Decatur, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

068
FXUS63 KILX 260246
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
946 PM CDT Tue Aug 25 2026

.KEY MESSAGES...

- Thunderstorm chances (30-50%) return as early as Wednesday
  afternoon, with damaging winds and hail as the main threats.
  Flash flooding is not a risk.

- Pleasant and dry weather is anticipated from Thursday into
  Saturday, followed by an approaching heat dome early next week
  that could drive heat index values up to 100 degrees.

&&

.UPDATE...
Issued at 944 PM CDT Tue Aug 25 2026

Some of the new CAMs are showing more light showers overnight
across more of central Illinois and into east central IL, and
also into the morning hours. So made some adjustments to add
some 20 pops in the forecast for parts of central and east
central IL. Rest of forecast looks, but did increase chance pops
for tomorrow late afternoon. Update should be coming soon.

&&

.DISCUSSION...
Issued at 1248 PM CDT Tue Aug 25 2026

Quiet and low-impact weather will persist through late afternoon
under the influence of a mid-level ridge axis and lingering surface
high pressure anchored over the eastern Great Lakes region.
Temperatures remain just below typical late-August normals,
with afternoon highs peaking near 80F beneath scattered high
cirrus.

The pattern transitions quickly tonight as low-level warm air
advection (WAA) and broad-scale isentropic ascent ramp up across
central Illinois. An amplifying shortwave trough over the
northern Plains will pivot eastward into the Upper Midwest
overnight, driving an increasing 30-35 kt 850 mb low-level jet
(LLJ). Moisture transport focusing along the nose of this LLJ
will drive a corridor of theta-e advection and synoptic forcing
across the CWA. Recent convective- allowing models (CAMs)
continue to trend more aggressive with nocturnal shower potential
overnight into early Wednesday morning. Forcing for ascent will
be further bolstered by a pre- frontal trough propagating into
western Illinois after Midnight before reaching the I-57
corridor toward daybreak. Expect scattered showers to focus
primarily north of the I-70 corridor through sunrise Wednesday.
The combination of expanding mid-level cloud cover, persistent
WAA, and sporadic precipitation will limit nocturnal
radiational cooling, holding overnight lows in the mid-to-upper
60s.

A more noticeable pattern shift arrives Wednesday afternoon as the
shortwave trough and associated surface cold front track into the
Great Lakes. Moderate boundary layer destabilization is expected
ahead of the advancing front as a narrow plume of low-70s dewpoints
pools over the region, helping yield MLCAPE values around 2500 J/kg
by mid-afternoon. Overlapping this buoyancy with ~30 knots of 0-6 km
bulk shear could favor organized multi-cell and semi-discrete
supercellular structures along and ahead of the boundary. Deep-
layer shear vectors oriented largely parallel to the initiating
frontal zone alongside forecast mesoscale soundings featuring
straight or subtle veer-back-veer hodographs point to a large
hail or damaging wind threat over an isolated tornado risk.

CAM coverage signals remain somewhat mixed Wednesday afternoon. The
primary upper-level vort max will be displaced well to our north,
leaving synoptic-scale forcing over central Illinois somewhat
poor and potentially limiting total storm coverage.

Furthermore, limited moisture return ahead of the front keeps
total QPF low. With forecast PWATs around 1.5 inches and
progressive storm motion, the flash flooding risk from
convective rainfall is quite low.

Following frontal passage Wednesday evening, cool and dry conditions
take hold for the end of the workweek into the weekend. An
elongated Central U.S. upper ridge will build east, driving an
expansive surface high pressure system and a continental polar (cP)
airmass across the Midwest and Great Lakes. Expect comfortable
conditions with afternoon highs hovering right around 80F
Thursday and Friday. Lowering dewpoints into the 50s will
support efficient radiational cooling overnight, dropping low
temperatures into the mid-to-upper 50s.

By early next week, medium-range global ensembles indicate a notable
warming trend taking shape. A strong upper-level trough making
landfall in the Pacific Northwest will amplify and nudge the central
CONUS ridge axis into the Midwest. The combined presence of a
588594 dam upper height dome and persistent low-level return
flow on its western flank will drive highs back into the upper
80s to low 90s. With dewpoints rebounding back into the upper
60s to around 70F, maximum heat index values could approach
triple digits by Monday and Tuesday.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 649 PM CDT Tue Aug 25 2026

VFR conditions will prevail at all sites through the
period...outside of any thunderstorms. High clouds are expected
tonight but lower mid clouds will move over the sites overnight
with possible high based showers. Even though possible, will not
add showers for overnight given if they occur, it will not
reduce cig or vis. After a lull during the morning hours, a cold
front will approach. More CU will develop ahead of the front in
the afternoon. Storms are possible, but based on timing of some
of the CAMS, think PROB30 only needed at SPI, DEC, CMI. However
will have VCTS at PIA and BMI. Chance of storms will be only in
afternoon.

Winds will be light and variable through the night then become
more southwest tomorrow morning. Winds become more southwest
around 10kts when the front approaches and then become west-
northwest behind the front. Wind speeds behind the front will
over 10kts.

&&

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

&&

$$

KEY MESSAGES...MJA
UPDATE...Auten
DISCUSSION...MJA
AVIATION...Auten