Watson, Illinois Weather Forecast Discussion

122
FXUS63 KILX 201049
AFDILX
Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
549 AM CDT Sat Sep 20 2025
.KEY MESSAGES...
- Unsettled conditions continue over the next week, with frequent
  chances for scattered showers and storms. Over the next 48
  hours, rain chances are highest during the evening/night, with a
  40-50% chance tonight, and a 50-60% chance Sunday night.
- A few storms could be strong to severe Sunday afternoon near and
  east of I-57. Strong wind gusts are the main concern.
- Over the next week, the best chance for total rainfall amounts
  to exceed 2" is in areas south of I-72/Danville (40-60% chance).
&&
.DISCUSSION...
Issued at 217 AM CDT Sat Sep 20 2025
*** TODAY ***
Unsettled conditions will persist through the weekend as a slow-
moving, negatively-tilted upper level trough is positioned just
northwest of IL. Early Sat AM, scattered storms west of the IL
River produced a subtle outflow boundary that was positioned from
Marshall to Sangamon counties as of 07z/2am, moving eastward in
time. While CAMs show little in the way of precip development
locally through the remainder of the pre-dawn hours, there are
various weak forcing sources between the proximity of the upper
low and its embedded impulses, the aforementioned subtle outflow
boundary, and some weak instability (about 500 J/kg) to work with.
Due to this, and the poor performance by the CAMs over the last
24 hours, opted to maintain a isolated to scattered precip mention
across most of the area through the pre-dawn hours and through
the day today.
A wing of WAA focuses across the area late this afternoon into
tonight and should lead to persistent scattered precip. PoPs are
higher this evening/tonight (40-50%) compared to during the day
(20- 40%). These showers/storms will be hit or miss but localized
areas of over 0.50" of rain could occur, however the probability
of exceeding such amounts at any one location (through Sun AM) is
just 10%.
*** SUNDAY ***
If taken at face value, CAMs suggest a relative lull in coverage
late Sun AM/early afternoon. Models show higher (mid 60s)
dewpoints present along/east of I-55 Sun PM, which combined with
cool temps aloft owing to the upper trough suggest modest
instability (1500- 2000 J/kg) developing by Sun PM. Soundings
appear fairly saturated, and there are questions as to how much
we`ll recover if there`s morning storms and/or subsidence left in
their wake. Deep layer shear is also fairly weak (in the 20-30 kt
range), but with modest mid-level lapse rates around 7 degC/km the
environment appears conditionally support of a marginal severe
threat across eastern IL Sun afternoon. The main concern would
seem to be pulse severe updrafts that produced severe wind gusts,
although the SARS analogs do correspond to a handful of severe
hail (0.75 to 2") events. Again, this is a conditional severe
threat, mainly near/east of I- 57, and the latest CAMs are pretty
sparse with the storm coverage Sun PM. Current forecast has a
40-50% chance of precip across E IL Sun PM. Precip chances are
once again expected to increase nocturnally, rising to 50-65% Sun
night owing to the arrival of yet another shortwave from the west.
*** NEXT WEEK ***
The upper troughing to our north will persist into the week,
keeping the unsettled pattern going with occasional precip
chances as upper level impulses pivot towards are area. The precip
chances are set to ramp up late Tues through Thurs as another
upper low ejects onto the central Plains and interacts with the
lingering upper troughing over the Great Lakes. The net result is
the formation of a deep, slow- moving upper low over IL and the
Midwest, with an associated sfc low favored to track across the
southern third of IL on Wed into Thurs.
While I`ve been skeptical of the medium range rainfall amounts due
to the ongoing drought, there is an undeniable and consistent
signal in forecast guidance for appreciable rainfall with this
mid/late week system. Within the warm sector of this system, PWAT
values in excess of 1.50-1.75" are possible, which combined with
strong synoptic forcing should lead to ample rainfall. There are
questions about the northward extent of rainfall from this system,
but at least south of I-72/Danville the latest Grand Ensemble has
a 40-60% chance of total rainfall over 2" through Thurs (as
opposed to 25-40% north of I-72/Danville). When looking at
ensemble clusters (scenarios created by grouping ensemble members
with similar upper air patterns), each cluster has total rainfall
amounts over 2" south of I-72/Danville, increasing confidence that
such amounts will be realized. Deterministic guidance also
appears supportive of this signal, as forecast soundings for SE IL
show deep saturation with skinny CAPE and a deep warm cloud layer
(in excess of 11-12 kft), all of which support efficient rainfall
rates. Finally, the ECMWF- based "Shift of Tails" is highlighting
non-zero values across areas south of I-72/Danville, indicating
that at least 10% of the ens members are depicting an outlier
event (above the 99th percentile of the model climatology for this
time of year). One of the bigger questions at this stage is what
track the sfc low takes. One near the I-70 corridor would favor
higher rainfall amounts across our area, particularly the
southeastern counties, while a track near the Ohio River Valley
would result in lesser precip totals for the ILX CWA.
In summary, the probability of 2+ inches of rain this week
increases with southeastward extent, with a meaningful,
consistent ensemble signal supporting such totals across areas
south of I-72/Danville (40-60% chance).
Guidance keeps the upper low over our area through at least
Friday, although differences become considerable in the evolution
of the sfc low or the timing of embedded shortwaves swinging
through our area. While confidence in the specifics is lower, I do
anticipate unsettled conditions and associated PoPs persisting
through the late parts of the work week as we remain beneath the
upper low/in the cyclonic sfc flow regime. Temps during this
period will be seasonable, if not a tad below normal, with highs
in the mid 70s Wed- Fri. A quick glance at the 8 to 14 day outlook
shows that above normal temps are favored (40-60% chance) for
late Sep-early Oct. Normal low temps during early Oct are in the
upper 40s, so it should still be a few weeks before we have to
give any thought to frost/freeze potential. On average, the first
freeze for central IL occurs in mid-October.
Erwin
&&
.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 547 AM CDT Sat Sep 20 2025
VFR conditions are favored through the period. Winds will be light
and variable this morning, becoming southwesterly for the
afternoon. Confidence remains low in the specific timing/location
of showers/storms during the period. Made two changes to the TAFs
from the previous forecast. First, introduced VCSH at all
terminals (except KPIA) this morning, as high-res models have
consistently shown development between 14-17z. Confidence in
thunder was too low to add during this time frame, but if these
showers do form thunder is a possibility. The other change made
was to expand the duration of the PROB30 showers during the
overnight hours, now encompassing 03-09z.
Erwin
&&
.ILX WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$

NWS ILX Office Area Forecast Discussion