092
FXUS63 KLOT 161927
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
227 PM CDT Sun Jun 16 2024

.KEY MESSAGES...

- Severe thunderstorms will sweep across northern Illinois this
  afternoon and evening, mainly along and north of I-80.

- Damaging winds and a brief tornado are the main threats. Flash
  flooding may occur if thunderstorms regenerate this evening
  over the Chicago metropolitan area.

- Hot and muggy conditions will prevail through the end of the
  week. Periodic chances for thunderstorms will continue, though
  many hours will remain dry.

&&

.DISCUSSION...
Issued at 227 PM CDT Sun Jun 16 2024

Through Tonight:

An impressively organized mesoscale convective vortex anchored
by a semi-circular outer-core of convection is currently lifting
from northeastern Missouri and into southeastern Iowa. Low-
level moisture continues to increase ahead of the MCV, with dew
points in the lower 70s now spreading toward the I-39 corridor
and Wisconsin state line. Mostly sunny skies have allowed for
highs to build into the upper 80s to around 90, which when
combined with the steepening mid-level lapse rates and
increasing low- level moisture, is contributing to some
2000-2500 J/kg of MLCAPE.

As broad southwesterly steering flow continues guiding the MCV
northeastward into the destabilizing airmass, coverage of
ongoing thunderstorms near the MCV will increase with perhaps a
rapid uptick near or west of I-39 sometime in the 2 to 4 pm
timeframe. Thereafter, convection will sweep across northern
Illinois, primarily along and north of I-80. Locally augmented
flow on the eastern side of the circulation (KDVN radar is
sampling some 50+kt of flow at 12kft) will provide a mesoscale
kinematic environment for the development of supercells and
bowing structures as convection matures across northern Illinois
this afternoon, with a threat for "upscale growth" into a
compact MCS as it crosses northern Illinois this afternoon and
evening. For this reason, the threat for a swath of damaging
winds (60 to 75 mph) and even brief tornadoes continues to
increase, focused near and north of I-80. Accordingly, the Storm
Prediction Center has introduced a Level 2/5 threat level for
severe weather this afternoon, mainly along and north of I-80.

Given the highly organized structure of the MCV, we remain
concerned for regenerating convection along the backside of the
circulation across northeastern Illinois after sunset. Such a
threat will come down to mesoscale details of whether a focused
zone of low-level confluence will develop at the intersection of
the western side of the cyclonic circulation and any continued
southwesterly low-level synoptic flow. Forecast confidence in
such a band of regenerating convection remains low, though it`s
something we can`t rule out. If radar and satellite trends begin
to hint at a mesoscale band of regenerating convection
materializing, we will consider issuing a short-fused Flash
Flood Watch for part of our area (likely just a few counties).

Borchardt


Monday through Sunday:

The main forecast focus for the week continues to be the hot
and humid conditions expected to impact the Great Lakes through
much of the week. The main driver for the heat will be a broad
upper ridge that will stall across the Mid-Atlantic which will
generate stout south-southwesterly flow over the area. This flow
will keep dew points in the mid to upper 60s to around 70 which
in combination with high temperatures likely to hover in the
low to mid 90s, will generate heat indices in the 95 to 105
degree range at times. The warmest days this week continue to
look like Monday and possibly Thursday depending on how quickly
the ridge breaks down. Though, the latest guidance has started
to show that dew points may mix down a couple of degrees Monday
afternoon which may help keep heat indices a bit lower as well.
Additionally, the high dew points will also aid in keeping
overnight lows in the mid to upper 70s through at least the
middle of the week which means little relief even after sunset.
Therefore, despite the aforementioned heat indices expected to
remain below our Heat Advisory criteria; we encourage anyone
with outdoor plans to practice heat safety by taking breaks from
the heat and staying hydrated.

Aside from the heat, there is also the potential for periodic
showers and thunderstorms through the week as well. However, the
confidence on coverage of showers and storms through Tuesday
remain low. The reason is because the ridge will create mid-
level height rises which should generate subsidence and work to
counteract the robust instability forecast to be in place. Given
the uncertainty on which parameter will win out, have decided
to maintain the 20% POPs for both Monday and Tuesday afternoons.

Heading into Wednesday and the later half of the week the
upper- level pattern is forecast to become more zonal as the
ridge gets squashed by a trough ejecting across the upper
Midwest. As this occurs, a frontal boundary is forecast to
develop over the upper Mississippi Valley and gradually drift
southeast into Wisconsin towards weeks end. However, exactly
when and/or if this boundary will actually get shoved through
northern IL remains very uncertain as guidance continues to
struggle as to when the ridge will actually break down.
Therefore, have made no changes to the advertised NBM forecast
which from Wednesday onward which offered 20 to 40 percent POPs,
highest across northwest IL near the aforementioned front.
Furthermore, high temperatures are currently forecast to remain
in the 90s through weeks end but these values could wind up
cooler if the front moves through sooner than advertised so stay
tuned.

Yack

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1226 PM CDT Sun Jun 16 2024

Key Aviation Impacts for the current TAF period:

- Gusty pre-storm southwest winds this afternoon

- Increasing confidence for thunderstorms to be expected at the
  Chicago terminals this evening/tonight. Main threat will be
  stronger, potentially severe, winds and heavy rain

- After lighter winds overnight, another chance for gusty
  southwest winds Monday afternoon

Skies are VFR with a some higher clouds moving overhead from
the storm system that is approaching tonight. Winds remain out
of the southwest. Gusts have generally been between 20 to 25
knots, though an occasional rouge gust up just less than 30
cannot be ruled out before the storm arrives.

As mentioned above, the threat for thunderstorms arriving at
the terminals has increased to the point that it is now
expected. The storm that is currently over Northern Missouri has
maintained itself and is tracking to the northeast. While the
TAF reflects the expected timing, it can arrive around KRFD as
early as 20Z, and the Chicago terminals around 22Z. While small
hail and a brief tornado cannot be completely ruled out, the
main threat will be strong and potentially severe winds, as well
as heavy rain. It is a large complex that will be seen coming
on radar and will move through the airspace through the night
and moving out over Lake Michigan away from the city after
midnight.

Winds will diminish behind the system as conditions dry out.
Temperatures will heat up tomorrow and with lingering moisture
around, a pulse shower or storm is possible, but the
probability is around 20 percent or less at this point therefore
not currently in the TAFs. Otherwise VFR conditions are
expected on Monday with chances for southwesterly wind gusts to
return in the afternoon.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Air Quality Alert until midnight CDT Monday night for ILZ005-
     ILZ006-ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-
     ILZ106-ILZ107-ILZ108.

IN...Air Quality Alert until 1 AM EDT Monday for INZ019.

LM...None.

&&

$$

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