102
FXUS63 KLOT 291556
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1056 AM CDT Sat Jun 29 2024

.KEY MESSAGES...

- Potential for isolated to widely scattered thunderstorms to
  redevelop this afternoon, mainly south of the Kankakee River
  Valley, a few of which could be strong to possibly severe.

- Dangerous swimming conditions with large waves expected late
  tonight through Sunday night at Lake Michigan beaches.

- Multiple chances for showers & storms late Tuesday through
  Friday, MANY dry hours between any bouts of rain

&&

.UPDATE...
Issued at 1056 AM CDT Sat Jun 29 2024

The main concern we will be monitoring into this afternoon is the
threat for isolated to widely scattered thunderstorm development
across my far southern counties in advance of an approaching cold
front. As of this writing, this surface cold resides near the
I-55 corridor, and is expected to shift southeastward across my
east central IL and northwestern IN counties through the peak
heating hours of the afternoon into early this evening. A very
moist low-level airmass (surface dew points in the mid 70s)
resides ahead of this front, and beneath a corridor of very steep
mid- level lapse rates associated with a capping EML. Filtered
sunshine across the prefrontal airmass into early this afternoon
will allow this moist lower-level airmass to heat and destabilize
under the capping inversion.

The destabilizing airmass will certainly conditionalize the
prefrontal airmass in my south for the development of
thunderstorms this afternoon along and ahead of the approaching
cold front. However, there remains questions as to the areal
coverage of any storms owing to the rather nebulous larger scale
forcing for ascent. Accordingly, the prospects of thunderstorm
development this afternoon may ultimately be tied to what looks
to be rather modest frontal convergence. Nevertheless, this could
be enough to lift parcels to their LFC, especially given the LFC
heights are rather low thanks to the very moist low-level airmass.
With this in mind, some isolated storm development will be
possible after 2 pm this afternoon.

Effective deep layer shear (up to 40 kt) will be favorable for
any storms that develop to become organized. Accordingly, there
is a conditional threat of locally strong damaging wind gusts
with any storms in my far south this afternoon. The main threat
area for these storms will largely be well south of the Kankakee
and Illinois River Valleys. The threat of storms will quickly end
after 7 PM this evening as the cold front clears the area.

KJB

&&

.DISCUSSION...
Issued at 344 AM CDT Sat Jun 29 2024

Through Sunday:

A subtle mid-level shortwave continues to pivot overhead this
morning which has maintained some scattered showers and
occasional thunderstorms across the area. This wave will
continue to move east into northern IN by mid to late morning
allowing the coverage of showers and storms to gradually
diminish from northwest to southeast. Therefore, dry conditions
are expected for the majority of the area today with decreasing
cloud cover. South-southwesterly low-level flow ahead of a cold
front, currently extending from western WI into central IA, will
keep temperatures feeling warm and muggy with readings in the
mid to upper 80s and heat indices peaking in the low to mid-90s
this afternoon.

Additionally, these warm and muggy conditions will also
increase instability ahead of the cold front this afternoon
which may support the development of some isolated
thunderstorms. Given that the cooler mid-level temperatures are
expected to remain closer to the base of a broad trough that
will be pivoting through the upper Midwest and northern Great
Lakes, it appears the most of the area should remain capped
which should keep storm chances low. However, a shortwave
impulse is forecast to develop from the decaying convection in
west-central KS and track into central IL this afternoon and
evening. If this impulse is able to maintain itself into central
IL as expected, then it may get close enough to the southern
quarter of our forecast area and provide sufficient lift to
overcome the aforementioned cap and thus generate isolated to
widely scattered thunderstorms.

Unfortunately, guidance continues to vary on where the highest
coverage of storms with this impulse will occur so this is
somewhat of a low confidence forecast. Though the aforementioned
environment (dew points in the low to mid-70s and 2000 to 3000
J/kg of MLCAPE) is enough cause to justify the addition of a 20
to 25 percent chance for thunderstorms for areas along and south
of the Kankakee River Valley this afternoon. If storms are to
develop in this area the most favored timeframe is between 4 PM
and 7 PM, but there is a chance storms could develop as early as
3 PM if the impulse arrives sooner. Furthermore, a few of these
storms may also have the potential to be strong to severe given
the 35 to 40 kts of effective shear overhead. The main threats
with any severe storms will be gusty winds upwards of 50 to 60
mph and hail up to quarter size.

Any thunderstorms that do develop this afternoon will come to
an end this evening as the aforementioned cold front pushes
through northern IL and northwest IN. Not only will this cold
front usher in cooler and drier air, but it will also generate
breezy northerly winds overnight into the day on Sunday. As a
result, notably cooler temperatures are expected for Sunday with
readings only topping out in the low to mid-70s with upper 60s
expected along the lakeshore. Additionally, these breezy north
winds will also aid in building waves at area beaches leading to
dangerous swimming conditions for tonight through Sunday night.
Therefore, a Beach Hazard Statement has been issued for Lake
IL, Cook, Lake IN, and Porter counties for this threat.

Yack


Sunday Night through Friday:

Monday`s weather looks gorgeous before we ease back into a
somewhat more active pattern the remainder of the upcoming work
week. Monday will be sunny with comfortable humidity and high
temps in the 70s.

By Tuesday, attention will turn to a positively titled upper
trough over the northern Plains. This trough is progged to track
eastward into Ontario by Wednesday afternoon with associated
surface low likely to track eastward across southern Canada.
Cold front trailing south from this low is progged to settle
south into northern Illinois later Tuesday night, eventually
laying out east-west somewhere in the area Wednesday. Generally
speaking, timing doesn`t look terribly favorable for convection
in our area, with frontal boundary and storms looking to arrive
later Tuesday evening into the overnight hours. Given
respectable shear and stout low level jet, its possible storms
could continue well into the night despite stronger synoptic
forcing progged to remain well north of the area with the upper
trough displaced well to our north.

Uncertainty grows Wednesday with low confidence on where the
surface front, likely augmented by convective outflow/cold
pools, will end up. Better chances of storms Wednesday will
probably be across our southern CWA or even south of the area
depending on how far south the effective boundary ends up.

Another trough is progged to dig into the northern Plains
Thursday afternoon before tracking east into the western Great
Lakes Friday. This could pull the boundary back north with
shower/storm chances increasing again Thursday into Friday. Too
far out to get into specifics on timing the better chances, but
certainly can`t rule out storms posing some challenges for
outdoor activities for the July 4th holiday.

- Izzi

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 554 AM CDT Sat Jun 29 2024

- Periodic of MVFR CIGS expected through mid-late morning
- Westerly winds expected to become gusty this afternoon,
  shifting to gusty northwest this evening

Patchy MVFR CIGS will move across the terminals this morning,
before scattering out this afternoon. As skies partially clear
this afternoon, should see westerly winds begin gusting to
around 20kt. Winds may briefly ease early this evening, but then
shift to northwest and pick up by mid-late evening as a cold
front moves across the terminals. Expect to see at least
occasional gusts tonight, particularly at ORD and MDW.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from late tonight through late Sunday
     night for ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement from late tonight through late Sunday
     night for INZ001-INZ002.

LM...Small Craft Advisory from midnight tonight to 4 AM CDT Monday
     for the IL and IN nearshore waters.

&&

$$

Visit us at weather.gov/chicago