FXUS63 KLOT 132014

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


- Scattered showers and storms are expected this afternoon,
  mainly across the western portion of the CWA. These could
  produce strong to severe downburst winds.

- A MCS may dive southeast from MN/WI into the area this
  evening into the overnight hours. Strong to severe winds are
  possible with this system.

- Additional rounds of convection, including the threat for
  severe thunderstorms and flash flooding, are probable late
  Sunday-Sunday night and late Monday-Monday night, with
  scattered thunderstorms possible for some on Tuesday.

- Hot and humid conditions are expected Sunday and Monday with
  peak heat indices over 100 degrees. Monday looks to be the
  hotter day with peak heat indices approaching 110F. A cold
  front passage on Tuesday will end the heat and humidity.


Issued at 314 PM CDT Sat Jul 13 2024

Through Sunday Night:

Current satellite imagery depicts a shortwave digging southeast
from MN/WI towards the area away from an ongoing MCV in
northern WI. Steeper mid-level lapse rates (7+ C/km) have been
advecting north/northeastward from northern MO through the
morning and early afternoon. The shortwave has provided enough
lift to overcome the dry mid-levels and some showers and storms
have begun across northwestern IL and southwestern WI. These
storms are expected to spread east through the afternoon, but
should remain confined to the western half of the CWA until this
evening. Given the 2000-3000 J/kg of MLCAPE that has developed
overhead, these storms could produce strong to severe downburst
winds and hail this afternoon despite the low-end bulk shear
values (20-25 kts). Given these shear values, storms should
remain scattered in nature and are not expected to grow upscale.

An impressive EML across the Northern Plains has allowed
convection to fire in MN is expected to congeal into a MCS
this afternoon. This MCS looks poised to dive southeast towards
the area this evening into the overnight hours. The most likely
timing for this system looks to be after sunset. Given this,
convection should be in a weakening phase as it reaches the
northern parts of the area. However, storms will still be
capable of producing strong to severe wind gusts. Uncertainty
still remains on how the exact timing and progression of this
system as the 12 and 18 Z guidance has not been in agreement
with the handling of the multiple rounds of storms and MCS's

One rather worrisome outcome for the storms overnight is outflow
setting up a scenario for convection to regenerate along the
nose of a 30-40 kt LLJ. This would lead to a quasi-training
event and the same areas being hit by storms multiple times.
With PWATs expected to be approaching 2", this leads to a threat
for localized flash flooding. Confidence in this actually
occurring is quite low at the current time, but the pattern is
favorable given a properly oriented outflow boundary.

Convection should be clearing out of the area by tomorrow
morning, setting the way for a hot and humid day on Sunday.
Temperatures are expected to climb into the low 90s with dew
points in the low 70s leading to peak heat indices pushing 100
degrees. Although hot, this falls below the criteria for a Heat
Advisory (peak heat indices of 105 degrees). However, if
convection carries over into the morning hours, this could
augment the high temperatures for the day, leading to cooler
peak heat indices.

Tomorrow afternoon, a warm nose around 900 mb should cap
convection across the area despite a reservoir of 2000-3000
J/kg of MLCAPE. Some guidance has a weaker capping inversion and
has convection being able to initiate. Although this scenario
looks unlikely, this too would work to limit high temperatures

Sunday night, yet another round of storms looks possible with an
embedded shortwave / vort max expected to move across the area.
This wave should move overhead through the late afternoon and
early evening. This sets the stage for more nocturnal showers
and storms overnight. Given the steep mid-level lapse rates (7-8
C/km) and ample low-level moisture another large reservoir of
MUCAPE (2500- 3000 J/kg) will build. A nocturnal low-level
inversion could keep storms from forming or could at least limit
the strength of these storms, however, the progression of
convection tomorrow night is still very unclear.


Monday through Saturday:

There is a scenario during the daytime on Monday, currently the
most likely one, in which the main story weather impacts wise is
the heat and oppressive humidity. That being said, our area being
precariously positioned on the northeast periphery of the mid-level
ridge with moderately brisk west-northwest flow means "convective
surprises" certainly can't be ruled out. Whether any surprises
would be in the form of remnant overnight convection holding on
longer or additional subtle impulses interacting with the
reservoir of extreme potential instability later in the morning
and afternoon is of very low confidence. Thus, we're only
featuring slight chance (~20%) PoPs, for the northeast 3/4 of the
CWA Monday morning, and for areas near and north of the IL and
Kankakee Rivers in the afternoon.

If heating can be maximized, progged 850 and 925 mb temps support
highs in the lower to mid 90s, likely highest over the heart of
the Chicago urban corridor, and a bit lower over the farming
areas, due to the effects of evapotranspiration (ET) wringing out
moisture from the crops at the expense of slightly lower temps.
Anticipating dew points at peak heating in the lower-mid 70s over
the Chicago metro (low 70s over the city) and mid-upper 70s
(possibly locally near 80F) over the farming areas due to
aforementioned ET effects. Forecast peak heat indices in the
100-110F range suggest much of the area flirting with or exceeding
local heat advisory (105+F heat indices) criteria. However, given
the lingering uncertainty regarding possible convective effects,
will handle heat headlines on a day by day basis.

The signal for more widespread convection is much stronger by
early Monday evening into the overnight. There has been decent
run to run consistency (and ensemble member and mean signal) for a
coherent and stronger mid-level impulse and speed max crossing
the mid and upper MS Valley intercepting the strongly unstable
(3-5k J/kg of MLCAPE) air mass in place. In addition, looking at
40+ kt of deep layer bulk shear supporting storm organization.
Conceptually speaking, the pattern seems to point toward mode
favoring MCS, possibly a severe forward propagating MCS, though
the differences between MCS and supercell/embedded supercell mode
can be subtle, and both modes are plausible as well. Most of the
area is in a level 2 of 5 (slight risk) severe threat per the SPC
Day 3 Outlook, and this appears reasonable. Damaging winds are the
preferred main hazard, with large instability and steep mid-level
lapse rates supporting large hail, and combo of strengthening
low-level shear and very moist profiles/low LCLs making
tornadoes a possibility as well.

Felt comfortable with the corridor of likely (~60%) PoPs near and
north of I-80 Monday evening sagging a bit south overnight. Very
high column moisture (PWATs 150-200% of normal) and the potential
for training/back-building convection also make flash flooding a
distinct threat to monitor for. Looking ahead into Tuesday, as is
common, there will be question marks on the position of the
effective frontal boundary given likely MCS activity overnight.
While the warm and humid air mass won't yet be dislodged, a cold
front will also be progressing southward and bringing in slightly
drier air from the north. Suspect that following the dissipation
of any lingering morning showers and thunderstorms, the best
chance for afternoon redevelopment near the cold front will be for
areas near and southeast of I-55. Forecast highs are in the mid-
upper 80s, except lower 80s near the lake from winds turning
onshore, and peak heat indices from the I-88 corridor in the
Chicago metro and points south and east in the mid-upper 90s.

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



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.






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