038
FXUS63 KLOT 211742
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1242 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 gusts 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

&&

.UPDATE...
Issued at 1142 AM CDT Tue May 21 2024

We continue to closely assess both observational and model
trends with the potential for significant severe convection in
the region this evening. The effective warm front has already
cleared all but far northeast Illinois while temps across much
of the remainder of the CWA are making a quick push at reaching
90F. Meanwhile, slight mid-level height rises and strong low-
level WAA will cap nearly the entire CWA through the late
afternoon. Cannot completely rule out that convection in
northeast Iowa clips northwest Ogle and Winnebago counties, but
trends support the entire CWA remaining dry through at least
6pm.

Convection has developed across south-central Nebraska just
ahead of the surface low, with the associated cold front and
negatively tilted trough becoming the primary focus for growing
convection across Iowa and northern Missouri this afternoon.
Early trends along with recent CAM guidance support a more
northward shift in the core of significant convection this
evening, with a notable focus now into southern Wisconsin. This
ties into a northwest trend for a more favorable kinematic
field, especially given the substantial displacement of the
surface low (expected to be over southeast MN) from our area.
This ultimately results in guidance producing a split of more
robust convection across a substantial swath of the CWA this
evening as dynamics are unable to overcome the remaining low-
level capping. However, as is often seen with highly dynamic
systems, guidance tends to decouple too quickly after sunset and
correspondingly dissipates convection faster than is typically
observed. We therefore expect that upscale growth of convection
will survive well into the forecast area this evening. Of
important note, decaying convection will still 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.

As a whole, we will stay the course with the current forecast
and overall messaging, but begin to more closely highlight areas
roughly north of a La Salle to Cook County line (and especially
closer to the WI line) for higher coverage of significant
severe potential. Otherwise, details on specific hazards remain
valid from the early morning forecast discussion.

Kluber

&&

.DISCUSSION...
Issued at 303 AM CDT Tue May 21 2024

Through Wednesday:

Early this morning our CWA is bisected by a warm front located
roughly between I-80 and I-88/290. There`s been a bit of
isolated convection popping up briefly to the north of this
front, but brunt of the convective activity is over Iowa and
Nebraska, where warm air advection/isentropic ascent is
maximized at the terminus of broad 35-55kt nocturnal low level
wind maxima. Low level jet is progged to remain west of the
Mississippi River through the morning hours, so while a few more
isolated showers or even a t-storm cannot be ruled out over our
northern CWA, the prospects for widespread organized convection
appear low through the remainder of the overnight into the
morning hours. We will continue to keep an eye on convection
over IA to make sure it doesn`t do anything funny, but all
indications are that this activity should move northeastward
into WI this morning.

The warm front will quickly move north into Wisconsin this
morning allowing unseasonably warm and relatively humid air to
overspread the CWA today. As convection shifts north into
Wisconsin this morning, blow off cirrus should decrease as well,
with it looking probable that we`ll see a considerable amount
of sunshine this afternoon. Given progged 925mb temps climbing
into the 23-25C range this afternoon, seems like 90F is within
reach across most of the CWA this afternoon. The very warm temps
and strong winds will promote deep mixing and will probably see
dewpoints mix out a bit this afternoon. The slight dropping of
afternoon dewpoints should help keep the warm sector capped with
chances of convection in the warm sector during the day less
than 15%.

Mid and upper level trough are progged to deepen quickly this
afternoon, taking on an increasingly negative tilt while moving
toward the upper Mississippi Valley. As this takes place, the
associated sfc low will quickly deepen as it lifts north into MN
this evening. All signs point toward a potentially significant
severe weather outbreak this afternoon into this evening across
the region. Impressive elevated mixed layer advecting
northeastward into the Midwest, juxtaposed above moderately
humid and very warm boundary layer will result in strong
instability. Meanwhile, wind fields through the atmosphere will
strengthen this afternoon into tonight in response to the
deepening cyclone, providing for very favorable shear profiles.

Initial convective development this afternoon will probably be
a mix of supercells and/or short line segments over western IA
into northern MO. Over time, due to strong linear forcing
anticipate this activity to congeal into a fast moving and
potentially dangerous squall line as it tracks toward the
Mississippi River early this evening. Nocturnal stabilization of
the boundary layer over our CWA this evening will be somewhat
stunted by the very strong southerly winds (potentially gusting
over 40 mph at times). These strong winds should temper the
decrease in MLCAPE and increase in MLCIN. Given the strong
synoptic forcing and expected organization of the QLCS, it is
reasonable to plan for and expect this dangerous squall line to
only weaken slowly as it moves across our CWA.

