396
FXUS63 KILX 022347
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
647 PM CDT Tue Jul 2 2024

.KEY MESSAGES...

- Showers and thunderstorms will develop upstream this afternoon
  and approach portions of central Illinois late this evening
  (starting between 8pm and 11pm). There is a Marginal Risk
  (Level 1 of 5) for severe storms mainly west of the Illinois
  River.

- Scattered thunderstorms (40-60 percent coverage) are expected
  Thursday afternoon and evening and may impact Independence Day
  festivities.

&&

.DISCUSSION...
Issued at 233 PM CDT Tue Jul 2 2024

This afternoon, trough of low pressure stretches from the OK/TX
Panhandle region into the western Great Lakes. Aloft, a shortwave
trough is noted lifting across the northern Great Plains. Near
and ahead of the surface trough, dew points pooling in the lower
to mid 70s are contributing to strong instability with MLCAPE
values around 3000 J/kg across much of Missouri, but values
quickly taper off to less than 1000 J/kg across west central
Illinois. Storms are expected to increase in coverage to our west
this afternoon as the aforementioned upper wave interacts with
these surface features. Seasonably strong deep layer shear will
support a threat for severe storms upstream. CAMs are in
reasonable agreement showing upscale growth into a QLCS with
storms entering the Illinois River Valley between 01-04Z / 8-11pm
CDT. Unfavorable diurnal timing as storms enter the local forecast
area and little if any advection of the stronger plume of
instability to our west means that storms should begin to decay as
they enter portions of central Illinois. 12Z HREF joint
probabilities (CAPE >500; CIN >-75, 0-6km shear >30) indicate high
probabilities (near 100 percent) across west central Illinois but
quickly falls to zero east of a Galesburg to Beardstown line. The
threat for severe storms will be greatest west of the Illinois
River Valley with the threat diminishing quickly between the
Illinois River and I-55 corridor. Given the expected storm mode,
damaging wind gusts will be the primary hazard. Marginal 0-3km
shear vectors around 30kt in magnitude oriented somewhat parallel
to the line of storms should keep the mesovortex tornado threat
low locally. Locally heavy rain is also a possibility. Cloud-
bearing layer flow runs parallel to the trough axis while Corfidi
vectors drop off to 5 kt or less and oppose the mean cloud-bearing
layer flow suggesting the potential for training and backbuilding
to the S/SW. 12Z HREF LPMM shows some swaths of 2-3 inches
entering the far western CWA, but similar to the severe threat,
dropping off quickly east of the Illinois River Valley.

Surface trough / cold front will slowly progress to roughly the
I-55 corridor around midday Wednesday. Moderately strong
instability is progged to develop ahead of the front Wednesday
afternoon, but the jet stream will be displaced to our north over
the Upper Midwest with a notable lack of any mid level support
resulting in sub-optimal deep layer shear. There is some
uncertainty on the location of afternoon thunderstorm development
due to outflow pushing the effective front further southeast.
Regional/global models are tending to show afternoon development
along the synoptic front across portions of central and east
central Illinois, while hi-res CAMs favor convection redeveloping
just southeast of the forecast area closer to the Ohio River
Valley. Regardless of location, storms may struggle to become
organized though strong instability and DCAPE values in excess of
1500 J/kg support the potential for damaging wind gusts with any
stronger storms. The chance for thunderstorms will persist mainly
south of I-72 Thursday night as the frontal boundary stalls along
the Ohio River Valley.

On Thursday, surface boundary remains stalled to our south while
aloft, a shortwave trough is progged to move across the region.
Shear and instability may be favorable for additional severe
storms Thursday afternoon, mainly in the vicinity of the
stationary front south of I-72. Storms will be possible north of
the front as well, but notably lower instability should keep the
severe weather threat in check. There is reasonable agreement
between models that the shortwave trough will move across central
Illinois around midday with a relative lull in precip activity
later in the day Thursday before a stronger wave move across the
region Thursday night into Friday. This gives some hope that some
locales should be able to squeeze in Independence Day festivities
Thursday evening, though it may be a narrow window, and at this
distance, forecast timing is still subject to some adjustments.

High pressure will build across the region over the weekend
resulting in primarily dry conditions across central Illinois.
Surface ridge axis will shift to our east early Sunday allowing
temps and humidity to build back up some on Sunday. Highs in the
lower to mid 80s Saturday will give way to upper 80s Sunday while
dew points creep back up to around 70F.

Deubelbeiss

&&

.AVIATION...  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 647 PM CDT Tue Jul 2 2024

A line of thunderstorms will approach the central IL terminals
from the west while weakening later this evening. Expecting storms
to reach KPIA around 03-05Z, KSPI and KBMI 05Z-07Z, and lower
confidence in timing eastward to KDEC and KCMI as storm
organization decreases markedly. The thunderstorm threat fades by
08Z-12Z, however lingering showers and a few thunderstorms remain
possible. Storms may re-develop after 18Z from around a KDEC-KCMI
line southeastward. Expect localized IFR-MVFR vsbys and MVFR cigs
in thunderstorms, as well as gusty and variable winds. A period of
MVFR cigs can be expected for several hours behind the storms, but
VFR conditions should return by 16Z-21Z as cigs lift and low
clouds start to dissipate. Winds S 12-20 kts with gusts 20-26 kts
this evening, decreasing and shifting to W after thunderstorms
pass through.

37

&&

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

$$