346
FXUS63 KLOT 172341
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
641 PM CDT Sat Aug 17 2024

.KEY MESSAGES...

- Scattered showers with isolated storms through this evening.
  Additional showers possible on Sunday.

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

- Quiet weather will prevail through the end of the week.

&&

.DISCUSSION...
Issued at 255 PM CDT Sat Aug 17 2024

Through Sunday Night:

A broad closed upper low continues to pinwheel over the western
Great Lakes with multiple embedded vorts/short waves rotating
around the low center. This has resulted in scattered to at
times numerous shower development across the area today, some of
which have been able to produce occasional lightning strikes.
Storm coverage has so far remained low owing to a warm layer in
the mid-levels limiting cloud top growth. The exception to this
has been when surface convergence has provided enough lift to
get past this warm layer (namely along the lake breeze and
outflow boundaries). Ongoing showers and storms will continue
to shift southeast with time as the first wave departs, with
the exception of within the vicinity of the lake breeze where
showers will persist. In addition, a second wave is diving south
out of eastern Wisconsin that will likely bring a renewed
increase in shower/storm coverage into far northeast Illinois
late this afternoon into the early evening.

While not especially high for the time of year, PWATs in the
1.5-1.7" range will support localized downpours beneath these
showers with visibilities dropping into the 1-2 mile range
beneath the heaviest showers. Gusty winds to 30-40 mph can`t be
ruled out under any of the taller showers/storms.

Tomorrow`s precipitation chances are a bit less clear, though
there remains a signal for a convergence axis to set up over
the lake and across northwest Indiana that may meander west into
portions of Illinois during the afternoon. This axis may serve
as a focus for renewed shower development during the day. At
this time the lightning potential looks even lower than it does
today so have left thunder out of the forecast with this update.
Temperatures will be depend on where the aforementioned surface
convergence boundary ends up with upper 70s to around 80 west
of the boundary and low to mid 70s east of the boundary
(favoring near the lakeshore and across northwest Indiana).

More details are included in the long term discussion below,
but persistent onshore flow will allow for building waves
overnight tonight along the Lake Michigan shoreline. This will
result in dangerous swimming conditions due to high waves and
rip currents at southern Lake Michigan beaches on Sunday.

Petr


Monday through Saturday:

A broad surface high pressure system is expected to shift into
the area on Monday. The tight pressure gradient ahead of this
feature will bring strong northerly winds with gusts in the
25-30 mph range through the day Monday and into the overnight
hours. These winds will generate large waves (5+ feet) and
strong rip currents, creating dangerous swimming conditions at
all southern Lake Michigan beaches through at least late Monday
night. Winds will start to diminish into Tuesday morning, though
the northerly component could still result in larger waves
persisting through the day Tuesday before a shift to southerly
winds Tuesday night completely axes the potential for dangerous
swimming conditions.

The expansive area of surface high pressure will slowly meander
across the Great Lakes region through the week, leading to an
extended period of quiet weather. High temperatures are expected
to be in the mid-to-upper 70s to start the work week with long
range ensemble guidance depicting a gradual warming trend
through the week as the upper level ridge pivots east. By next
weekend high are expected to be in the upper 80s with some areas
pushing 90 degrees. Dew points are expected to hang out in the
mid 50s through the extended period bringing lower humidity
levels back to the area while skies remain mostly sunny through
the week.

Carothers

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 618 PM CDT Sat Aug 17 2024

Aviation Forecast Concerns:

- Winds expected to become briefly easterly at KORD this
  evening as combined outflow/lake breeze shifts west.

- Scattered SHRA, isolated TSRA in vicinity of Chicago terminals
  early this evening (TSRA mainly for KGYY).

- Period of MVFR ceilings late tonight into Sunday morning.

- Breezy north winds with gusts 20-25 kts Sunday, becoming
  north-northeast at KORD and KMDW in the afternoon.

- Isolated to scattered SHRA possible for Chicago terminals
  Sunday.

An upper level closed low continues to spin while drifting very
slowly eastward across lower MI early this evening. A series of
smaller weak disturbances continue to rotate through the flow
on the west side of the main circulation, helping to maintain
isolated to scattered SHRA/TSRA across the western Lakes region.
These should diminish/end this evening with the loss of diurnal
warmth and associated low-level instability. Isolated TSRA have
been most common along a couple of convergent boundaries across
the southern parts of the Chicago metro area, with KGYY the
main terminal most likely to have TSRA in the vicinity.
Otherwise, a combined lake breeze/outflow boundary lingers from
west of KUGN to east of KORD and west of KMDW at this time, with
indications (and high-res model support) that it will move west
and bring a brief period of northeast or easterly winds to KORD
at some point early this evening, before winds settle back to
light northwest later tonight.

Various model guidance remain in good agreement in developing
MVFR ceilings across the area after midnight tonight, lingering
Sunday morning before eventually lifting to VFR early in the
afternoon. With the area remaining in cyclonic flow west of the
slowly departing upper low, a few isolated to widely scattered
showers are again possible for mainly KORD, KMDW and KGYY during
the later morning and afternoon hours. Any minimal thunder
threat appears significantly lower than today. Northerly winds
are expected to become breezy after sunrise, with gusts 20-25 kt
common especially for terminals closer to Lake Michigan. Winds
will likely shift north-northeast at KORD and KMDW during the
afternoon.

Ratzer

&&

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

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

LM...Small Craft Advisory from 7 AM Sunday to 4 AM CDT Tuesday for
     the IL and IN nearshore waters.

&&

$$

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