477
FXUS63 KLOT 191758
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1258 PM CDT Thu Sep 19 2024

.KEY MESSAGES...

- Today will feature highs in the 80s, low humidity levels, and
  a lake breeze.

- A few showers or thunderstorms may sneak into northern
  Illinois toward daybreak Friday. Additional showers and storms
  may develop during the afternoon along and east of I-55.

- A pattern change is expected this weekend into early next week
  with the arrival of several waves of showers and storms
  (favoring Sunday and Monday).

&&

.DISCUSSION...
Issued at 213 AM CDT Thu Sep 19 2024

Through Wednesday:

National water vapor imagery shows a wound-up low pressure
system lifting northward into southern Saskatchewan, a second
low pressure system drifting southeastward along the California
Coast, and a broad remnant cyclonic circulation along the
Atlantic Seaboard. A recent surface analysis depicts a deep
plume of low-level moisture extending from the Gulf of Mexico
through the central Plains and into southern Manitoba and
southwestern Ontario, east of a surface cold front arcing
southeastward from the aforementioned surface low in southern
Saskatchewan. Meanwhile, the Great Lakes remain within a
relative stagnant zone in the surface-to-upper- level pattern,
which will delivery yet another day of uninteresting weather.
Today will be similar to the past week with highs in the upper
80s, a lake breeze, and low humidity levels. A few clouds should
dot the sky this afternoon and evening, closing out what will
likely be the last meteorologically quiet day for a while.

Tonight, the passage of a subtle upper-level shortwave will
encourage the explosive development of thunderstorms along the
southeastward moving cold front in southeastern Minnesota,
western Wisconsin, and northeastern Iowa. With time, the
thunderstorms are expected to grow upscale into a cluster or two
while moving southeastward toward northern Illinois. With the
convection seemingly likely to outrun the cold front and
moisture axis, it should begin to wane while moving into
Illinois toward daybreak Friday. With that said, areas near the
Wisconsin state line or near I-39 may see periods of showers and
hear perhaps a clap of thunder Friday morning.

After daybreak on Friday, the moisture axis will slide through
northern Illinois and northwestern Indiana leading to noticeably
more humidity conditions. Compared to 24 hours ago, model
guidance has slowed the arrival and passage of the cold front,
now favoring the afternoon hours. Based on how fast morning
clouds (from the decaying showers) erode and when the front
actually moves through our area, there now appears to be a
window of time for the redevelopment of showers and
thunderstorms along the front, favoring areas along and east of
I-55. While low-level shear looks pretty NIL along the front,
the nose of a 100kt 250mb upper-level jet will poke into our
area behind the front providing some 40 to 50kt of straight-line
deep-layer shear. When combined with around 1000 J/kg of MLCAPE
and DCAPE, any sustained storm along the front Friday afternoon
will have the opportunity to develop mid-level rotation
supporting a threat for both damaging wind- and hail-producing
splitting supercells. In all, felt there was enough of a signal
for a localized severe weather threat to collaborate an
introduction of a "Day 2" Level 1/5 threat level area generally
along and east of I-55, valid for Friday afternoon. (Keep in
mind that a Level 1/5 threat level equates to a 95% chance that
there -won`t- be severe weather within 25 miles of any given
point).

This weekend, the upper-level trough responsible for the system
along the California Coast will eject into the Plains and
Midwest. While spread in the ensemble space has decreased
appreciably in the past 24 hours, there remains uncertainty in
exactly how the trough will evolve and when it will arrive owing
to opportunities for phasing with other upper-level shortwaves
near the US/Canadian border. With that said, a signal is
emerging that a surface low may develop and track close to our
area sometime in the Sunday to Monday timeframe, supporting
waves of much-needed rainfall. Even if not drought-busting,
ensemble guidance continues to tick upward in the range of total
rainfall amounts with EPS 75th percentile QPF values (which are
probably more representative than the mean or 50th percentile
values given thunderstorms will be in play) now exceeding an
inch across most of our area. And, while certainly not the only
scenario out there, we will have to keep an eye on the timing
and track of the surface low particularly in the Monday
timeframe as several individual ensemble members depict an
evolution that can support severe weather in our area.
Regardless, the message at this time is to expect waves of
showers and thunderstorms sometime from this weekend into early
next week.

After the system early next week, ensemble model guidance is
locked in on seasonable temperatures (highs in the low to mid
70s, lows in the upper 50s to lower 60s) and continued
opportunities for rainfall to close out the month.

Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1255 PM CDT Thu Sep 19 2024

Key Messages:

- Slow inland push of lake breeze today at ORD/MDW (22-23Z)
- Period of SHRA possible overnight (30% chance of TS at RFD,
  15% at ORD/MDW/DPA)
- Low chance of a period of MVFR stratus Friday morning
- Northwest wind shift behind a cold front Friday afternoon

A lake breeze is evident in surface observations and on radar
imagery early this afternoon and has been slower to push inland
compared to the past few days. Have trended the east wind shift
at MDW a bit later to 22Z based on latest guidance and
maintained 23Z at ORD. Otherwise expect winds to remain SSW
through the afternoon. As we head toward sunset all sites will
gradually trend SSE to SE to at times variable overnight,
remaining light.

Focus then turns to the potential for a period of showers (and
possibly an embedded thunderstorm or two) likely in a decaying
phase with eastward extent. Accordingly, maintained a PROB30
group for TSRA at RFD but left TS out of the Chicago area
terminals with this update. Can`t rule out a few lightning
strikes persisting into the metro, though confidence remains too
low for a formal TAF mention and left the PROB30s as SHRA.

Depending on the coverage of showers, wind directions could be
quite variable overnight, but generally expect them to maintain
a southerly component. In the wake of the showers there is a
signal for a period of stratus development Friday morning. For
now have introduced a scattered 1000-1500 ft layer due to
lingering model variability in cloud coverage.

A cold front is then progged to move through during the late
morning into the afternoon hours turning winds WNW.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...None.

&&

$$

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