Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 211036
Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
536 AM CDT SAT MAY 21 2016
.SHORT TERM... (Today and Tonight)
Issued at 308 AM CDT Sat May 21 2016
Large scale amplification will proceed this period and the
deepening upper trof in the eastern U.S. will keep a surface high
pressure system locked in across the MS Valley today and tonight.
With the exception of some patchy fog early this morning, weather
will be rather tranquil during this time frame. Highs today will
be warmer than yesterday owing to considerably more sunshine
although still below average. Lows tonight will be seasonably cool
with low dew points and light winds resulting in good radiational
.LONG TERM... (Sunday through Friday)
Issued at 258 AM CDT Sat May 21 2016
The upper ridge across the Plains will shift into the MS Valley on
Sunday however the blocky nature to the downstream pattern will
keep surface features in slow motion. Thus the surface high will
be only slowly retreating and will continue to exert some
influence, especially across eastern MO into IL. Mainly this will
slow the onset of southerly low level flow, better WAA and moisture
return. Temperatures however will continue to moderate, and highs
on Sunday will finally be more typical of late May. Progression
will continue on Monday-Monday night, albiet slowly, with low
level WAA and moisture transport underway into central and eastern
MO during the day, and into IL Monday night as southerly flow
becomes better established in the wake of the retreating high. The
combination of these advections and several disturbances within
the southwest flow aloft will bring increasing chances of showers
and thunderstorms from west to east, with the highest threat in
central and northeast MO.
Southwest flow aloft will continue Tuesday-Friday keeping a
steady stream of weak disturbances traversing the area, as well as
maintaining relatively undisturbed steep mid level lapse rates.
These lapse rates will contribute to a persistent broad and
unstable warm sector. No well defined frontal penetrations are
evident at this point, with the disturbances aloft, the south-
southwesterly LLJ, and mesoscale features influencing what will
be continuous thunderstorm chances.
.AVIATION... (For the 12z TAFs through 12z Sunday Morning)
Issued at 529 AM CDT Sat May 21 2016
High pressure continues to sink into the mid Mississippi Valley.
This combined with the cyclonic flow around the large low pressure
area to the east will keep a light northerly wind flow across the
area. VFR stratus flowing south across Illinois will stay east of
most locations. Some high clouds flowing over the upper high
pressure area will flow over COU.
Specifics for KSTL: VFR stratus deck lies just east of STL. It is
dropping south but would not be surprised to see a couple hours of
VFR ceilings this morning. Otherwise continued VFR conditions
through the period.