473
FXUS63 KLOT 171549
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1049 AM CDT Sat Aug 17 2024

.KEY MESSAGES...

- Scattered showers today and tomorrow with a 20-30 percent
  chance of thunderstorms this afternoon into early evening.

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

&&

.UPDATE...
Issued at 1049 AM CDT Sat Aug 17 2024

Overall the forecast remains in good shape this morning. A broad
closed upper low continues to pinwheel over the western Great
Lakes today with multiple embedded vorts/short waves rotating
around the low center and move across the area. The first of
which has brought an increase in shower coverage already this
morning from the IL-WI stateline to the I-80 corridor. These
will continue to drift east southeast with time through early
afternoon. A brief lull in activity across far northeast
Illinois is possible before the next wave moves into the area
which could result in a renewed increase in shower coverage
through early evening. Did opt to trend the thunder potential
down a bit. Forecast soundings show a warm layer in the mid-
levels beneath the freezing level which could limit cloud depths
through early-mid afternoon and hence reduce lightning
potential, at least initially. There are signs that this layer
could weaken toward late afternoon early evening as the next
vort pivots into the area and could support a window of better
thunder potential across northeast Illinois (30%) prior to
sunset.

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.

Petr

&&

.DISCUSSION...
Issued at 321 AM CDT Sat Aug 17 2024

Through Sunday:

The upper-level low continues to slowly progress eastward
across the northern Great Lakes this morning with a pair of
shortwave disturbances pivoting across WI. As of this writing,
the shortwaves are not generating much more than widespread
cloud cover across WI which will gradually spill into northern
IL and northwest IN through the morning. As diurnal heating
commences after sunrise instability is expected to increase with
most guidance in good agreement on at least 500 to 700 J/kg of
MLCAPE developing this afternoon despite the cloud cover. This
instability in combination with the broad forcing from the
aforementioned waves passing overhead will be sufficient to kick
off some scattered showers and isolated thunderstorms, mainly
during the afternoon and early evening hours. Otherwise, expect
mostly cloudy skies and seasonably cool temperatures in the
upper 70s to around 80 degrees.

Heading into tonight, showers and any thunderstorms should
taper shortly after sunset as the diurnal instability wanes.
However, guidance has began to latch onto the idea that some
lake effect showers (and perhaps a storm or two) may persist
through the night. While instability profiles are rather modest,
ample moisture will be in place along with lingering ascent
from the upper low. Therefore, have decided to introduce some
20-30% POPs into the forecast for tonight, primarily for Lake,
IL and Porter counties, where this potential seems to be
maximized. Elsewhere, dry conditions are expected for tonight
with overnight lows dipping into the lower to mid-60s areawide.

The upper low will continue to meander across Lower Michigan
and into the eastern Great Lakes on Sunday as a surface high
begins to build into the Midwest. While the potential for
another round of scattered showers is possible Sunday afternoon,
the drying mid-levels should limit the longevity and coverage
of showers to mainly areas near Lake Michigan. Aside form the
linger rain chance, more sunshine is expected for our Sunday
with high temperatures remaining in the upper 70s to around 80.

Additionally, the approaching high will also increase winds,
especially over Lake Michigan, as the pressure gradient
tightens. These breezy northerly winds (gusts of 20 to 25 mph)
will act to quickly build waves at area beaches throughout the
day on Sunday resulting in a high risk for rip currents and
dangerous swimming conditions. Given that the signal for these
elevated winds and waves has been very persistent in guidance
for several forecast cycles, have decided to hoist a Beach
Hazard Statement with this forecast package for Lake IL, Cook,
Lake IN, and Porter counties beginning Sunday morning through at
least Monday night. Thus, any beach goers this weekend should
remain out of the water and obey any beach restrictions.

Yack

Sunday Night through Friday:

An expansive surface ridge of high pressure extending from the
Midwest northward across Ontario early next week is expected to
foster a persistent period of enhanced northerly winds down the
length of Lake Michigan Sunday through late Monday. The net
result of this will be the development of dangerous, to
potentially life threatening swimming and boating conditions
along the southern shores of Lake Michigan on Sunday as waves
build in excess of 5 feet. Expect these conditions to persist
through at least Monday night before abating winds and waves
result in gradually improving conditions into Tuesday.

Outside of the dangerous swimming and boating conditions on
Lake Michigan through Monday night, a few day stretch of quiet
weather is likely next week. This as our area ends up locked
along the eastern periphery (western periphery) of an upper-
level ridge (trough) axis centered across the Plains (eastern
CONUS). With this pattern expected to favor northerly lower-
level flow and large scale subsidence our area through mid-week,
seasonably cool weather conditions (highs 70s, lows in the 50s)
can be expected through midweek.

Later next week, there are signs that the upper-level ridge axis
over the Plains will begin to migrate eastward towards the western
Great Lakes. As this occurs, it appears temperatures are expected
to be on a warming trend (readings back into the 80s again) late
in the period. Ensemble forecast guidance even hints that
conditions could turn downright hot (temps in the 90s) in our
area for the final week of August under this upper ridge.

KJB

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 624 AM CDT Sat Aug 17 2024

Aviation Forecast Concerns:

- MVFR ceilings this morning.

- Isolated to widely scattered showers and thunderstorms again
  this afternoon into the early evening, especially for Chicago
  metro terminals.

- Low-confidence potential for winds to shift northeast for
  Chicago terminals later this afternoon/evening.

A deck of MVFR CIGs have developed across the area. Expect these
CIGs to persist for a few hours this morning, before diurnal
improvement gradually pushes bases back into the VFR category
sometime around midday.

As we head into this afternoon, an upper level low will
continue to drift slowly east-southeast across the western
Great Lakes. As it does, a couple of smaller mid-level
impulses wrapping around its western periphery will be steered
right across northeastern IL this afternoon. These should each
help foster at least isolated to widely scattered diurnal
showers and thunderstorms this afternoon, especially across
eastern WI/northeast IL including the Chicago terminals. While
TSRA coverage may not be very high, forecast soundings do
indicate enough instability during the afternoon and early
evening hours. For this reason, we opted to convert the going
PROB30 mention of TSRA to a tempo group from 21 to 00z this
afternoon.

Westerly winds this morning are expected to become northwest to
north-northwest during the day as the surface low gradually
progresses eastward into towards Lake Huron. A couple of models
suggest a shift to northeast is possible for KORD/KMDW/KGYY
later this afternoon as a lake breeze tries to develop and push
inland. However, confidence in this remains very low at this
distance. It appears the ability of this wind shift to make it
all the way inland to ORD and MDW may ultimately become
dependent upon the ability for any outflow from this afternoons
convection to drive the lake breeze boundary inland. Given the
low confidence with this wind shift at this stage, we have opted
to leave a formal mention out of the TAF at this time.

KJB

&&

.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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