438
FXUS63 KILX 270839
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
339 AM CDT Thu Jun 27 2024

.KEY MESSAGES...

- Thunderstorm activity returns to the region Friday into
  Saturday, bringing an attendant severe weather and flash flood risk.

- Conditions become seasonably hot and humid as we enter July,
  with additional rainfall chances evident.

&&

.DISCUSSION...
Issued at 1229 AM CDT Thu Jun 27 2024

Shallow, patchy fog has developed across portions of east central
and southeast Illinois early this morning. This is largely where
the cloud/precip shield lingered through yesterday afternoon, with
areas to the north receiving enough sunshine to help dry things
out from recent rainfall. Short term guidance (e.g. GLAMP & HRRR)
has a reasonable handle on the areal footprint of the patchy,
dense fog, which keeps it confined to areas south of a Springfield
to Paris line. There, visibility may occasionally fall below one-
half mile , and we have covered this potential hazard with a
Special Weather Statement.

After a dry and pleasant Thursday, both temperature and humidity
will increase on Friday ahead of an approaching cold front. Short-
term, hi-resolution guidance resolves a decaying MCS pushing into
west central Illinois by late Friday morning beneath a hearty
shortwave and increasing 500-mb flow. Despite these favorable mid-
level features, the MCS will have run well-out ahead of the
moisture/instability axis by the time it reaches the Mississippi
Valley. Thus, thunderstorm prospects are meek through midday
Friday.

Confidence of storms increases Friday evening as strong shortwave
energy pivots across the Upper-Mississippi Valley and helps drive
a surface cold front into northern Illinois. The LLJ will become
an important driver of nocturnal thunderstorm activity as it
brings in the necessary moisture, convergence, instability and
shear, but model guidance is consistent in keeping the LLJ axis
along/north of the IL River Valley for much of the night,
eventually drifting southward along I-72 but receding by Saturday
morning. This suggests that much, if not all, of the convective
activity is confined to areas north of I-72 Friday night. These
storms will mostly be elevated, and with only marginal MUCAPE and
strong dCIN in place, the severe hail/wind potential is low.

Perhaps the greater severe risk comes Saturday afternoon in areas
south of I-72 where mid 70s dewpoints pool ahead of the cold front
and contribute to moderate instability (MLCAPE 2500+ J/kg),
juxtaposed to modest deep-layer shear (30 kts). This arbitrary
environment could become upset by convective debris left over from
upstream activity Friday night, but negating this possibility,
organized convection is certainly plausible Saturday afternoon
along the cold front and would be accompanied by an attendant
flash flood risk (mean wind roughly parallel to front and 99th %
PWATS).

A substantial cool down will occur behind the departing cold front
as modified Canadian high pressure builds into the Great Lakes
region. Afternoon temperatures on Sunday and Monday, as told by
the deterministic NBM, are only in the upper 70s across a good
chunk of central Illinois. This stretch of cooler weather will be
short-lived, however, as the axis of the upper heat dome pushes
across the region Monday night and summons a stretch of upper 80s
to low 90s temperatures for the remainder of the week. Depending
on where the periphery of the 594mb high sets up, central Illinois
may become susceptible to ring-of-fire convection -- perhaps
daily. This is likely why the NBM is blanketing the region with
chance (40%) PoPs Tuesday - Thursday.

MJA

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1147 PM CDT Wed Jun 26 2024

Clear skies expected overnight as high pressure drifts toward the
area. Diurnal cumulus and increasing high clouds will take place
Friday, however conditions will remain VFR. Enough pressure
gradient tonight should keep light winds going through the night,
helping to prevent any fog formation. Winds N 3-6 kts overnight,
becoming E-NE around 10 kts by 18Z.

37

&&

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

$$