905
FXUS63 KLOT 081722
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1222 PM 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.

&&

.UPDATE...
Issued at 1145 AM CDT Mon Jul 8 2024

Looking at convective trends for this afternoon, the current
forecast appears to be on track with scattered showers and
isolated to scattered thunderstorms. Given a similar airmass as
yesterday but without the morning activity generating outflow
boundaries to kick off additional showers and storms, it is hard
to see why thunderstorms today would be more widespread than
yesterday. Even so, with copious moisture, weak shortwaves on
the east side of the upper trough to our west, and a slowly
approaching frontal zone, there should be plenty to support the
development of showers this afternoon once a small low level
inversion mixes out. CAPE values peaking in the 1000-1500 J/kg
range with 0-6 km shear values generally at or below 40 kts
would suggest more of a disorganized pop-up nature to the
storms. Severe weather is not a primary concern apart from some
locally gusty winds, though the possibility of localized
flooding will also need to be monitored.

Lenning

&&

.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 /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1221 PM CDT Mon Jul 8 2024

The primary aviation weather concerns for the 18Z TAF period
include:

* Isolated showers and thunderstorms this afternoon and evening.

* A potential for a prolonged, soaking rain from the remnants of
  Hurricane Beryl beginning as early as Tuesday afternoon.

SW winds around 10 kt with gusts to near 20 kt will persist for
the rest of the daytime. Isolated to widely scattered showers
and thunderstorms are anticipated this afternoon and early
evening. It looks like most areas should actually remain dry,
but a brief vsby reduction will be possible underneath any of
these storms. Given the low coverage and little impacts
expected, felt a general VCTS was appropriate for the potential.
Precip should fizzle away during the earlier half of the
evening.

The SW winds will go light, near or below 5 kt, this evening and
veer throughout the night, possibly going variable at times.
We`ll begin Tuesday with light NEs that will veer to more of an
easterly during the afternoon.

Confidence is growing in the remnants of Hurricane Beryl
trekking into the Midwest and impacting portions of northern
IL Tuesday into Wednesday. While the better precip chances look
to arrive closer to Tuesday night, rain could find its way over
the Chicago sites as early as Tuesday afternoon. Depending on
the path the storm takes, parts of Chicagoland could be in for a
prolonged period of soaking rainfall. The greatest potential
for this resides south and southeast of the terminals. However,
we remain in that sweet spot where little shifts in the storm`s
track will mean big changes in expectations. Impacts to
operations will become more clear as the storm approaches over
the next day.

Doom

&&

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

&&

$$

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