-- Discussion containing changed information from previous version are highlighted. --
926
FXUS63 KLOT 222343
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
643 PM CDT Sat Jun 22 2024

.KEY MESSAGES...

- Potential for strong to severe storms with damaging winds,
  torrential downpours and localized flooding this afternoon
  near the Wisconsin state line and area-wide this evening.

- Breaks in heat and humidity are expected Sunday and Monday,
  and again Wednesday through the end of the workweek.

- Tuesday may feature dangerous heat and/or severe weather in
  the general region.

&&

.MESOSCALE...
Issued at 625 PM CDT Sat Jun 22 2024

A bit of a complex set-up regarding our severe and convective
potential tonight.

Well defined shortwave trough and an associated 40-50kt mid
level jet streak is tracking eastward into the western/upper
Great Lakes region. Several intense storms, including tornadic
supercells, are ongoing over southwest Wisconsin east of the
weak sfc low and associated cold front. These storms will likely
remain north of our CWA or at worst potentially nearing the
IL/WI border.

Farther south into eastern IA, we`re seeing attempts at
convective initiation, but thus far storms have been struggling
to become sustained and really take off. These attempts at
convection are ahead of the cold front and in area of weaker low
level convergence, but seem to reaping the benefits of the
large scale ascent associated with the shortwave trough.

An axis of low-mid 70 degree dewpoints has pooled along and
ahead of the cold front from northern Missouri into northwestern
IL and southern WI. While low/mid level lapse rates are weak
and limiting instability somewhat, the very moist boundary layer
is resulting in an axis of MLCAPE values of 1500-2000 J/kg over
eastern IA into northwest IL. The mid-level jet streak is
resulting fairly long hodographs favorable for supercells.

While shear vector orientation would tend to favor an evolution
to a linear convective mode, the 18z sounding from DVN was very
moist and not particularly favorable for large cold pool
development. Given the attempts at convection are ahead of the
cold front in eastern IA are occurring in an area of weaker
convergence, it is possible that if any of these storms could
become maintained, then it is possible they could remain
discrete for couple-few hours this evening as they track into
northwest IL.

Low level jet is expected to ramp up this evening and result in
rather large, curved low level hodographs. If discrete
supercells are ongoing into this increasingly strong low level
shear environment this evening, then there could be a window of
a greater tornado threat into mainly our western CWA during the
early-mid evening hours.

Another, possibly more likely scenario, is that these storms
ahead of the cold front fail to take off with our better
convective threat delayed later and closer to the cold front, in
which case the tornado threat would be lesser and confined to
mainly brief QLCS tornado threat with any line segments that are
more north- south oriented and perpendicular to the mean flow.

Still anticipating that convection will eventually evolve into a
QLCS, along sagging cold front or amalgamating cold pools.
Assuming evolution to QLCS occurs mid-late evening, then it could
begin to lay out a bit more east-west with some regenerative
development leading to an increasing threat of flash flooding
later this evening. Given the extremely high PWATS and very high
freezing levels, storms will be extremely heavy rain producers.
If we can identify any potential higher threat area for training
cells, then a short-fused flash flood watch may be needed.

Izzi

&&

.DISCUSSION...
Issued at 309 PM CDT Sat Jun 22 2024

Through Tonight:

Forecast attention this afternoon continues to focus on the
expected development of severe thunderstorms to our west-northwest
across northeastern sections of IA. Satellite imagery continues
to depict breaks in the cloud cover across this area within the
vicinity of a west-to-east oriented stationary frontal boundary.
Heating of a very high theta-e airmass along and south of this
boundary is resulting in rapid destabilization. This was noted in
the recent 18z RAOB from DVN. Continued heating of this airmass
over the next hour or two will likely continue to erode what
little cap still exists, thus allowing for the initiation of
thunderstorm development.

As noted in previous discussions, the kinematic environment
across the area will be favorable for the development of organized
severe convection, with supercells likely becoming the initial
mode across northeastern IA late this afternoon. While a few
tornadoes and instances of hail will be possible with this
supercellular activity for a couple hours late this afternoon,
amalgamating outflows are expected to transition the predominant
storm mode into east-southeastward moving organized linear
clusters across northern IL this evening. Rather poor mid-level
lapse rates with eastward extent over northern IL in combination
with a very high PWAT airmass (up to 2.25") suggests the threat of
hail will remain rather low outside of any well developed
supercell structures.

