803
FXUS63 KLOT 122337
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
637 PM CDT Fri Jul 12 2024

.KEY MESSAGES...

- While majority of the weekend should be rain free, there could
  be a period or two of showers/storms this weekend

- Some severe potential exists with any convection this weekend
  into early next week, but confidence in timing/details is low
  and will hinge on mesoscale details

- Increasingly hot and humid conditions expected through Monday,
  heat indices over 100 degrees probable Sunday and Monday
  afternoons

- Shower/storm chances increase again Monday night and Tuesday
  associated with cold front that will bring cooler/less humid
  weather for middle-end of next week

&&

.DISCUSSION...
Issued at 236 PM CDT Fri Jul 12 2024

Through Saturday Night:

With an upper-level trough moving off to the east, quiet weather
is expected through the remainder of the day today. Dry air in
the mid- levels will work to limit any chance of showers or
storms this evening. Light northeasterly/easterly winds will
turn southeasterly late this evening. Temperatures in the
mid-80s this afternoon will cool into the mid-60s overnight
tonight. Warmer and more humid conditions are expected on
Saturday with high temperatures around 90 degrees and dew points
climbing into the mid 70s.

Steeper mid-level lapse rates (nearing 7 C/km) moving into the
area tomorrow morning, juxtaposed above an increasingly moist
boundary layer will result 2000-3000 J/kg of MLCAPE during the
afternoon. The arrival time of these lapse rates could kick off
some elevated convection in the morning hours tomorrow given any
kind of lifting mechanism. Although bulk shear is not expected
to be the greatest (20-25 kts), the large CAPE values could lead
to strong to severe storms. Although this scenario does seem
like a possibility, most model guidance keeps things dry through
the morning before convection initiates in the afternoon and
early evening due to persistent WAA to overcome the weak capping
that will be in place with a warm nose around 900-850 mb. Have
opted to go with slight chance (15-25%) PoPs in the morning with
chance (30-40%) PoPs in the afternoon to account for these
scenarios.

A shortwave over the Northern Plains along with steep EML lapse
rates is expected to kick off convection in Minnesota tomorrow
morning. This will congeal into a MCS, which is expected to
advance southeast towards the CWA tomorrow afternoon/evening. It
will most likely follow a track along the instability gradient
and along the edge of the EML, which looks to set up just
northeast of the area. However, a right hand turn into the area
or the instability gradient setting up across the northeastern
portion of the area looks like a distinct possibility, which
would allow the storms to propagate through the area. However,
uncertainty still remains high on how this system will move and
behave. Wherever it does pass, looks likely to see strong to
possibly severe storms given the large MLCAPE values and
marginal shear (20-25 kts). Model guidance is also all over the
place on the timing of this system with some models moving it
through the area as early as 00Z and others as later as 06Z.
This could play a role in how strong the storms are when they
get with the later scenario likely having weaker storms due to
the lack of diurnal heating. Have carried chance (30-40%) PoPs
through the overnight hours. Either WAA induced showers and
storms or a strengthening of ongoing convection also look
possible on the nose of a 35-40 kt LLJ overnight Saturday night.

Carothers

Sunday through Friday:

Tricky forecast Sunday through Monday as upper ridge over the
western U.S. attempts to build eastward toward the Mississippi
Valley, allowing hot and humid air mass to overspread the area.
The tricky part of the forecast is potential for occasional MCS
activity potentially disrupting the heat and humidity and posing
risk of locally heavy rainfall and potentially severe weather.

Guidance continues to suggest that a series of low amplitude
shortwave troughs will ride the west-northwesterly flow from the
northern Plains into the western Great Lakes. Unfortunately, the
low amplitude nature of these waves results in particularly low
predictability in guidance, especially more than a day in advance.
In addition, it is likely that these subtle waves will become
significantly augmented convectively, another process that is not
generally well handled in guidance, especially beyond 24 hours.

Assuming no disruptive MCS or MCS debris, then Sunday should be
breezy, hot, and humid with highs in the 90s and heat indices
likely near or above 100 degrees. There is some signal in
guidance suggesting a slightly better chance of convection Sunday
night, followed by another potential break during the day Monday.
Again, timing is highly subject to change given the predictability
challenges in these regimes.

Once again Monday, if lingering cloudiness or convection from
prior night`s MCS isn`t an issue, then high temps should once
again soar well into the 90s with afternoon heat indices over 100
degrees. Heat headlines could be needed for Sunday and/or Monday,
but until confidence increases in convection not interrupting
the heat, will not be issuing any headlines.

A more formidable and amplified synoptic trough is progged to dig
southeast into the upper Mississippi Valley and western Great
Lakes Monday night into Tuesday. This should help drive a cold
front southeastward across the area with additional shower and
thunderstorm chances Monday night into Tuesday. Stronger flow
aloft associated with the amplifying trough should provide for
more favorable shear profile for severe weather potential Monday
night and Tuesday, but there are still a lot of mesoscale details
yet to be worked out.

Cooler, less humid, and drier conditions are expected during the
middle and later part of next week as Canadian high pressure
settles into the region.

- Izzi

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 637 PM CDT Fri Jul 12 2024

Only forecast concern is the chance of thunderstorms Saturday
afternoon and Saturday evening.

Light easterly winds are expected to become light southeasterly
tonight and likely light and variable for many locations. Winds
will become southerly Saturday morning with speeds increasing
to 10kts. A lake breeze is expected Saturday afternoon, but is
also expected to remain near the lake, and possibly not affect
even GYY.

Convective trends are uncertain for Saturday afternoon into
Saturday evening, though chances for thunderstorms will slowly
be increasing during this time period. There are a few different
possible scenarios, none of which have any higher confidence
than the other. While isolated thunderstorms are possible as
early as mid Saturday afternoon, the most pessimistic scenario
is thunderstorms develop across southern WI Saturday evening and
move into northeast IL and continue to redevelop through early
Sunday morning. Confidence remains too low for thunder mention
with this forecast. However, some thunder mention will likely be
needed with later forecasts. cms

&&

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

&&

$$

Visit us at weather.gov/chicago