101
FXUS63 KLOT 191132
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
632 AM 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 /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 632 AM CDT Thu Sep 19 2024

Forecast concerns include...

Lake breeze/southeast wind shift mid/late afternoon.
Chance of showers early Friday morning with thunder possible.

No significant changes to the current forecast. Calm winds will
become light/southeast over the next few hours and then are
expected to become south/southwest by mid/late morning with
speeds into the 6-9kt range. A lake breeze will move inland
this afternoon shifting winds easterly though there is still
some uncertainty for when it will reach ORD/MDW and timing may
need to be adjusted. Even if the lake breeze doesn`t reach
ORD/MDW, winds will turn back to the southeast early this
evening, for the entire area. Winds will then turn back to the
south/southwest early Friday morning.

Scattered showers and at least a few thunderstorms are expected
to move across eastern IA and into northwest IL early Friday
morning. This activity is expected to be weakening as it moves
across northern IL during the predawn hours. Prob shower mention
for the Chicago terminals still looks on track but added prob
thunder mention at RFD. Still low confidence for thunder at RFD
but this area has the best chance. Some showers may linger past
sunrise Friday morning, but after daybreak Friday looks mainly
dry. cms

&&

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

&&

$$

Visit us at weather.gov/chicago