373
FXUS63 KILX 050443
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
1143 PM CDT Thu Jul 4 2024

.KEY MESSAGES...

- Dry conditions will prevail for July 4th activities this
  evening.

- The best chance for thunderstorms (40-50% chance) will focus
  along and south of a Paris to Shelbyville line late tonight.

&&

.UPDATE...
Issued at 840 PM CDT Thu Jul 4 2024

01z/8pm surface analysis shows a cold front extending from far
western Wisconsin to Oklahoma. A broken line of convection has
developed along the northern part of the boundary from Wisconsin
into central Iowa. These storms are being maintained by a modestly
unstable airmass with MLCAPEs of 1000-1500J/kg and strong upper
support from a vigorous short-wave trough over Minnesota/Iowa.
Meanwhile further south along the front, robust clusters of
thunderstorms have developed from southwest Missouri into Oklahoma
thanks to a much more unstable environment with MLCAPEs of
2500-3500J/kg. In between the two areas of convection, the
boundary is currently inactive immediately west of central
Illinois. Cannot rule out a stray shower as the front pushes
eastward overnight, but think most locations north of the I-70
corridor will remain dry. Further south, some of the storm
clusters currently ongoing across southwest Missouri may lift
northeastward ahead of the cold front and spill into the SE CWA
toward dawn Friday. Have therefore maintained 40-50 PoPs
along/south of a Paris to Shelbyville line late tonight. Overnight
lows will drop into the middle 60s behind the front across the
Illinois River Valley, but will remain in the lower 70s
along/south of I-70.

Barnes

&&

.DISCUSSION...
Issued at 230 PM CDT Thu Jul 4 2024

This afternoon, a shortwave trough is noted lifting from central
Illinois into northeastern Illinois with scattered showers
occurring, mainly north of a Macomb to Paris line. Latest RAP
suggests very weak instability is in place, less than 200 J/kg but
forecast soundings show the instability does poke up above -10C
at times and an isolated storm could not be ruled out. As upper
wave continues to move northeast, subtle shortwave ridging will
move in behind and should keep conditions dry late this afternoon
through much of the evening.

Late this evening and overnight, deep/closed H5 low is progged to
dig across the Upper Midwest while attendant surface cold front
sweeps across central Illinois overnight into early Friday
morning. Showers and a few storms will accompany these features,
spreading back into the Illinois River Valley likely around or
just after midnight tonight, then across the remainder of central
Illinois overnight. A narrow plume of weak instability (MUCAPE
around 300 J/kg) will support a few storms, but the severe threat
appears low given the unfavorable diurnal timing and weak
instability. Precip amounts will be light overnight across central
Illinois. NBM 75th percentile generally runs less than a tenth of
an inch north of I- 70, with up to around a quarter inch south.

Behind the departing cold front, slightly drier (low to mid 60)
dew points will begin to advect across central Illinois along
with slightly cooler temps. Expect highs in the upper 70s to low
80s across the area Friday afternoon. Despite the drier air,
steep low level lapse rates and lingering low level cyclonic flow
should drive some cold-air stratocumulus development Friday
afternoon. Forecast soundings suggest instability may be several
thousand feet deep allowing a few light showers or sprinkles to
develop, especially near and north of I-74. Otherwise, dry weather
is expected to prevail Friday and continue into Saturday as high
pressure spreads from the central Great Plains Friday across the
mid Mississippi Valley Saturday.

Heat and humidity begin to work back across central Illinois
Sunday as the ridge axis shifts to our east allowing return flow
to overspread the area. Precip chances will return early next week
as a deep upper trough slowly moves east across the region.

Deubelbeiss

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1143 PM CDT Thu Jul 4 2024

VFR conditions will prevail through the entire 06z TAF period.
Winds will initially be S/SE at around 5kt, then will veer to SW
after midnight as a cold front approaches from the northwest.
Once the front passes, winds will become W/NW and gusty on
Friday...with forecast soundings and numeric guidance suggesting
gusts in the 20-25kt range. FEW-SCT clouds between 5000 and
8000ft will persist through tonight, then will clear out after
FROPA. With a pocket of cold air aloft dropping southeastward out
of the Northern Plains, steep lapse rates will develop by peak
heating Friday. NAM Cu-rule indicates SCT-BKN diurnal clouds at
3500-4000ft during the afternoon.

Barnes

&&

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

$$