265
FXUS63 KLOT 291950
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
250 PM 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 Tuesday through Friday,
  many dry hours between any bouts of rain

&&

.DISCUSSION...
Issued at 250 PM CDT Sat Jun 29 2024

Through Sunday night:

A cold front continues to shift southeastward across my east
central IL and northwestern IN counties this afternoon. The threat
of isolated thunderstorms will continue with this front in my
southeast through early evening (~7 pm). Conditions will then cool
off tonight as breeze northwesterly winds usher in a much cooler
and less humid airmass for the second half of the weekend. Expect
temperatures to bottom out late tonight in the mid to upper 50s
outside of the urban areas.

While some lower clouds may move in off the lake early Sunday
morning, conditions are expected to turn mostly sunny for the
afternoon. This will set up a very good weather day for the region
through the afternoon with seasonably cool inland temperatures
into the low to mid 70s expected. Slightly cooler conditions can
be expected near the lake. The only real weather related hazard
for Sunday will be the gusty northerly winds and the associated
large waves (4 to 8`) expected to develop tonight on Lake
Michigan. These high waves will create dangerous (potentially life
threatening) swimming conditions at area Lake Michigan beaches
through Sunday night.

KJB


Monday through Saturday:

July will start on an atypically comfortable note on Monday with
highs in the 70s and dew points in the 40s to low 50s due to
expansive high pressure over the Great Lakes. Onshore winds
will keep lakeshore locations a bit cooler than inland.
As the surface high pressure exits on Monday night, increasing
cloud cover and warm air advection (WAA) will result in lows
about 10 degrees warmer than those of Sunday night.

Large scale forcing (a positively titled upper trough over the
northern Plains) will interact with an MUCAPE plume across the
upper MS Valley and points west to result in elevated convection
developing Monday night. Any convection that outpaces the
instability axis may push into far northwestern portions of the
CWA in a weakening state toward dawn on Tuesday. Into Tuesday
morning, the instability axis may make enough eastward progress
for scattered showers and isolated to scattered thunderstorms to
ooze southeast into areas north of I-80 in Illinois. The
highest chance of thunderstorms is favored to be over roughly
the northwest 1/3 or so of the CWA.

Following the likely dissipation of any lingering Tuesday AM
convection as capping increases, a warm front will lift north
through Tuesday PM. This will result in a return to typical
July warmth (mid-upper 80s away from IL north shore) and dew
points back up into the 60s, along with breezy southerly winds
gusting up to 25-30 mph. Any widely scattered afternoon showers
or thunderstorms able to develop would probably be across the
far northern CWA.

The earlier mentioned upper trough will track eastward into
Ontario by Wednesday afternoon, with its associated surface low
likely to track eastward across southern Canada. Cold front
trailing south from the low pressure is forecast to push
southeastward across northern Illinois Tuesday night. The
timing still doesn`t look favorable diurnally speaking for
robust convection in our area, with the cold frontal boundary
and showers/storms looking to arrive late Tuesday evening into
the overnight. However, given respectable deep layer bulk shear
and a stout low level jet, it`s possible scattered thunderstorms
could persist well into the night despite the stronger synoptic
forcing remaining well north the area.

Tuesday night`s cold front will likely lay out east-west
somewhere in the area on Wednesday, perhaps favoring near our
southern CWA. If convective outflow and cold pool development
augment the effective frontal position far enough south, current
forecast PoPs up to the I-88 corridor or so will likely be too
far north. Conceptually, the better chances of storms Wednesday
will probably be across our southern CWA or even south of the
area. Outside of the uncertain chances for any showers and
thunderstorms, Wednesday will feature highs in the 85-90F range
with afternoon dew points in the upper 60s to low- mid 70s
(highest south).

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. It
remains too far out to get into specifics on timing the better
chances for convection, but certainly can`t rule out storms
posing some challenges for outdoor activities for the July 4th
holiday. Even if part of the 4th is affected by rain/storms,
hopefully a favorable window opens up for evening fireworks
displays.

The system cold front should sweep across the area Thursday
night into Friday, potentially placing better chances of any PM
thunderstorms primarily southeast of I-55. A drier and somewhat
cooler air mass will then push into the area for the start of
the first weekend of July.

Castro/Izzi

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1214 PM CDT Sat Jun 29 2024

Broken MVFR CIGs have been persisting through the morning at all
terminals. These CIGs have scattered out at KRFD and will do the
same at KORD, KMDW, and KDPA over the next hour or so. They may
hang on just a smidge longer at KGYY, but shouldn`t persist to
far into the afternoon. Winds have turned westerly and will
continue to veer to northwesterly this afternoon while also
becoming gusty due to the tightening pressure gradient. The
gusty winds will continue through the overnight hours and
through the majority of the day tomorrow, turning northerly.

Tomorrow some low clouds over the lake will push slightly inland
likely bringing MVFR CIGs back to KGYY. These low CIGs should
stay confined along the lakeshore, keeping the other four
terminals in VFR conditions. However, there is a chance that
these clouds may drift a little further inland, which would
bring KORD, KMDW, and KDPA into MVFR conditions as well.

Carothers

&&

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

IN...Beach Hazards Statement from midnight CDT 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.

&&

$$

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