209
FXUS63 KILX 110635
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
135 AM CDT Thu Jul 11 2024

.KEY MESSAGES...

- Through 9am, patchy fog will result in localized visibilities
  under 1 mile, which could increase the risk for motor vehicle
  incidents.

- There is a 30-40% chance for showers and thunderstorms this
  afternoon, posing a lightning risk to those participating in
  outdoor activities.

- There is a 60-80% chance for heat indices over 100 degrees west
  of I-55 on Sunday, with higher chances area-wide on Monday.
  This will increase the risk for heat-related illness in
  vulnerable populations.

- Scattered thunderstorms (30% coverage) are expected Saturday,
  and again late Monday into Wednesday. The timing and location of
  any severe weather risk remains unclear at this time.

&&

.DISCUSSION...
Issued at 130 AM CDT Thu Jul 11 2024

Early this Thursday morning, showers and thunderstorms continue to
weaken over east-central Illinois, with some lingering mid level
clouds in their wake. These will offset radiational losses through
morning, though in breaks between these clouds there will be some
patchy fog around after the recent rains; Galesburg`s visibility is
already down to 1 1/2 miles and Peoria recently (though briefly)
dropped to 5 miles. Mainly driven by the NAMNest and ARW, HREF
advertises 20-35% chances for visibilities dipping below 1/2 mile
between 5 and 9am, but thereafter radiational warming will result in
this boundary layer moisture getting mixed out.

Beneath the upper level trough this afternoon, cool advection and
widespread diurnal cu will keep us a few degrees below normal for
mid July with highs in the low 80s. Near and east of the surface
low, meso-a scale uplift and 1000-1500 J/kg SBCAPE will promote a
few more pop-up storms this afternoon and evening mainly east of the
IL River where we`ve got 30-40% chances in the forecast. Being
diurnally driven, those storms will dissolve late evening,
simultaneously exiting our area to the east for a period of dry
weather tonight through tomorrow night when each CAM is precip-less
over central Illinois.

Heading into Saturday, the ECMWF and GFS depict a subtle shortwave
trough attempting to round the northeast periphery of the ridge
building into the region from the west, and suggest storms will
develop during the morning across a portion of the ILX CWA amidst
overrunning/warm advection. While instability will be on the rise
Friday night into Saturday when the NAM suggests 2500+ J/kg of
MUCAPE overspread the region as mid level lapse rates steepen to
around 7C/km, there`s not a strong signal for shear in the global
models with LREF mean only giving us around 20 kt of it between 0m
and 500mb. While one shouldn`t expect widespread or significant
severe weather Saturday, some locally severe hail seems plausible,
though its potential will be limited a bit by warm cloud layer
depths around 4km with Donovan heights around 40kft.

Given the lack of wind shear, convective outflows are likely to
trigger more storms through the afternoon on Saturday which could
keep our highs confined to the mid 80s, near NBM`s 25th percentile.
Sunday, however, unless the deterministic CMC (unlike the GFS and
ECMWF) is correct in its depiction of a morning MCS diving southeast
into the area, temps are likely to become warmer as the ridge builds
into the region and capping inhibits convective potential. It`ll
certainly be a humid day, especially if much of the area gets
respectable rainfall totals from Saturday`s storms. LREF mean
dewpoint temps reach 76-77 degrees by Sunday afternoon, with around
15% of membership bringing values to 80 or higher. If high temps in
fact reach the low to locally mid 90s as forecast by NBM mean (which
may be a degree or three high if we have such humid low levels),
heat indices would be in the 100-110 degree range during the
afternoon - highest west of I-55. While LREF probabilities for heat
indices surpassing 105 (local Advisory threshold) are only around
10- 15%, we think the chances might actually be a bit higher,
nearer 50- 70% west of I-55 and 25-40% east.

Monday continues to look like the hottest day, with forecast highs
running about 2 degrees warmer than Sunday (92-95, vs 89-94, degF)
and LREF probabilities for dew points of 80 deg or warmer also
higher at 20-40%. Based on these forecast temps and dew points,
afternoon heat indices would reach the 104-114 degree range area-
wide, necessitating a Heat Advisory (70+% chance). However,
confidence is not high enough to issue one at this juncture due to
potential for convection to offset the heat as the upper level
jet remains too close for comfort and each global deterministic
model, though varying from run to run in precise timing and
location, develops storms at least near our area.

Global models and their ensembles exhibit considerable spread in
the timing of the ridge breaking down and a trough diving into the
region from the northwest, though LREF mean shows 500hPa heights
lowering across our area gradually (perhaps due to model timing
differences) by midweek. The global models suggest the upper
trough`s center will pass to our east, giving the region only a
glancing blow with its cool front stalling somewhere near the
area. This would suggest uncertainty in three key things: (1) when
the area cools down, (2) how much we cool down, and (3) timing of
convection. Depending on frontal timing, Monday, Tuesday, and/or
Wednesday we`ll need to be on the lookout for severe storms given
just how unstable the airmass will be. Given model spread in
timing of fropa, LREF mean SBCAPE decreases from 4000+ J/kg Monday
to ~3500 Tuesday, to ~1500 Wednesday. Before the cold front
arrives, one shouldn`t be surprised if on any given day we have
over 5000 J/kg SBCAPE on our raob, with the most warm and humid
day currently slated to be Monday. While forward-propagating MCS
activity would be favored near and north of the region late
Saturday into Monday, the approach of a cool front, an upper
trough, and its associated enhanced shear (35-45 kt 0-6km, if you
believe the GFS) sometime between late Monday-Wednesday would
favor more of an elongated, broken line of storms which could
feature a conditional tornado threat. We`ll continue to monitor
and reassess with forthcoming model iterations and, of course, our
suite of (upstream) observational data.

Bumgardner

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1145 PM CDT Wed Jul 10 2024

Showers and thunderstorms will continue to taper off with most of
the area dry overnight outside of an off chance for an isolated
storm. Scattered clouds will be in place overnight, but if any
substantial clearing occurs, then fog may develop, especially
along the I-74 corridor which saw multiple rounds of precip
Wednesday. Winds will be light and variable overnight, then set up
out of the NE Thursday. Scattered showers and storms are possible
again Wednesday afternoon and evening, with the greatest coverage
expected across east central Illinois.

Deubelbeiss

&&

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

$$