250
FXUS63 KLOT 112014
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
314 PM CDT Thu Jul 11 2024

.KEY MESSAGES...

- Isolated to scattered showers/storms this afternoon and early
  evening, with the greatest coverage south of I-80. Gusty winds
  and locally heavy rainfall the main threats.

- Patchy fog development possible south of I-80 tonight.

- Hot and humid conditions expected early next week. Heat
  indices will likely approach or exceed 100 degrees.

- Periodic thunderstorm chances Saturday through mid next week,
  including the potential for a couple nocturnal/early AM
  t-storm complexes.

&&

.DISCUSSION...
Issued at 223 PM CDT Thu Jul 11 2024

Through Friday:

Isolated to scattered showers and thunderstorms will develop this
afternoon in the presence of broad upper troughing interacting
with several surface boundaries. The main area is expected to
focus generally south of I-80 in the vicinity of a diffuse
quasistationary boundary, along which dewpoints have pooled in the
mid and upper 60s. Instability looks like it wanes with northward
extent owing to fairly extensive morning cloud cover and shower
activity, and it seems like some suppressive effects from ongoing
subsidence may also curtail additional thunderstorm activity, at
least on a widespread basis. North of the Wisconsin state line,
instability increases once more where mid-level temperatures are a
bit colder, so suspect that additional widely scattered activity
will percolate this afternoon. Slow southwesterly cell motions
could bring some of this activity into our far northwest locales
later this afternoon. Severe weather is not expected, but gusty
winds and brief torrential downpours will be possible with any
activity. Slow cell motions (under about 10 mph) may lead to very
localized pockets of minor flooding.

Otherwise, activity will slowly diminish with time this evening
although we may not entirely get rid of isolated, festering
showers as broad ascent on the southern flanks of a well-defined
vort max and modest divergence along the cyclonic shear side of an
upper jet linger in the region. Patchy fog development is possible
as well, with soundings looking most supportive south of I-80
where low-level flow looks weakest amidst slightly deeper near-
surface moisture.

Generally quiet conditions are expected on Friday as subsidence on
the backside of a departing upper wave/trough spreads overhead.
Can`t totally rule out a few showers developing with heating near
the lake breeze, but soundings look pretty dry aloft with some
notable 500-600 mb warming which should further act to suppress
afternoon activity.

Carlaw

Friday Night through Thursday:

Following a quiet and seasonably mild Friday night, the first
potential episode of convection in the humid and at times stormy
pattern expected this weekend through early next week may
arrive Saturday morning in parts of the area. This will occur
in concert with a warm front lifting north, and a large
instability reservoir along with a mid-level impulse advancing
eastward across the mid and upper MS Valley. With very dry mid-
level air in place early Saturday being shunted eastward with
the arrival of the MUCAPE reservoir, have chance (30%) PoPs for
elevated convection in the western 1/2 or southwest 2/3 of the
CWA, tapering quickly to the east and northeast due to the west
to east instability gradient.

Large scale forcing mechanisms will be lacking through the
afternoon following the dissipation of morning elevated
convection, with a warming EML base increasing capping. For
these reasons, the afternoon may end up with isolated
thunderstorms (slightly better chance south of I-80?) at most
with a potential for mainly dry conditions. Highs on Saturday
will be in the mid to upper 80s, except slightly cooler lower
80s along the Illinois shore due to lake influence backing winds
east- southeasterly. Dew points in the upper 60s to lower 70s
at peak heating will yield peak heat indices in the low-mid 90s,
a harbinger of things to come Sunday through Tuesday.

Expansive mid-upper 500s DAM 500 mb ridging centered over the
inter-mountain west and mean troughing centered over Hudson Bay
will set up a hot and humid northwest flow aloft locally in the
Saturday night-Tuesday morning timeframe. Unlike previous
occurrences this warm season of overnight arriving convection
weakening quickly as instability diminishes, the impressive
instability reservoir (3000-4000+ J/kg of MUCAPE) will largely
remain in place through the period. This entails a period of
"ring of fire" conditions, where our position on the northeast
flank of the ridge puts us in the threat zone for southeastward
moving convection, including potentially well-organized severe
MCSs, and episodes of flash flooding due to PWATs up in the
~1.75-2" range at times (150-200% of normal). Also concerning
regarding flooding potential is a west-southwesterly low-level
jet feeding into southeastward moving convection, which is a
pattern that can favor backbuilding and training.

Aside from operational and ensemble members hinting at the
likelihood of this type of activity, specific timing
predictability in these types of patterns are commonly low
confidence. This maxes out PoPs only in the chance (30-40%
range) during windows of MCS favorability at this lead time.
The first window may be Saturday night, with moderate deep layer
shear up to 30-35 kt present, followed by Sunday night into
early Monday when 40+ kt of deep layer bulk shear is forecast.
Monday night into early Tuesday could end up being another
window for parts of the area as well.

Classic in these type of patterns, there will be many dry
hours, and a relative daytime lull/capping Sunday and Monday PM
will support highs in the lower to mid 90s with dew points in
the 70s yielding peak heat indices flirting with or exceeding
heat advisory (105+F) criteria. Maturing cropland will likely
contribute an evapotranspiration component to the dew point
temperatures on Sunday and Monday, so wouldn`t be surprised to
see upper 70s (locally higher) Td realized at observation sites
in farming areas.

While Tuesday will again be humid (upper 60s-mid 70s Td),
overnight-early Tuesday convection may shunt the effective
boundary south and force the zone of any afternoon thunderstorm
development primarily south of I-80. Temperatures will be a
notch below Sunday and Monday, in the upper 80s to lower 90s,
yielding heat indices in the 90s to around 100F, highest
southern CWA. The last few model cycles have pointed toward the
instability axis being shunted well south by a true cold front
passage Tuesday night into Wednesday to bring a cooler, less
humid, and mainly dry day on Wednesday, with these conditions
continuing into Thursday.

Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1254 PM CDT Thu Jul 11 2024

There are no significant aviation weather concerns for the
current TAF period.

A couple of spotty showers could redevelop near RFD this
afternoon, but confidence in this occurring was too low at TAF
issuance to warrant having a formal shower mention of some sort
in the RFD TAF. A stray shower or two near GYY also couldn`t be
ruled out through tomorrow morning, but confidence in that is
similarly low.

Additionally, patchy fog may develop in the area overnight, but
the likelihood of notable visibility reductions occurring at
any of our TAF sites is low. Otherwise, expect VFR conditions
and light winds under 10 kts out of a predominantly easterly to
northeasterly direction throughout the TAF period.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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