497
FXUS63 KLOT 081130
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
630 AM CDT Mon Jul 8 2024

.KEY MESSAGES...

- Chance (30 to 40 percent) for scattered thunderstorms this
  afternoon in advance of an approaching cold front. Torrential
  downpours and locally gusty winds may accompany these storms.


- Post-Tropical Depression Beryl will move through the Ohio
  River Valley or Lower Great Lakes region Tuesday night into
  Wednesday night with a corresponding threat for heavy rain and
  gusty winds.

- Heat and humidity levels will increase toward the end of next
  week and continue into the weekend.

&&

.DISCUSSION...
Issued at 337 AM CDT Mon Jul 8 2024

Through Tuesday:
The primarily forecast challenges today will once again revolve
around the coverage of showers and thunderstorms, particularly
this afternoon through early evening along an approaching cold
front.

Water vapor imagery early this morning depicts the presence of
several weak remnant MCV/mid-level perturbations streaming
northeastward across IA and IL in advance of an approaching upper
trough centered across the Mid-Missouri Valley early this morning.
There have been a few short lived showers pop up from time to
time with the IL impulses, but by far most of the convective
activity has remained to our west in IA where a slightly better
thermodynamic environment resides. This MCS has been in a
weakening phase for the past couple hours, and this trend is
expected to continue as it moves into the less favorable
thermodynamic environment east of the Mississippi.

While a few isolated showers cannot be totally ruled out this
morning, it appears most areas will remain dry through the
morning. Accordingly, this will allow the seasonably moist
airmass overhead to destabilize through the day, thus priming the
environment for afternoon thunderstorm development ahead of an
approaching cold front. Uncertainty continues to be below average
with the overall coverage of afternoon storms across the area.
However, it appears likely (60%+) that at least widely scattered
storms will develop in advance of an approaching cold front and
mid-level trough after 2 or 3 PM this afternoon.

The overall threat for severe weather today is expected to
remain low, owing to overall poor mid-level lapse rates. However,
stronger mid-level flow (~40 kt at 500mb) is expected in advance
of the approaching mid-level trough axis this afternoon, which
could result in support for a few more organized storms.
Accordingly, we cannot rule out a couple of locally strong wind
gusts with some of the storms. Otherwise, aside from lightning,
the main threat from any storms this afternoon will be torrential
downpours given the high PWAT environment.

The threat of storms will wane from northwest to southeast into
early this evening with the passage of the front. This should set
up another rather quiet night tonight before rain associated with
the post tropical remnants of Beryl begin to stream northward
into central parts of IL and IN on Tuesday. Guidance continues to
support a track into the Ohio Valley by Wednesday morning, which
would result in better chances of rain coming Tuesday night into
Wednesday across our far southern areas. See more on this
below.

KJB


Tuesday Night through Sunday:

The remnants of the Beryl is expected to move northward into
the Ohio River Valley on Wednesday. Ensembles have sped up and
shifted the track of the center of the storm ever so slightly to
the south. It is now expected to pass slightly east of St.
Louis and follow the Ohio River east of the Mississippi River
before heading toward Cleveland. Even though the central core
will be south of the forecast area, it will still make for
impacts locally, While there is still some uncertainty with how
it will phase with the long wave trough to the north, the update
to the track has resulted in trimming back PoPs for areas
northwest of I-55, while maintaining "likely" PoPs for areas to
the south and east. With tropical moisture increasing
precipitable water amounts to around 2 inches, 2 to 4 inches of
rain is possible for areas well southeast of I-55. It was
decided to maintain thunder given models suggesting weak
instability present, though it is not expected to be predominant
through the time period. The main concern with this system will
be the threat for flooding due both in part to the amount of
rain expected as well as the potential for continuous rain over
the same area. Over the course of Tuesday night through
Wednesday, areas from Ford County, Illinois through Porter
County, Indiana are under a level 2 out of 4 for excessive
rainfall.

Winds will turn to the northeast ahead of the system and turn
to the north by Wednesday afternoon. While the strongest winds
will be associated with the central core of the system to the
southeast away from our area, the gradient will tighten and
provide wind gusts between 20 to 25 mph on Wednesday. Given the
good northerly fetch over the lake and strong winds, there is
gaining confidence on Wednesday for developing dangerous
swimming conditions. If waves are choppy enough, there is a
chance for the conditions to persist into Thursday morning.

After the system exits on Thursday, synoptically the long wave
trough over the Great Lakes will remain, though gradually weaken
through Saturday as a ridge of high pressure aloft builds over
the Sierra Nevada. Depending how quickly (or slowly) the trough
weakens and lifts to the northeast, there is a slight chance for
some showery activity on Thursday. But gradually better high
rises and dry air will creep in from the west. Warmer air aloft
will slowly move in with deterministic models suggesting 850 mb
temperatures could creep into the 20 to locally up to 25 degree
range over the weekend. However, ensemble models are suggesting
that warmest 850 temperatures will not arrive until late on
Sunday. In addition, this pattern of a trough lifting into
Canada and a ridge to the west puts more northwesterly flow over
northern Illinois. Some of the ensembles have dry conditions
through the weekend, while a few develop a larger convective
system that could move through and thereby inhibit maximum
temperatures. All that being said, warmer temperatures are still
expected to move in this weekend with reach at least the low
90s. How warm they get (and their subsequent heat indices)
remains `tbd` with so much uncertainty.

DK

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 630 AM CDT Mon Jul 8 2024

- Intermittent showers through the early morning

- A chance for showers and thunderstorms Monday afternoon,
diminishing into the evening

Scattered showers have developed within a stronger region of
moisture. With nothing to inhibit its northeastward movement,
-SHRA have been added to the Chicago terminals. Lightning is
being closely monitored but it has not appeared yet in this
cluster of showers and confidence remains low. As for the rain
in Iowa mentioned at the 06Z AFD, it did start to disintegrate
as it approached to Mississippi River, but it is still holding
itself together, so VCSH was kept to the northwest for a couple
more hours.

Winds are expected to remain out of the southwest through the
TAF period. While their may be the occasional gust in the teens,
most winds should be 10 knots or less.

Models are starting to come into agreement of a southwest to
northeast boundary developing in the afternoon. While there is
lower confidence in the exact timing and placement, it should be
enough for showers and thunderstorms to have a chance to
develop on it. For now, -SHRA and VCTS was added to the TAF due
to the lower confidence for lightning specifically at the
terminals, but will be re-evaluated as the events today unfold.
The chance for precip and thunder diminish quickly as the sun
sets.

DK

&&

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

&&

$$

Visit us at weather.gov/chicago