909
FXUS63 KILX 010147
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
847 PM CDT Tue Apr 30 2024

.KEY MESSAGES...

* Band of showers/thunderstorms is expected to decay as it moves
  into our CWA tonight, with highest shower/storm chances near the
  IL River Valley.

* Above average temperatures expected through the next 7 days,
  warmest temps Thursday with highs expected to be well into the
  80s.

* Another round of showers and thunderstorms expected Thursday
  night into Friday morning.

&&

.UPDATE...
Issued at 847 PM CDT Tue Apr 30 2024

A broken line of convection continues to track eastward across
Iowa this evening. While the storms were originally rooted in an
instability axis characterized by MLCAPEs of 1500-2500J/kg, they
are beginning to outrun their energy source as the airmass further
east is much more stable. Latest SPC mesoanalysis shows MLCAPEs
of only 50-100J/kg along/west of the Illinois River...and even
lower further east. CAMs have consistently shown the storms
weakening considerably as they spill into the Illinois River
Valley toward midnight, and see no reason this trend will not be
realized. As a result, think scattered showers/thunder will impact
locations west of the I-55 corridor between 11pm and 2am...with
the activity diminishing to just a few showers as it reaches I-55
after 2am. Current forecast has a good handle on the situation and
requires no major update at this time.

Barnes

&&

.DISCUSSION...
Issued at 244 PM CDT Tue Apr 30 2024

Water vapor imagery early this afternoon depicts a vigorous,
negatively tilted shortwave trough moving east across the
northern Plains toward the Upper Mississippi Valley. Strong/severe
thunderstorms are forecast to develop over western/central IL
late this afternoon in association with this trough. Axis of
moisture and instability with this system is rather narrow and
while it will likely advect eastward toward the Mississippi River
later this afternoon and evening, it seems likely that it will get
narrower and eventually pinched off with convection likely to
outrun the better instability as it moves into western Illinois.
In addition, the upper trough will pass north of the area with no
real height falls and strong synoptic ascent progged to pass north
of the CWA.

Given all of this, the expectation is that storms will be in a
weakening phase by the time they reach the Mississippi River or
very soon there after. The greatest threat of severe weather will
likely remain west/northwest of our CWA. Interestingly, there has
been a consistent signal in CAM guidance depicting the potential
for strong to potentially damaging wake low winds tonight as this
area of convection decays moving into Illinois. Conceptually, this
would make sense as wake lows tend to form when adiabatic warming
on the backside of a convective complex is unable to be offset by
evaporative cooling from precip (precip shield progged to
weaken/dissipate over IL). There`s a high degree of uncertainty
on how well CAMs can forecast a rare scenario like this. While the
CAM guidance has mostly been across northern IL and perhaps our
far northwestern CWA with the strongest winds, it is worth noting
that in addition to the highly uncertain ability to accurately
predict the occurrence of this mesoscale phenomena, there would be
a high degree of spatial uncertainty about where it could occur.
Given this, our northern CWA is close enough to warrant monitoring
for a potential period of strong wake low driven winds tonight.

Weak cold front will clear the area early Wednesday morning with
clearing/partial clearing and another day of above average
temperatures and comfortable humidity expected Wednesday. Front
is progged to return north as a warm front Wednesday night, but
strongest forcing and moisture return will lie west of our CWA.
Have maintained some chance pops for showers/storms Wed night into
early Thu morning, especially across the IL River Valley for this
potential scenario.

Another vigorous trough is expected to emerge over the northern
Plains Thursday with the associated surface lo lifting north into
the Upper Mississippi. This should place central and southeastern
Illinois in the warm sector of the cyclone with strong southerly
winds likely driving temps into at least the mid 80s. While likely
shy of daily records, these temps are unseasonably warm and more
typical of June than early May.

Cold front will sweep across the area Thursday night into early
Friday morning. Medium range guidance has been trending gradually
quicker with the frontal passage and it would appear that the
best chances of showers and storms will be Thursday night. Have
maintained some low pops Friday afternoon, particularly across
southeastern IL where some afternoon convection could redevelop
if the front doesn`t clear these areas in time.

Above average temperatures are expected through the weekend into
early next week. Generally looks like most of the weekend will be
dry with better shower/storm chances returning early next week as
western long wave trough reloads and ejects another shortwave
east into the mid-section of the nation.

- Izzi

&&

.AVIATION...  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 657 PM CDT Tue Apr 30 2024

Broken line of convection from KDSM to KMCI will push eastward and
gradually diminish later this evening as it encounters a more
stable airmass across Illinois. Based on radar timing tools and
the latest HRRR, have opted to include predominant thunder at KPIA
from 05z-07z. Further east, have only carried showers at the
remaining terminals overnight. Winds will initially back to SE at
around 10kt this evening, then will veer to SW after the passage
of the dissipating convection. Winds will then swing around to the
W/NW by Wednesday morning.

Barnes

&&

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

$$