273
FXUS63 KLOT 110820
AFDLOT

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

.KEY MESSAGES...

- Scattered non-severe thunderstorms again today, primarily
  south of I-80.

- 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 320 AM CDT Thu Jul 11 2024

Through Friday Night:

A mid-level trough axis centered across eastern IA early this
morning, is expected to gradually slide eastward over northern IL
through the day. As it does so, several embedded smaller scale
impulses are expected to pinwheel into the area through the day.
Each of these impulses is likely to foster at least some isolated
to widely scattered showers and thunderstorms today, particularly
to the south of I-80, in closer proximately to where a surface
boundary resides. Similar to yesterday, mid-level lapse rates
and deep layer shear are expected to remain poor, which will
largely curtail the threat for severe thunderstorms today.
However, some locally gusty winds and very heavy downpours may
accompany these storms, particularly during the afternoon.
Significant widespread hydro concerns are not anticipated,
though rather slow storm motions (towards the east at 10-20 mph)
could result in some very localized heavy rainfall totals in
excess of an inch.

Widely scattered showers and storms may linger across
eastern/southeastern sections of the area into tonight as the
mid-level trough axis gradually shifts into northwestern IN.
However, the coverage of storms is expected to wane with time
overnight, and end altogether prior to daybreak Friday morning.
Shower and thunderstorm chances then going into the daytime
hours of Friday look rather low (20% or less), owing to rising
mid-level heights and associated capping in the wake of the
departing mid-level trough.

Temperatures today will be a hair cooler than those on Wednesday,
owing to more cloud cover and cooler temperatures under the mid-
level trough. Accordingly, inland temperatures are expected to
top out in the lower 80s, while onshore northeasterly flow keeps
areas along the lakeshore in the 70s. On Friday, conditions will
warm a bit with more sunshine. Temperatures are thus forecast to
top out in the mid 80s inland, with a bit cooler conditions
expected along the lakeshore areas again.

Surface high pressure will remain over the area through Friday
night, making for a quiet and seasonably mild night.

KJB

Saturday through Wednesday:

Forecast thinking has not changed beyond Friday night. For more
on the long term period please reference the previous
discussion below.

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/KJB

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1235 AM CDT Thu Jul 11 2024

Key Aviation Impacts for the current TAF period:

- Slight chance for reduced vis overnight, but confidence
  remains low

- Around a 20-30 percent chance for light showers late morning

Showers and thunderstorms are ongoing over the Mississippi
River and in Central Illinois, but drier conditions over
northeastern IL and northwestern IN. Some mid to high level
clouds are slowly passing over, but precip is not expected
overnight. The main question will be with regards to lower vis
and the potential for isolated or patchy fog developing. The
ground is certainly saturated from yesterday`s rain and winds
are expected to be light and variable, if not calm. The main
question is how cool temperatures will get. The prevailing
thinking is that the clouds overhead will help keep conditions
warm enough to prevent fog development, but isolated 4 to 6 SM
is possible; however, the better chances are farther inland away
from the lake (and most TAF sites).

As the upper level disturbance moves east through Thursday,
there is a chance for isolated showers to crop up in the late
morning to early afternoon. TAFs were kept dry because a.) the
probability is less than 30 percent and b.) they should be
momentary and not of long duration.

As mentioned winds will be weak through the overnight
increasing out of the northeast to 10 knots or less through the
day. Cloud cover will diminish through the day with chances for
clearer skies Thursday night as winds diminish once again.

DK

&&

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

&&

$$

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