Thomasboro, Illinois Weather Forecast Discussion

530
FXUS63 KILX 211049
AFDILX
Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
549 AM CDT Sun Sep 21 2025
.KEY MESSAGES...
- An unsettled pattern continues with periods of showers and
  storms throughout the next week. The highest rain chances are
  tonight into Monday morning (60-80% chance).
- There is a marginal risk (level 1 of 5) of severe storms this
  afternoon. Strong wind gusts are the main concern, although
  isolated hail up to the size of quarters could also occur.
- Through the end of the week, the potential for heavy rainfall
  remains highest south of I-72/Danville where there is a 40-70%
  chance for total rainfall amounts of 2", and a low chance
  (10-20%) for amounts over 4".
&&
.DISCUSSION...
Issued at 230 AM CDT Sun Sep 21 2025
*** TODAY - MONDAY MORNING ***
Unsettled conditions persist as a broad, `double barrel` upper low
sits over the Upper Midwest early Sun AM. Weak low-level WAA as
well as some fgen focused near the IL/IN border keep some pockets
of showers/storms ongoing as of 0715z/215am CT. Into the day, the
21.00z HREF has PoPs around 20%, but CAM coverage has often been
too low in this pattern so opted for PoPs as high as 30-40% across
much of the area this afternoon.
I maintain a degree of concern that a conditional severe threat
exists this afternoon. Storms yesterday produced isolated 1" hail
reports, and even a tornado (it appears there was influence from a
storm scale boundary with that one), and the parameter space for
today appears marginally more favorable than yesterday. Forecast
soundings show CAPE values of 1500-2000 J/kg, and while deep layer
shear is lackluster (20-25 kts), some soundings do show cloud
layer shear approaching 30 kts. DCAPE values of 1000 J/kg and mid-
level lapse rates of 7-7.5 degC/km support near-severe wind/hail
potential, respectively. Despite the tornado that occurred
yesterday, still think that potential is low today (non-zero but
less than 2%) as there`s nothing jumping out in guidance to
suggest a meaningful potential for tornadoes. NadoCast machine
learning guidance has the probability of a tornado at less than
1%. An SPC marginal risk (level 1 of 5) encompasses much of the
CWA today (for the wind threat).
Temps are forecast to reach the low/mid 80s again today, but
finally cool back to seasonable values on Mon (mid/upper 70s) and
remain near those values for the rest of the work week. Precip
coverage will increase into the late evening/overnight hours as
the next shortwave rotates towards the area, with high probability
(over 80% chance) that areas along and east of the IL River see
rain tonight. The HREF LPMM suggests localized rainfall amounts
could reach 1- 1.5". The probability of exceeding a half inch of
rain is highest along/south of I-72/Danville (40-70% chance). The
timing of this wave suggests showers/storms will linger into the
Monday morning commute across eastern IL (70-80% chance east of
I-55).
*** MONDAY AFTERNOON AND BEYOND ***
Mon night into Tues, the upper pattern will be characterized by
an upper trough over the Great Lakes (slowly shifting east) and
our next wave digging towards the central Rockies. In between
these two systems, guidance hints at various weak impulses
pivoting around or ahead of the primary upper lows, which along
with occasional periods of low-level WAA should keep
showers/storms percolating. Eventually, these two waves are
expected to merge, forming an expansive upper low right over IL
that slowly shifts towards the Ohio or mid-TN Valleys by the
weekend.
This mid-week system will keep the rain chances going through the
latter half of the work week, with PoPs/accumulation chances
increasing with southeastward extent due to the expected track of
the sfc low being just south of the ILX CWA. Latest ensemble
guidance maintains a 40-60% chance for 2" of total rainfall in
areas between the I-72 corridor and the I-70 corridor, with a
60-70% chance south of the I-70 corridor, which is where the
latest ECMWF- based "Shift of Tails" is highlighting potential for
an outlier rainfall event. There is even a low chance for total
rainfall to exceed 4" (10-20% south of I-72/Danville). The latest
deterministic QPF from the NBM has started to tighten the expected
rainfall gradient, ranging from 0.75" to 2" across the northern
half of the area and 2-3" across the southern half. This trend is
consistent with the last several days of ensemble signal in
suggesting the highest rainfall totals will generally be south of
I-72/Danville. (All of the aforementioned probabilities/amounts
include the expected rainfall tonight/Mon AM).
The severe storm potential appears low with the mid-week wave, as
central IL is on the north side of the sfc low track and the joint
probability of exceeding 500 J/kg of CAPE and 25 knots of deep
layer shear during the Wed-Fri period is less than 40% area-wide.
That being said, it would not be a surprise if there were some
cold air funnels, particulary on Thurs as the upper low and
associated vorticity are positioned directly over IL.
Into the weekend, the positioning of the system should lead to a
gradual lowering of rain chances locally, but we can`t yet rule
out rain across eastern IL this weekend. The latest blended
guidance has a 15% chance for rain along/east of I-57 on Sat, with
PoPs increasing as you move east into IN. If the system trends
slower (as closed upper lows often do), PoPs would likely
increase/expand westward for the weekend. Eventually, there is a
signal for a broad, sfc high pressure to settle into the
Midwest/Great Lakes behind this system, and while the exact timing
is uncertain, this should provide a break from the daily rain
chances.
Erwin
&&
.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 549 AM CDT Sun Sep 21 2025
Periods of MVFR vis are possible at the terminals during the first
hour or two of the period, but dense fog is expected to stay west
of the IL River and not impact the terminals. Periods of scattered
showers and storms remain the main concern this period. Coverage
should be sparse through the morning and afternoon. Contemplated
adding a thunder mention to the PROB30 group, but given the
expected limited coverage opted against. A more widespread waves
of showers and embedded storms will arrive late evening, moving in
from the southwest after 03z. Precip chances will then persist
through the rest of the period.
Erwin
&&
.ILX WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$

NWS ILX Office Area Forecast Discussion