969
FXUS63 KLOT 171956
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
256 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 /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 110 PM CDT Sat Aug 17 2024

Main Concerns:

- Scattered SHRA and isolated TS through early evening with
  lower VSBY in heavier SHRA and any storms

- Possible wind shift to northeast near the lake late this
  afternoon

- Lower CIGs tonight into Sunday

- Gusty northerly winds on Sunday, isolated SHRA possible

Scattered small footprint SHRA will continue to pinwheel across
the area through this evening, with isolated TS at times. Aside
from the activity near GYY as of this writing, it appears the
next best chance for TS over the Chicago metro will be late this
afternoon as another stronger disturbance moves overhead.
Though confidence is on the lower side, opted for a VCTS mention
with this next window and maintained a short TEMPO TSRA until
sunset. Some model guidance continues to depict a wind shift
near the lake this afternoon and tied this potential to the
TEMPO group. Will continue to monitor observational trends.
Winds will otherwise vary from west to northwest with occasional
gusts near SHRA.

Lower CIGs will redevelop tonight, likely later than
explicitly mentioned in the TAF. Can`t rule out patchy MVFR
CIGs. MVFR CIGs should eventually erode on Sunday. Additional
isolated SHRA are possible, with no mention in this issuance.
Gusty northerly winds, strongest near the lake (20-25 kt gusts)
will develop by mid to late Sunday morning and continue through
the afternoon.

Castro

&&

.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 10 AM Sunday to 4 AM CDT Tuesday for
     the IL and IN nearshore waters.

&&

$$

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