230
FXUS63 KLOT 211642
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1142 AM 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 638 AM CDT Tue May 21 2024

Key Aviation Impacts for the current TAF period:

- Low clouds may spill into Chicago terminals from the north,
  but due to low confidence kept ORD/MDW VFR in the short term

- Strong southerly winds develop late morning. Gusts will
  gradually ramp up from 20 to 35 knots through the afternoon,
  potentially occasional gusts over 40 knots during the evening.

- A strong and potentially severe line of storms is likely to
  arrive after 00Z and move east over the area

- MVFR cigs expected overnight behind the line of storms as well
  as gusty southwesterly winds

Dense fog developed over KUGN this morning and pushed southward
that dropped KPWK to 3SM and OVC003. It started to encroach on
KORD and lightly at KMDW, but the sun rose and started to
provide a little bit more mixing. There is a chance that a FEW
lower clouds could move into the Chicago terminals, but given
the lower confidence ORD and MDW were kept at VFR.

Winds will be out of the SE to start and become out of the south
with increasing wind gusts through the day 20-25 knots and
around 35 knots in the late afternoon. As a squall line pushes
through, winds will flip WSW with the potential for gusts
greater than or equal to 40 knots during the evening.

This line of storms has the potential to be severe where
localized stronger westerly winds gusts, hail, and maybe a
brief tornado will be possible as it passes. Current models have
dry conditions before and after it, so there is no VCTS/VCSH
prevailing. The timing for the line is within the TEMPO which
was slowed slightly based on latest guidance and can be adjusted
in later TAF packages.

Low confidence in any lingering showers behind the line. Some
models have it but kept the TAFs dry for now. Main impacts after
the line will be from cigs and winds. MVFR conditions from cigs
2000 feet or higher are expected behind the line. Additionally,
west southwest winds are expected to remain strong. Kept gusts
around or just under 25 knots at this issuance, but it would not
be surprising if there was a secondary "pop" of winds behind the
line between 04Z-12Z where gusts went back over 30 knots.
Eventually, VFR skies should return Wednesday morning, but
breezy southwesterly winds will remain.

DK

&&

.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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