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482
FXUS63 KILX 222049
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
349 PM CDT Sat Jun 22 2024

.KEY MESSAGES...

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- A decaying line of thunderstorms will push into the area
  tonight. Hazardous weather will not be widespread, but a few
  locations will be susceptible to torrential rainfall,
  lightning, and pockets of gusty thunderstorm winds.

- Afternoon temperatures will average about 5 degrees warmer than
  climatology over the next week, with the hottest day of the
  period coming Tuesday when heat index values could exceed
  triple digits.

- Additional rainfall chances (greater than 50% probability) exist
  by the middle of next week with an attendant severe weather risk
  looming.
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&&

.DISCUSSION...
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Issued at 151 PM CDT Sat Jun 22 2024

A quick check on features this afternoon reveals a compact mid-
level shortwave trough pushing across the Upper-Mississippi
Valley, with an attendant cold front extending from Green Bay to
Omaha to Dodge City. A narrow corridor of low-to-mid 70s sfc
dewpoints have pooled ahead of the this front, and convective
initiation is already occuring across portions of NE Iowa. Steep
low-level lapse rates and sufficient surface heating out ahead of
these features should help erode any remaining cap, while a mid-
level speed max (40-50 kts @ 500 mb) nosing in from the west helps
augment deep layer shear. The net effect will be clusters of
organized thunderstorms blossoming and pushing southeastward along
the cold front. CAMs remain in excellent agreement that such
activity will hold together as it sinks into central and southeast
Illinois late this evening.

An attendant severe weather risk remains in tact tonight, mainly
in areas north of a Beardstown-to-Bloomington line where the axis
of a 30-40 kt LLJ will be positioned. Damaging thunderstorm wind
gusts, associated with a mature cold pool, is the main concern by
the time the line reaches our forecast area.  The expectation,
though, is for convective activity to steadily fade overnight as
the upper forcing (shortwave) becomes displaced too far northeast,
the low- level jet axis gradually narrows and pinches off while
convergence along the front weakens. In other words, thunderstorms
will become less organized with southeast extent, and areas south
of I-72 may not see much thunder at all.

Hydro has the potential to steal the show tonight, with a few CAM
members (notably the ARW, NAM Nest, and NSSL WRF) offering localized
amounts up to 3" anywhere north of that Beardstown-to-Bloomington
line. While this won`t be the rule by any means, it`s certainly a
plausible outcome given a healthy LLJ and PWATs along the front
that exceed the 99th percentile of mid-June climatology.

A marginal cool down is anticipated Sunday in the wake of frontal
passage, with afternoon highs mostly in the mid-to-upper 80s. This
will be short-lived, though.

By Monday, mid-level heights will steadily increase beneath a
plume of hot air surging northward out of the western Gulf.
Temperatures on Monday will eclipse 90 degrees in many areas, and
if the deterministic NBM gets close to verifying, Tuesday will
max out in the mid-to-upper 90s ahead of the next frontal zone
with heat index values potentially exceeding 100 degrees. Tuesday
is not necessarily a slam dunk Heat Advisory day, though, with
multiple failure modes in play. Chief among them is a nocturnal
MCS. While this signal is mostly muted in ensemble and multi-
model guidance, the global deterministic do show some semblance
of this possibility with a few shortwaves lurking upstream across
the Corn Belt.

Otherwise, our attention is focused on the frontal passage itself
Tuesday night into Wednesday. Given the very moist and unstable
air mass ahead of the front, its passage may be accompanied by an
attendant severe weather and hydro risk.

Another brief cool down is then anticipated Thursday into Friday
as modified Canadian high pressure settles over the Great Lakes
region. Afternoon highs will mostly be in the upper 80s during
this period. Nevertheless, another signal for hot and increasingly
humid weather returns late next week and into the weekend --
perhaps marking a return to mid 90s heat and an additional round
of thunderstorm activity.

MJA
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&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1140 AM CDT Sat Jun 22 2024

Low-level winds will increase ahead of the cold front,
occasionally gusting to 25 kts this afternoon. Mid-level clouds
will also increase in coverage as the front approaches from the
northwest. Confidence remains high enough to mention TSRA at
KPIA/KBMI by 03z, with less confidence at KSPI/KDEC/KCMI prompting
the PROB30 by 06z. Showers and thunderstorms will gradually decay
overnight with southeast extent, but MVFR/stratus will develop
behind the cold front and overspread KPIA/KBMI early Sunday
morning. VFR conditions then return to all terminals by Sunday
afternoon as drier air mixes in from the west.

MJA

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$