283
FXUS63 KLOT 251948
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
248 PM CDT Wed Sep 25 2024

.KEY MESSAGES...

- Patchy shallow ground fog again tonight into early Thursday.

- Windy on Friday and Friday evening, which will also cause
  large waves and dangerous conditions on Lake Michigan.

- Diminishing chance for any meaningful rain Friday night
  through early next week.

&&

.DISCUSSION...
Issued at 248 PM CDT Wed Sep 25 2024

Through Thursday Night:

Following a pleasant afternoon today, the only item of note
tonight will be the potential for patchy shallow ground fog once
again. The main limiting factors will be the very shallow
inversion height, and modest northeasterly flow above it, as
well as dry air above the inversion. However, with overnight
temperatures dipping down into the upper 40s (locally mid 40s)
to lower 50s outside of the heart of the Chicago metro, these
temps are generally at or below the afternoon crossover temps.
Calm winds, clear skies, and a bit more moisture due to recent
rain, should support patchy ground fog development. It seems
like a less favorable setup than last night, so the coverage of
any dense fog should be more limited.

Thursday will be another seasonably warm day after any lingering
early morning fog erodes, with mostly sunny skies and highs in
the 75-80F range away from the lake, and lower 70s along the
immediate shore. High clouds well ahead of Helene`s remnants
will filter in from the south Thursday night and keep overnight
temperatures propped up a bit vs. tonight`s.

Friday through Wednesday:

The main change in the forecast message for Friday is the
increasing likelihood of strong northeasterly winds, especially
south of I-80 and along the Lake Michigan shore.

Helene`s strong extratropical remnants, possibly still
technically partially tropical into Friday morning, will track
to the Ohio Valley by Friday evening, still at 990 mb or a bit
less. Given the unusually strong surface low pressure butting up
against 1015+ mb high pressure over the Great Lakes, the
pressure gradient will quickly tighten through the day on
Friday.

Exceptionally dry air in the 700-500 mb layer and down to the
low levels will be tough to erode. This will not only slow the
northward progress of (and eat away at) the northwest fringe of
the precip shield, but also present supportive conditions for
efficient mixing, as the drier low levels will keep lapse rates
fairly steep. Model forecast soundings today were in solid
agreement depicting 35-45 kt flow atop the mixed layer, highest
in our southern CWA, closest to the center of Helene`s remnants.
The wind field should peak in magnitude in the mid afternoon
through the early evening hours.

There`s some uncertainty on how thick the mid and high cloud
cover will be north of Helene`s remnants, which could modulate
how warm temps get, how much dew points mix out, and how
effectively the higher momentum aloft can be tapped into. With
that said, the signal was consistent across the guidance today
(along with showing a general uptick in the robust EPS suite) to
support ramping up wind speeds to 15-25 mph and gusts into the
35-40+ mph range. The best chance for any gusts approaching 45
mph advisory criteria is, as noted earlier, primarily pegged for
our southern CWA. Occasional gusts up to 40 mph appear probable
near the lake, though there it`s looking like more of a
stronger sustained wind off the frictionless lake, and gusts not
much higher than the sustained winds. A Marine section will be
added below with details on the large waves and dangerous
conditions expected on the lake.

The interesting "fujiwara" interaction and then merger with a
larger upper low does still look likely to take place with
Helene`s remnants, but limiting factors for appreciable rain in
our area discussed the past few to several days have not
materially changed. Strong mid-level ridging extending across
the northern Great Lakes will likely be a road-block and prevent
the gradually weakening, but still very large extratropical
cyclone, from getting much farther north than the general Ohio
Valley area.

With very dry air needing to be eroded and stronger forcing held
to our south, some scattered showers (30-40% PoPs) focused
southeast of I-55 may be the best we can do Friday night. The
potential showery regime over the weekend into early next week
is trending toward more of a occasional light rain showers and
sprinkles setup Saturday and Sunday, with the best (but still
low/~30%) chances of measurable rain in our southeast half or
third of the CWA. Saturday will be another breezy, but not quite
as windy day as Friday, with winds further stepping down on
Sunday as the cyclone to our south steadily weakens.

Seasonably warm daytime temperatures are expected over the
weekend, in the 70s, and possibly approaching 80F in our far
western CWA on Sunday. A cold front will likely sweep across the
area in the Monday PM-Tuesday timeframe, bringing a more
substantial cool-down from near summer-like warmth to start the
work week. Signs do not currently point to this being an
anomalous cool-down for this time of year into mid next week, so
it looks to remain comfortable by day and chilly overnight. It
does appear we`re headed into another well below normal precip.
stretch, particularly with dwindling chances of beneficial rain
this weekend.

Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1159 AM CDT Wed Sep 25 2024

The only minor concerns are:

- Timing a light east/northeast wind shift with a lake breeze
  late this afternoon/evening at the Chicago-area sites.

- Potential for shallow BR/FG tonight/Thursday morning, mainly
  near RFD.

Fair weather cumulus with bases near 040-050 will continue to
develop today. VFR conditions will prevail at the terminals.
Northwesterly winds will become east/northeast this afternoon at
ORD, MDW, and GYY with the passage of a weak lake breeze.
Timing at ORD/MDW remains unchanged from previous forecasts.

Tonight, winds will become calm/light and variable. Some
shallow BR/FG will again be possible, with the strongest signal
at this time near RFD. Some potential this could extend towards
DPA and GYY, but confidence is too low to include a mention with
the outgoing 18z TAFs at this time.

Thereafter, winds will become northeasterly through Thursday
morning.

Carlaw

&&

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

&&

$$

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