Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 092326

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
626 PM CDT Fri Jun 9 2023

.SHORT TERM... (Through Saturday) Issued at 221 PM CDT Fri Jun 9

Surface high pressure currently centered over **Illinois** will
continue to move southeast into the Appalachians tonight. Its
influence will keep skies mostly clear and winds light tonight,
opening up the potential for efficient radiational cooling to take
place again tonight. Saturday morning lows are expected to fall into
the 50s and low 60s, warmer than last night, but still slightly
below average in most locations.

Surface winds will veer to southerly tonight as the surface high
treks southeast. This will allow warm air and low-level moisture
from the Gulf of Mexico to advect into the Mid-Mississippi Valley.
Because of this, highs on Saturday will be a few degrees warmer than
those of today, reaching the mid to upper 80s in in most of Illinois
and in east-central Missouri.

In the upper levels, a shortwave trough will move through the
central Plains and into Missouri Saturday afternoon as another
trough drops south into the Upper Midwest. The shortwave is expected
to provide upper level forcing for convection mainly in western
Missouri during the afternoon, with the chance for showers and
thunderstorms in our CWA increasing after 00z. Despite this, central
and northeast Missouri do have the chance to see showers and
thunderstorms Saturday afternoon, though severe chances appear low.
Most deterministic guidance is keeping mean SBCAPE values on
Saturday afternoon and early evening under 1000 J/kg, with the HREF
90th percentile just barely reaching 1000-1100 J/kg in only portions
of central Missouri. Deep layer shear is unimpressive as well, with
only 25 to 30 kts present in the area.


.LONG TERM... (Saturday Night through Next Friday) Issued at 221
PM CDT Fri Jun 9 2023

The chance for showers and thunderstorms increases areawide
beginning Saturday evening and lasting through Sunday as a cold
front associated with the upper level trough in the Upper Midwest
sinks south into the region. This front will provide forcing for low-
level moisture to interact with producing widespread showers and
thunderstorms during this timeframe. Most locations still have a
high potential of seeing at least 0.5 inches of rain. Ensemble
guidance has remained consistent in showing around a 70% chance of
this occurring in locations outside of our far southern counties.
These counties, however, have a higher potential at seeing
thunderstorm activity ahead of the front Sunday afternoon which
could produce locally heavier amounts.

The potential for severe thunderstorms in these areas is present,
albeit low, as mean MUCAPE values may exceed 1500 J/kg. 0-6 km shear
will also be marginally better than that of Saturday afternoon,
ranging from 30 to 40 kts. However, the window for severe weather
will be more focused to the south of our CWA as the front wipes out
any instability during the mid-afternoon to early evening.

Otherwise, Sunday will be an unseasonably cool day with highs in the
70s for the majority of the CWA.

Temperatures will moderate during the early week, and conditions
look dry under the influence of upper level and surface ridging. The
potential for showers and thunderstorms returns to the forecast mid-
week, as deterministic guidance continues to depict an upper level
shortwave riding into the area. Although the potential is present,
there are too many deviations in ensemble and deterministic guidance
to discuss the details at this point.



.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 610 PM CDT Fri Jun 9 2023

Mainly dry and VFR conditions are expected through at least mid
morning on Saturday. Then scattered showers and thunderstorms
will move into central Missouri, so have started VCSH at both COU
and JEF at 17Z. There will also be some chance for showers and
thunderstorms at UIN after 21Z at UIN and the St. Louis area
terminals after 03Z, but confidence isn`t high enough to include
in the the TAF at this time. Any of the showers and thunderstorms
may cause MVFR or possible IFR ceilings and visibilities in brief
downpours. Otherwise, winds will be light and variable through
tonight before turning out of the south on Saturday morning around
7 knots.





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