087
FXUS63 KLOT 161739
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1239 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 1239 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 Saturday:

Going into Monday, the upper-level ridge will remain locked in
place, allowing for ample sunshine to further warm the newly
arrived air mass. High temperatures are thus likely to end up
being a few degrees warmer on Monday than they will be today.
There`s still some uncertainty as to how much dew points will
mix out on Monday, with HRRR forecast soundings suggesting that
the 850-700 mb layer will be quite a bit drier than it will be
today, possibly allowing for more efficient mixing to take place
and scour out dew points more readily. Nevertheless, continued
warm/moist advection off of southerly winds and
evapotranspiration should offset this mixing to some degree and
keep dew points from plummeting lower than the mid 60s,
affording us another humid day. The degree of mixing that occurs
shouldn`t have a substantial effect on heat indices as lower
dew points would mean that air temperatures would likely end up
being a little higher, but our latest forecast grids have peak
heat indices getting to about as high as 100 degrees, with some
potential for them to end up being as high as 105 degrees if dew
points struggle to mix out. Either way, it looks like we will
fall shy of meeting Heat Advisory criteria.

It remains possible that isolated to scattered pulse-type
thunderstorms could be seen tomorrow afternoon. Most CAMs are
quite bullish on thunderstorms developing and expanding in
coverage as new convection bubbles along the outflow that they
spit out. However, large-scale forcing for ascent looks pretty
nebulous tomorrow, and it appears that we will see slight
geopotential height rises as well, so the thinking is that the
CAMs are a little overcooked with their output, and have
continued to carry just slight chance PoPs for now with the
expectation that their coverage should remain isolated to widely
scattered at best in our CWA. Nevertheless, wherever storms do
develop, their rain-cooled outflow will provide temporary local
relief from the heat and humidity, but there will also be some
potential for strong to perhaps locally damaging downburst winds
to occur as deep updraft cores collapse in an environment
characterized by meager deep layer shear.

The northern fringes of a deeper moist plume advecting off the
Gulf of Mexico will arrive in the area on Tuesday, and PWATs look
to build towards 1.75-2 inches through the day. At least some
increase in mid-level cloud cover looks to accompany this
advancing moist plume, and this may curtail mixing a bit compared
to Monday. This should result in slightly higher dewpoints and
lower high temperatures, driving peak heat indices in the mid to
upper 90s. Somewhat gustier southerly winds may help take some of
the edge off though, and this is reflected with slightly lower
wet bulb globe values compared to Monday. Large scale forcing
still won`t be significant, but low-amplitude vort lobes within
the southwesterly steering flow will likely continue parading
around the periphery of the sprawling east coast high. Isolated to
widely scattered mainly afternoon shower/storm chances will
continue, as a result.

Through Tuesday night, a southward-advancing cold front to our
north looks to be pretty active across Iowa, Minnesota, and parts
of Wisconsin with widespread convection. While it`s possible some
of this activity tries to ooze into our northwest into Wednesday
morning, thickness gradients and LLJ orientations seem to point to
any upwind propagating portions of MCSs building preferentially
into southern Iowa, while the forward propagating components scoot
across central/northern Wisconsin.

On Wednesday, the aforementioned cold front will have sagged
further southward into southern Wisconsin as outflow from
overnight activity pushes closer to our region. With diurnal
thunderstorm activity igniting in a bit closer proximity, higher-
end chance PoPs across northwest Illinois seem appropriate during
the afternoon. The thermodynamic/kinematic parameter space
suggests pulse convection will be the norm, with heavy rain (due
to slow cell motions) and gusty winds as the main threats, if
activity manages to sneak south of the Wisconsin state line. With
the core of the parent upper ridge expected to be situated near
Long Island, highs on Wednesday will probably be near where they
were on Tuesday although there is a decent spread across the
guidance suite. The latest gridded forecast shows heat indices
generally in the mid and upper 90s again.

On Thursday, guidance suggests the east coast high may attempt to
build farther west which would increase thicknesses locally and
support a higher ceiling on air temperatures compared to the
previous two days. However, it`s possible the portion of the
quasistationary boundary/cold front across southern Wisconsin
starts to move southward, with some acceleration as a lake
breeze/lake-enhanced boundary. Latest indications are this
probably wouldn`t occur until later in the day and for a small
part of our forecast area across northeast and northern Illinois,
but there`s quite a bit of uncertainty regarding this evolution.
Ahead of wherever the front ends up during the afternoon, eyeing a
potential for temperatures to surge a bit within the thickness
bump ahead of the synoptic front, along with dewpoints to locally
pool which could result in a corridor of notably higher peak heat
indices. Pinpointing where (and if) this will set up remains too
unclear to try to paint this in the grids for Thursday, but
something we`ll be keeping an eye on.

Looks like another toasty day on Friday as the front lifts back
northward, but we`ll eventually push a reinforcing front through
here on Saturday or Sunday. This will bring increased chances for
thunderstorms to the area along with a cool down into early next
week.

Regarding heat headlines: while this continues to look like an
extended stretch of anomalously warm and humid conditions, it
still looks like we`ll end up largely under local Heat Advisory
criteria with peak heat indices generally near or under 100
degrees. However, if these values creep up a bit more,
particularly Wednesday and Thursday, there could be an argument
for an advisory due to the extended nature of 100+ values. We`re
not seeing a strong enough signal for this right now, however.

Ogorek/Carlaw

&&

.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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