Severe Thunderstorm Warning 317, NWS St. Louis, MO

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1040 PM CDT Tue Aug 1 2023

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 328 PM CDT Tue Aug 1 2023

Our primary focus through Wednesday night continues to be two main
rounds of heavy rainfall tonight and Wednesday, leading to
hydrologic impacts including flash flooding and flooding on smaller
rivers and streams. A Flash Flood Watch is in effect tonight through
early tomorrow morning, with an additional watch possible for
Wednesday night once details become clearer.

Water vapor imagery currently shows an upper-level ridge remaining
positioned over the western U.S., with continuing northwesterly flow
on the eastern periphery of the ridge over the CWA. Rain from early
this morning associated with forcing from the low-level jet
interacting with a boundary over southwestern Missouri has died with
the weakening of the low-level jet, offering the CWA a much needed
break ahead of what will be several rounds of impactful, heavy
rainfall.

With the ridge having shifted slightly westward, guidance consensus
is that the effective boundary will have shifted slightly
northeastward and the nose of the low-level jet will be focused more
eastward over portions of eastern Missouri tonight compared to last
night. This will put the focus for heavy rainfall more into east-
central Missouri, where training thunderstorms will produce rainfall
amounts of up to 5 inches with localized higher amounts in the
heaviest corridor per the 12z HREF. For this threat, we have issued
a flash flood watch valid tonight into Wednesday morning. It is
unlikely that the entire watch area will see these higher amounts,
but where they do occur, possibly significant, localized flash
flooding is possible as well as quick rises on smaller rivers and
streams. Additionally, effective shear of around 40 kts paired with
around 1,000 J/kg of MUCAPE late this evening and early tonight
mostly over northeastern Missouri will leading to a threat of
elevated supercells capable of mainly isolated large hail and
possibly a damaging wind gust in the strongest of storms. Confidence
in this threat is low, and if it does occur, it will be isolated.

As the low-level jet weakens Wednesday morning, rain chances will
gradually taper off, similar to how Tuesday morning went. However,
guidance consensus is that enough of the low-level jet will be
maintained to feed an unstable airmass over the Lower Missouri
Valley late tomorrow afternoon into the early evening. Then, a
subtle shortwave riding the ridge will provide lift among 1,500-
2,000 J/kg of MLCAPE and 35-40 kts of 0-6km bulk shear, leading to a
potential for multicell and supercell thunderstorms. There is some
uncertainty as to if these storms will be elevated or high based, as
model soundings show varying degrees of this. This will impact the
degree to which storms will be severe: If storms end up being more
elevated, large hail will be the primary threat, but if storms end
up being more surface based, large hail and damaging winds will be
possible as well as a tornado or two given a cyclonic curvature seen
in low-level hodographs.

Of greater concern late tomorrow evening and overnight is another
round of heavy rainfall. The area of focus for this heavy rainfall
will depend on how the late afternoon/early evening round of storms
transpire and where the nose of the low-level jet focuses - both
factors remaining uncertain at this time. If the focus for rain ends
up being more focused in Illinois where not much rain has fallen
lately, then an event of similar proportions as expected tonight is
possible. However, if the focus for rain ends up being further west
along the Mississippi River and points west where recent rainfall
has lead to saturated soils, a high-impact flash flood event is
possible.

Elmore

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 328 PM CDT Tue Aug 1 2023

Thursday morning, guidance consensus is that upper-level ridge will
start breaking down as the low-level jet veers southeastward across
the CWA. However, there are differences in the placement of the nose
of the low-level jet, leading to uncertainty in how quickly rain
will move out of the area Thursday morning. If the nose is able
remain over the CWA, rain chances and flooding concerns will linger
well into Thursday morning, mainly in our Illinois counties. A more
easterly low-level jet will push the focus for rainfall eastward out
of the CWA.

As the ridge continues to break down into the weekend with zonal
flow setting up over the CWA across boundaries generally oriented
west-east, active weather will remain in place, though details on
timing and placement of any rain chances remains uncertain. However,
plenty of dry time is expected, and the threat for heavy rainfall
will be limited relative to what we will have experience earlier in
the week.

Late Saturday into Sunday, ensemble consensus is that a shortwave
will move into the Central Plains on the southern periphery of a
deepening trough that will dig into the eastern U.S. early next
week. However, ensembles are split between the speed of this
shortwave and its associated surface low, as well as the track of
the low. A slower and more northerly track of the low would favor
better thunderstorm chances, with some storms possibly strong, over
the area during the weekend ahead of a cold front. A more southerly
and quicker solution would favor more of a blend of rain and storm
chances and limit storm strength ahead of the front. The spread
among guidance prevents me from having confidence in either solution
at the moment, with the going forecast being a blend of the two.

With the ridge breaking down, regular rain chances and convective
debris, and the passage of the cold front late in the period,
confidence is high in temperatures remaining at or just below normal
through the period. This is reflected well in ensemble means, which
show a gradual slight decrease in temperatures through the period.

Elmore

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1034 PM CDT Tue Aug 1 2023

An active 06Z TAF period is expected at most local terminals.
Strong thunderstorms remain likely to develop across the region
during the overnight hours and persist through at least the early
morning hours. These thunderstorms will almost certainly produce
heavy rain and frequent lightning (along with visibility/ceiling
reductions), with some potential for strong surface winds and hail
as well. For this first round of thunderstorms, COU and JEF are
the most likely terminals to be directly impacted. However, all
terminals may see at least some thunderstorm activity by tomorrow
morning.

A break in storms is likely tomorrow afternoon, although some
scattered showers will likely persist. Another round of
thunderstorms is likely tomorrow evening and overnight, with less
certainty regarding the terminals most likely to be impacted.
In spite of these uncertainties, similar hazards can be expected
with any storms that develop tomorrow night.

BRC

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch until 10 AM CDT Wednesday for Audrain MO-Boone MO-
     Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Knox MO-Lewis MO-Madison MO-Marion MO-Moniteau MO-
     Monroe MO-Montgomery MO-Osage MO-Ralls MO-Reynolds MO-Saint
     Francois MO-Shelby MO-Warren MO-Washington MO.

IL...None.
&&

WFO LSX

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