774
FXUS63 KLOT 012336
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
636 PM CDT Mon Jul 1 2024

.KEY MESSAGES...

- Dangerous swimming conditions Tuesday morning for the Lake
  County (IL) shoreline.

- Typical July warmth and humidity through the week with
  periodic chances for showers and storms Tuesday through
  Friday

- Showers and storms are expected to move across the area
  Tuesday night through Wednesday morning. Some storms could be
  strong to severe.

&&

.DISCUSSION...
Issued at 304 PM CDT Mon Jul 1 2024

Through Tuesday:

Convective debris clouds continue to filter over the region as
a surface high drifts eastward across the Great Lakes region
this afternoon. Expect partly to mostly cloudy skies tonight
combined with steady S/SSE flow to support temps in the upper
50s to low 60s. With a slight onshore flow increasing over
southern Lake Michigan tonight, dangerous swimming conditions
will develop from building waves and long-shore currents along
Lake (IL) County beaches from around sunrise through late
Tuesday morning.

While a NW to SE oriented arc of low-level isentropic ascent
will lift across northern Illinois late tonight through sunrise
Tuesday, it appears moisture will be insufficient for
saturation and realization of elevated instability from marginal
mid-level lapse rates. Have put slight chance of thunderstorms
in the forecast roughly north of a Mendota to Chicago line from
around 3-7am. Effective shear around 30 knots with low MUCAPE
suggests the primary threats will be limited to perhaps very
small hail and some sporadic gusts under 40mph.

An area of convection is expected to develop across northwest
KS and southwest NE late this afternoon, with the remnant
convective wave progged to lift ENE across southern Wisconsin
late Tuesday morning into the mid-afternoon. Marginal lapse
rates around 6-6.5 C/km, poor deep layer moisture profiles, and
stagnant if not slightly rising mid-level heights all point
toward a notable weakening trend with any residual convection as
it shifts toward eastern Iowa Tuesday morning. However,
continued low-level isentropic ascent extending southward into
far northern Illinois supports maintaining a chance of showers
and some embedded thunderstorms north of a Dixon to Evanston
line through the afternoon hours on Tuesday.

Kluber


Tuesday Night through Monday:

Convection will be ongoing across Iowa and into western Illinois
Tuesday evening as a cold front pushes east through the
Mississippi River Valley. This convection will advance towards
the CWA through the evening and into the overnight hours. Dry
air in the low levels and poor mid-level lapse rates (5-6 C/km)
will limit instability from building in our area. This will
result in the storms to be weakening as they reach the western
portion of the CWA, however some storms producing severe weather
is not out of the question given deep layer shear values of
40-45 kts. SPC has placed the far western portion of our CWA in
a level 1 out of 5 risk of severe weather for this threat.

The cold front should pass through the area sometime early
Wednesday morning with shower and storm chances dropping off
through the morning for most of the area. Wednesday afternoon is
expected to be dry for most areas though a few lingering
showers are possible. Despite the frontal passage temperatures
will still climb into the mid-to-upper 80s with dew points in
the low 70s making conditions hot and humid. Model guidance
depicts the front slowing down and eventually stalling out just
south of the area through central Illinois. A couple of the
models show the possibility of the front actually stalling out
within the southern third of our CWA. If this scenario were to
play out then showers and storms would likely continue through
the day on Wednesday, though they would primarily be confined to
areas south of I-80.

On Thursday cyclogenesis in the lee of the Rockies will latch
onto the stalled front and allow it to slowly meander back to
the north as a warm frontal boundary. Modest instability
(1000-2000 J/kg of MLCAPE) and deep layer shear (25-35 kts) will
develop behind the front, in the open warm sector of the
developing low pressure system. Thus, depending how far north
the warm frontal boundary is able to traverse, this could put
the area under the gun for another round of showers and storms
on Thursday. For now have gone with likely (50-70%) PoPs across
the southern CWA with chance (30-50%) PoPs north of I-80,
however, as previously mentioned, this is highly dependent on
the ability of the boundary to move northward.

Chance (30-40%) PoPs will once again be over the entire area on
Friday as the cold front of the low pressure system moves across
the area. A plume of modest (6-7 C/km) mid-level lapse rates
and instability (1000-1500 J/kg of MLCAPE) are expected to
precede this frontal passage, bringing the potential for
thunderstorms. However, uncertainty still remains in the timing
and progression of this front and system.

Moving into the weekend, dry conditions are expected behind the
cold front with temperatures in the low-to-mid 80s. Chances
for showers and storms return late Sunday into Monday as there
is a signal for a shortwave to move across the area, although
uncertainty is quite high in how this will play out.

Carothers

&&

.AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 636 PM CDT Mon Jul 1 2024

Forecast concerns include...

Chance for showers Tuesday morning.
Gusty south/southeast winds Tuesday.
Chance for showers/isolated thunderstorms Tuesday night.

Current scattered showers across northeast IA are expected to
move east/northeast tonight into far northern IL and southern
WI. This activity will be moving into a drier airmass and some
weakening is expected though some showers may affect northwest
IL and RFD overnight through mid morning Tuesday. Thunder
chances appear low and no mention for now. Much of this activity
will dissipate as it approaches the Chicago terminals later
Tuesday morning and confidence is too low to include mention in
this forecast. There will be another chance of showers and
possibly thunderstorms later Tuesday evening into early
Wednesday morning. The bulk of any activity that forms is
expected to remain southwest of the terminals.

East/southeast winds this evening will become more southeasterly
tonight and south/southeasterly Tuesday morning when gusts will
increase into the lower 20kt range. Winds may turn more
southerly by Tuesday evening, when gusts are expected to
diminish. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from 5 AM CDT Tuesday through Tuesday
     morning for ILZ006.

IN...None.
LM...Small Craft Advisory from 5 AM to noon CDT Tuesday for
     Winthrop Harbor to Wilmette Harbor IL.

&&

$$

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