257
FXUS63 KILX 182022
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
322 PM CDT Fri Apr 18 2025

.KEY MESSAGES...

- Scattered thunderstorms this evening will be capable of large
  hail and damaging wind gusts. There is a level 2 of 5 (slight)
  risk for severe weather. Most likely timeframe will be between
  5 pm and midnight, though the risk will linger south of I-70.

- Some training thunderstorms this evening into tomorrow may
  result in localized flooding, with the highest risk near and
  south of roughly I-70.

&&

.DISCUSSION...
Issued at 136 PM CDT Fri Apr 18 2025

An active weather pattern will continue across central and southeast
IL through the upcoming weekend, with a period of repose earlier
next work week. An area of low pressure will lift northeast into the
Great Lakes this evening, with a trailing, diffuse cold front
crossing portions of the region tonight. In the seasonably warm and
humid airmass ahead of that front, scattered strong thunderstorms
will fire this evening, bringing heavy rain, gusty winds, and even
some hail to parts of central and southeast IL. That front will
become quasistationary late tonight into tomorrow, and as a series
of waves ride along it, support multiple periods of additional rain
showers. Sunday, another system will approach from the west,
resulting in some chance for showers during the day and a better
opportunity for storms during the evening. Surface high pressure is
then progged to settle into the district Monday into Tuesday,
favoring a period of dry and benign weather.

***** Severe weather *****

Mid-level, convective remnant clouds continue to stream across the
region today, though we`ve gotten some breaks of sunshine which has
begun to slowly erode the strong capping inversion apparent on our
observed sounding from 12z/7am. Horizontal convective rolls (HCRs)
have developed near and west of roughly I-57, suggesting some
shallow instability beneath the layer of CIN. This all points to a
severe weather risk where and when that cap breaks, allowing the
development of deep convection. High PWATs and mid level dry air
point to water-laden downbursts capable of damaging wind gusts with
any storm this evening. In addition, the EML advected into the
region from the High Plains has resulted in 8-9 C/km mid-level lapse
rates per SPC`s mesoanalysis, conducive to fast parcel accelerations
in the hail growth zone and at least some hail risk. Storms will
likely continue for a bit after dark (and the consequent waning of
instability) given 50-60 kt of deep layer shear. At this time, the
severe risk is slated to be greatest between 5 pm and midnight,
though it could linger a bit longer south of I-70.

While conditional, another severe weather risk may materialize on
Sunday as a surface low tracks somewhere to our west and drags a
sharp cold front through a portion of our CWA. Low level directional
shear will be relatively strong near the triple point with that
system, and LREF ensemble mean brings several hundred J/kg MUCAPE
into the region ahead of the front, so we could wind up with a low
CAPE/high shear severe weather set up. At this time, however,
confidence remains low in exactly where that risk will be greatest.
SPC`s Day 3 update includes a level 1 of 5 (marginal) risk for
severe weather west of roughly I-57 and a level 2 of 5 (slight)
across several of our southwest counties.

***** Heavy rainfall *****

According to SPC`s mesoanalysis, PWATs across central/southeast IL
are sitting in the 1.1 to 1.2 inch range, and HREF mean brings these
up to 1.4 to 1.6 inches over the next couple hours - suggesting the
environment is primed to generate efficient rainfall-producing
storms. 50-60 kt of largely unidirectional cloud bearing layer (LCL
to EL) shear will promote fast storm movement which generally limits
flooding potential, though with one caveat: flooding can still occur
where repeated rounds of storms result from training. While this
could occur anywhere across the ILX CWA this evening, it appears
most likely near and south of roughly the I-70 corridor overnight
tonight into Saturday morning - when and where we have the heaviest
rain totals in the forecast, in line with HREF whose LPMM features
pockets of totals exceeding 2 inches. Given our 6h FFG from the RFC
is running 2-2.5 inches, flooding should be the exception and not
the rule this evening into tonight.

Tomorrow`s hydrological concerns will be largely driven by the
magnitude and location of the heaviest rain totals that occur this
evening through tonight. The CAMs generally depict a steady light
rain (south of I-72 during the morning, area-wide during the
afternoon) with perhaps a couple periods of moderate rainfall driven
by prolonged, weak forcing aloft atop the trailing boundary
lingering across our area. This suggests the flash flooding risk is
low (less than 10%), though any ongoing flooding from tonight`s
storms is unlikely to abate; in addition, any swollen creeks and
streams could receive enough additional water to finally overflow
their banks. Rain should diminish tomorrow evening across areas
mainly north of I-70 as low level east-northeasterly flow increases
in advance of the incoming system, importing a slightly less humid
airmass into the region.

Our mercurial models are still struggling to resolve the evolution
of that system, though middle of the road guidance suggests a
surface low will track across MO and into eastern IA Sunday into
Sunday night, with PWATs ahead of it surging to nearly 1.5 inches
once again. We may have some showers in the WAA wing ahead of it,
however the best forcing for ascent will be along the cold front
which should be moving relatively quickly. At this time, rainfall
amounts Sunday evening range from a quarter inch to an inch -
highest west of I-55 - and this we should be able to handle without
flooding issues aside from swollen rivers which will still be on the
rise from tonight and tomorrow`s rains.

Bumgardner

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1251 PM CDT Fri Apr 18 2025

Day cloud phase distinction satellite imagery shows convective
debris clouds drifting across the Prairie State, with horizontal
convective roll clouds around FL040 across much of central IL.
HREF guidance suggests a 20-40% chance resulting ceilings fall
below 3,000 ft, which will be something to watch throughout the
remainder of the afternoon though conceptually cloud bases should
rise during the afternoon as mixing deepens.

The bigger concern will be thunderstorms this evening into
tonight. High resolution model guidance still is not in consensus
with timing of storms in any given location, but the most probable
4-hour timeframe was used for a PROB30 group at each airfield.
Any storm could generate strong wind gusts and hail, in addition
to frequent lightning. Outside of storms, south-southwest winds
will gust to 30 kt at times this afternoon, easing slightly this
evening and veering to west-northwest behind a cold front
overnight. Continued showers and storms are anticipated especially
at SPI, DEC, and CMI overnight and into Saturday morning, where
there will be a 50-70% chance for MVFR ceilings.

Bumgardner

&&

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

$$