241
FXUS63 KLOT 131719
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1219 PM CDT Sat Jul 13 2024

.KEY MESSAGES...

- Today will be more humid than yesterday, with peak afternoon
  heat indices in the mid to upper 90s.

- Several waves of thunderstorms may occur from this afternoon
  into Tuesday with threats for severe weather and flash
  flooding.

- Sunday and Monday may feature dangerous heat, though the
  timing and coverage of thunderstorms may limit the intensity
  and coverage of the heat.

&&

.UPDATE...
Issued at 1052 AM CDT Sat Jul 13 2024

Current radar imagery shows an ongoing MCS moving from eastern MN
into WI. A mid-level shortwave is associated with the MCS and is
expected to dig to the southeast and into the CWA this afternoon. A
plume of steeper mid-level lapse rates (7+ C/km) has been slowly
working its way northeastward out of MO and into southeastern IA and
western IL. Satellite imagery shows some ACCAS associated with
this area of steeper lapse rates. As the steeper mid-level lapse
rates continue to advect northeast and are intercepted by the
shortwave, current thinking is that there may be enough lift to
initiate some convection across northwestern IL and the western
half of the CWA this afternoon. Timing of this looks to be late
afternoon (21-22 Z) through the early evening. Many of the 12Z
CAMs (particularly the 3kNAM, WRF-ARW, and WRF-NSSL) seem to
have caught onto this scenario playing out as well. Although
confidence is a bit higher in this scenario than anything else
at this point, confidence remains low in this scenario actually
playing out. Thus have opted for chance (20-30%) PoPs for the
western portion of the CWA through 00Z.

Any storms that develop this afternoon could become strong to
severe given the large reservoir of instability (2000+ J/kg
MLCAPE) that is expected this afternoon. Relatively weak shear
values (20-25 kts) should facilitate more of a pulse/multi-
cellular storm mode today (as opposed to more
organized/supercellular convection), with locally damaging
downburst winds looking like the main severe threats.

Moving into the evening, thinking remains the same in that some sort
of ongoing convection will move into the area, though guidance
continues to be in disagreement on the timing and progression of
these storms.

Carothers

&&

.DISCUSSION...
Issued at 305 AM CDT Sat Jul 13 2024

Through Sunday:

Regional water vapor imagery depicts several clusters of
thunderstorms across the northern Plains, each apparently tied
to subtle vort maxes or upper-level shortwaves along the
northern periphery of an expansive summertime ridge centered
over the central US. In addition, several ephemeral areas of
mid-level ACCAS are evident on nighttime microphysics imagery
along the implied eastern edge of an expansive EML plume, with
one such area in southern Minnesota and another in west-central
Iowa. Ahead of the thunderstorm clusters and beneath the EML
plume is an expansive reservoir of deep low-level moisture
arcing from the Gulf Coast to southern Manitoba along the
southwestern side of a surface high centered over the eastern
Great Lakes. Taken altogether, the surface to upper-level
pattern is quintessential of summertime.

Today, the surface high pressure system in the northeastern
United States will drift further away, allowing for the plume
of low-level moisture to slosh northeastward into northern
Illinois and northwestern Indiana. Today will hence feel
noticeably more humid than yesterday, with peak heat indices
poised to climb into the mid to upper 90s. An initially weak
surface pressure gradient should support the development of a
lake breeze, though inland penetration may be stunted by mid-
afternoon as synoptic southwesterly winds increase in magnitude.

With several clusters of thunderstorms already ongoing well
upstream of our area, chances for some sort of convectively-
augmented vort max, MCV, or shortwave, not to mention remnant or
reinvigorating convection itself or some combination thereof,
to propagate southeastward into our destabilizing airmass and/or
activate the eastward-spreading EML plume seems inevitable.
Indeed, within the myriad of forecast scenarios advertised by
successive runs of CAMs is a commonality of at least one but
potentially several episodes of thunderstorms in our general
region in quick succession, perhaps as early as later this
morning but more likely from this evening and through the
overnight hours. There is always a degree of inherent low
forecast confidence in these types of patterns, but felt there
was enough of a signal to cautiously increase PoPs from around
20% from late morning to the 40 to 50% range from sunset onward.
(If I had to pick which CAMs that appears most realistic, the
00Z NAM3/HRDPS looked pretty reasonable).

