Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 071807

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
107 PM CDT Fri Aug 7 2020

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 403 AM CDT Fri Aug 7 2020

For today, an upper-level ridge centered over the Southern Plains
will build eastward into the mid-Mississippi River Valley. In
conjunction with this feature, low-level southerly flow will
advect warmer temperatures and higher humidity into the CWA. Both
afternoon highs and dew points will be roughly 5 degrees greater
than what was experienced on Thursday. Despite the increase in
low-level moisture, guidance has greater instability remaining
west of the CWA, and with the lack of forcing, I anticipate that
conditions will remain dry.

For Saturday, convection associated with a disturbance across the
Central Plains will begin pushing into northern and central MO early
in the morning, primarily impacting the western and northern portion
of the CWA around sunrise. With better instability and forcing
remaining just west of the CWA, my confidence in this convection
reaching eastern MO is low, but I currently can not rule that
scenario out. With the loss of forcing, CAMS have this convection
dissipating around 14z. For temperatures, the warming trend will
continue. NAEFS and EPS guidance has 850 mb temperatures in the
upper-teens, which usually translates to highs in the low-90s. Cloud
cover from the previously mentioned early morning convection will
help to limit heating somewhat, so I anticipate highs toping-out in
the upper-80s to near 90. If the morning convection does not occur
as forecast, the CWA could see afternoon highs a couple degrees
warmer than currently forecast. This scenario could also allow for
the possibility of diurnally-driven convection in the afternoon.


.LONG TERM...  (Saturday Night through Thursday)
Issued at 403 AM CDT Fri Aug 7 2020

The upper-level pattern at the beginning of the period will be
marked by a building ridge centered over the southern U.S.
South/southwesterly mid- to low-level flow in response to this ridge
will continue to advect warmer temperatures and deeper moisture into
the CWA. NAEFS and EPS guidance has 850 mb temperatures in the low
20 degree C range for Sunday and Monday, translating to surface
temperatures in the low- to mid 90s. Guidance consensus is that dew
points will be in the low- to mid-70s, and when combined with the
forecasted surface temperatures, heat index values will likely reach
100 degrees or higher for many locations in the CWA.

What could mitigate this degree of heat is convection. For early
Sunday morning, guidance has convection developing across the CWA at
the nose of a low-level jet. With the dissipation of the jet,
convection will wain shortly after sunrise. During the afternoon,
guidance generally has at least 2,000 J/kg of MUCAPE across the CWA,
but no apparent large-scale forcing. However, with that level of
instability, I can not rule out isolated convection tied to peak
heating Sunday afternoon. With the convection coverage currently
anticipated, a wide-spread impact on afternoon highs is not
expected. For Monday, guidance consensus continues to position a
front just north of the CWA. While this front will not pass through
the CWA, pooling moisture ahead of it may produce enough cloud cover
and convection to help curb afternoon highs on Monday.

Beyond Monday, zonal flow aloft takes hold over the CWA. Guidance
has several disturbances embedded in this flow passing through
the mid- Mississippi Valley region. Convection associated with
these disturbances will potentially impact temperatures, and this
has lead to notable spread in surface temperatures seen in GEFS
and EPS guidance. While spread may be high at the moment, ensemble
consensus is that temperatures will generally be several degrees
cooler than what will have been experienced earlier in the period.



.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1224 PM CDT Fri Aug 7 2020

VFR conditions are expected through the remainder of Friday, with
convection staying southwest of the terminals. Starting around
10Z Saturday morning, confidence is increasing that showers and
thunderstorms will effect KCOU and KUIN. Due to higher uncertainty
in terminal impacts, showers with VCTS are in the TAFs. If
thunderstorms are more widespread, flight conditions could
deteriorate in more intense storms. Multiple cloud decks and
scattered showers/thunderstorms could linger across the area
through 18Z Saturday as the system slowly weakens. Uncertainty is
high with regards to timing and location of these secondary


VFR conditions will prevail through daybreak Saturday. Showers and
thunderstorms are expected in central Missouri in the morning,
but should remain west of the terminal. Remnant energy from
earlier convection could redevelop showers and thunderstorms
around or at KSTL after 18Z Saturday. As of now, confidences is
not high enough to include in the TAFs.





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