656
FXUS63 KILX 131733
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
1233 PM CDT Sat Jul 13 2024

.KEY MESSAGES...

- Hot and humid conditions are expected through Monday, with
  afternoon heat indices over 100 degrees on Sunday and Monday.

- Periodic storm chances exist as disturbances track from the
  northern Plains to the upper Midwest through Monday. Locally,
  the severe weather potential from these storms is marginal
  (level 1 of 5).

- A cold front moves through the area on Tuesday, providing
  another chance for storms and then ushering in much cooler air
  for the latter half of the work week.

&&

.UPDATE...
Issued at 1033 AM CDT Sat Jul 13 2024

A large cluster of thunderstorms that developed near Kansas City
late last night continues to drop southeastward through central
and western Missouri this morning. Further east, scattered
convection has developed along the E/NE periphery of the cluster
and is slowly pushing eastward into west-central Illinois.
1515z/1015am radar imagery shows a few showers near western
Schuyler County. CAMs generally keep this activity along/west of
the Illinois River through midday and this seems reasonable given
only slow eastward progress noted on radar. Skies are currently
sunny across central Illinois: however, NAM CU rule strongly
suggests SCT-BKN diurnal Cu this afternoon. With strongest upper
support remaining well to the north across Wisconsin and far
northern Illinois and no well-defined focusing mechanism in place,
think any convection that forms will be isolated and disorganized
today. Main weather story will be the building heat as air
temperatures climb into the upper 80s and lower 90s and peak heat
index readings approach 100 degrees.

Barnes

&&

.DISCUSSION...
Issued at 309 AM CDT Sat Jul 13 2024

Little change is expected in the pattern over the next few days,
as upper level ridging remains anchored over the southwestern US,
while occasional disturbances round the ridge and track from the
Dakotas towards the upper Midwest. This pattern will result in
increasing heat and humidity across central Illinois, with
periodic storm chances, although the higher PoPs will generally be
to our north.

While the worst of the heat is expected to be Sunday and Monday,
today won`t be a picnic either, with high temps climbing to near
90F and dewpoints in the low 70s, resulting in peak heat indices
of 95- 100 degrees. In this hot and humid airmass, weak low-level
WAA could kick off a few scattered showers and storms. Some CAMs
keep the area completely dry, but there was enough of a signal to
warrant slgt chnc (15-20%) PoPs through much of the daytime hours.
Moderate-to- high sfc-based instability is expected to develop
(2000-3000 J/kg of SBCAPE) by this afternoon, which could support
some robust updrafts if storms do develop, but the environment
will be very weakly sheared (under 20 knots of deep layer shear).
This would limit storm organization, but result in a set up where
if storms they quickly become outflow dominant, and then that
outflow subsequently kicks off new storms, and so on. Additional
storm chances exist during the overnight hours, as CAMs depict a
convective complex currently present over the Dakotas continuing
southeast through the day today, reaching N IL by the evening and
perhaps moving into the northern half of the ILX CWA, likely after
midnight (Sat night into Sun AM). Given the continued presence of
elevated (non-surface based) instability, these storms could pose
a threat of severe hail/wind (level 1 of 5 risk).

Temps continue to climb through Monday, with highs in the low 90s
on Sun, then mid 90s on Mon. Dewpoints remain in the low to mid
70s, pushing peak heat indices to near or above heat advisory
criteria (105 degrees) both days. Low temperatures in the mid 70s
both nights mean the nighttime hours will offer little relief.
Advisories will likely need to be considered with the next
forecast package, but the potential for cloud cover/precip from
MCS activity continues to represent one possible mode of failure
for this heat forecast.

The good news is that this heat wave is relatively short-lived,
with a cold front progged to reach the area late Monday night
into Tuesday. This front is expected to produce thunderstorms,
some of which could be severe, and the northern half of the CWA is
once again in a marginal risk (level 1 of 5) for late Monday
night. As that front continues to progress south across the area
during the day Tuesday, the environment ahead of it is expected to
be quite unstable. However, an early look at the global models
suggest the stronger 500-mb flow (and thus the stronger shear)
will be confined to the post-frontal areas, which would limit the
severe potential somewhat. For now, the CSU-MLP has a 15% contour
across portions of the ILX CWA Tuesday. Following the frontal
passage, below normal temperatures are expected from Wednesday
into the weekend (highs in the upper 70s or low 80s), along with
mostly dry conditions.

Erwin

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1233 PM CDT Sat Jul 13 2024

Main aviation forecast challenge will be the potential for
convection late tonight. CAMs are in disagreement...with the HRRR
depicting a dry scenario and the NAM/WRF-ARW suggesting a cluster
of storms dropping southeastward out of the Upper Midwest toward
dawn Sunday. Given synoptic pattern featuring strong upper ridge
over the 4-corners region and weak W/NW flow across the
Midwest/Great Lakes, think the NAM/WRF-ARW have the right idea. As
such, have included a PROB30 for thunder at KPIA between 09z and
12z and further SE at KCMI between 11z and 14z. Given high degree
of uncertainty, have opted not to mention thunder at KSPI/KDEC at
this time. Winds will initially be SW at around 10kt this
afternoon, then will back to S and decrease to less than 10kt this
evening. With the pressure gradient tightening, SW winds will
increase to 10-15kt by Sunday morning.

Barnes

&&

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

$$