086
FXUS63 KILX 040516
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
1216 AM CDT Sat May 4 2024

.KEY MESSAGES...

- Near daily chances for showers and thunderstorms over the next
  week as a series of fronts advance across the region. The
  Tuesday through Wednesday period looks particularly active,
  with medium confidence in thunderstorms capable of producing
  all severe weather hazards.

&&

.UPDATE...
Issued at 917 PM CDT Thu May 2 2024

The surface ridge axis has made eastward progress into Illinois
this evening, helping to nudge high clouds eastward into Indiana
as well. The back edge of the clouds is mainly covering counties
along the Indiana border as of 9 pm, with additional clearing
expected in the next few hours. Light and variable winds are
expected overnight, under the weak pressure gradient. The main
concern is for a possible need to expand the patchy fog across
more of our counties than just our far southeast 6 counties
south of I-70. There doesn`t appear to be enough support in the
high res guidance at this point to do that, so will leave the
fog coverage as is. Only minor updates were done to sky and wind
grids. The remainder of the forecast database remains on track.

Shimon

&&

.DISCUSSION...
Issued at 127 PM CDT Fri May 3 2024

Current GOES WV imagery depicts a strong upper-level low
positioned over western Ontario with more subtle shortwave energy
ejecting out of the Great Basin. Model guidance remains in excellent
agreement that shortwave energy lifts into the Northern Plains and
Upper Midwest regions overnight, ultimately helping to force a
synoptic cold front into western Illinois Saturday afternoon.

A plume of low-to-mid 60s dewpoints is progged to advect northward
ahead of the cold front, and SBCAPE values should be able to achieve
1000-1500 J/kg with CINH eroding in the presence of steep 0-3 km
lapse rates (> 7.5 C/km). This all adds up to likely (> 60% chance)
thunderstorm development Saturday afternoon along and ahead of the
cold front.

Coverage of Saturday storms could be limited by somewhat
underwhelming deep-layer shear profiles, with much of the area at
or below 30 kts in the vicinity of the cold front. Updrafts may
struggle to become organized or struggle to stay organized,
especially some of the early updrafts, and recent CAMs hint at
this as well. Nevertheless, CAPE/Shear profiles are just good
enough to support the potential for an isolated severe hail and
wind threat.

As storms move toward and east of I-55 Saturday evening, the
severe potential should gradually fade. This will largely be due
to the loss of daytime heating and the lack of a LLJ to help
augment instability. This thinking lines up well with SPC`s Day 2
Marginal Risk, which ends the threat near the I-57 corridor.

Flooding should not be a concern with the Saturday storm event.
NBM Mean QPF is around 0.25", and this guidance only offers a 10%
chance that rainfall exceeds 0.75", which is well below 3-hour
flash flood guidance.

The rest of the weekend through Monday should be fairly low drama,
with the main frontal zones displaced just to our south. Attention
quickly turns to the Tuesday - Wednesday timeframe as persistent,
deep southwest flow envelops the region ahead of an elongated
upper-level low. While we`re running lean on details at this
timeframe, the synoptic pattern (kinematics/thermodynamics) being
modeled supports a multi- day severe weather threat across
portions of the Plains and Midwest. This strong signal in the
global models is also reflected in the analogs and machine
learning tools for severe weather potential.

MJA

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1216 AM CDT Sat May 4 2024

VFR conditions will prevail in the near term through mid afternoon
on Saturday, under surface high pressure. A non-zero chance of light
fog at our eastern TAF sites of DEC/CMI, but better chances of
MVFR/IFR fog should remain south of I-70. A weakening line of
thunderstorms is projected to impact (70% chance) the western TAF
sites (PIA/SPI/BMI/DEC) during the late afternoon through mid-
evening time frame, with primarily a 2-3 hour window of thunder.
More isolated thunder chances (20%) at CMI.

Shimon

&&

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

$$