Aside from the damaging wind threat, the near record high PWATs
will also support instances of extremely heavy rainfall with the
storms, with rain rates of 1 to 2"+ per hour likely. For this
reason, even a brief period of training storms could result in
some localized 2 to 3"+ amounts before the storms sag southward
through the evening. While we still cannot rule out the need for a
short fused flash flood watch for western parts of the area, we
will continue to run with the current ESF (hydrological outlook)
for areas generally along and north of I-80 in IL.

The heavy rain and severe weather threat will gradually end from
northwest to southeast later this evening into the overnight as
the clusters of storms sag southeast out of the area.

KJB


Sunday through Saturday:

Forecast focus for the extended is on two time periods, Tuesday
and Friday.

Before then, lingering showers will be possible Sunday morning
as the cold front departs the area and isolated showers will be
possible Sunday afternoon into Sunday evening, but confidence is
very low and maintained a dry forecast for now. Cooler and
slightly drier air will spread across the area on Sunday.
Lowered high temps to the lower/mid 80s but these are still
above some of the cooler guidance. Winds will turn northerly
over the lake, which will keep temps cooler near the Lake
Michigan shore.

Low temps by early Monday morning likely to be in the lower/mid
60s for much of the area. Dry and mostly sunny for Monday as
highs likely to be back into the mid to possibly upper 80s. A
lake breeze will turn the lake shore areas cooler for the
afternoon.

Much of Monday night will likely be dry but by early Tuesday
morning, there is considerable disagreement/spread among the
models and their ensembles. There is one solution of morning
convection either over or near the cwa which continues through
midday Tuesday, as it moves south. The other solution keeps the
local area dry through early/mid afternoon Tuesday and then as a
cold front approaches, convection develops along/ahead of the
front and continues into Tuesday evening. While this time period
has consistently been shown in the models for convection for
the local area, either solution seems plausible and there is
little to support one over the other at this time. Strong to
severe storms along with heavy rain would be possible in either.

When precipitation develops, assuming it does, will have a big
impact on temps. Morning convection will keep temps cooler and
while they`ll still likely recover into the afternoon, unlikely
to get past lower 90s. If Tuesday morning is dry, southwest
winds will likely tap into the stronger winds aloft, with gusts
into the 30 mph range, pushing highs into the mid 90s, perhaps a
few locations reaching upper 90s. The strong winds and deeper
mixing will likely keep dewpoints in the upper 60s/near 70. This
would result in heat indices in the upper 90s to around 100.

There is the potential for thunderstorms to linger or continue
into Wednesday morning, though if they do, it would likely be
south of I-80 and departing through mid morning. Dry conditions
are then expected through at least Thursday evening. Cooler and
less humid conditions are expected Wednesday and especially on
Thursday, along with cooler conditions near Lake Michigan with
northeast winds.

Friday has been another time period the models have been showing
the potential for thunderstorms, again with little confidence
for timing. Another cold front will moving across the area
during this time period. Similar to Tuesday, there is the
potential for both morning and afternoon/evening. cms

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
-- Changed Discussion --
Issued at 643 PM CDT Sat Jun 22 2024

Forecast concerns include...

Scattered thunderstorms this evening through early morning.
Gusty southwest winds this evening.
Mvfr cigs overnight/Sunday morning.

Scattered thunderstorms are expected to develop over northwest
IL early this evening and move east/southeast across the Chicago
terminals later this evening into early Sunday morning. Timing
continues to slow with the latest guidance and additional timing
tweaks may still be needed as trends emerge. While strong/gusty
winds will be possible with the strongest storms, heavy rain
will also be possible with ifr/lifr cigs/vis possible with the
strongest storms. Lingering showers may continue overnight.

Prevailing mvfr cigs are likely after the storms move southeast
of the area. Guidance has settled mainly on low mvfr cigs and
ifr cigs look unlikely. Mvfr cigs will continue into mid/late
Sunday morning and then scatter out by early afternoon.

Southwest winds will remain gusty this evening, into the lower/
mid 20kt range though may slowly diminish through the evening.
Winds will turn westerly overnight and then shift to the
northwest Sunday morning. Gusts into the 15-20kt range will be
possible overnight into Sunday morning. cms
-- End Changed Discussion --

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 1 AM CDT Sunday for the IL and IN
     nearshore waters.

&&

$$

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