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FXUS63 KILX 010106
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
806 PM CDT Sun Jun 30 2024

.KEY MESSAGES...

- There is a 50-60% chance for thunderstorms west of I-55 Tuesday
  night, and 50-70% chance for much of central and southeast
  Illinois both Wednesday and Thursday. However, confidence is low
  in both severe potential and specific timing of storms at any
  given location.

- Seasonably warm and humid conditions will occur outside of
  thunderstorms. Wednesday looks like the warmest day, with the
  highest chance (40-60%) for heat indices eclipsing 100 degrees
  near and south of I-70.

&&

.UPDATE...
Issued at 806 PM CDT Sun Jun 30 2024

High pressure centered near the MI upper peninsula will remain
firmly in control of the weather tonight, promoting clear skies
and light northeast winds. Dewpoints range from the upper 40s to
mid 50s, and look to be even lower just off the surface according
to the 00Z KILX sounding, and should pose little resistance to
falling temperatures overnight. Have therefore nudged lows for
tonight down slightly, expecting temperatures to range from around
49 at Champaign and Danville, to around 53 at Jacksonville and
Flora.

37

&&

.DISCUSSION...
Issued at 142 PM CDT Sun Jun 30 2024

Under the influence of surface high pressure, tranquil and
seasonably cool conditions continue across central Illinois this
Sunday. As the surface high settles over the Great Lakes tonight,
winds will veer to easterly and ease, leading to rapid radiational
heat losses in the absence of clouds with forecast lows in the low
to mid 50s; in fact, some sites may fall into the upper 40s,
according to NBM`s 20-35% probabilities for sub 50 degF Monday
morning lows (highest northeast of a Peoria to Effingham line).

As heights continue to build into the region tomorrow, large scale
subsidence will favor a continuation of dry conditions which, for
most of the area, should last into late Tuesday if not Wednesday.
Temperatures will return to near normal Tuesday and above normal by
Wednesday as southwesterly flow gradually imports to the area a
warmer and more humid low-level airmass.

Deterministic models and their ensembles, while in excellent
agreement in the upper level pattern, have varying solutions for the
evolution of the weather maker slated to approach our area late
Tuesday into Wednesday - not surprising given differential heating
boundaries from upstream convection will serve as triggers for more
convection...and global models simply lack the resolution to handle
those features well. Not to infer a model cannot be consistently
wrong, I would like to point out there`s some consistency, from one
iteration to the next, in the deterministic ECMWF`s lightning
activity forecast, which would suggest storms upstream across Iowa
weaken as they enter our CWA and finally fall apart near the I-55
corridor Tuesday night, while more develop south of I-72 on
Wednesday, with renewed storms perhaps making it a tiny bit further
north Wednesday night into Thursday. Conceptually, and in accordance
with ensemble guidance, this seems like a reasonable scenario. Mean
500mb heights from the LREF decrease a bit over our region early
Wednesday when the deterministic models show a cold front pushing
into the area, but then they hold steady as the front stalls and
potentially lifts slightly northward heading into Thursday. The area
of greater than 30% LREF joint probabilities for more than both (1)
30 kt 0m-500mb shear and (2) 1500 J/kg SBCAPE stays west of our area
Tuesday night*, extends south of I-72 on Wednesday, and then
remains roughly southwest of a Peoria to Terre Haute line on
Thursday. Nonetheless, the precise location of the boundary will
dictate both daily high temps, humidity, and the location, timing,
and strength of thunderstorms. Since this, being a function of
multiple waves of convection, is generally not well-resolved by
global models, we can`t get too specific during this forecast
package. However, we`re likely to have seasonable warmth and
humidity in parts of central and southeast Illinois each day, and
a conditional threat for severe thunderstorms - confidence in
which remains low (5-10%) at any given location.

*Deterministic guidance suggests that while sufficient SBCAPE values
for convection will be displaced to our west Tuesday/night, MUCAPE
will increase as the LLJ noses into our western counties (mainly
near and west of the IL River).

Eventually, global ensembles would suggest a trough will at least
graze our area to the north, although the magnitude and precise
timing of any cool down is a subject of considerable model
variability still. By mid next weekend, the LREF mean portrays west-
northwest flow across the region, with weak troughing across the
Great Lakes and a ridge building in the West. Most members of the
GEPS and ENS have negative PC1s (principle components) for EOF1,
suggesting less troughing over the Great Lakes and less ridging
across the Great Basin/West than the mean of the LREF depicts,
while the highest concentration of GEFS membership is in cluster 3
which suggests just the opposite. The deterministic forecast
takes the middle of the road and has seasonable temps and 10-25%
chances for showers/storms all weekend.

Bumgardner

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 604 PM CDT Sun Jun 30 2024

High pressure drifting eastward through the Great Lakes Region
will bring mostly clear skies tonight, with gradually increasing
high and mid clouds Monday. VFR conditions will continue. Winds
NNE 6-12 kts this evening, becoming E-NE 4-8 kts by 06Z. Winds
further shifting to SE by 18Z with speeds 8-12 kts.

37

&&

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

$$