Decatur, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

697
FXUS63 KILX 162314
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
614 PM CDT Sat May 16 2026

.KEY MESSAGES...

- There is a Marginal Risk (level 1 of 5) for severe weather
  through tonight. The highest probability for thunderstorms
  with damaging wind gusts will focus across the Illinois River
  Valley from late evening into the overnight hours.

- Windy and very warm conditions are expected Sunday and Monday
  as highs surge well into the 80s.

- The next significant chance for convection will come into the
  picture Monday into Tuesday as a slow-moving cold front
  approaches from the west.

&&

.DISCUSSION...
Issued at 250 PM CDT Sat May 16 2026

*** Scattered Thunderstorms Tonight ***

An outflow boundary created by early morning convection across
north-central Illinois became the primary forcing mechanism for
additional showers and thunderstorms along the I-72 corridor
midday today. This activity has subsequently weakened and
shifted just south of I-72...with 19z/2pm radar imagery showing
showers focused along a Quincy to Terre Haute line. Due to
extensive cloud cover, MLCAPEs across much of central Illinois
are currently 1000J/kg or less. Meanwhile further south toward
the I-64 corridor where sunshine has been more prevalent,
MLCAPEs range from 1000-2000J/kg. Scattered cells are beginning
to form in the moderately unstable/weakly sheared environment
from Saint Louis eastward to Evansville where a few strong
storms with gusty winds will be possible into early this
evening.

The greater focus for additional convection will be further west
across Missouri into southern Iowa where an even more unstable
airmass characterized by MLCAPEs of 2000-3000J/kg exists. As the
nocturnal low-level jet strengthens from northern Texas to
southwestern Iowa later this evening, thunderstorms will develop
west of the Mississippi River. Some of this activity will spill
into the Illinois River Valley as it lifts northeastward, posing
a gusty wind risk from roughly 11pm to 5am. Have therefore
focused the highest PoPs (60-70%) across the Illinois River
Valley tonight. Further east, several CAMs suggest a few cells
forming along the outflow boundary as it lifts northward
overnight. Since this is even further away from the low-level
jet energy, am not expecting widespread or organized convection
along/east of I-55.


*** Severe Weather Risk Monday into Tuesday ***

All models show central Illinois firmly in the warm sector on
Sunday, as S/SW winds gusting 25-30mph push afternoon highs into
the middle to upper 80s. Things get more interesting during the
Monday/Tuesday time period as a significant trough and its
associated cold front come into the picture. The 12z May 16
models are in much better agreement concerning the timing of
this process...with both the GFS and ECMWF showing FROPA Tuesday
afternoon/evening. Given this trend, confidence is growing that
significant thunderstorm chances will likely hold off until
Monday night at the earliest. While the current forecast
features likely to categorical PoPs Monday afternoon, think
these will eventually be lowered...as the primary focus for
widespread storms will initially be much further west across
eastern Kansas/western Missouri immediately ahead of the front.
This convection will spill into the Illinois River Valley Monday
night, potentially posing a damaging wind risk. The storms will
likely decrease in intensity overnight as they push eastward
across the I-55 corridor. The most recent SPC outlook highlights
areas west of I-57 with a 15-30% chance for severe.

With a slower FROPA expected, the likelihood of convective
re-development across parts of central Illinois on Tuesday
continues to grow. While it is still too early to pinpoint the
exact development zone, most models and conventional wisdom
suggests it will be along/east of I-55. SPC shows a 15-30%
chance for severe along/east of the Illinois River, and this
seems reasonable until minor timing discrepancies and mesoscale
details are better resolved.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 613 PM CDT Sat May 16 2026

A warm front will lift north across the region overnight. There
is some variability in the wind direction currently in the wake
of thunderstorms earlier today, but should eventually favor a
SE direction with approach of the warm front then will veer to
southerly with gusts increasing into the mid 20 kt range Sunday.
Showers and storms are expected along the warm front over
Missouri and Iowa tonight, with eastward extent of the storms
possible into portions of west central or central Illinois.
Confidence is highest (around 30 percent) for PIA to see precip
overnight.

&&

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

&&

$$

KEY MESSAGES...Barnes
DISCUSSION...Barnes
AVIATION...Deubelbeiss