Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 261141

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
641 AM CDT Tue Sep 26 2023

.SHORT TERM... (Through Wednesday)
Issued at 309 AM CDT Tue Sep 26 2023

Key Messages:

1. There is a low chance (15-30%) of scattered showers and
thunderstorms this afternoon increasing to around 60% tonight.

2. A storm or two will be capable of small hail and wind gusts up to
45 mph, but confidence in severe weather remains below 5%.
Therefore, we are not publicly messaging the SPC Day 1 Marginal
Risk over our south-central Illinois counties.

3. Confidence is high (90%) in above normal temperatures through
early next week.

An cutoff low is slowly spinning south-southeastward over the
Midwest per recent water vapor imagery, with its trajectory
generally pointed toward the Ohio Valley. South of the cutoff over
the CWA, a wind shift from north to south over the center of the
area highlights a weak boundary. This boundary is expected to slowly
move eastward through the morning and early afternoon.

This boundary may serve as a focus for convection this afternoon
across south-central Illinois and southeastern Missouri, though
confidence in this is low (30% or less) as convergence along the
boundary is weak, and upper-level support from the cutoff will be
displaced to the north. Additionally, spread among hi-res guidance
regarding dew points leads to uncertainty in if the boundary layer
will be too dry to support convection. This spread in dew points has
lead also to uncertainty in how unstable the atmosphere will become.
A majority of hi-res guidance per the 00z HREF supports SBCAPE
values of 500 J/kg or less, with the minority showing values around
1,000-1,500 J/kg. Given these factors and better forcing from the
cutoff remaining just north of the area during a majority of peak
heating, confidence is very low (less than 5%) in severe storms
occurring this afternoon and early evening.

If convection is able to form this afternoon and early this evening,
0-6km bulk shear of 20-25 kts will support a brief, strong updraft
or two capable of isolated small hail and gusty winds around 45 mph.
If SBCAPE values on the higher end of the possible range are
realized, this would support an isolated severe thunderstorm

As the cutoff moves over the area later this evening into the early
overnight hours, upper level forcing will increase particularly over
south-central Illinois, leading to a medium chance (40-60%) for
scattered showers and thunderstorms. This chance will overspread the
area overnight primarily along and south of I-70 as a weak low-level
jet ramps up and provides isentropic ascent. Instability will have
waned, but there may be enough MUCAPE (200-500 J/kg) to support a
threat for small hail. As the low-level jet winds down early
tomorrow morning, rain chances will begin to decrease.

Tomorrow, as the cutoff slowly continues southeastward into the Ohio
Valley, it will continue to support the low chance (20-40%) for
scattered showers and thunderstorms mainly over southwestern
Illinois. Weak northerly to northwesterly flow behind the departing
cutoff will support cooler air advecting into the CWA. This will
simply lead to temperatures topping out just above climatology
during the afternoon, with highs in the upper 70s to low 80s - the
coolest day within the foreseeable future.


.LONG TERM...  (Wednesday Night through Monday)
Issued at 309 AM CDT Tue Sep 26 2023

While there is some variability among ensemble clusters in the
extended range concerning the upper-level pattern, each cluster
shows some degree of ridging over the Midwest. In turn, temperatures
among ensembles range approximately 5-10 degrees between the 25th
and 75th percentile. While this leads to low confidence in exactly
how warm temperatures will be at the end of the week into early next
week, the 25th percentile is roughly 5 degrees above climatological
normals, leading to high confidence (90%) in above normal
temperatures through this portion of the period. The placement of
the ridge is not favorable for rain chances, with 10% or less of
global ensemble members showing measurable rainfall during this



.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 638 AM CDT Tue Sep 26 2023

Confidence is high in VFR flight conditions through today and into
tonight. As early as this afternoon, there is a very low chance
for scattered showers and thunderstorms near KUIN; however, the
probability and potential coverage of showers and storms is so low
that I have continued to leave mention of them out of the TAF for
now. Another round of scattered showers and thunderstorms is
expected tonight near the KSUS, KSTL, and KCPS terminal. Right
now, confidence in direct impact is low. If direct impact is to
occur at any of these three terminals, brief drops in visibility
are possible.





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