rain Friday night,
especially southeast 1/3 or so of our CWA. Helene is already a
very large hurricane and is forecast by the National Hurricane
Center to strengthen right up until landfall along the Florida
Panhandle coast. As Helene makes landfall, it is forecast to
interact favorably with the cut off mid-upper level low over
that`s parked over the lower Ohio Valley. The center of Helene
is forecast to move rapidly northward across western Georgia
tonight, before turning northwest and moving into the lower
Ohio Valley on Friday as it merges with the cut-off low. The
favorable interaction with cut-off mid-latitude upper low is
progged to cause the sfc low associated with Helene to weaken
MUCH slower than is typical for a landfalling hurricane. By
early Friday evening, operational GFS and ECMWF both have
sub-990mb in the vicinity of Evansville, IN, (KEVV) with a
number of their ensemble members sub-980mb. For reference, the
record low SLP for KEVV for both September and October is 993mb,
so this is progged to be a record deep low pressure for this
area of the country. The ensemble mean (which is usually higher
than what verifies due to location differences in the ensemble
spread) SLP from the ECMWF ensembles is 989mb, which is greater
than 6 standard deviations below average!

Even with a relatively weak sfc high to the north, the pressure
gradient with this very large and record deep surface low is
likely to be very impressive. Thankfully, given the forecast
track, the strongest winds appear likely to pass south of our
CWA. Even with that, the potential for 40-50 mph gusts looks
rather high south of I-80 and especially the farther south you
get. Briefly window of gusts to 50+ mph cannot be ruled out even
as far north as the Highway 24 corridor. Still too far out for a
wind headline, but a wind advisory will likely be needed for at
least our southeastern CWA. Guidance has continued to trend
deeper with this low and higher with the resultant wind field.
While this isn`t currently expected, should that trend continue,
then there could even be a potential for damaging wind gusts
(58mph+) getting into our extreme southeastern CWA. As the
system becomes vertically stacked Friday night, the surface low
should begin to fill/weaken with the wind fields following suit.
The strongest winds look to be Friday afternoon and early
evening, likely dropping off significantly overnight Friday
night into Saturday morning.

Rain band from this system looks like it should pivot
northwestward into our southeastern CWA Friday evening. That
initial band will likely start to dissipate as it encounters
drier air over the central and northern portions of our CWA,
though some light showers or sprinkles could make it as far
north as the Chicago metro area later Friday night.

Saturday-Monday:

Big closed mid-upper level low should slowly fill and meander
east or northeastward. While it certainly doesn`t look like the
weekend will be a rain out by any means, can`t rule out a band
of two of light showers or sprinkles pivoting around the
northwestern side of this upper low and providing portions of
our CWA a shower or sprinkle this weekend. Highest chances of
seeing a shower or two will be east of I-55 with lesser chances
to the northwest. NBM pops came in with 20-40% pops with a
gradient from northwest to southeast, so really couldn`t improve
on those much, but will continue to message the weekend as
mostly dry with seasonable temperatures.

Tuesday onward:

Below average forecast confidence coming out of this blocking
pattern, which also tends to be a time when skill scores of the
medium range models are the lowest. It does appear as though a
northern stream shortwave trough will be the kicker that
finally pushes the upper low east out of the area. GFS and ECMWF
both have a cold front moving across the region in the Tuesday-
ish time frame associated with this shortwave. The upper low to
our south out ahead of this trough should shut down any return
moisture from the Gulf, so the frontal passage will probably be
dry. Big difference in the ECMWF/EPS and the GFS/GEFS regarding
the depth of that trough and how quickly it moves east of the
area, which will have implications for how quickly temps rebound
later next week. Both models would favor dry conditions for the
week as it looks now.

- Izzi

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1207 PM CDT Thu Sep 26 2024

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

* Strong NE winds Friday into Friday night

ENE winds near 10 kt will persist through the remainder of today
subsiding to below 10 kt tonight. The remnants of Hurricane
Helene will work into the Tennessee Valley tomorrow morning
which will drive quite a wind response here across the Midwest
on Friday. NE winds look to begin gusting not long after dawn,
near 20 to 25 kt at first before building into the afternoon.
Regular gusts to 30 to 35 kt appear rather likely but it`s very
possible that we could find ourselves closer to around 40 kt.
Conditions at RFD are expected to be marginally lighter than
across Chicagoland. It looks as though the afternoon should see
the strongest winds, but they will be slow to subside during the
evening. Expect VFR throughout the period.

Doom

&&

.MARINE...
Issued at 311 AM 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 later Friday will result in a
strong pressure gradient well north and up to far southern Lake
Michigan. While 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 of 35-40kt. If in later forecasts
the low trends weaker, it is possible that winds will stay below
gales, but the threat of gales appears high enough to justify a
gale watch. The gale watch will probably be converted to a gale
warning or small craft advisory later today or this evening.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Watch from Friday afternoon through late Friday night for
     Wilmette Harbor IL to Michigan City IN.

&&

$$

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