459
FXUS63 KLOT 220530
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1230 AM CDT Wed May 22 2024

.KEY MESSAGES...

- Severe thunderstorms likely this evening, with potential for
  widespread, and potentially significant, damaging winds and
  tornadoes

- Non-thunderstorm winds could gust over 40 mph from the south
  this evening, then from the southwest in the wake of the
  storms later tonight

- There will be dry breaks at times, but an active pattern for
  showers and storms returns Friday through early next week

&&

.MESOSCALE...
Issued at 1022 PM CDT Tue May 21 2024

Recent radar and satellite trends new convection filling in just
east of the Mississippi River from just east of Quad Cities south
to just east of Quincy. These storms are seemingly forming on a
pre-frontal convergence axis evident on the velocity data on DVN
WSR-88D.

Evening RAOB from ILX and recent ACARs soundings from Chicago
show presence of stout capping inversion in the warm sector,
however strong ascent in association with the approaching upper
trough is progged to continue lifting/cooling the inversion this
evening resulting in weakening cap/inhibition. RAP objective
depicts an axis of 1000-2000 J/kg of MLCAPE over western IL, which
should largely maintain itself as strong winds limit nocturnal
cooling and dewpoints remain steady or even rise a bit. Near term
RAP suggests that continued ascent should continue to erode the
inhibition as the axis of strongest instability shifts eastward
coincident with the cold front and upper trough.

Assuming inhibition does indeed erode as progged, then recently
developing convection along the convergence axis would have the
potential to maintain itself and even intensify as it spreads
eastward into our CWA. While it isn`t clear this will be the case,
should sustained, strong convection maintain itself wind profiles
remain exceptionally favorable for damaging winds and even a
tornado threat. While convection hasn`t been particularly intense
in our CWA up to this point, given the exceptionally favorable
wind profiles and at least the potential for weakening inhibition
it is still a little early sound the all clear on the severe
threat tonight.

Strong synoptic winds have been gusting to and in some cases over
45 mph, so going to shoot out a short fused wind advisory.

- Izzi

&&

.DISCUSSION...
Issued at 327 PM CDT Tue May 21 2024

Through Wednesday:

All eyes are on developing convection along a cold front
extending from western Iowa through far eastern Kansas this
afternoon. Low-level capping from previous WAA and some PBL
mixing/drying continues to support dry conditions through at
least 6-7pm in our area. Meanwhile, the spread in potential
solutions from near-term CAM guidance is far higher than desired
at this point, with guidance ranging from little/no convection
over most of the CWA to an expansive squall line with
significant damaging winds and an increased tornado threat well
into the forecast area. Explosive growth of convection across
eastern Kansas and far northwest Missouri supports maintaining a
more aggressive approach with organized convection surviving
well into the area, so we again made only some tweaks to the
overall forecast and messaging with this update.

Overall expectations are for the ongoing upscale growth of the
convection along the KS/MO line, ultimately developing a robust
cold pool that will erode remaining capping and maintain
organized convection across the MS River and into at least the
western CWA around the 7-8pm window. Convection should then
gradually weaken with eastward extent as the line becomes
farther removed from the prime kinematic environment to the
north. With that said, the highest coverage of potential
significant winds favors areas roughly north of a line from
LaSalle to Cook counties and especially in the northwest CWA
toward the Rockford area.

All severe hazards are possible as the convection crosses the
area this evening, but favors a transition to primarily damaging
winds with eastward extent. Any convection, particularly
linear/bowing segments and especially any QLCS, will support a
damaging wind threat with the potential for swaths of
significant wind gusts (75mph+) through the evening. Even
decaying convection will support a significant damaging wind
risk owing to substantial low-level shear profiles and a pre-
convective pool of DCAPE up to around 1500J/kg.

The tornado risk will remain highly conditional on both storm
evolution and coverage, with the most notable threat occurring
with any QLCS/bowing structures owing to an impressive low-level
wind profile with highly elongated hodographs and only a weakly
decoupled nocturnal PBL. In fact, there may be a brief increase
in tornado potential during the mid-evening as EHI values
maximize with the increasing LLJ and slight increase in low-
level moisture. As noted in earlier discussions, strong
tornadoes (EF2) cannot be ruled out with any well-formed line
segment.

