259
FXUS63 KLOT 020603
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
103 AM CDT Tue Jul 2 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 /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 103 AM CDT Tue Jul 2 2024

Forecast concerns for the terminals...

- Periods of showers tonight through Tuesday morning, especially
  near RFD

- Breezy south-southeast winds Tuesday afternoon

- Another round of showers and thunderstorms Tuesday evening and
  overnight with a cold front


Showers continue to develop ahead of an upper-level disturbance
that is pivoting into western Iowa. However, the copious amounts
of dry air in the sub-cloud layer are preventing many of the
showers over northern Illinois from reaching the surface. The
expectation is for this dry air to gradually erode across
northern Illinois as the disturbance gets closer around daybreak
which should allow a better coverage of showers to be observed
through Tuesday morning, especially for areas in and around
RFD. While some guidance continues to suggest that the shower
coverage should make it into ORD, MDW, and DPA; confidence on
this occurring remains low due to aforementioned dry low-levels
and lack of more defined forcing with southward extent. Though,
there was at least enough of a signal in guidance to justify the
inclusion of a VCSH mention at ORD, MDW, and DPA for a few hours
Tuesday morning.

Any showers that do materialize Tuesday morning are expected to
diminish by 18z at the latest leaving dry and VFR conditions for
the rest of the afternoon. However, winds will become more
south-southeasterly and increase in speed with gusts in the
20-25 kt range expected through the afternoon. Gusts will
subside Tuesday evening as directions become due south and
eventually southwesterly ahead of a cold front.

Finally, another round of showers and thunderstorms is expected
to develop along the cold front Tuesday evening and persist
through the overnight hours. While guidance continues to vary
on the magnitude and southward extent of the better instability,
confidence is sufficient to introduce PROB30s for thunder at
RFD, ORD, and MDW. Note DPA and GYY do not have any mention for
this second round due to it being beyond their respective TAF
periods, but will likely need to add with next issuance.
Regardless of the extent of thunder, light to moderate showers
are expected at the terminals into Wednesday morning as the
front moves through. Additionally, there is also the potential
for some MVFR ceilings (and visibilities) with the
showers/storms Tuesday night.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from 5 AM early this morning to noon
     CDT today for ILZ006.

IN...None.
LM...Small Craft Advisory from 5 AM early this morning to noon CDT
     today for Winthrop Harbor to Wilmette Harbor IL.

&&

$$

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