721
FXUS63 KILX 290204
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
904 PM CDT Fri Jun 28 2024

.KEY MESSAGES...

- Thunderstorms will bring beneficial rain to most of central
  Illinois late tonight. The risk for severe weather remains low
  (only around 5%).

- There is only a 20-30% chance for thunderstorms Saturday. Once
  again, the severe risk is low (around 5%).

- Drier, cooler, and less humid conditions evolve Saturday night
  through Monday; ideal for outdoor activities.

- The relief from the heat and humidity will be brief as it makes
  a return by the middle of next week along with daily chances
  for rain starting Wednesday lasting into the holiday weekend.

&&

.UPDATE...
Issued at 904 PM CDT Fri Jun 28 2024

Updates this evening to decrease PoPs as an outgoing shortwave
drifts off to the northeast with light showers tapering off behind
the feature. Also, lowered overnight PoPs.

A couple of bands of showers have been organizing closer to a
cold front from the upper Midwest to the southern Plains, with
the stronger activity taking place west of I-35 nearest the
surface cold front. Mesoanlysis products based on the RAP model
depict an axis of strong instability around 4000 J/kg just ahead
of the cold front, trailing off to a couple hundred or less along
the Mississippi River. A couple of 00Z high resolution model runs
just coming in are following a trend of weakening pre-frontal
convective bands shifting into central IL overnight, as well as
keeping activity more scattered and disorganized. May need
additional updates later to delay onset of higher PoPs, which
currently shift into the area around midnight.

37

&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 258 PM CDT Fri Jun 28 2024

At 230pm, day cloud phase distinction satellite shows scattered mid
(mostly supercooled water, as the freezing level is >14,000 ft!) and
high-level (ice) clouds across central Illinois, with scattered
convection upstream across southeast Iowa where low level moisture
transport is strong ahead of a shortwave trough. Current suite of
CAMs suggests this wave of thunderstorm activity will gradually
weaken as it becomes displaced from better instability and shear in
west central/northwest Illinois, though this forecaster wasn`t
confident enough to completely eradicate all mention from the
forecast this evening; hence, 25-50% PoPs remain northwest of I-55
this evening.

The better chance (60-80%) for precip will arrive later tonight, as
storms developing closer to the upper-level forcing drift into our
area while simultaneously weakening. It`s uncertain whether we`ll
wind up with any hydrological or convective (wind, hail) issues
tonight, but the CAMs continue to advertise the greatest
thermodynamics to our southwest where storms developing along the
northern periphery of the instability axis/LLJ across central/east
Missouri may rob our region of any appreciable CAPE. Nonetheless,
our morning sounding revealed a PWAT of 1.62 (near the
climatological 90th percentile), and given continued warm/moist
advection those values may surge to nearly 2 inches overnight -
more than enough for efficient rain producers (especially given
the depth of the warm cloud layer). NBM suggests the greatest
chance (20-35%) for rain amounts surpassing 1 inch will be
generally west of a Galesburg to Taylorville line, where some
guidance even depicts some highly- localized 2+ inch totals.
Precip loading could also contribute to downward momentum
transport fostering gusty winds in the heaviest cells, especially
if we struggle to decouple given continued mixing via the LLJ. In
general, the severe risk appears low, with SPC maintaining just a
level 1 of 5, marginal (5%), risk for severe weather.

That holds for tomorrow as well, though tomorrow the reason for the
marginal risk is slightly different. Recovery of instability (HREF
mean is 1500-2500 for SBCAPE) in our area is well forecast by the
12z CAMs, though none of them are particularly aggressive in
redevelopment of storms along/ahead of the cold front. If storms
should develop, the unstable airmass would favor severe wind/hail
with them, though at this point only about 20-30% of guidance shows
redevelopment during the afternoon. We`ll be keeping a close eye on
model trends for this activity.

Tomorrow evening, quite the impressive cold front for this time of
year will sink southward across central Illinois, bringing a quick
drop in humidity (dewpoints are forecast to drop 15+ degrees, from
mid 70s to upper 50s, in the span of 1-2 hours) and increase in wind
(northwest gusts are forecast to reach 30+ mph). By Sunday morning,
our northwest counties are likely to drop into the mid-upper 50s,
while south of I-70 some spots might still be hovering around 70.

Bumgardner

.LONG TERM...  (Sunday through Next Friday)
Issued at 258 PM CDT Fri Jun 28 2024

A brief stretch of unseasonably cool and dry weather is on tap for
Sunday and Monday as a strong mid to upper level ridge will be
building to our west over the Great Plains. Unfortunately, the
pleasant stretch of weather will be short lived as the ridge axis
passes through by Monday evening, placing us back under a southerly
flow regime once again by Tuesday. A weak mid level shortwave, that
is expected to traverse across the Upper Midwest Tuesday night into
Wednesday, may provide enough forcing aloft to support some rain.
With the depth of moisture still in question, especially east of I-
55 in Illinois, only chance PoPs remain for most of the area on
Wednesday. The 00z LREF members are in good consensus that the same
upper level ridge that will bring us our cool down Sunday and Monday
will setup over the SE CONUS, with 500mb heights of 594dm. The
current placement of this 594dm heat dome puts us under the "ring of
fire", giving us 30-50% PoPs daily from Wednesday through the end
of the extended.

With deep northwesterly flow aloft, along with rising heights,
Sunday and Monday will be dry and unseasonably cool with widespread
highs in the 70s. The coolest night in the next 7 days will be on
Sunday night when lows are expected to drop into the 50s, bringing a
springtime like chill back to the area. With southerly flow
returning by Tuesday, temps will be on the rise with highs reaching
the upper 80s to lower 90s on Tuesday and Wednesday along with
dewpoints reaching the lower 70s. The combination of heat and
humidity will result in heat indices nearing or exceeding 100
degrees in some locations on Tuesday and Wednesday, especially south
of I-70 in Illinois on Wednesday. Confidence in temps Thursday and
Friday is lower due to the uncertainty in the strength and position
of the nearby heat dome by the end of next week.

Peine

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 642 PM CDT Fri Jun 28 2024

Scattered showers will continue for an hour or two to start the
TAF forecast period, but there is little chance of significant
impact in terms of low cigs, vsbys, or thunderstorms. After a few
hours, a line of thunderstorms is expected to organize to the west
and sweep into the area by 05Z-07Z, and exit the central IL
terminals by 09Z-12Z. This will likely be accompanied by brief
MVFR cigs and vsbys. Following the departure of these
thunderstorms, a period of MVFR cigs is expected from around
15Z-21Z before low cloud cover lifts and scatters out. Winds S
around 10 kts, shifting to W after 15Z.

37

&&

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

$$