Springfield, Illinois Weather Forecast Discussion

040
FXUS63 KILX 220752
AFDILX
Area Forecast Discussion
National Weather Service Lincoln IL
252 AM CDT Mon Sep 22 2025
.KEY MESSAGES...
- Scattered showers and embedded thunderstorms will continue this
  morning, mainly in areas south of I-72/Danville.
- The unsettled pattern will continue with daily rain chances
  through at least Thursday. The best chance of rain is Wednesday
  (60-80%). New rainfall amounts of over 2" are possible, with
  the best chance being south of I-70 (40-70%).
- Fog development, some of which could be dense, is possible
  tonight into the Tuesday morning commute. The best chance for
  visibility below a half mile is east of I-55 (40-70%).
&&
.DISCUSSION...
Issued at 249 AM CDT Mon Sep 22 2025
The unsettled pattern continues, although there are some periods
of lesser precip coverage anticipated over the next 36-48 hours.
At 06z/1am Monday, some areas of light rain lingered across the
ILX CWA. Further southeast, a more expansive area of
showers/storms was located beneath an area of upper level
divergence, and this feature is expected to pivot northeast into
this morning, resulting in increasing rain chances that are
highest with southeast extent (20- 40% along I-72, increasing to
50-80% southeast of a Shelbyville-to- Paris line). The rain
chances will gradually diminish across these areas into this
afternoon, after which point much of the CWA should see a break in
the rains as we will be positioned in between upper level waves.
This break in the precip should continue through most of Tues AM.
*** FOG POTENTIAL LATE TONIGHT - TUESDAY AM ***
The potential for dense fog will need to be monitored tonight into
Tues AM. While some high clouds could stream overhead in advance
of the next wave, HREF sky cover keeps ILX CWA mostly clear, which
combined with light winds will favor radiational cooling. The
airmass, which features mid 60s or higher dewpoints, won`t change
much between now and late tonight. Multiple guidance sources are
highlighting potential for fog, although not necessarily in the
same location. The HRRR in particular has output a broad swath of
dense fog across the eastern half of the CWA in multiple model
runs now. HREF probabilities for vis below a half mile are highest
(50- 70%) east of I-55, which seems logical given the expected
location of additional rainfall between now and then. Added patchy
fog mention to the official forecast, and later updates may be
needed to increase the fog coverage if current forecast holds.
*** RAIN CHANCES CONTINUE ***
While much of Tues morning looks to be dry, a slow cold front
sagging southward across IA is progged to approach the NW ILX CWA
Tues afternoon/early eve. Central IL will still be positioned
between two upper lows at this time, one over the Great Lakes and
another emerging onto the lee of the Rockies, but impulses ahead
of these respective waves will act to increase synoptic forcing.
So between the synoptic forcing and the approaching front,
anticipate shower coverage increasing Tues PM. High-res models
depict a weak sfc low (perhaps a remnant MCV) embedded within the
frontal zone, positioned near the Quad Cities. While weak, this
sfc low is an area where locally backed sfc winds could enhance
the wind shear, and moisture pooling along the front could
increase the instability and lower the cloud bases (LCLs). This
will be a feature to monitor for a localized severe threat
(including tornadoes) Tues afternoon NW of the IL River, although
this feature could stay just NW of the ILX CWA until daytime
heating is lost. Storm Prediction Center Outlooks do not include
the ILX CWA for Tues, suggesting the tornado (hail/wind) threat is
less than 2% (5%).
*** MID-WEEK SYSTEM ***
Guidance still shows a mid-week system, with a sfc low tracking
from the TX Panhandle to the Ozarks to either the Ohio River
Valley or north-central IN. Overhead, an expansive upper low will
be present from Wed through at least midday Thurs. PoPs from this
system are highest Tues night through the day on Wed, ranging from
around 40% NW of the IL River to 50-60% east of the IL River to
70-80% south of I-70. The latest rainfall forecast follows a
similar NW to SE gradient, with 0.5" to 1" forecast NW of the IL
River and anywhere from 1.25" to 3" along and south of
I-72/Danville. While these amounts are high, the parameter space
continues to look supportive.
Ensemble members for SE IL suggest PWAT values could exceed 2" at
times, which is quite rare for late September per the ILX sounding
climatology. 90th percentile PWAT values for this time of year
are around 1.5". Forecast soundings continue to show a period of
prolonged deep saturation, with skinny CAPE and a deep warm cloud
layer (over 13kft) present in those soundings, both of which
suggest efficient rainfall rates. It would not be a surprise if,
as high-res models start to capture this event, guidance starts to
suggest localized amounts exceeding 4" to 5". This potential,
albeit a low one, is already apparent in some of the coarser
ensemble members, with NBM data indicating a 5-15% chance for over
5" south of I-70. Again, the sfc low track will be key here, as a
track closer to the Ohio River Valley would keep the highest PWAT
air south of the ILX CWA (precluding such high rainfall amounts
from being realized in our coverage area). The latest WPC
excessive rainfall outlooks only contain marginal risks for
portions of the ILX CWA, which seems reasonable as the antecedent
dryness/drought and associated low streamflow will make it quite
difficult for any flooding issues to arise during this event.
By Thurs, models race the sfc low off to the eastern Great Lakes,
although cyclonic sfc flow and upper level troughing will persist
across IL. The expectation is that dryer (lower PWAT) air will
advect into the region during the day Thurs, so while a chance of
precip remains in the forecast (20-40%), any such activity should
be scattered and on the light side (new rainfall less than 0.25").
The NBM has a 20-30% chance for rainfall amounts exceeding 0.25"
Thurs- Thurs night, although this is likely due to timing
differences in the sfc low rather than the currently forecast
light showers on Thurs. Given the vorticity (spin) from the upper
low, can`t rule out funnel cloud development on Wed-Thurs,
although mid-level lapse rates are uninspiring both days.
*** LATE WEEK UNCERTAINTY ***
Forecast confidence decreases considerably Fri into the weekend as
models/ensembles vary wildly in the evolution of this upper level
system. Solutions range from the system never becoming cut off
from the primary jet stream and quickly being swept off to the
east, to becoming cut off and meandering/retrograding within the
mid-MS Valley for several days. Not only is there spread between
models, but individual models have been inconsistent from run to
run. With that in mind, I don`t have much meaningful to add to the
forecast discussion for this weekend beyond the fact that the
forecast will depend heavily on how this upper low evolves.
Erwin
&&
.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1208 AM CDT Mon Sep 22 2025
A slow moving upper low over the Upper Midwest will continue to
provide some periodic chances for showers and storms across
central Illinois with continued low confidence in specific timing
and placement. A decaying cluster of rain is moving across
central Illinois late this evening with more showers and storms
upstream across Missouri and southern Illinois. This next wave of
showers will approach the I-72 corridor Monday morning, but is
favored to stay just south of the terminals. Despite the better
precip chances staying just south, MVFR ceilings are expected to
overspread the I-72 corridor for a few hours Monday morning. A
few additional showers and storms may form Monday afternoon, with
precip favored near and south of the I-72 corridor and covered by
a prob30 group.
Deubelbeiss
&&
.ILX WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$

NWS ILX Office Area Forecast Discussion