944
FXUS63 KLOT 141713
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1213 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.

- Very warm to hot and very humid this afternoon and hot and
  very humid on Monday. Tomorrow (Monday) will likely be the
  hottest day of the pair, and will likely need a Heat Advisory.

- Additional rounds of thunderstorms are expected Monday evening
  through Tuesday with continued threats for severe weather and
  flash flooding.

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

&&

.UPDATE...
Issued at 1213 PM CDT Sun Jul 14 2024

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. We`re finally seeing the
organized morning MCS exit southeast into central and northern
Indiana. In the near term, monitoring the elevated convection
that redeveloped from southwest Wisconsin into northwest
Illinois. With lingering cold pool effects over our area, it`s
unclear if these storms will maintain or intensify (leaning
toward weakening), but did add in PoPs for isolated to widely
scattered coverage over portions of the northwest CWA through
early afternoon.

Aside from the clouds associated with that cluster, cloud cover
will steadily erode through the early to mid afternoon.
Lingering cold pool effects from morning convection greatly
delayed the heating process and also brought dew point
temperatures lower than previously forecast to be at mid day
today. Given the cooler temps (70s) and dew points (upper 60s
to around 70F) as of this writing, lowered the high temperature
forecast to the upper 80s to low 90s. Still expecting dew point
temperatures to recover to well into the 70s later this
afternoon, supporting peak heat indices in the mid 90s to low
100s, highest for the southwest 1/2 or so of the CWA.

Turning back to the thunderstorm forecast, 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
4-5 PM. Will need higher PoPs than in the official gridded
forecast this evening as the MCV translates eastward. 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 opted to cancel the previous Flood Watch
but will likely need to issue a new one this afternoon for a
chunk of the area. 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.

Castro

&&

.DISCUSSION...
Issued at 322 AM CDT Sun Jul 14 2024

Through Tuesday:

On the heels of a robust cluster of overnight thunderstorms, yet
another cluster of storms is surging down the Mississippi River
along the edge of the EML plume. While the next cluster of storms is
moving into a relative minima of instability, a steady feed of
instability via a 925/850mb low-level jet should allow it to reach
at least the Wisconsin state line and likely into the northern-most
row or two of counties of northern Illinois around or after
daybreak. Given the areas of heavy rainfall that fell
overnight, including significant flash flooding that continues
around Rockford at press time, opted to issue a Flood Watch for
counties along and north of I-88 through 1 PM.

Behind the morning round of storms, a period of dry weather is
anticipated into the early afternoon. Attention turns firmly toward
yet another round of thunderstorms this evening. Regional radar
imagery shows a derecho racing across southeastern South Dakota. The
latest radar trends indicate that the gust front is now well ahead
of the reflectivity, which is not surprising given the system is
well south of the upper-level jet and source of deep-layer shear
displaced to the north. As a result, the derecho should begin to
weaken as it surges into northwestern Iowa later this morning. With
that said, there appears to be a highly organized MCV within the
core of the system, which will approach northwestern Illinois by mid-
afternoon and ignite the next round of storms. With low- to mid-
level shear augmented in the vicinity of the circulation, any storm
may be severe with damaging winds, damaging hail, and even a
tornado. In addition, flash flooding may occur as well especially if
thunderstorms favor far northern Illinois once again.

With the threat for two rounds of storms today, confidence in the
temperature and heat index forecast is quite low. In scenarios
with minimal convective coverage and a footprint of outflow,
highs should climb into the lower 90s. When combined with
dew points in the low to mid 70s, afternoon heat indices would
top off in the 100-105F range. I gave serious consideration to
issuing a Heat Advisory, but simply didn`t have high enough
confidence in widespread 105F+ readings today. Now, it does
appear that there will be a break in the waves of thunderstorms
during the day on Monday, which gives higher confidence in heat-
headline criteria being archived. Indeed, highs in the low to
mid 90s and dew points in the mid to upper 70s will support peak
heat indices of 105 to 110 areawide. Additional rounds of
storms are expected Monday night through Tuesday, which will
include the threats for severe weather and flash flooding.

Borchardt

Tuesday night through Sunday...

Following the cold front clearing our southern CWA Tuesday night
into Wednesday morning, mean troughing will set up over northeastern
North America while powerhouse 500 mb ridging rebuilds over the
western US. This should bring an extended period of dry, cooler and
less humid conditions Wednesday PM onward through next weekend.

Castro

&&

.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 until 1 PM CDT this afternoon for ILZ003-ILZ004-
     ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ103-
     ILZ104-ILZ105.

IN...None.
LM...None.

&&

$$

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