-- Discussion containing changed information from previous version are highlighted. --
177
FXUS63 KLOT 220022
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
722 PM CDT Tue May 21 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...
-- Changed Discussion --
Issued at 718 PM CDT Tue May 21 2024

The severe weather threat is quickly moving into northwest
Illinois early this evening as a vigorous storm system
continues to intensify into the Upper Midwest. Current
expectations suggest the primary severe weather threat through
10 pm will be across northwestern parts of the area from the
western Chicago metro and points westward into the Rockford
area. This does not mean Chicago is out of the woods yet, but
given the quickly northeastward shifting dynamics, thinking is
the highest coverage of storms will track northeastward
northwest of Chicago, at least through 10 pm. Some low threat
for severe storms still exists into the immediate Chicago area,
though it would not be until after 10 pm.

With the above in mind, a tornado watch was just recently issued
for portions of the area in collaboration with the SPC. Strong
damaging wind gusts in excess of 75 mph are expected with the
approaching storms. There will also be an increasing threat for
a few tornadoes after 8 pm as the low-level shear ramps up with
an increasing low-level jet with sunset this evening.

KJB
-- End Changed Discussion --

&&

.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 /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 619 PM CDT Tue May 21 2024

Key aviation messages:

- Strong S to SSW winds this afternoon with gusts to 30-40kt
- Line of TSRA expected to move through late evening. Medium to
  high confidence in a period of severe winds at KRFD, lower
  confidence in the strength of winds at KORD/KMDW.
- Line may become broken as it approaches KORD/KMDW, but medium-
  high confidence in thunder
- Strong SW to W winds develop in wake of TSRA with gusts to
  30-37kt

There are no big changes to the 0z TAF cycle. Gusty south winds
will continue ahead of the line/broken area of thunderstorms now
approaching the Mississippi River.

Confidence is high in these storms maintaining themselves into
KRFD and KDPA. Confidence is also high in near severe or severe
level winds (60 mph) reaching KRFD, with chances diminishing
toward the Chicago terminals. Therefore holding gusts to 50 mph
at ORD/MDW for now, and we will monitor how well the southern
portion of the line of storms holds together. More will
certainly be known in the coming hours.

Also, the current timing for TS at ORD/MDW does appear to be
closer to 03z, but if any individual cells develop ahead of the
current activity - it could be closer to 0230z or so, and this
will be an area we will keep a close eye on as well.

Gusty SW to W winds in the mid 30 kt range will continue in the
late evening and early overnight hours. MVFR conditions will
arrive, though in pockets overnight into early Wednesday, but
fairly low confidence in arrival timing and how long these will
last. Winds will be closer to W then SSW on Wednesday with gusts
more so in the 20 kt realm than the 30 kt like today.

KMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
IL...None.
IN...None.
LM...Gale Warning until 4 AM CDT Wednesday for the IL and IN
     nearshore waters.
-- End Changed Discussion --

&&

$$

Visit us at weather.gov/chicago