Decatur, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

505
FXUS63 KILX 191935
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
235 PM CDT Sat Jul 19 2025

.KEY MESSAGES...

- Thunderstorm activity both tonight and Sunday night will be
  capable of intense rainfall, with an associated threat of flash
  flooding. The storms Sunday night will be occurring in areas
  that saw intense rainfall earlier on Saturday.

- A heat wave will impact the area from Tuesday through the end of
  the week, with the worst conditions Wednesday and Thursday.
  Chances of the heat index reaching higher than 110 degrees on
  these days are around 40-50%.

&&

.SHORT TERM...  (Through Monday Night)
Issued at 235 PM CDT Sat Jul 19 2025

MCS which has been impacting the forecast area much of the day is
finally waning. Hefty rain accumulation of 2-4 inches occurred in
a stripe from near Macomb southeast through Decatur, with a
separate area of 4-6 inches near Flora. The activity has pushed
the effective boundary well into southern Illinois, where new
thunderstorm development is taking place. For the remainder of the
afternoon, lingering showers and isolated thunderstorms south of
I-72 will continue to diminish, and some clearing will spread into
areas northwest of the Illinois River.

Main concern for this part of the forecast will be with additional
nocturnal MCS activity both tonight and Sunday night. A weak
surface boundary will set up near or just south of I-80 this
evening, drifting southward a bit overnight. Synoptic models
show the main MCS activity developing over Iowa and eastern
Nebraska this evening, then tracking across the northern third of
Illinois overnight into Sunday. However, a few of the high-res
models suggest some scattered activity this evening roughly near
the I-74 corridor, so some 20-30% PoP`s will be included in that
area for the evening hours. With precipitable water values in the
2-2.3 inch range, and warm cloud depths around 14kft, very heavy
rain remains a concern tonight. Since the I-74 corridor missed out
on the soaking rains today, flood headlines will be held off for
now, but this will need to be watched closely.

Development of Sunday afternoon and evening activity will depend
on where the boundary from the overnight convection lays out, but
highest PoP`s will be maintained during the day near I-74. Thinking
is that the threat Sunday night will be further south into central
Illinois, with little chance of recovery for the areas that were
dumped on today. Flood headlines will also need to be considered
for Sunday night by future shifts.

Extent of the clouds/rain will complicate temperatures on Sunday
as well, but the areas south of I-72 should stay dry enough that
heat index values reach 100-105 in the afternoon. With a more
widespread cloud cover and lingering rain Monday, temperatures
should be held down into the mid 80s in most areas.

Geelhart


.LONG TERM...  (Tuesday through Next Saturday)
Issued at 235 PM CDT Sat Jul 19 2025

Focus during this portion of the forecast remains on the intense
heat. Ring-of-fire pattern will begin in earnest over the central
U.S. on Monday, with a ridge axis and the center of upper level
high pressure over the lower Ohio Valley by early Wednesday. Core
of the heat wave still looks to be on Wednesday and Thursday,
with widespread afternoon heat index values of 105-115, especially
if dew points get up to near 80F (plausible with the amount of
rain we`re concerned with over the next couple of days). WPC heat
index guidance shows about a 40-50% chance of 110+ on both
Wednesday and Thursday. Late in the week, the ridge gets more
elongated from west to east, slowing any substantial cooling
trend, and heat index values over 100 still appear likely.

MCS activity will be shunted north next week as the ridge builds,
but as we get toward Friday and Saturday, the flattening of the
ridge will bring the rain threat southward into our area.

Geelhart

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1224 PM CDT Sat Jul 19 2025

Thunder potential continues for another hour of two from KSPI-
KCMI, as weakening convection continues to push east-southeast.
MVFR/IFR conditions have been most prevalent with the rain, but
improvement to VFR is likely at all sites before mid afternoon.

Later into the forecast period, concern shifts to another
convective cluster that should move southeast out of Iowa
overnight. Some uncertainty as to the exact track, which will
dictate the potential for another round of MVFR ceilings, but
KPIA/KBMI have the highest threats at this time. Will include
PROB30 groups for both sites late tonight into Sunday morning for
this potential.

Geelhart

&&

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

$$