643
FXUS63 KLSX 200906
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
406 AM CDT Fri Sep 20 2024

.KEY MESSAGES...

-Scattered showers and thunderstorms (20 to 30% chance) are
 possible through this evening. An isolated strong to severe storm
 over southeast Missouri and southwest Missouri is possible this
 afternoon and this evening.

-Several rounds of showers and thunderstorms (50-90% chance) and
 locally heavy rainfall are expected between Saturday and Monday night,
 with the best chance north I-70 between Saturday night and
 Monday morning. Isolated strong to possibly severe storms and
 locally will be possible on Sunday.

-Temperatures are expected to be above normal through tomorrow,
 but then back to near normal starting on Monday.

&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 405 AM CDT Fri Sep 20 2024

The latest surface analysis is showing a front extending from
central Iowa through northwest Missouri into northwest Oklahoma.
Thunderstorms that had earlier developed across Iowa have shown a
gradual weakening trend as they have moved southeast into less
instability early this morning.  I have continued with lows PoPs (20-
30%) through today as the front will continue to move east across
the CWA including early this morning when convergence increases on
the nose of the low level jet.  Chances will continue this afternoon
into this evening over southwest Illinois into southeast Missouri
when MLCAPES are expected to climb into the 2000-3000 J/kg range
with deep layer shear up to 30 knots.  This would support one or two
strong to severe storms capable of producing downburst and perhaps
some large hail.

There will be a better chance (40-70%) on Saturday morning when
showers and thunderstorms develop along a warm front that will move
through the area during the day.  Both the NAM/GFS are showing a 35
knot low level jet developing over the Plains late Friday night that
will bring the best chance for showers and thunderstorms into
central Missouri on Saturday morning.

High today will once again be around 90 degrees with 850mb
temperatures around 20C and sufficient sunshine.  Highs tomorrow
will be mainly 80s and with the coolest reading where there be the
most clouds and rain.

Britt

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 405 AM CDT Fri Sep 20 2024

The unsettled weather pattern will continue into early next week as
the medium range models are showing the upper ridge that has been
over the central CONUS shifting to the east and the low in the
Southwest lifting out and moving across the Midwest.  The highest
PoPS (70-90%) will be Saturday night into Sunday when a initial
shortwave trough will move out across the area as cold front moves
back southeast across the area.  There will be a few strong to
possibly severe storms given CAPE/shear parameter space and some
locally heavy rainfall is also possible as PWATS will be near 2".
There will also be likely PoPs (60-70%) going Sunday night into
Monday night as the main upper trough and attendant cold front moves
across the area.  Chances for rain the rest of the week are low as
the LREF is showing a large area of high pressure moving across the
area with most members (<15%) not producing any precipitation after
Monday night.

Temperatures will fall back closer to normal behind the cold front
beginning on Sunday and staying there most of next week.

Britt

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1045 PM CDT Thu Sep 19 2024

It is becoming more evident that any potential for showers and
thunderstorms will originate out of Iowa late tonight into early
Friday morning. However, how long thunderstorms survive is in
question as support is limited the further southeast of ongoing
development. Latest trends look as though any activity that makes
it into KUIN will be delayed by an hour or two, which is reflected
in this update. Guidance has continued to show a void in activity
over central Missouri with the far southwest tail of the
convection skimming north of KCOU, but weakening as convection
moves southeast into a relatively more stable airmass. VCTS was
maintained at KCOU but the window was limited to just a couple of
hours. I felt comfortable enough to pull VCTS out of KJEF given
the latest trends. Held onto VCTS over the metro terminals with a
slight delay in arrival and also reduced the timeframe by an hour
as convection looks weak and scattered through early Friday
morning.

The most notable chance was the addition of VCTS at metro
terminals Friday afternoon/early evening. This may end up being
the best chance with better instability available through peak
diurnal heating. As stated in earlier forecasts, potential for
direct impacts are low, but could result in a brief period of
MVFR vsby. Otherwise, VFR conditions are forecast the majority of
the period at all sites.

Maples

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX