979
FXUS63 KLOT 142021
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
321 PM CDT Sun Jul 14 2024

.KEY MESSAGES...

- Additional clusters of storms are expected through this
  evening, with continued threats for severe weather and flash
  flooding.

- Hot and very humid on Monday with heat indices of 100-110F and
  a Heat Advisory likely needed for at least portions of the area.

- Additional rounds of thunderstorms are expected late day
  Monday-Monday night (area wide) and on Tuesday (mainly I-55
  and southeast) with continued severe and flash flood threats.

- The pattern finally breaks Tuesday night into Wednesday with a
  return of tranquil conditions late Wednesday onward.

&&

.DISCUSSION...
Issued at 320 PM CDT Sun Jul 14 2024

Through Monday Night:

Through Tonight:

A complex convective scenario will unfold later this afternoon
through the evening as a well defined MCV from overnight severe
convection tracks across the region. In the near term, widely
scattered convection continues to percolate across portions of
northeast Illinois and northwest Indiana. However, in a still
less favorable thermodynamic and marginal deep layer wind shear
environment, these showers and isolated thunderstorms have thus
far (thankfully) not shown any signs of intensification. Should
this continue to be the case, the main threats through about 5
PM CDT with this activity will be localized lightning strikes
and downpours.

the presence of the
MCV over Iowa moving into the region during peak heating and
large instability certainly adds to concern for a scattered
severe weather threat, along with flash flooding. Given the
current location of the MCV, suspect that scattered
thunderstorms will initiate over the northwest CWA as early as
4-6 PM. Bumped up PoPs to likely (~60-70%/numerous storms type
coverage) this evening. Deep layer shear will be on the
increase (up to 35 kt if not more), so initial storm mode may be
mixed supercellular and multi- cellular clusters. There should
be a tendency for cold pool development and transition to bowing
segments with time this evening.

Hazards wise, initial supercells may be capable of all hazards,
though extent of any tornado threat would be driven by how much
low-level shear is increased by the enhanced flow aloft
associated with the MCV, as LCLs will certainly low enough.
Despite the high moisture and freezing levels, initial
supercells will also be capable of producing large hail due to
steep mid-level lapse rates. Overall greatest threat, especially
into the evening should be for pockets of damaging winds. With
the LLJ ramping up in concert with MCV flow enhancement, may
also need to monitor for mesovortex and QLCS tornado
development.

Finally, last but not least, given the significant flooding
in Rockford and nearby across far northern Illinois last night,
plus very high moisture, flash flooding continues to be a
noteworthy concern. We issued a new Flood Watch for all of our
counties except Livingston, Ford, Iroquois, Newton, Jasper, and
Benton. The potential for significant flooding may be somewhat
less if propagation tied to cold pools and the progression of
the MCV clears deep convection east of our area quicker, by
midnight or a bit after. The faster propagation may be most
favored once the organized clusters of storms/possible MCS reach
our far southern CWA counties. With the sandy soils of the
Kankakee Valley a bit less vulnerable to flash flooding and the
above in mind, opted to leave out our far southern counties, but
can`t rule out needing to extend the watch this evening.

Monday-Monday Night:

Potentially dangerous heat and humidity will be followed by yet
another round of thunderstorms in the late afternoon through the
evening and early overnight. This round will be accompanied by a
threat for significant severe weather (damaging winds and
tornadoes) and flash flooding. The remainder of the discussion
for this period will be issued shortly.

Castro

Tuesday through Sunday:

Tuesday-Tuesday Night:

Showers and storms may be ongoing Tuesday morning, mainly across
the southern portion of the CWA. A cold front passage Tuesday
morning will keep high temperatures in the mid-80s with onshore
flow limiting them to the upper 70s near the lake. This cold front
will be draped across or just south of the southern portion of
the CWA Tuesday afternoon. With 2000-3000 J/kg of CAPE expected to
build along the frontal boundary and steep mid-level lapse rates
(7-8 C/km), thunderstorms are expected to form during the
afternoon. These storms should be limited in extent to the far
southern/southeastern portion of the CWA (southeast of I-55)
before exiting to the south and southeast. These storms pose a
threat of strong winds and hail.

Carothers

Wednesday and beyond...

We`ll eventually get into a break from this active stretch towards
the end of the week. However, based on the incoming guidance,
can`t entirely rule out a few showers and thunderstorms on
Wednesday afternoon as a trailing shortwave speeds across the
Great Lakes. Forecast soundings suggest there will be a little
lingering boundary layer moisture, and perhaps sufficient
destabilization to realize some modest lingering MLCAPE. The
blended guidance showing low-end chance PoPs east of about I-55/57
seem reasonable at this range as a result. If storms do manage to
fire within sufficient lingering low-level moisture, sufficient
shear exists to yield a low-end risk for a few stronger/more
organized cores.

As it stands right now, Thursday through the upcoming weekend are
looking nice and quiet as sprawling high pressure begins to build
over the region. In the process of building this high, however,
breezy northerly winds down the lake may yield a brief period of
dangerous swimming conditions/rip currents Wednesday night into
Thursday. Other than that, however, conditions look outstanding
into the weekend with comfortable dewpoints, near to below-normal
temperatures, and daily lake cooling.

Carlaw

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1213 PM CDT Sun Jul 14 2024

Key Aviation Messages:

- Widespread thunderstorms are expected late this afternoon into
  the evening. These storms will have an accompanying threat for
  damaging winds, in addition to reduced VSBY and CIGs.

- Another round of thunderstorms are expected tomorrow afternoon
  and evening.

Discussion:

The primary concerns through the TAF period are the multiple
chances for thunderstorms through tomorrow afternoon. Radar
and satellite imagery show the convection from this morning
moving out of the area. There are some showers and storms in
northwest IL that are headed towards KRFD and will be arriving
shortly. Current thinking is that this convection will dissipate
before it reaches the terminals as it interacts with the
remnant cold pool from this morning`s convection, however, can`t
rule out the chance for some VCTS this afternoon.

Another round of strong to severe storms is expected this
evening across the whole area, with the main hazard expected to
be damaging winds. Most guidance suggests a start time of
around 23Z at KRFD with an hour lag for KORD, KMDW, and KDPA and
another hour lag for KGYY. These storms will decrease visibility
and CIGs through the evening and into the overnight hours.
Winds will likely get a little squirrelly as the storms pass
through as well. Similar to this morning`s convection, it is not
out of the question that winds turn easterly/northeasterly
behind the storms, however, they should turn back to a
southwesterly component shortly after the storms move move out
of the area.

Things should be quiet through the remainder of the night with
southwesterly winds prevailing through the end of the TAF
period. Tomorrow afternoon, there is yet another chance for
widespread storms. Best timing looks to be around 00Z, however,
some guidance has storms initiating in the mid-afternoon. Thus,
confidence is currently low in the timing, but have covered the
possibility of afternoon initiation with a PROB30 at KORD and
KMDW.

Carothers

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch through late tonight for ILZ003-ILZ004-ILZ005-
     ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-
     ILZ021-ILZ023-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Flood Watch through late tonight for INZ001-INZ002.

LM...None.

&&

$$

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