988
FXUS63 KILX 200507
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
1207 AM CDT Sun Apr 20 2025

.KEY MESSAGES...

- There is a level 2 of 5 (slight) risk for severe weather
  Sunday. All severe hazards are in play (tornadoes, damaging
  winds, large hail). Most likely timing would be 5pm to midnight.

- Thunderstorms tomorrow evening may also produce torrential
  rain. If there is training of storms, localized flash flooding
  may occur. The greatest risk will be near and west of roughly
  I-55.

- Chances for showers and storms return towards midweek. At this
  time, the chance for severe weather appears low (less than 5%).

&&

.DISCUSSION...
Issued at 130 PM CDT Sat Apr 19 2025

An elongated quasistationary front stretching from the Ozarks
into the Upper Ohio Valley remains the focus for repeated waves of
showers and a few thunderstorms this afternoon. Fortunately, the
remnant outflow boundary from this early morning`s thunderstorm
activity has shunted appreciable surface based instability south
of the ILX CWA, limiting potential for severe weather through the
afternoon.

Scattered showers will continue through tonight, mainly south of the
I-72 corridor (I-74 east of Champaign). A lower theta-E airmass will
be advected into the region from the east-northeast late this
evening as low level winds increase in advance of the next system: a
compact, negatively-tilted, fast-moving shortwave lifting from the
Southern Plains into Iowa.

***** Severe Weather Risk Sunday *****

12z CAMs indicate the boundary will be pulled back northward as a
warm front during the afternoon and evening tomorrow, landing
somewhere near I-74 before the cold front sweeps through from the
west during the mid-late evening timeframe. While cloud cover will
be in abundance tomorrow (which prompted us to lower high temps a
couple degrees), all indications are the capping inversion will be
rather weak and thus much easier to overcome than last evening`s
CIN. In addition, outflows from ongoing convection approaching the
region from the southwest should provide sufficient lift for storms
to overcome any cap in place and continue through our CWA amidst
1000-1500 J/kg MLCAPE. That being said, there will be 3
opportunities for severe weather tomorrow afternoon through tomorrow
night:

The first opportunity would be for something to form in the warm
sector during the late morning-late afternoon timeframe. This is
conditional on any ongoing storms or remnant outflows in our area
tomorrow morning, as something will be needed to overcome the
shallow cap, and it may be difficult for surface heating to do
that given all the low clouds that will be lingering. These
storms could go severe given 50-60 kt of 0-6km bulk shear and
again upwards of 1000 J/kg MLCAPE, especially if they can latch
onto the warm front where low level helicity will be maximized
(HREF mean is 250-400 m^2/s^2 along that boundary); this may prove
difficult as bulk shear vectors are pointed towards the east-
northeast, while the warm front will be oriented west-northwest
to east-southeast...though right movers in any splitting
supercells might be able to.

The second opportunity for severe weather will arrive as isolated
cells (ahead of the main line) developing upstream across MO and
lifting across our area during the late afternoon-evening timeframe.
The 12z HRRR, in particular, suggests one such supercell will ride
along/west of I-55 from 6-9pm, producing large hail and strong wind
gusts. Given LCLs generally less than 600m and the aforementioned
prolific low level helicity further increased (some guidance has
over 500 m^2/s^2 in the 0-1km layer) by the LLJ, any such isolated
supercell could also produce a tornado. Nonetheless, one iteration
of a CAM is hardly the basis for a forecast; yet it does give us a
glimpse into the realm of possibilities and heighten our awareness
for such a scenario. We`ll be watching closely.

The third opportunity for severe weather will come with the cold
front during the latter portion of the evening (8-9pm across our
west) into the overnight (midnight-1am east), as a line of showers
and embedded thunderstorms sweeps through the CWA. Storm mode would
suggest a lower risk for hail with this wave, but wind will be a
concern. The QLCS tornado risk is not entirely clear at this point,
as we typically want 0-3km line-normal shear magnitude > 30 kt and
guidance seems to suggest those shear vectors back to a more SSW to
NNE orientation right along and with the line of storms. The GFS
suggests anywhere from 40-50 kt of 0-3km shear with the line. For
reference, our minimum angle off of line normal required to meet the
30 kt line-normal requirement (for QLCS tornadogenesis) would be
41 deg with 40 kt of total shear and 53 deg with 50 kt of total
shear; obviously, this makes a big difference, so this is another
scenario we`ll be keeping a careful eye on heading into tomorrow
evening.

***** Flash Flooding Risk Sunday *****

A few of the CAMs suggest 6h precip amounts may exceed 2 inches
where any training of storms sets up - most likely with cells ahead
of the main line. While this will be localized, it does approach our
6h FFG, suggesting some flash flooding could occur. For what it`s
worth, WPC maintains a level 1 of 4 (marginal) risk for excessive
rainfall. The chance will be highest near and west of I-55.

***** Gusty Winds Sunday Night *****

We`ll experience a period of strong west-southwest winds Sunday
night behind the cold front The nocturnal timing should limit
mixing somewhat, though strong mesoscale subsidence favored in the
wake of the cold front may prove sufficient to bring down a
couple rogue gusts over 40 mph according to Bufkit soundings
(30-35 mph will be more common). Deeper mixing but a weaker
pressure gradient will make for still low confidence in gust
potential during the day Monday, but we did raise gusts to 30-40
mph across our northeast counties where it`s common for winds to
surpass the high end of the ensemble envelope during these cold
advection regimes.

***** Thunderstorm Chances Return Midweek *****

Behind the cold front, conditions will turn drier on Monday and
should remain too dry to support more precip until perhaps Tuesday
evening or night when an ill-defined mid level disturbance may spark
a couple showers and storms north of roughly a Macomb to Gibson City
line. GFS forecast soundings suggest a few hundred SBCAPE, and a
deep mixed layer to around 750-800mb which may allow for some gusty
outflow winds with any storms. The weak instability and lack of
appreciable shear, however, suggest the risk for organized severe
weather is low at this point.

Generally speaking, ensemble guidance suggests zonal flow continues
across the region through the remainder of the work week, though
individual global models suggest there may be an additional
disturbance or two capable of bringing some precip chances (20-40%).
Temperatures are slated to remain above normal each day with
forecast highs well into the 70s. In fact, NBM`s probability for
highs over 75 degF is greater than 50% for most/all of the CWA each
day Tuesday through Thursday (highest at 70-90% Wednesday).

Bumgardner

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1207 AM CDT Sun Apr 20 2025

As a warm front lifts northward overnight, winds will trend toward
the east/southeast by early morning and become fairly gusty
through the rest of the forecast period (mainly 20-25 knots). A
lowering of ceilings is expected to accompany the passage of this
feature, and chances of MVFR conditions are high (>80%) for about
3-5 hours around midday into mid afternoon.

Getting into the evening, a line of showers and thunderstorms will
quickly move eastward, generally in the 02-05z time frame, though
the intensity will wane as it gets close to the Indiana border.
Ahead of this line, a strong low level jet around 50 knots will
set up over eastern Illinois, and LLWS has been introduced for
KBMI/KDEC/KCMI.

Geelhart

&&

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

$$