Decatur, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

666
FXUS63 KILX 250656
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
156 AM CDT Fri Apr 25 2025

.KEY MESSAGES...

- Scattered showers and storms today will generate heavy rain,
  resulting in a low (5%) risk for localized ponding of water in
  low-lying and urban areas.

- Temperatures will trend cooler tomorrow when forecast highs are
  in the 60s, but rebound quickly early next week. There is a
  30-50% chance high temperatures exceed 85 degrees area-wide
  Monday, and similar chances south of I-70 on Tuesday.

- There is a 15% chance for severe weather Monday night west of
  the IL River, and elsewhere on Tuesday. This corresponds to a
  level 2 of 5 (slight) risk.

&&

.DISCUSSION...
Issued at 153 AM CDT Fri Apr 25 2025

Mid level water vapor satellite shows clusters of thunderstorms
drifting across the Great Plains early this morning. Ahead of a weak
mid level disturbance, flow across the Prairie State is transporting
moisture northeastward with up to around 500 J/kg MUCAPE fueling a
couple isolated showers and storms.

***** Showers and Storms Today *****

As the mid level shortwave lifts over the region later today, a weak
surface low will meander east across central IL, sparking a few
additional showers and storms which will end northwest to southeast
this evening. An area of 60+ degF dewpoints, limited in northward
extent by a lake breeze which pushed south off of Lake Michigan last
evening, is expected to expand northward to roughly the I-74
corridor with and ahead of that surface low by early afternoon. This
seasonably rich low level moisture will support anywhere from 500-
1500 J/kg of SBCAPE which in forecast soundings looks pretty tall
and narrow (low NCAPE values) due to poor mid level lapse rates
(HREF mean is 6-6.5 C/km). As a result, deep warm cloud processes
will be conducive to moderate-heavy precip with storms, which will
also be moving slowly. The HREF LPMM suggests some highly localized
pockets of 2+ inches of rain may occur beneath the heaviest showers
and storms, which could lead to brief nuisance flooding. That being
said, not everyone will get in on these showers; a blend of NBM and
HREF suggests 12 hour PoPs from 7am-7pm will range from 40-50% west
of I-55 to 50-70% further east. As the surface low pulls away
towards the east this evening, its trailing cold front, bolstered
east of I-57 by another Lake Michigan lake breeze, will shift
southward across the ILX CWA. Northerly winds in its wake will gust
20-25 mph, making for a chilly evening as temperatures fall into the
50s.

***** Cooler Saturday and Sunday *****

Winds should stay up enough to keep temperatures from hitting rock
bottom tonight into tomorrow morning, but the continued cool
advection will hold highs a bit cooler tomorrow as well. Forecast
highs are generally in the mid 60s, which, though cooler than what
we`ve grown accustomed to this past week, are only 1-3 degrees below
seasonable normals. In addition, 00z HREF guidance suggests it`ll be
a mostly sunny day with winds easing in the evening as surface high
pressure settles in from the north. With the lack of cloud cover and
light winds, Sunday morning will be cooler with NBM giving a 30-50%
chance for lows below 40 along the I-74 corridor; this will bear
watching for frost potential, though when we have frost 2m temps
usually are in the mid 30s and confidence is lower (10-25%) in that
happening at this time. Low level flow veers to southeasterly on
Sunday behind the departing surface high, and 1000-500mb thickness
increases ahead of a deepening trough in the Great Basin, spelling
the beginning of a warming trend for us. Highs will kiss 70 again in
many places across central IL, especially west of the I-55 corridor.

