300
FXUS63 KILX 040457
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
1157 PM CDT Mon Jun 3 2024

.KEY MESSAGES...

- There is a 20% chance for a thunderstorm west of I-55 this
  evening. If a storm develops, it could produce locally gusty
  winds and hail.

- There is a 40-50% chance for thunderstorms tomorrow afternoon.
  The strongest storms could produce locally damaging wind gusts
  and hail.

&&

.UPDATE...
Issued at 812 PM CDT Mon Jun 3 2024

A weakly sheared environment is present across much of MO/IL this
evening. Earlier this afternoon, thunderstorms that developed
over E MO produced an outflow boundary that has steadily
progressed northward, kicking off additional scattered showers and
storms. The lack of shear is resulting in updrafts being short-
lived, and while some showers/storms will percolate near that
aforementioned outflow boundary (along a corridor from the SE tip
of Iowa towards Flora/Olney) for another hour or two, the
expectation is that these will remain sub-severe and wane as
diurnal heating is lost. CAMs continue to suggest another round of
showers/storms develops late tonight along the leading edge of a
low-level jet, reaching southern portions of the ILX CWA after
sunrise, but then weakening thereafter. No significant changes
were made to the gridded forecast at this time, just PoP edits in
the first few hours to better reflect the short-term precip
evolution.

Erwin

&&

.DISCUSSION...
Issued at 107 PM CDT Mon Jun 3 2024

At 18z/1pm, SPC`s mesoanalysis suggested 1500-2500 J/kg SBCAPE
across central Illinois, but effective bulk shear under 20 kt and
lack of a forcing mechanism suggests we might just escape without
storms today. The 12z NAMNest and FV3 suggest something might
clip our west where one/more convective outflow boundaries could
stall, so we`ll have one eye on the radar, but at this point
convection is expected to stay to our northwest and southwest
until tomorrow. In the meantime, we`ll enjoy an afternoon of
summer-like warmth as temps continue to climb into the upper 80s
and dewpoints hover about 20 degrees lower.

The deterministic models bring some sort of meso-low/MCV out of
the Central Plains and into the region tomorrow morning/early
afternoon, with CAMs varying considerably in their depiction of
convection firing near it. The HRRR and ARW have storms developing
across our west and central CWA, while the FV3 has storms mainly
east of I-57 and the NAMNest has them split across our area - some
storms near/south of I-72 and others west of the IL River. Given
there will be more shear tomorrow in the vicinity of this MCV,
storms would have the potential to be a bit more sustained, though
instability is less certain and will ultimately depend on whether
any breaks in cloud cover can materialize ahead of it. Sometimes
these MCVs can produce "sneaky" severe weather events, so we`ll
have one eye on things. The output from the CAMs certainly
suggests a threat for localized damaging wind gusts and hail with
storms - wherever they track - tomorrow afternoon, but widespread
and/or significant severe weather is not anticipated. In terms of
temps tomorrow, highs should be generally a few degrees cooler
than today given (1) convection, (2) cloud cover, and (3)
increasing low level moisture with WAA ahead of the cold front
slated to arrive tomorrow night. High temps will vary considerably
by location (NBM`s 10th-90th range is mid 70s to mid 80s) for
these reasons, though we`ve got low 80s in the forecast area-wide
due to lack of confidence in the exact placement of
convection/clouds.

Then attention turns to another round of possible showers and storms
along the cold front tomorrow night. However, depending on how
overturned the atmosphere is from afternoon storms with the
aforementioned MCV, there may not be much instability leftover to
fuel convection along the front - especially with it moving into the
area so late. We will have a LLJ maintaining warm/moisture transport
conducive to at least marginal elevated instability, but with even
the 90th percentile sfc-500mb wind shear from LREF at only ~30-35 kt
along the front it`s not like we`re going to have so much shear that
it makes up for the lack of instability and results in widespread
storms. In fact, the 12z CAMs are suggesting the upstream line of
storms will break apart and much of the area could miss out on rain
altogether tomorrow night. Near/east of I-57, however, renewed
development is a possibility (see the 12z NAMNest) Wednesday should
the front slow down at all. We`ll keep an eye on that, but for now
have lowered PoPs slightly tomorrow night into Wednesday; we may
need to further lower those in forthcoming forecast updates if the
trend toward more sparse convection along the front continues.

Once the cold front clears the area, surface high pressure will
build in to hopefully keep us dry for a while. However, with temps
aloft cooling considerably (GFS has 850mb temps dropping to sub 2
degC across northern Illinois late Thursday), we can`t completely
rule out a surface based instability driven shower or storm
Wednesday through Friday; coverage would be sparse at any rate, so
we`ve kept PoPs around 10% (i.e., sub-mentionable) through that time
period.

This weekend into early next work week, ridging will attempt to
build into the region in the wake of the upper trough departing
slowly to the northeast, resulting in cool and stable northwest
flow. Models generally keep the ridge-riding MCSs to our southwest,
but we`ve maintained slight (20%) chances for precip early next week
for the possibility one of them sets up far enough northeast to clip
our area.

Bumgardner

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1156 PM CDT Mon Jun 3 2024

VFR conditions are anticipated through the period, along with
southerly winds that gust to around 20 knots during the day.
Scattered thunderstorms are expected from mid-afternoon into the
evening. An intial wave of storms should devleop around 19- 20z
and move from south to north across the area (between 20-00z).
The evolution becomes less certain between 00-06z, and while t-
storms could occur during that period confidence became too low to
keep a prolonged TS mention in the TAFs.

Erwin

&&

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

$$