Synoptically enhanced southerly low level jet is expected to
increase to 45-50kt early this evening in advance of the squall
line. These powerful LLJ winds are progged to extend down to
500-1000m agl, creating extreme low level shear. So even with
only modest CAPE (500-1000 J/kg), very large, curve low level
hodographs and strong 0-3km shear will be favorable for
mesovortex tornadoes embedded within this QLCS. Given the
extreme nature of the shear, cannot rule out a strong (EF2) QLCS
tornado or two, particularly over our western CWA where the
line may encounter slightly stronger instability.

In addition to the non-trivial tornado threat, the extremely
strong wind fields will provide an opportunity for this squall
line to produce widespread damaging winds, potentially locally
in excess of 75 mph. The thermodynamic environment this squall
line encounters will be gradually becoming less favorable as it
reaches our eastern CWA and northwestern Indiana. That should
result in some weakening of the QLCS, however, it is not
uncommon for these type of strongly forced squall lines to
continue to pose a damaging wind threat well into more hostile
thermodynamic environments, particularly when background
kinematic fields are as strong as they will be this evening.

Behind this squall line, a cold front will sweep across the
area overnight with strong and gusty southwest winds expected.
It is possible that there could be synoptic winds flirting with
wind advisory criteria both in the warm sector this evening as
the LLJ ramps up, and again for a couple/few hours behind the
cold front. Given some uncertainties and in the interest of not
clouding the main message (the severe wx threat), opted to hold
off on a wind advisory at this time.

Drier, though still very mild, air mass will result in a pretty
delightful day across the area Wednesday.

- Izzi

Wednesday Night through Monday:

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 /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1240 PM CDT Tue May 21 2024

Key aviation messages:

- Strong S to SSW winds this afternoon with gusts to 30-40kt
- Line of severe TSRA expected to move through late evening
- Strong SW to W winds develop in wake of TSRA with gusts to 30-40kt

The surface warm front has lifted well north of the terminals early
this afternoon with S to SSW winds prevailing. Wind speeds and gusts
are expected to steadily increase over the next few hours with gusts
in excess of 30kt becoming common by mid afternoon. Can`t rule out
gusts as high as 40kt at times. This will present crosswind
challenges for east to west oriented runways. Also a brief
return to SSE (~170 degrees) can`t be ruled out ahead of the
line of storms as it approaches.

Will continue monitoring upstream convection across east
central IA, though the current trajectory suggests this activity
will remain just north of RFD and accordingly have opted to
keep the afternoon dry for all sites with VFR cumulus expected
areawide beneath increasing coverage of cirrus.

No changes to TSRA timing were made with this update with a
line of strong to severe thunderstorms expected to move across
the area roughly between 01Z-06Z earliest at RFD and latest at
GYY. There is a recent trend in guidance where the line of
storms may begin to decrease in coverage as it moves across the
area; however, even if this were to occur the threat for severe
thunderstorms remains, some potentially significant (especially
near RFD) where accordingly the gusts in the TEMPO group have
been increased to 50kt. The primary hazard with these storms
will be damaging winds, though the potential for hail and brief
tornadoes also exists.

In the wake of the storms, guidance continues to suggest winds
remain strong out of the SW to W overnight. There may be a period
where gusts exceed 40kt for 1 hour period though confidence in
this remains too low to include with this update and will likely
be handled tactically based on upstream observations as storms
begin to move into Illinois later this evening.

Other than patchy MVFR stratus potentially lingering overnight,
especially out toward RFD, conditions quiet down heading into
the daytime hours of Wednesday with steady WSW to W winds and
gusts in the lower 20kt range expected through the afternoon.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Dense Fog Advisory until 1 PM CDT this afternoon for Winthrop
     Harbor to Wilmette Harbor IL.

     Small Craft Advisory from 3 PM this afternoon to 6 AM CDT
     Wednesday for Winthrop Harbor to Northerly Is. IL.

     Small Craft Advisory until 6 AM CDT Wednesday for Northerly
     Is. IL to Michigan City IN.

&&

$$

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