514
FXUS63 KLOT 161952
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
252 PM CDT Fri Aug 16 2024

.KEY MESSAGES...

- Scattered thunderstorms north of I-80 mainly this evening, a
  few of which may be strong to severe (wind and hail threat).

- Scattered showers and isolated to widely scattered t-storms on
  Saturday, with isolated to widely scattered showers, and
  perhaps a few isolated t-storms on Sunday.

- Long duration of dangerous swimming conditions and hazardous
  boating conditions on southern Lake Michigan Sunday-Monday.

&&

.DISCUSSION...
Issued at 252 PM CDT Fri Aug 16 2024

Through Saturday Night:

To be issued shortly.

Sunday through Friday:

The mid-upper level low pressure that will bring diurnally
driven showers and some storms on Saturday will shift into the
eastern Great Lakes on Sunday. While large scale forcing will be
steadily diminishing (mid-level height rises), weak embedded
impulses on the west side of the upper low, lingering mid-level
moisture, and modest destabilization inland of an advancing
lake shadow may yield the development of isolated to widely
scattered showers (highest PoPs 20-40%).

Shower activity should favor inland areas east of I-39 in the
early to mid afternoon. Mid-level moisture will likely quickly
dry out by the late afternoon, causing lingering showers to
dissipate. Regarding thunder chances, generally poor mid-level
lapse rates (5.5C/km or less) are forecast, so with diminishing
forcing and warmer air aloft, expectation is a few isolated
thunderstorms (~20%/slight chance) at most inland. Notably, the
instantaneous lightning flash density product from the 12z ECMWF
kept almost our entire area devoid of any implied lightning.

The slow movement eastward of the weak surface low pressure
associated with the upper low, seasonably strong expansive high
pressure building in from the north and northwest, and relatively
cool air over the warm lake (modest lake induced instability)
will result in an extended period of full fetch brisk northerly
winds Sunday through Monday. Expecting to need a Beach Hazards
Statement and Small Craft Advisory for this period barring a
significant forecast change.

A psuedo-blocky pattern will develop next week, keeping us in
dry northerly mid-level flow aloft between strong ridging over
the southwest and pronounced troughing over northeastern North
America. This will bring dry weather, comfortable temperatures
(slightly cooler by day lakeside), and low humidity through the
work week. Late summer heat may finally make inroads into the
region during the last work week of August.

Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1242 PM CDT Fri Aug 16 2024

Aviation Forecast Concerns:

- Widely scattered SHRA and possible TS early this evening.

- MVFR ceilings early tomorrow morning.

- Another round of widely scattered SHRA tomorrow afternoon.

VFR conditions with west-southwest winds gusting to around 20 knots
will develop and remain into the early evening hours before
diminishing and becoming more westerly. A mid-level disturbance will
move over the area from the northwest early this evening. This may
produce a round of widely scattered TSRA early this evening. Low
confidence in coverage leads to a VCTS mention. Slightly better
coverage north of the Illinois/Wisconsin border.

Guidance continues to show MVFR ceilings moving in from the
northwest that will enter early tomorrow morning. Ceilings should
lift to VFR by mid-morning. Another disturbance around the periphery
of the upper level low over Michigan will bring more widely
scattered showers to the area by tomorrow afternoon with a
possibility of a thunderstorm.

Kraujalis

&&

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

&&

$$

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