583
FXUS63 KLOT 102039
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
339 PM CDT Wed Jul 10 2024

.KEY MESSAGES...

- Scattered non-severe thunderstorms in the area this afternoon
  and again tomorrow afternoon.

- Hot and humid conditions expected early next week. Heat
  indices may 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 339 PM CDT Wed Jul 10 2024

Through Thursday Night:

Convection has sprouted this afternoon along a surface
convergence axis extending from near Freeport, IL to near
Danville, IL. Convective coverage should continue to increase
heading into this evening as large-scale forcing for ascent
increases with the approach of a mid-level disturbance currently
twirling its way across southern Minnesota and into Iowa, with
outflow boundaries also helping to kick-start new convection. A
consistent signal in recent runs of both the HRRR and WoFS
lends relatively high confidence in the convective footprint
remaining maximized generally near and west of a Rockford, IL to
Watseka, IL line, but radar and satellite observations suggest
that at least isolated showers and possibly storms remain
probable through this evening farther to the east.

Mediocre mid-level lapse rates and deep layer shear should
prevent any of today`s storms from becoming severe, and a warm
nose near 600 mb will also likely play a role in curtailing
overall thunderstorm coverage and longevity, particularly east
of the aforementioned line. That being said, the strongest
convective cores can still be expected to produce locally
torrential rainfall, small hail, and gusty winds (possibly up to
around 45-50 mph). With the relatively slow storm motions and
presence of multiple surface boundaries, wouldn`t be entirely
surprised if any favorable cell-boundary interactions led to an
isolated funnel cloud or two being spotted somewhere -- though
any actual touch-downs occurring would be highly unlikely.

The same air mass will remain in place into tomorrow, and thus
similar conditions to today can be expected. High temperatures
will likely end up being in the low 80s/near 80F at most
locations, though onshore flow should cause locations closer to
the lakeshore to remain in the 70s. The aforementioned
Minnesota/Iowa disturbance will likely be near or directly over
our forecast area come tomorrow, and with similarly moist near-
surface conditions remaining in place, isolated to scattered
diurnal convection thus is likely to blossom once again. Current
thinking is that convective coverage tomorrow may not be quite
as widespread as it should be today, and it appears that it may
end up focusing near and south of I-80, but the QPF spread in
the latest suite of forecast guidance warrants carrying at least
a slight chance PoP mention across our forecast area.

Once again, lackluster mid-level lapse rates and deep layer
shear should curtail a threat for severe weather, but we`ll have
to keep an eye on the potential for localized minor flooding
with the expectation that any storms that develop will be
slow-moving, efficient rainfall producers. The likelihood of
any hydrologic concerns arising would be greatest wherever the
rainfall amounts from today and yesterday were highest. Also
couldn`t rule out some additional funnel cloud funny business
given the likelihood of a mid-level vorticity maximum gyrating
directly over our CWA.

Ogorek


Friday through Wednesday:

Friday will be primarily quiet and seasonably warm (mid 80s
inland), except cooler (mid-upper 70s) near the lake. Following
the exit of mid-upper level low pressure into the central and
eastern Great Lakes, very dry air aloft and persistent mid-
level height rises of 4 DaM/12 hour should effectively cap most
of if not all diurnal convective development. With lingering
higher dew points south of I-80 (upper 60s to around 70F),
erosion of MLCINh and subtle convergence may be enough for
widely isolated showers and maybe a thunderstorm, if any
updrafts can survive pronounced dry air entrainment. Surface
high pressure will remain over the area through Friday night,
making for a quiet and seasonably mild 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, probably
in a weakening state. This will occur in concert with a warm
front lifting north and a large instability reservoir advancing
eastward across the mid and upper MS Valley. Large scale forcing
mechanisms will be lacking through the afternoon, though any
convectively induced impulses, and convergence tied to lingering
outflow and the northward advancing warm front may be enough for
widely scattered afternoon pulse-type thunderstorms (30-40% PoPs).
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 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).

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, prior to stronger deep
layer shear being 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 possibly mid 90s with dew points in the 70s yielding
peak heat indices flirting with heat advisory (105+F) criteria.

Tuesday afternoon will be similar thermally to Sunday and
Monday, though perhaps less capped with a bit better chance of
widely scattered PM thunderstorms as a weak cold front advances
south. It`s unclear whether the appreciable instability axis
will be shunted far enough south by Wednesday to limit another
round of convection, especially south of I-80, as the cold front
takes on back door characteristics. What is of higher confidence
is an end to the hot and humid conditions, with forecast high
temps on Wednesday in the low-mid 80s inland and upper 70s near
Lake Michigan.

Castro

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1208 PM CDT Wed Jul 10 2024

Aviation weather concerns are:

- Lake breeze with a NE wind shift this afternoon

- Scattered showers and storms developing this afternoon. Main
  TS potential INVOF RFD.

- Some lingering showers/TS possible through the evening, but
  diminishing in coverage.

- Spotty MVFR/IFR cigs and MVFR BR overnight.

Winds are varying between NW and NE at the Chicago-area
terminals, and will become predominantly northeasterly through
20-22z with the arrival of a lake breeze. Scattered showers and
some thunderstorms will develop through the afternoon, although
the greatest coverage is expected generally west of the c90
TRACON. Have introduced a VCTS at RFD to account for this
potential. It`s possible that showers develop on the lake
breeze, so have continued the VCSH mention for the Chicago-area
sites into the early evening.

Activity may be slow to diminish this evening with a disturbance
pivoting overhead, but a gradual diminishing trend and loss of
lightning is expected.

With a moist airmass in place, some MVFR/VFR BR is possible
along with spotty MVFR/IFR cigs. Latest thinking is that
MVFR/IFR conditions may remain too spotty to justify formal
mentions in the TAF at this time given lingering mid and high-
level cloud cover through the night. Guidance and observation
trends will be monitored, however.

Light/variable winds overnight will trend NE to southeasterly on
Thursday morning.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Beach Hazards Statement until 10 PM CDT this evening for
     INZ001-INZ002.

LM...Small Craft Advisory until 10 PM CDT this evening for the IN
     nearshore waters.

&&

$$

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