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410
FXUS63 KILX 212046
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
346 PM CDT Fri Jun 21 2024

.KEY MESSAGES...

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- Daily afternoon temperatures will average about 5 degrees warmer
  than climatology over the next week, with the hottest day of
  the period coming Tuesday. Respect the summer heat.

- Periods of showers and thunderstorms are forecast Saturday
  evening and again by the middle of next week as a pair of fronts
  drop through the region. There is potential for hazardous
  weather to accompany either or both of the frontal passages.
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&&

.DISCUSSION...
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Issued at 1253 PM CDT Fri Jun 21 2024

Synoptic Overview:
A series of mid-level shortwaves will eject across the Plains and
Upper Midwest over the next 36 hours and help dampen what has been
dominant ridging over the eastern U.S. Of these shortwaves, the
chief player will be the one pushing across the Upper Mississippi
Valley into Lake Superior Saturday evening, as it will help drive
a surface cold front through central Illinois. The net effect will
be a period of showers and thunderstorms Tuesday evening, with a
decaying trend as activity moves southeastward overnight. And,
while most models blow the front south of I-70 by midday Sunday,
any delay could result in renewed, short-fused storm development
across our southernmost counties.

A nominal cool down is anticipated Sunday and Monday as a second,
well- defined shortwave trough digs across the Great Lakes region,
but heights are expected to steadily increase Monday night into
Tuesday as a surge of Gulf air advects northeastward into portions
of the Southern Plains and Mid-Mississippi Valley. This pattern
will set the stage for what could be the hottest day so far this
year by Tuesday. Additionally, while some spatial and temporal
discrepancies exist within global deterministic guidance, there is
good semblance of strong mid-level shortwave energy digging
across the Upper-Midwest Tuesday evening into Wednesday,
ultimately forcing a cold front into central Illinois. If trends
continue, this will be another period to watch for showers and
thunderstorms to develop.

A brief cool down will occur Thursday in the wake of frontal
passage. Despite surface high pressure centered over the Great
Lakes and Ohio Valley regions, central Illinois will be positioned
beneath northwest flow, with a few shortwaves lurking upstream --
especially as we get into Friday. Such a set-up could yield
additional shower and thunderstorm activity late next week.


Mesoscale Details:
A relatively narrow corridor of mid 70s dewpoints, extending along
a Kansas City to Quad Cities line, will pool ahead of the cold
front on Saturday afternoon, becoming the focus for severe storms
as a mid- level shortwave and its attendant speed max nose across
the Upper/Mid Mississippi Valley and help augment CAPE/Shear
profiles. As storms blossom and push southeastward along the front
Saturday night, they will be maintained by a 40-kt LLJ. However,
as instability gradually wanes and the shortwave becomes displaced
further from the area, thunderstorm activity along the front will
eventually decay. This trend is well captured among the 12z hi-
resolution models, which signals organized convection fading
before reaching Interstate 72.

As far as timing goes, storms should enter the Illinois River
Valley sometime between 8-10 PM CDT and noticeably dissipate
across the region between 12-2 AM CDT. As outlooked by SPC, gusty
thunderstorm winds will be the primary severe weather hazard --
owing to the potentially healthy/mature cold pool.

While PWATs are forecast to exceed the 99th percentile along the
front, the expectation is for storms to have enough forward
motion to mitigate training -- at least in these parts of
Illinois. And so while torrential downpours and frequent CG
lightning could be on display, flash flooding is not as big of a
concern for us.


MJA
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&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1157 AM CDT Fri Jun 21 2024

A damping ridge aloft will help keep terminals at VFR through this
TAF period. High clouds will increase later this evening into
Saturday as a slow- moving cold front approaches from the
northwest. Low- level winds will remain from the south through
this evening, becoming southwest and increasing late Tuesday
morning ahead of the front.

MJA

&&

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

$$