602
FXUS63 KILX 171937
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
237 PM CDT Fri May 17 2024

.KEY MESSAGES...

- Isolated showers and storms (15-30% chance) east of the Illinois
  River through sunset this evening. Severe thunderstorms are not
  expected.

- Fog development is likely tonight across eastern IL, with a 40-70%
chance of visibility falling below 1 mile east of I-55.

- Severe thunderstorms are possible next week on Tuesday and/or
  Wednesday. The exact timing remains uncertain at this range.

&&

.DISCUSSION...
Issued at 231 PM CDT Fri May 17 2024

** THROUGH SUNDAY **

A weakly forced pattern exists across IL this afternoon, with the
state sitting between lower sfc pressure in the northern Plains
and weak sfc high pressure over the mid-Atlantic. Vis sat shows
two corridors of more agitated Cu development at 140pm/1840z, one
from Quincy to Bloomington, and another from Shelbyville to Paris.
Forecast soundings suggested 1500-2000 J/kg of SBCAPE developing,
but with very weak shear (less than 20 knots), and that is well
aligned with the latest RAP-based mesoanalysis. Given the weak
forcing those two aforementioned corridors of enhanced Cu will be
the favored area for any storm development over the next few
hours. Given the weak shear, these will be short-lived, and that
matches what we`ve seen with the few cells that have developed
thus far. As we progress later into the afternoon, the forcing
could become a little more nebulous, with a collapsing cell
producing outflow that kicks off new cells, and so on. With the
potential for that kind of messy evolution, maintained a 15-30%
PoP east of the IL River through sunset, after which point any
lingering cells will quickly diminish.

Another round of fog development is possible tonight, including
some dense fog, owing to a combination of light winds and mostly
clear skies. Guidance has been consistent in showing the chance of
fog increasing with eastward extent. 12z HREF guidance had a
40-70% chance of vis falling below 1 mile east of I-55 tonight,
while the available runs of the HRRR that go out far enough show
dense fog development near/east of I-57.

For the weekend, central IL will be positioned between a sfc low
over south-central Canada and a weak shortwave moving through the
lower TN Valley. In between these two systems, upper level height
rises occur over IL, resulting in mostly dry conditions and
steadily warming temps. CAMs do show storm development Sat
afternoon along the cold front associated with that Canadian sfc
low, but it will be positioned across WI and IA, and the activity
fades in the evening with the loss of heating before reaching the
ILX CWA.

Previously, there had been high confidence that Sunday would be
dry area-wide, but now guidance is showing storms developing over
east CO Sat night and progressing east along the instability
gradient, which puts it on a track towards west-central IL by Sun
afternoon, if it holds together. For now, the NBM introduced
15-20% PoPs which seemed reasonable given the uncertainty in the
evolution of a convective complex at this range. Shear values look
quite weak across our area Sunday, with deep layer shear values
again below 20 knots.

Temperatures do become quite warm on Sat-Sun, with highs in the
mid to upper 80s which is about 10-15F above normal. Record highs
for Sunday (May 19) are around 92-93F, and at this point it looks
unlikely that we`ll breach those thresholds.

** EARLY NEXT WEEK **

A trough is set to amplify over the western US and send a low
racing towards from the central Plains to the upper Midwest on
Tuesday. Multiple days of gulf moisture advection in advance of
this system will help push dewpoints near or into the low 70s. The
warm-sector associated with this system continues to appear quite
volatile, but mesoscale details and timing uncertainties
remaining regrading the svr threat. From an ensemble perspective,
the latest GEFS shows a 70-90% chance of exceeding 2000 J/kg of
SBCAPE. CWASP (a parameter which combines numerous environmental
factors related to severe weather into one index) is depicting
values over 70 west of I-55 Tues afternoon. For reference, most
EF2+ tornadoes occur in environments with CWASP values over 70.

It needs to be emphasized that CWASP is just a summary of the
environment, it does not factor in whether or not storms will be
present in that environment. In that regard, it does look like the
latest guidance has trended a bit slower with the cold front,
such that the higher threat may be focused in areas west of the
ILX CWA Tues afternoon. The previous forecast from the NBM had
PoPs Tuesday afternoon around 60%, but the latest forecast ranges
from 30% along I-70 to 60% NW of the IL River. PoPs across the ILX
CWA peak during the Tues night time frame (60-80%). It`s too
early to say how the severe threat could evolve into the Tues
night timeframe, but perhaps the diurnal timing could be our
saving grace from a higher-end severe weather outbreak. The flip
side to a slower system is that it could mean a greater portion of
the area is at risk for severe storms on Wednesday. With several
days of convective activity needing to evolve between now and
then, confidence in placement of storms on Wednesday is low, and
SPC did not introduce a Day 6 Outlook area in this morning`s
forecast.

In summary, there is a chance for severe thunderstorms both
Tuesday and Wednesday - with the latest guidance suggesting
precip/severe chances increase with northwestward extent Tues
afternoon, then storms push into the ILX CWA Tues evening/night.

High temps remain in the 80s through Wed, then cool into the 70s
for the latter half of the work week following FROPA.

Erwin

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1213 PM CDT Fri May 17 2024

Widespread diurnal cumulus have developed across the terminals,
ranging from 2.5-4kft currently. The expectation is that any MVFR
clouds bases will gradually lift to VFR as we progress into the
afternoon. Early Saturday morning (09-13z), fog development is
possible across eastern IL, with the best chance of impacts being
at KDEC/KCMI. Dense fog is possible, but wasn`t confident enough
to tempo in 1/4 mi vis just yet. Winds will be light with a
southerly component through the period.

Erwin

&&

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

$$