660
FXUS63 KILX 051659
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
1159 AM CDT Fri Jul 5 2024

.KEY MESSAGES...

- Isolated (30% coverage) thunderstorms will exit eastern Illinois
  mid to late morning, with the next chance (40-50%) for scattered
  storms arriving Sunday and lingering into Tuesday.

&&

.UPDATE...
Issued at 935 AM CDT Fri Jul 5 2024

An occluded sfc low continues to drop SE over the upper Midwest,
positioned over west-central WI at 0930am/1430z. The associated
cold front has pushed to near the I-57 corridor. Ahead of the
front, skies are mostly cloudy and winds have a southerly
component. While some showers were present earlier this morning,
these have since pushed east into Indiana. Behind the front, skies
are clear and winds out of the northwest. The clear skies will be
relatively short- lived, however, as a stratocumulus field
associated with the occluded low pushes into the area. The only
adjustment to the forecast at this time was to increase cloud
cover based on the upstream satellite presentation as well as the
latest HREF guidance.

Erwin

&&

.DISCUSSION...
Issued at 113 AM CDT Fri Jul 5 2024

Two areas of thunderstorms continue approaching central Illinois at
this early hour: one lifting quickly across the Ozarks via warm
advection that noses into the Prairie State south of I-70, and
another being dragged along slowly by a seasonably sharp cold front
across central and eastern Iowa. While most locations in central
Illinois will be missed by each of these waves, we`ve maintained 20-
40% PoPs to account for an isolated shower/storm along the cold
front near and north of I-72, while PoPs are a bit higher (40-50+%)
further south where the most persistent of the Ozarks storms
could make an appearance near dawn. Meanwhile, patchy fog has
developed across our east, where dewpoint depressions are
generally less than 2 degF thanks to light winds and a lack of
appreciable cloud cover. CAMs suggest this will continue at least
the next several hours, perhaps becoming locally dense (HREF probs
for sub 1/2 mile vis briefly climb to 20-30%), though the
incoming convective clouds may help offset radiational cooling
near dawn.

Looking at the corridor of maximum dew point and theta-E spread from
the HREF, it appears the cold front will reach the I-57 corridor mid-
late morning and finally exit the CWA to the east-southeast sometime
early afternoon, by which time our PoPs come to an end everywhere
except along and north of I-74. There, low level cyclonic flow and
steep low level lapse rates amidst the drier airmass may prove
sufficient for a stray shower or two, although the better chance
will definitely be further north where NAM and GFS forecast
soundings appear more favorable. It`ll be a breezy Friday by July
standards as momentum is transferred well to the surface via
convection given the aforementioned steep low level afternoon lapse
rates, with gusts reaching the 25+ mph range if forecast soundings`
winds at the top of the mixed layer materialize.

In the wake of the upper low, surface high pressure will build into
the Ozarks, Midwest, and Ohio Valley heading into the weekend,
theoretically keeping the area dry. However, a couple mid-upper
level shortwaves lifting northeast across the Plains may graze us
with showers/storms Sunday into Monday, with the highest chances
across our west; there`s not going to be much in the way of
instability or shear which should maintain a low (<5%) severe risk,
though the feistiest cells might bring some gusty (mainly sub-
severe) winds to the surface given the dry mid level air in place
and steep low level lapse rates contributing to efficient
evaporational cooling and hence negative buoyancy of downdrafts.
Sometime Tuesday into Wednesday, the deterministic models suggest
the parent upper level trough will shift east toward New England.
However, the Midwest should generally remain beneath weak and stable
flow heading through mid-late week as a ridge of high pressure
becomes anchored across the West.

Bumgardner

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1159 AM CDT Fri Jul 5 2024

A cold front has pushed through central Illinois leaving a modest
west to northwest breeze in its wake. Winds will gust to around 20
kt today, then diminish around sunset. VFR conditions are likely
through most of the period, though some MVFR ceilings could spread
south to near the I-74 corridor Saturday morning.

Deubelbeiss

&&

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

$$