Decatur, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

146
FXUS63 KILX 041040
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
540 AM CDT Fri Apr 4 2025

.KEY MESSAGES...

- A Flood Watch is in effect for areas along and south of a
  Sangamon to Vermilion County line through Sunday morning. The
  greatest potential for heavy rain exists is of I-70, where there
  is a 25% chance of exceeding 5" of additional rainfall. The
  heaviest rains are expected Friday afternoon through Friday
  night.

- There is a slight risk (level 2 of 5) of severe storms this
  afternoon into tonight, with scattered large hail the primary
  hazard. A tornado is possible during the late evening (roughly 9
  PM to midnight) south of I-70 (5% chance).

- A hard freeze (lows below 28 degrees) still appears likely
  Monday night (50-90% chance). The cold temperatures will
  threaten any sensitive early season vegetation.

&&

.DISCUSSION...
Issued at 310 AM CDT Fri Apr 4 2025

The pattern starts off highly amplified, with a slow-moving upper
trough present over the western US. At the sfc, a front extended
from south-central TX to cntrl TN and continued off to the NE.
Multiple impulses will lift along the frontal zone through the
weekend, resulting in several periods of showers/storms locally
and making heavy rain/flooding the top concern this forecast
period. MRMS 48-hour rainfall estimates show 1.25-3" has fallen
south of I- 70, with totals more variable north of I-70 (ranging
from less than a half inch in some spots to over 2" in others).

*** Heavy Rain/Flooding ***

The expectation remains that heavy rain chances increase with
southward extent, but there is uncertainty over exactly how far
north the precip gradient will be located. A very moist airmass is
expected to overrun the frontal zone resulting in precip pushing
into the ILX CWA by late this morning or early afternoon.
Precipitable water (PWAT) values are forecast to be around 1.4" in
Lincoln, which would be near the max of the ILX sounding
climatology, and further SE (south of I-70) in the primary
moisture plume PWAT values could exceed 1.8". Forecast soundings
show a skinny instability profile (500-1000 J/kg of MUCAPE)
present above a sharp warm nose, which combined with a warm cloud
layer in excess of 12 kft deep and the highly anomalous PWAT
values will be supportive of efficient rainfall rates. The latest
HREF (04.00z) has a 70-90% chance of over 2" of rain south of I-70
through Sat eve, and the NBM indicates a 25% chance of over 5" of
rain. Current 6-hr flash flood guidance is around 2-3" across SE
IL, and those values could decrease as this prolonged event
unfolds.

A tight gradient exists in the rainfall probs, such that the HREF
probability of exceeding 2" of rain is less than 25% north of
I-72. However, some models, namely the NAM suite, produce
localized amounts over 4" as far north as I-72, so did opt to
expand the flood watch to include areas from Sangamon to Vermilion
County and southward.

There is a slight risk (at least 15% chance) of flash flooding
within 25 miles of a given location across the flood watch area
both today and Saturday. A moderate risk (at least 40% chance) of
flash flooding clips the far southern portions of the ILX CWA both
days, highlighting the increasing potential of flooding impacts
with southward extent. In terms of main stem river flooding, river
forecast ensembles indicate at least a 30% chance for moderate
flooding across the ILX hydrologic service area (HSA) south of
I-70. Just to the south and east of the ILX HSA, there are several
rivers with at least a 30% chance of major flooding. For the
latest official river forecasts, refer to www.water.noaa.gov.

*** Severe Storm Potential ***

Through most (if not all) of Fri/Fri night, these showers/storms
will be located north of the sfc front. As noted above, modest
elevated (non-sfc based) instability will increase into Fri PM
with MUCAPE values around 500-1000 J/kg. With a strongly sheared
environment, this could support scattered instances of large hail.
With a stout inversion and saturated profile (limiting the
evaporative cooling within downdrafts), damaging winds are less of
a concern. Tornadoes are not expected with any of activity north
of the front, but it`s worth noting that a few models (namely the
NAM suite) show the front surging northward late Fri eve just far
enough to provide a brief window (roughly 9pm - midnight) where
sufficient sfc based instability could advect northward to support
tornadoes given the dynamic wind fields (0-1 km shear of 50
knots). A 5% tornado risk was added for areas south of I-70.

While additional precip is expected on Sat (mainly SE of I-72),
the front will be well south and models show little to no
instability present, to the point that most of the thunder mention
was removed from the forecast during the daytime hours Sat. Thus,
no svr storms are expected locally during that period.

*** Extended Forecast ***

Temps trend cooler through the weekend, with highs struggling to
reach the 50s by Sun, which is about 10 degrees below normal. A
trough digging into the Upper Midwest early next week combined
with a trailing Canadian sfc high will usher in an additional push
of cold air. These below normal temps will provide a few chances
for below freezing low temps Sunday night (60-70% chance east of
I-55, lower to the west) and Monday night (greater than 90%
chance). There is potential for a hard freeze (below 28F) Monday
night (70-90% chance north of I-70, 50-70% chance to the south of
I-70). These cold nights will threaten any sensitive early season
vegetation. As that aforementioned sfc high slides east, a gradual
warming trend is expected from the middle of next week into next
weekend.

Erwin

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 540 AM CDT Fri Apr 4 2025

Widespread showers will overspread the area from south to north
around 18-19z, along with ceilings lowering to MVFR. Showers will
continue through much of the afternoon and evening, and a few t-
storms are possible at the I-72 terminals (KSPI/KDEC/KCMI) during
the evening. Considered a PROB30 for -TSRA mention, but opted
against including thunder mention in the TAF at this time.
Ceilings lower to IFR during the evening and are expected to stay
IFR through the remainder of the period. Shower coverage should
start to diminish after 06z. Winds will be easterly during the
day, gradually turning to north-northwesterly overnight.

Erwin

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Sunday morning for ILZ044>046-051>057-
061>063-066>068-071>073.

&&

$$