In terms of the threat for severe weather today and tonight,
pattern recognition of a deep plume of moisture-laden
instability displaced south of the upper-level jet supports
outflow-dominant clusters with locally severe winds (e.g. not a
widespread severe weather threat). Perhaps more concerning would
be the threat for flash flooding, given repeated/reinforcing
outflows would orient perpendicular to the increasing nocturnal
low-level jet and associated replenishing feed of 1.75"+ PWATs.
Such a regime can support repeated rounds of convection over the
same area, if not outright backbuilding, leading to a focused
zone of copious rainfall amounts greater than 4". At this point,
the level 1 to 2 threat for damaging winds (consistent with
isolated to scattered reports of wind damage) advertised by SPC
looks appropriate. The Level 1 threat level for flash flooding
(consistent with isolated reports) is appropriate for now, but
may need to be increased in tandem with the issuance of a Flood
Watch if confidence in repeated rounds of thunderstorms tonight
increases.

On Sunday, the upper-level ridge and associated pool of warm 925-
850mb temperatures is expected to build eastward setting the
stage for potentially dangerous heat. With prospects for at
least remnant convective debris and remnant outflow lingering
through the morning, not to mention regenerating convection as
the low-level jet veers, confidence in the high temperature
forecast is lower than average. In forecast scenarios where the
convective footprint is minimized, high temperatures in the low
to mid 90s and rising dew points into the mid 70s would support
heat indices in the low to mid 100s across part if not all of
the area. However, in forecast scenarios where the convective
footprint is higher, the threat for dangerous heat would be
lowered. We`ll take it one day at a time in this pattern, and
consider the need for a heat headline once we have a better
picture of the coverage of (or are outright dealing with)
thunderstorms tonight.

Borchardt


Sunday Night through Friday:

Sunday evening and night, a low-mid level shortwave disturbance
is progged to track right over northern IL providing another
potential for some nighttime showers and storms. While a few
isolated storms may go up earlier in the day, the better chances
arrive in the evening and overnight as the system moves across
the area, although coverage remains largely uncertain. An EML
overspreading the region will park an impressive pool of MUCAPE
ahead of the system. However, forecast soundings develop a
stout, diurnally-driven capping inversion amid poor low level
lapse rates during the evening just prior to the wave moving in.
Plus, the strength of the shortwave and resultant quality of
the forcing is also disputed which makes it difficult to
decipher what roll the cap will play. Overall, the trend among
guidance appears to be toward a cluster of storms moving across
northern Illinois late Sunday, although there is still plenty of
support for less. If forcing proves ample enough for parcels to
take full advantage of the EML, some stronger storms will
certainly be possible with the threat for gusty winds and heavy
downpours.

Oppressive heat looks to carry over from Sunday into Monday as a
pronounced upper high will lock a hot, humid airmass over the
central US early next week. High temperatures are expected to
tag the lower and middle 90s with afternoon heat indices near or
over 100F. Most models are thinking that an influx of low-mid
level dry air behind the overnight convection will provide
plenty of sunshine throughout the day. However, if cloud cover
ends up struggling to clear out post-convection earlier in the
day, temperatures could prove slightly cooler than expected.

The heat on Monday in conjunction with the EML still in place
will build a great deal of instability across the region
during the day. Dry low-mid level air and a general lack of
forcing look to keep our CWA mostly quiet during the day. A cold
front will drop south through northern Illinois on Tuesday.
This looks to drive a complex of showers and storms through the
CWA beginning Monday evening into the night. The time of day
should work in our favor with instability decreasing some during
the evening ahead of the potential. However, still-noteworthy
instability and increasing shear beneath the encroaching trough
base could result in strong to severe convection ahead of the
front during the evening and night. Additional thunderstorms are
expected to form ahead of the front on Tuesday as the boundary
works south through the CWA. The pattern should settle down
after Tuesday with the upper trough axis progged to move through
early Wednesday and height rises and weak flow aloft to follow
into late next week.

Doom

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1219 PM CDT Sat Jul 13 2024

Aviation weather concerns are:

- Low-confidence potentials for TS late this afternoon/early
  evening and again overnight.

- Breezy/gusty southwesterly winds on Sunday.

The main focus today is on the potential for isolated to
scattered TS late this afternoon and evening, and then again
overnight. Overall, the TS/precipitation forecast through
midday is low confidence. Latest indications are that an area of
isolated to scattered showers/storms may develop this afternoon
across northwest Illinois and scoot eastward through the early
evening. Chances, however, remain under 50 percent, and too low
for VCTS or TEMPO TS mentions at any of the terminals. Chances
are the "highest" at RFD, but still under thresholds justifying
a formal TS mention. If activity were to develop, the main time
for on-station impacts at the Chicago-area sites is about
23-03z, and a few hours earlier than that INVOF RFD.

The next chances appear to be overnight, but are contingent on
activity currently across central Minnesota developing further
into a mature convective complex. If this were to occur, another
TS potential exists from late this evening into the early
morning hours on Sunday.

While additional chances for storms exist beyond early Sunday
AM, chances realistically are too low to carry additional PROB30
groups. Sunday may remain largely precipitation-free, with the
next round of potential storms arriving in the evening.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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