839
FXUS63 KILX 292001
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
301 PM CDT Sat Jun 29 2024

.KEY MESSAGES...

- There is a 30% chance for thunderstorms along a cold front this
  evening. The strongest of the storms will pose a risk for
  locally damaging wind gusts and torrential rain leading to
  ponding of water in urbanized areas.

- Seasonably hot and humid conditions make a return Wednesday-Friday
  with heat indices likely (50-70%) reaching 100 degrees or
  greater for locations south of I-72 on Wednesday.

- Multiple chances for thunderstorms also return with a 40-50%
  chance on Wednesday and a 60-70% chance on Thursday. Outdoor
  activities could be impacted, especially due to the threat of
  lightning.

&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 257 PM CDT Sat Jun 29 2024

Our morning thunderstorms have now departed to Indiana, with a bit
of capping building in their wake to limit convective potential for
at least the next couple hours. however, given 2500-3500 J/kg MLCAPE
(highest across our northeast) and PWATs near/over 2 inches per
SPC`s mesoanalysis, water-laden microbursts capable of torrential
rain, small hail, and especially localized damaging wind gusts
remain a concern through evening with the strongest cells that
develop along the southeastward-moving cold front currently situated
near the I-55 corridor. This front will become more organized as it
surges southward late this evening, bringing a sharp gradient (and
rapid change with time at any given location) in theta-E (nearly 1
degK/km) and uptick in northwest winds which could gust close to 30
mph. Went with a blend of HRRR and NBM for overnight lows, which are
forecast to fall into the 50s northwest of the Illinois River but
remain close to 70 south of I-70 where the front will not arrive
until between roughly midnight and 3 am.

The continued cool advection will prevent temps from rising above
the 70s tomorrow for most, if not all, of the CWA, making it feel
more like mid May than late June and offering a much needed reprieve
from the oppressive humidity. Temps dip further tomorrow night as
winds drop off and in the absence of clouds radiational cooling is
optimized. Around 60-80% of HREF membership brings Monday morning
lows into the 40s outside of urban areas, though NBM, which is
calibrated to climatology, suggests those chances are closer to
10- 20% with the ensemble mean in the low 50s; we`re going to
maintain forecast lows closer to NBM mean for now, but wouldn`t be
surprised if we wind up on the cooler side of things if
tomorrow`s highs don`t overachieve (which sometimes happens with
so much sunshine) and we wind up with both as little wind and as
few clouds as currently forecast.

Bumgardner

.LONG TERM...  (Monday through Next Saturday)
Issued at 257 PM CDT Sat Jun 29 2024

On Monday, a mid to upper level ridge will be centered over the area
with its associated surface high over the Great Lakes region. This
will lead to a clear, calm, and somewhat chilly start to Monday with
temperatures in the low 50s to upper 40s, while highs will only
reach the upper 70s Monday afternoon. As the mid to upper level
ridge axis passes through, our area will return to a deep southerly
flow regime, leading to the return of summertime heat and humidity
by Tuesday. A vast majority of the LREF guidance depicts the center
of this ridge (with 500mb heights of 596dm) settling over the SE
CONUS next week, placing our region on the northern edge where waves
of shortwave energy will traverse.

A weak upper level trough will pass along the northern Great Plains
and deamplify as it progresses eastward just north of our region on
Wednesday. The shortwave energy and its attendant, weak, cold front
approaches from the NW on Wednesday, bringing with it the next
chance for some rain, especially in the northern portions of the
area where there is more sufficient column moisture present. Some of
the deterministic model guidance depicts this weak cold front
stalling out near our area Wednesday into Thursday, likely due to
the 596dm ridge to our SE keeping the front from progressing to the
SE of our region. The stalled front will essentially act as a runway
for any waves of precip or convection that traverse around the 596dm
ridge, potentially leading to flooding concerns depending on where
this sets up at on Wednesday. Wednesday still appears to be the
warmest day next week with highs in the lower 90s and heat indices
nearing 100 degrees south of I-72, where the later frontal passage
is at. However, high temps on Wednesday will be highly dependent on
the exact timing of this weak cold front, which at this time is
uncertain.

Perhaps the better and more widespread chance of rain will be on
Thursday (the 4th of July) as shortwave energy pivots around the
596dm ridge into the CWA, resulting in likely PoPs for most of the
area. Uncertainty regarding the strength and position of the strong
ridge remains after Thursday next week as an upper level trough may
dig into the Upper Midwest, potentially deamplifying the upper
level ridge. Regardless, our current forecast has highs in the 80s
and lows in the 60s, but given the current ensemble spread there
is low confidence this far out.

Peine

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1241 PM CDT Sat Jun 29 2024

Scattered thunderstorms have developed early, courtesy of a weak
boundary and a bit more sunshine than expected which eroded our
capping inversion. DEC and CMI have already reported
thunderstorms, which should be moving out of the area in the next
hour or two. Additional development is less certain given the
strengthening cap further west, though wind shear will be
increasing along the cold front as it moves southeast through the
area late this evening. Given expected limited coverage (30-50%)
along that front and continually shifting model output, we`ve
included a prob30 for a few hours (roughly 22z Sat to 02z Sun, or
5-9pm CDT) this evening to message that potential, which is
highest once again at CMI and DEC. Behind the front, an abrupt
shift and uptick in winds is forecast with frequent gusts near or over
20 kt expected for several hours before winds gradually become
more steady around 10-13 kt toward Sunday morning.

Bumgardner

&&

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

$$