696
FXUS63 KILX 142008
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
308 PM CDT Mon Jul 14 2025

.KEY MESSAGES...

- Increasing warmth and humidity will bring heat indices into the
  triple digits Wednesday afternoon, and possibly (30% chance)
  again on Thursday. This will increase the risk of heat illness
  for vulnerable populations.

- Daily chances (30-50%) for thunderstorms will exist Tuesday
  through the weekend. Confidence is low on potential for severe
  weather, though storms may produce heavy rain. There is a level
  1 of 4 (marginal) risk for excessive rainfall each day Tuesday
  through Thursday.

&&

.DISCUSSION...
Issued at 306 PM CDT Mon Jul 14 2025

***** DRY THROUGH TONIGHT *****

A quasistationary frontal boundary across far southern IL remains a
source of convergence for isolated showers early this afternoon.
WPC, in line with what we`re seeing in highres guidance, suggests
this boundary will stay roughly in place through early tomorrow
morning, and then slowly lift northward towards the I-72 corridor
during the day. Hence, most of our neck of the woods should stay dry
through tonight, although diurnal cu have been producing localized
(5-10% coverage) sprinkles which has been added to the forecast
the next couple hours.

***** DAILY STORM CHANCES TOMORROW THROUGH THE WEEKEND *****

As the aforementioned front lifts north tomorrow and south-
southwesterly flow increases, guidance from both HREF and NBM
suggests we`ll have a virtually uncapped 1800-2500 J/kg SBCAPE
which will lead to isolated afternoon storms forming from subtle
differential heating boundaries. We maintain 40-60% PoPs tomorrow,
with coverage maximized late afternoon-early evening when
outflows from early storms have thoroughly percolated. The limited
shear suggests the probability of organized convection capable of
producing severe weather is low (less than 5%, per SPC`s latest
outlook), however deterministic models generally agree on PWATs
climbing to 1.9-2.1 inches and warm cloud layer depths surpassing
12,000 ft which would suggest that, given the anticipated slow
storm motions (cloud-bearing layer winds ~ 25 kt), heavy rain
leading to localized flash flooding is at least a small risk.

Ahead of a more potent shortwave across the Upper Midwest, low level
warm air advection will increase Wednesday into Wednesday night
leading to slightly steeper lapse rates (6-6.5 C/km vs 5-5.5 C/km)
and consequently more robust instability (50-70% probability of >
3000 J/kg SBCAPE, per NBM). However, forcing for ascent will be well
upstream, and it remains unclear whether storms forming there will
make it this far southeast given weak shear (20-30 kt) and waning
diurnally driven instability after sunset. The better chance may
actually be Thursday, but this will depend on where thunderstorm
outflows push the effective boundary Wednesday night; areas near and
south of the I-72 corridor look to stand the highest chance of
destabilizing on Thursday at this time. Shear still looks marginal
ahead of the front, but - as will be the case with any storm that
manages to fire in the warm sector Wednesday - high PWATs, steep low
level lapse rates, and prolific mid level dry air apparent on
forecast soundings suggests downburst winds will be a risk with
the tallest storms and any bowing segments that manage to develop.

Middle of the road guidance suggests the cold front should stall
south of the area Friday, offering a repose from the warmth and
humidity. Ensemble guidance increasingly diverges, however, heading
into the weekend, on whether or not (and when) the boundary lifts
back northward as a warm front - potentially serving as the focus
for convection, along with ushering in a return to hot and humid
conditions.

***** HOT AND HUMID MIDWEEK *****

Additionally, heat index values ahead of the front (and outside of
storms) both Wednesday and Thursday will creep up to near triple
digit values as the combination of moisture advection from the
southwest and agricultural evapotranspiration causes dewpoints to
climb into the upper 70s. Humidity should decrease behind the front
Thursday night into Friday, but is slated to eventually return
either late this weekend or early next work week by which time the
more bullish guidance suggests surface dewpoints may start with
the number 8.

Bumgardner

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1249 PM CDT Mon Jul 14 2025

The moisture from this morning`s dense fog was deep enough to
linger until midday as an MVFR cumulus layer across portions of
central IL. Visible satellite loops are not showing much clearing
in those clouds through 1730z. However, we still expect a gradual
improvement as the sun works to mix out some of that moist layer,
per the ILX 12z sounding. All sites should see VFR cloud conditions
by 20z. VFR conditions should prevail until Tuesday morning,
although pockets of MVFR fog will be possible between 09z and 13z
tomorrow morning. Confidence is not high enough to add those
conditions at this point, but it will bear watching.

A stationary front will lift north toward I-70 tomorrow morning,
helping to increase MVFR clouds from south to north and increase
rain potential for our southern TAF sites. SPI and DEC will have
the better chances for sprinkles as early as 15z, but we left off
precip for the 18z update.

Winds will remain light through tomorrow, with a general southerly
flow pattern prevailing.

Shimon

&&

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

$$