***** Breezy and Warm Monday *****

The approach of a developing lee cyclone will tighten the surface
pressure gradient across the area on Monday as high pressure slowly
shifts east. Southerly winds will thus increase with gusts over 30
mph common during the afternoon and evening - highest across our
west. A 50-60 kt LLJ intensifies over the northwest portion of the
CWA Monday evening, though forecast soundings suggest a capping
inversion will shield the surface from the strongest winds aloft.
Nonetheless, if mixing is as deep as the GFS suggests (a function
of cloud cover), Bufkit mixing techniques would suggest some
sporadic gusts of 40-45 mph may occur. Temperatures in this strong
warm air advection (WAA) regime will soar into the 80s area-wide,
and with enough sunshine we may surpass 85 degF in many spots
(NBM chances are 30-50%, highest southwest).

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

The biggest concern during the forecast period at this point is
severe weather Monday and Tuesday. It remains a conditional threat,
and since the system responsible for our potential storms is
currently off the California Coast the global models are
understandably in some disagreement on specifics; of course, they
also lack the resolution to capture upstream convection which will
inevitably have implications for our local storm risk. SPC maintains
a broad 15% risk across our northwest counties for Monday-Monday
night, and the rest of the area Tuesday (which seems prudent taking
all of the extended global guidance together), though ultimately who
sees severe storms on which day will be a function of the cold front
position and upstream convection which of course are not going to be
clear that far out.

The past several iterations of the deterministic GFS and ECMWF
suggest the cold front will be positioned through the Great Plains
and Upper Midwest - well upstream - on Monday, with abundant
instability ahead of it. LREF mean MUCAPE runs from 1800 to 2500
J/kg across our CWA (with even higher values to our west), though
forecast soundings suggest this will be rooted near 700mb and atop a
robust cap. Frontal forcing and cooling temps aloft should erode the
cap upstream, though that would be a more difficult feat for us to
achieve this far east. Consequently, we should stay dry during the
daylit hours Monday. Eventually, upstream storms will make their way
towards us from the west/northwest, and if their outflow is strong
enough to overcome our CIN they may continue through a portion of
our area Monday night (particularly if the more progressive model
solutions materialize). Eventually, the lower shear environment
should cause the storms to slow down and fall apart, though they may
end with some heavy rain given near record PWATs between 1.3 and 1.5
inches.

At some point Tuesday, storms will refire along the cold front,
though the upper low and more favorable kinematics for severe
weather will be well displaced to the north by that time. In
addition, low level shear vectors will be oriented more parallel to
the cold front on Tuesday, favoring a linear storm mode and a lower
tornado threat compared to Monday evening-night. When and where
Monday night`s storms fall apart, as well as the timing of the cold
front, will be key in precisely where Tuesday`s wave of storms fire;
however, the risk seems highest across our southeast counties.

***** Rest of the Week *****

Since the cold front will be growing increasingly displaced from its
forcing as it shifts southward late Tuesday into Wednesday, it`ll
slow down and eventually stall somewhere across the Ohio/Tennessee
Valley Region. Consequently, we can`t quite guarantee a dry
period through the end of the work week at this point though we
don`t expect a wash out as the prolonged period of continued PoPs
might imply. If and when it becomes clear that that boundary will
clear the area, PoPless periods of the forecast will reflect it.
Highs the second half of the work week are currently forecast to
run near climatological normal for the close of April/beginning of
May - around 70 degrees - though these will also be a function of
the front`s position. We`ll continue to monitor.

Bumgardner

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1231 AM CDT Fri Apr 25 2025

A surface low will lift northeast across the area today, bringing a
few chances for scattered showers and storms. Activity noted over
eastern Missouri early this morning will lift north through mid
morning, potentially impacting a few of the the terminals. However,
the best chance for thunderstorms will be near/east of I-55 this
afternoon ahead of a cold front. Winds will shift to the northwest
this afternoon as the front moves through then veer to the north
later tonight and increase in speed.

Low stratus will develop tonight, with guidance favoring MVFR
ceilings mainly at KPIA and KBMI prior to 06Z. HREF probabilities
for <3k ft ceilings are between 40-60% at the previously mentioned
airfields, with low chances south of there.

NMA

&&

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

$$