973
FXUS63 KILX 181916
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
216 PM CDT Mon Aug 18 2025

.KEY MESSAGES...

- Scattered thunderstorms (20-50% coverage) are possible the rest
  of today and tonight. The severe risk is marginal (level 1 of 5)
  with isolated damaging wind gusts and locally heavy rain the
  primary hazards.

- Heat and humidity will begin to be shunted to our south through
  the day Tuesday. While some triple digit heat indices remain
  possible Tuesday afternoon mainly south of I-72, a more
  comfortable and seasonal air mass will overspread central
  Illinois by Wednesday and persist through the weekend.

&&

.DISCUSSION...
Issued at 203 PM CDT Mon Aug 18 2025

A strong upper level ridge will remain over the lower and middle
Mississippi Valley today, while at the surface, high pressure
stretches from the lower Great Lakes across the upper Ohio Valley.
Hot and humid conditions persist in the low level southwesterly
flow across central Illinois today with triple digit heat indices
common this afternoon and a heat advisory remains in effect into
this evening. Convective evolution through the remainder of today
and tonight remains uncertain, but showers and storms remain in
the forecast. Latest RAP shows 2500-3500 J/kg MLCAPE across all
of Central Illinois this afternoon, though deep layer shear is
weak. 0-6km shear vectors range from roughly 15 kts in the
northern counties down to around 5 kt in the south. The weak
shear should limit a more organized severe threat across central
Illinois, but the moderately strong instability and DCAPE values
around 1300 J/kg suggest that strong downburst winds will be
possible from the strongest storms. In addition, slow storm
motions are expected in the weak steering flow under the influence
of the upper ridge. This could lead to some pockets of locally
heavy rain. 12Z HREF produces a few localized areas of 2.0+ inches
of rain. In terms of timing, storms are ongoing across northern
Illinois with a trailing outflow boundary approaching the Illinois
River Valley late this afternoon. The better convergence remains
well to our north, but the forcing may be sufficient to kick off a
few isolated storms this afternoon. Despite their poor handling
of ongoing convection, CAMs continue to show an increase in storm
coverage this evening into the overnight hours in response to
increasing mid level warm air advection.

On Tuesday, upper level ridge will begin to retrograde towards
the Desert Southwest as a shortwave trough digs across the Great
Lakes. At the surface, a weak cold front will sag south across
central Illinois through the day. Depending on the overall
coverage of storms tonight, and if storms are able to congeal
enough to develop a cold pool, this will have implications for
both temps/heat indices and convective chances Tuesday as it
helps push the effective cold front across the area a little
faster. Given the overall uncertainty on the evolution of storms
through the rest of today and tonight, and the potential for a
cold pool to help push the effective front to our south early
Tuesday, will hold off for now on additional heat headlines. The
southern portions of the CWA have the best chance of needing an
additional Heat Advisory. Temps are expected to top out in the
mid/upper 80s across the northern CWA to lower 90s south but
humidity levels will remain elevated with dew points still running
well into the 70s. Slightly lower dew point air will gradually
filter into central Illinois Tuesday night into Wednesday.
Additional showers and storms will be possible through the day
Tuesday along the slow moving cold front, though some of the CAMs
with a more pronounced cold pool system overnight suggest the
storm threat may be confined to areas south of I-72.

By Wednesday, the cold front will be to our south and high
pressure will build across the Great Lakes. Locally, dew points
will drop back into the upper 60s and afternoon highs will remain
in the 80s areawide. This more seasonal air mass is expected to
persist through the remainder of the week and into the weekend.
Temps may dip even more over the weekend with highs in the 70s for
portions of central Illinois behind another cold front.

Deubelbeiss

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1247 PM CDT Mon Aug 18 2025

While there is a small (10-20%) chance of an isolated shower or
storm as a weak outflow boundary enters central IL this afternoon,
high pressure should mitigate coverage of storms. Later this
evening, as instability is peaking and wind shear slowly increases
from north to south with a shortwave dropping out of the Upper
Midwest, storm chances will increase. For this reason, PROB30
groups for thunder were added to each of the terminals for the
period of greatest overlap in storm timing from the convective
allowing models (CAMs). Early Tuesday morning, there may be some
shallow ground fog and/or IFR ceilings at PIA, BMI, or CMI if
enough breaks occur in cloud cover after any rain, however
probabilities appear too low (at around 20%) at this juncture to
introduce a mention in the TAFs.

Bumgardner

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 8 PM CDT this evening for ILZ027>031-036>038-
040>057-061>063-066>068-071>073.

&&

$$