400
FXUS63 KILX 211100
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
600 AM CDT Sat Sep 21 2024

.KEY MESSAGES...

- Patchy dense fog east of I-55 early this morning will lift by
  mid morning.

- One more hot day lies ahead for central and southeast IL with
  highs in the upper 80s and lower 90s, as more humid conditions
  return this afternoon as a warm front lifts back north.

- Isolated showers and thunderstorms are possible this afternoon
  and evening. Greater rain chances materialize later tonight
  through Monday evening with showers and chance of thunderstorms.
  There is a 50-70% chance of 1-2 inches of rain across central
  Illinois during that time period, with 2-3 inches possible in
  southeast IL.

&&

.DISCUSSION...
Issued at 345 AM CDT Sat Sep 21 2024

Areas of fog had developed past few hours east of I-55 with
patchy dense fog from Champaign to Decatur to Taylorville east
where more moist air present with low to mid 60s dewpoints and
very light to calm winds. Some cirrus clouds arriving from the
west has been keeping the dense fog from getting too widespread
while less high clouds further east over Indiana to allow more
widespread dense fog formation and they have a dense fog advisory
until 8 am cdt. Have increased fog wording in eastern IL until 9
am, and will continue to monitor dense fog coverage and dense fog
advisory possibility early this morning. A special weather
statement will be issued for the patchy dense fog in eastern IL
early this morning with visibilities restricted to a quarter mile
at times.

A frontal boundary over southeast IL and into sw IL, will lift
back northeast across central IL during this afternoon. Dewpoints
to increase back up into the upper 60s and lower 70s while highs
reach the upper 80s and lower 90s, with afternoon heat indices
peaking back in the 90s. Limited forcing today, but MLCAPES rising
to 2k j/kg over western/sw CWA by mid/late afternoon with wind
shear values of 20-30 kts could support a few strong storms
western and sw CWA from mid afternoon into early evening for hail
and gusty winds. SPC day1 outlook officially keeps marginal risk
nw of CWA over much of Iowa, nw MO and central/western WI closer
to a cold front.

This cold front progged by CAMs to track se into nw IL late
tonight and moving into central IL during Sunday afternoon. This
system has stronger forcing and will produce more widespread
showers and a chance of thunderstorms arriving late tonight into
Sunday. SPC day2 outlook has marginal risk of severe storms sw of
St Louis over central/southern MO. Cooler highs Sunday in the mid
to upper 70s central IL and around 80F in southeast IL. Frontal
boundary will be near southeast IL Sunday night and Monday as
another system moves along it and likely brings another round of
showers and a few thunderstorms with the heavier rains focused in
southeast IL during that time frame. SPC day3 has marginal risk of
severe storms over far southern IL from Carbondale to Evansville
south on Mon/Mon night. Precipitatable water values of 1.5-2
inches are expected Sunday into Monday so heavier rain showers are
possible at times. Between 1-2 inches of rainfall expected over
CWA Sunday through Monday night with 2-3 inches possible in
southeast IL where rain likely linger into Tue morning. Even
cooler Monday with highs in the lower 70s over much of central IL
with mid to upper 70s in southeast IL.

Models diverge with handling upper level pattern during mid/late
week so stayed close to model blend with this forecast package.
Generally dry conditions forecast for Tue night through Thu with
below normal temperatures with highs in the low to mid 70s and
lows in the low to mid 50s from Tue night through rest of the
week. There is potential for tropical system to move northward
over the Gulf of Mexico and into the central Gulf coast or
southeast States late next week. Consensus of models keep its
moisture south and southeast of central and southeast IL. Highs
get closer to normal for late September in the upper 70s on
Friday and next Saturday.

07

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 600 AM CDT Sat Sep 21 2024

Patchy fog lingering at KDEC will see tempo IFR/LIFR conditions
until around 13-14z. After that, scattered mid level clouds will
dominate the area into this evening. Calm winds early this morning
will increase from the southeast this morning, and south to
southwest this afternoon up to 10 kt. A broken line of
thunderstorms is forecast to develop over west central IL this
afternoon, with these storms possibly impacting KPIA-KSPI before
fading, so icluded a PROB30 group there. Late tonight a more
widespread area of showers will move in from the west. Some
thunder could accompany this but confidence impacting a terminal
is too low to include in TAFs.

25

&&

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

$$