930
FXUS63 KLOT 282333
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
633 PM CDT Fri Jun 28 2024

.KEY MESSAGES...

- Periods of showers decreasing in areal coverage for a period
  this evening.

- Increasing potential for a period of thunderstorms tonight
  after 11 pm, some could be strong and produce locally heavy
  rainfall.

- Dangerous swimming conditions expected on Sunday at Lake
  Michigan beaches with strong onshore winds.

- Cooler and dry Sunday and Monday, then warmer and more humid
  for the remainder of the coming week with shower and
  thunderstorm chances returning on Tuesday.

&&

.DISCUSSION...
Issued at 257 PM CDT Fri Jun 28 2024

Through Saturday Night:

An area of showers continues in advance of a mid-level impulse
shifting eastward into northwestern IL. While a majority of this
activity is now mainly north and west of Chicago, it is likely
that these showers will work their way back into most of the
Chicago metro area for a couple hours late this afternoon as the
parent mid-level disturbance shifts east across far northern
IL. While a few thunderstorms are currently ongoing across parts
of southeastern IA with this activity, conditions remain less
favorable for storms with eastward extent due to poor lapse
rates. For this reason, the threat of thunderstorms across
northern IL is expected to remain low (<20%) through early this
evening. The coverage of these showers should diminish for a few
hours this evening as the more persistent showers departs to
our east. While this will allow for the potential for a few
hours of dry time this evening, the warm air advective pattern
we are may still may foster some occasional isolated to widely
scattered showers this evening, so we cannot guaranty it will be
rain free everywhere.

A second round of showers (along with a better chance of
thunderstorms) is then expected with another impulse shifting into
the area late this evening (likely after 11 pm) into the
overnight hours. Strengthening theta-e advection along an
intensifying 40 kt southwesterly low-level jet ahead of this
disturbance will help steepen mid-level lapse rates through the
evening, thus supporting a better chance of thunderstorms with
this overnight activity. A low end threat for a few strong storms
with hail and strong wind gusts exists with these overnight
storms given the favorable kinematic environment. However, the
best threat for any severe storms continues to be favored to our
west-southwest.

The strong theta-e advection tonight will also support very high
precipitable water values (in excess of 2", or 200% of normal)
developing into the area. Conditions will thus be supportive of
very efficient and high rainfall rates (1+" per hour) with these
overnight storms. This could result in some localized instances of
minor flooding. However, the storms do look to be rather
progressive, which should limit their duration at any given
location. Nevertheless, with high rain rates, even a short period
of training storms could result in localized flooding, so this is
something that will need to be monitored.

A surface cold front will shift across the area Saturday morning.
A few showers and storms may linger until this front passes,
especially for areas south of I-80. However, the threat of storms
is expected to wane from north to south following the frontal
passage. We will have to keep an eye on the speed of the cold
front into Saturday afternoon, however, as it will likely foster
renewed storms later in the day. At the present time it appears
this may occur southeast of our area, though a slower frontal
timing could result in some strong storms developing into south of
my far southeastern counties Saturday afternoon. Otherwise, expect
a rather warm day across the area, with temperatures topping out
well into the 80s.

KJB


Sunday through Friday:

Sunday will see surface high pressure and subsidence working
into the area bringing seasonably cool temperatures and clear
skies. Temperatures are expected to be in the low-to-mid 70s
with high 60s along the lakeshore. With the low pressure system
moving off to the east and broad high pressure building to the
west, a tight pressure gradient will generate strong, gusty
northerly winds through the day on Sunday. With winds being
oriented along the fetch of Lake Michigan, large waves and
strong currents will develop, creating a high swim risk at all
Southern Lake Michigan beaches through Sunday night.

Monday looks to be another pleasant day as the high pressure
system moves across the area. Highs will be in the mid-to-
upper 70s with light winds. Late Monday, winds will turn
southerly as the high moves off to the east promoting warm,
moist advection on Tuesday. This brings a chance of showers into
play on Tuesday, although marginal mid-level lapse rates (5-6
C/km) and weak large scale forcing currently look unfavorable
for widespread thunderstorm development and severe weather.

More favorable conditions for thunderstorms should arrive
Tuesday night into Wednesday as the shortwave moves out and
more broad trough with a surface cold front pushes into the area
bringing better lapse rates and a more robust shear profile.

Following this shortwave, the pattern appears to become more
zonal with several shortwaves propagating through. This brings
routine chances of showers and storms to the area through the
remainder of the week. There is also a scenario where
Wednesday`s frontal feature stalls out in or around the CWA,
which would bring a more focused potential for showers and
storms during a portion of the busy 4th of July period.

Carothers

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 632 PM CDT Fri Jun 28 2024

Key Aviation Impacts for the current TAF period:

- Scattered shower activity will continue, but there could be
  more breaks and drier spots through midnight

- Thunder chances increase around midnight through early
  Saturday morning with rain rates that may reduce vis down to
  IFR conditions

- Shower chances continue through Saturday morning diminishing
  by midday.

- Winds become westerly with chances for gusts 20-25 knots
  Saturday afternoon

A large swath of rain is currently moving eastward over the air
space. All lightning activity diminished as it crossed the
Mississippi River, so thunder is not expected in the short
term. As it passes, more breaks in the rain are expected
through just before midnight; however, there could still be some
occasional showers, so VCSH was maintained in the TAFs.

The forecast remains on track for better instability to move
west to east over the region and provide the risk of -TSRA
during the overnight period, around 06Z for Chicago terminals,
and slightly earlier out around KRFD. Cloud bases are expected
to lower to just above 3000 feet, but more impactful will be the
potential vis reductions from the higher rain rates that can
create IFR conditions.

After the thunder chances diminish, showery activity is
expected to continue through mid Saturday morning. Current model
guidance has the frontal passage between 12Z and 15Z.
Potentially, that could result in enough forcing to allow for a
thunderstorm to develop along the front, but the chances are
diminishing below 30 percent, so for now, the PROB30 group for
thunder was removed, but will be monitored closely through the
night.

Showers are expected to gradually diminish after 16Z allowing
for much of the airspace to have drier conditions through
Saturday. While MVFR conditions for lower cigs were added to
KRFD, they were left out of the Chicago terminals. Winds will
become more southwesterly through the overnight and by midday
Saturday become more westerly. Better mixing Saturday afternoon
should allow for some wind gusts to develop between 20-25 knots.
Winds should diminish after 00Z with winds becoming
northwesterly.

DK

&&

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

&&

$$

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