Behind the convection just ahead of and behind the convection
overnight, strong and gusty south to southwest synoptic winds
will potentially gust to 40+mph for several hours, with some
gusts possibly even reaching 50mph.

Cooler and dry conditions are expected Wednesday behind this
system.

Kluber


Wednesday Night through Tuesday:

Models are starting to key in on the clearer conditions
allowing for better heating to develop Thursday and potentially
Friday. Previous forecasts capped temperatures in the 70s
through the weekend, but now it looks like there is a better
chance for temps to return to the 80s for much of the area for
Thursday and Friday as a warm front lifts northward from
Central Illinois (more on this in a second). Unmentionable PoPs
south of I-80 along and south of the warm front were kept in the
forecast with only a few low end slight chances (15 to 20
percent) on Thursday afternoon. With little capping, perhaps a
pulse shower pops up in southern Ford or Iroquois Counties, but
confidence is low and kept the forecast mostly dry through
Friday morning.

A weaker (relative to the short term forecast above) upper
level trough will move out of the Plains and toward Ontario
Friday night into Saturday. The associated surface low still
gradually move northward through Iowa on Friday, lifting the
previously mentioned warm front with it before a cold front
sweeps across the area in the evening. Notably, models have
sped of the progression of this system from a Saturday morning
event to now more of a Friday event. Model soundings are
starting suggest better mid level lapse rates with sufficient
instability aloft. With additional forcing from the cold front,
showers and thunderstorms have a chance to develop. One
limiting factor is the projected wind shear values along the
line do not look very impressive, limiting the severe
potential. Additionally, the forecast kept likely PoPs for
Friday afternoon for areas southwest of I-39, but only chance
PoPs for the rest of the area because it is currently expected
to move east over the area after sunset and therefore have
weaker instability without the diurnal heating component. But
given how models have changed, there is a lot of uncertainty in
the exact timing of the frontal passage.

Other than lingering showers Saturday morning, brief ridging
should allow for a slightly drier Saturday afternoon through the
start of Sunday. Both Euro and GFS ensembles are suggesting two
weak waves next week, one moving northward from St. Louis
Sunday night, as well as a slightly deeper one over the
northern Plains arriving over Illinois sometime between Monday
and into Tuesday. While there will certainly be breaks, it is
hard to argue with the NBM`s minimal slight chance PoPs through
next week. While PoPs are higher as models take their best
guess at timing at this range of the forecast will probably need
to be adjusted in the coming days. The main takeaway is that
regardless of the magnitude, it looks like a fairly active
pattern will remain in place through the end of the forecast
period.

DK

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1230 AM CDT Wed May 22 2024

Key Aviation Impacts for the current TAF period:

- Strong southwesterly wind gusts expected through Wednesday
  afternoon

- MVFR cigs expected overnight

The thunderstorm threat has fizzled out allowing the TAFs to
return to dry. However, strong south to southwesterly wind
gusts are prevailing. Obs are reading the strongest gusts
between 30 to 40 knots at the time this discussion was
published. There is lower confidence for 40+ knots to continue,
but the TAFs were updated to show a slower progression down into
Wednesday morning to the 25 knot gust range. Persistent
southwest wind gusts around 25 knots is expected through the day
with stronger gusts not expected to end until 00Z.

Additionally, cig observations behind the front over in Iowa are
down between 1500-2500 feet. Those clouds are expected to move
over the terminals to create MVFR cigs overnight and into early
Wednesday morning. After daybreak, cigs will lift for VFR skies.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Wind Advisory until 2 AM CDT Wednesday for ILZ004-ILZ005-
     ILZ006-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-
     ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-
     ILZ108.

IN...Wind Advisory until 2 AM CDT /3 AM EDT/ Wednesday for INZ001-
     INZ002-INZ010-INZ011-INZ019.

LM...Gale Warning until 4 AM CDT Wednesday for the IL and IN
     nearshore waters.

&&

$$

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