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407
FXUS63 KLOT 271641
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1141 AM CDT Thu Jun 27 2024

.KEY MESSAGES...

- Today will be beautiful with highs in the low to mid 70s, low
  humidity levels, periods of sunshine, and a northeasterly
  breeze. Cooler conditions can be expected along the lakeshore.

- Dangerous swimming conditions will prevail at all Lake Michigan
  beaches throughout the day.

- Thunderstorms are expected Friday night into Saturday, with a
  conditional severe weather threat areawide late Friday night
  and generally southeast of I-55 Saturday afternoon.

&&

.UPDATE...
-- Changed Discussion --
Issued at 1137 AM CDT Thu Jun 27 2024

High pressure lingering over the lower Great Lakes should
continue to create dry conditions today over northeastern
Illinois through the afternoon. High clouds should increase this
afternoon as warm air advection increases over the western Corn
Belt.

NWS Chicago
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&&

.DISCUSSION...
Issued at 253 AM CDT Thu Jun 27 2024

Through Friday:

A surface high pressure system will continue slowly meandering
through the Great Lakes and deliver a spectacular late June day.
Mostly sunny skies, low humidity levels, a northeasterly
breeze, and seasonably cool highs in the mid to upper 70s will
prevail. Closer to the lakeshore, onshore flow will hold highs
in the mid to upper 60s.

The only forecast element today worthy of additional
explanation today is waves at lakeshore beaches. This afternoon,
the center of the surface high pressure system is expected to
be located in Lower Michigan. Meanwhile, the difference between
the marine and land airmasses will grow to around 10 or 15
degrees. As a result, the low-level pressure gradient along the
southern tip of Lake Michigan should become locally packed and
support a continuous feed of sustained 15 kt winds. Waves thus
appear poised to remain between 3 and 4 feet through much of the
afternoon across the southwestern section of Lake Michigan. For
this reason, felt it would be prudent to extend the Beach
Hazards Statement through 10 pm tonight from northern Cook to
Porter counties.

Tonight, winds will turn southwesterly in response to the
development of and eastward movement of aggregate lee pressure
troughing in the Plains. Clouds will thicken as upper-level vort
maxes approach from the west. Overnight lows should range from
the upper 50s to lower 60s (which would make for the coolest
nighttime temperatures since around June 14th or 15th).

Tomorrow, several upper-level shortwaves will pass through the
Great Lakes ahead of a more substantial upper-level trough
propagating through the northern US. With forecast soundings
depicting a deep (10kft+) layer of dry air through the morning
hours, any approaching radar echoes should be largely virga with
perhaps a 10-20% for a few sprinkles making it down from time
to time. By early afternoon, top- down saturation should allow
for occasional scattered showers. With that in mind, tomorrow
afternoon does not look like a washout. In fact some areas may
remain completely dry. Meager mid-level lapse rates will
preclude a mention of thunder in our forecast through at least
Friday evening.

Borchardt

Friday Night through Wednesday:

Strong low-level moisture transport below a plume of modest
mid- level lapse rates across the mid and Upper-Mississippi
River Valley will advect toward the area Friday night. Low-level
WAA/isentropic ascent and strong effective shear suggests a
threat for a few strong to severe elevated storms with large
hail late Friday night through daybreak Saturday. A cold front
approaching on Saturday will gradually end thunderstorm chances
from northwest to southeast through the day. Impressive mid-
level flow for late June will yield a kinematic environment
favorable for discrete supercell structures with large hail
congealing into multicell clusters with damaging wind across
primarily the southeast half of the CWA Saturday afternoon. This
scenario will be conditional upon the absence of any upstream
convective debris Friday night as well as any further slowing of
the frontal speed. Finally, 2"+ PWATs advecting into the area
Friday night into Saturday will support a localized flash flood
risk, though recent forecast trends showing a more progressive
pattern during this window have limited a more expansive flood
risk as this time.

A mid-level ridge and broad surface high will drift eastward
across the Great Lakes region Sunday and Monday, promoting dry
and seasonably cool conditions under mostly clear skies.

Modest mid-level lapse rates and higher low-level moisture will
advect toward the mid-Mississippi River Valley on Tuesday ahead
of a couple fast-moving mid-level waves within the western
flank of the departing ridge. While current expectations are
that forcing will out-run a more favorable low-level moisture
environment Tuesday afternoon and evening, a severe thunderstorm
risk exists into at least the northwest half of the CWA if low-
level moisture arrives earlier than currently forecast. A
trailing cold front may then stall somewhere between the
forecast area and Ohio River Wednesday into Independence Day,
supporting generally chance PoPs for showers and storms in this
period.

Kluber

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1208 AM CDT Thu Jun 27 2024

VFR conditions are expected through the period. NE winds 10-15
knots through early evening will gradually veer SSE around 10
knots or less tonight.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
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IL...Beach Hazards Statement until 4 PM CDT this afternoon for
     ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement until 4 PM CDT this afternoon for
     INZ001-INZ002.

LM...Small Craft Advisory until 3 PM CDT this afternoon for the IL
     and IN nearshore waters.
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&&

$$

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