974
FXUS63 KLOT 270819
AFDLOT

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

.KEY MESSAGES...

- Windy conditions with northeasterly gusts 50 mph+ expected
  this afternoon and evening, especially near the lake and south
  of I-80.

&&

.DISCUSSION...
Issued at 319 AM CDT Fri Sep 27 2024

Through Saturday:

Hurricane Helene continues its northward trek across Georgia
this morning, with land-based sites still reporting gusts to
around 100 mph as of 3am CT. The fast forward motion around 25
mph and expected NNW curve will result in Helene entering
Tennessee by mid-morning. Transition into a post-tropical system
will quickly occur as the system absorbs a remnant upper-level
low currently centered near Memphis. This will allow the new
system to maintain an usual level of strength this far north
into the mid-latitudes, with guidance overwhelmingly showing a
sub-985 hPa low reaching near Evansville late this afternoon.
While a slowly strengthening 1015 hPa surface high over the
northern Great Lakes is far from anomalous for this time of
year, the growing pressure gradient will support a period of
strong winds across the forecast area, especially the south
half, this afternoon and evening.

Substantial cirrus cover within an expansive area of diffluence
ahead of the upper-level low early this morning is expected to
persist through the day and may subdue some deeper mixing
through the day. With that said, steepening low-level lapse
rates late this afternoon into the evening concurrent with the
arrival of the maximum pressure gradient under thick upper-level
cloud cover still favors winds to perform on the higher end of
guidance. No significant changes were made to the previous
forecast, with gusts to 50mph+ expected south of the Kankakee
River Valley. It still cannot be ruled out that some sporadic
rogue gusts reach 60mph roughly south of a Paxton to Rensselaer
line late afternoon into mid- evening. No changes were made to
the going Wind Advisory, but did consider including Lake/Porter
counties for areas along the lakeshore. If winds show a
strengthening trend through the day, these two counties and
possibly other portions in the southern Chicago metro may need
to be added to the advisory.

As for precip, conditions should remain dry for the vast
majority of the CWA through the daylight hours, with the
exception of a band of decaying showers surviving far enough to
brush the extreme southern CWA this morning. A more formidable
decaying rain band should affect portions of the southeast CWA
this evening, but much of the area will again remain dry during
this time.

Increasing low-level baroclinicity on the northwest flank of
the tropical airmass will promote a weak upper-level jet above a
subtle area of low to mid-level frontogenesis across northern
Illinois late tonight into Saturday. Deep-layer moisture may be
lacking this far north, but with significant agreement in
guidance depicting a narrow band of rain developing within this
region, have increased PoPs (and corresponding QPF given
expected warm-rain) to the likely (60-70%) range centered
roughly near and southeast of the I-55 corridor through at least
Saturday morning, with gradually diminish PoPs through the
afternoon.

Kluber

Saturday Night through Thursday:

Shower chances will continue (mainly east/southeast of I-57) into
Monday as the remnants of Helena remain nearly stationary across
the lower Ohio Valley while weakening. What`s left of Helena will
finally get ejected east of the region Monday night as a long
wave trough over southwestern Canada digs into the Upper Midwest
and sends a surface cold front southeastward across our area. A
cooler and much drier airmass (dewpoints potentially in the 30s
and 40s) in the wake of this front will then filter into the area
for Tuesday into Wednesday as a surface high tracks eastward
across the Mid-Mississippi Valley region. This airmass looks to
remain rather progressive, however, as winds should shift back to
a southerly direction again Wednesday night into Thursday
following the eastward passage of the surface high. Accordingly,
moderating temperatures appear to be a good bet for a day or two
period later in the week ahead of what looks to be another
approaching cold front late in the week. Precipitation chances
beyond Monday will remain very low at <10%.

With the passage of the cold front into early Tuesday, high
temperatures both Tuesday and on Wednesday look to struggle to
reach 70 degrees. Meanwhile, overnight temperatures Tuesday night
look to become downright chilly, with readings potentially
bottoming out in the low to mid 40s outside Chicago and inland
from the lake. Moderating temperatures on Thursday could then push
readings back well into the 70s to around 80. 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.

KJB

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1248 AM CDT Fri Sep 27 2024

Main forecast highlights through the period include:

- Strengthening northeasterly winds after daybreak today, with
  frequent gusts up around 35 kt expected this afternoon into
  the evening.

- Lowering MVFR CIGs with a threat for showers after 09Z
  Saturday.

The remnants of Helene will shift north-northwestward into
central and western KY later today and tonight. As it does,
northeasterly winds are expected to ramp-up across our area through
the morning in response to a rapidly intensifying surface
pressure gradient. While the strongest, and more, damaging
winds are largely expected to remain south of the main Chicago
area terminals, frequent gusts up to around 35 kt are expected
this afternoon through at least early this evening before
speeds begin to gradually ease later tonight.

Higher-level VFR cloud cover is expected overhead today.
However, later tonight (after 09Z Sat) low-level moisture will
begin to increase overhead. This is likely to foster the
development of a deck of lower MVFR CIGs late tonight into
Saturday. There will also be an increasing threat for a period
of scattered showers across the Chicago area terminals during
this period as the remnants of Helene track into western parts
of KY. With the chances for these showers increasing towards the
end of the current 30 hour ORD and MDW TAF period, we have
opted to add only a PROB30 mention for the last few hours (09 to
12Z Sat) at this time.

KJB

&&

.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 7 AM CDT this morning through
     Sunday morning for ILZ006-ILZ103-ILZ104.

     Wind Advisory from 1 PM this afternoon to midnight CDT tonight
     for ILZ023-ILZ032-ILZ033-ILZ039.

IN...Beach Hazards Statement from 7 AM CDT this morning through
     Sunday morning for INZ001.

     Wind Advisory from 1 PM CDT /2 PM EDT/ this afternoon to
     midnight CDT /1 AM EDT/ tonight for INZ010-INZ011-INZ019.

     Beach Hazards Statement from 10 AM CDT this morning through
     Saturday evening for INZ002.

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

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

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

&&

$$

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