Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 241800

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
100 PM CDT Thu May 24 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 331 AM CDT Thu May 24 2018

A surface ridge extended from a high over the lower Great Lakes
across Missouri and Illinois early this morning.  Winds were light
and variable and dewpoint depressions were near zero in some areas,
but high clouds were helping to prevent fog development this
morning.  Will continue to monitor these trends through sunrise.

Otherwise, the surface ridge will move off to the east today and the
atmosphere will become unstable again this afternoon across central
Missouri. Will go with a slight chance of thunderstorms in areas
around KCOU and KJEF where the cap will erode briefly this
afternoon.  Expect this chance to wane with loss of daytime heating.
Chances will increase again after midnight over southeast and
central Missouri as there will be some low level moisture
convergence ahead of a upper level trough that will approach the
area from the central Plains.

Temperatures today once again will climb well above normal.  Have
gone above MOS guidance given the warm start to the day.  Lows
tonight will be in the lower to mid 60s.


.LONG TERM...  (Friday through Wednesday)
Issued at 331 AM CDT Thu May 24 2018

Little has changed in the forecast for Friday into the middle of
next week.  The upper level ridge currently over the Midwest will
have weakened and moved east by Friday morning...this will allow a
shortwave to dig into the Mississippi Valley Friday afternoon and
evening.  The wave will deepen over the Mississippi Valley and link
up with a Gulf low pressure system this weekend.  The wave is then
forecast to cut off into a closed low over the northeast Gulf Sunday
or Sunday night and an upper level ridge will build over the Midwest
with the low drifting up into the south central or southeast U.S. in
a rex-block configuration.  The GFS and ECMWF are very consistent
run-to-run with this scenario.  Additionally, the GFS still brings
the low up over Florida and Georgia Sunday evening into Monday while
the ECMWF is a little slower and further west with the low over
Louisiana.  Both models open the low up into a wave by 12Z Wednesday
morning and eject the wave northeast into the Ohio Valley through

The Mid Mississippi Valley will be under the influence of mid/upper
level high pressure for the majority of the period (except for
Friday).  850mb temperatures will be in the 18-21C range which mixes
down to mid 80s to low 90s.  This is a bit warmer than previously
forecast.  With warmer daytime highs and increasing humidity, the
overnight lows will likewise increase a bit generally in the upper
60s to low 70s.  Of course, temperatures will be highly influenced
by any thunderstorm activity through the medium range.  Speaking of
thunderstorms...the only low level boundary that`s apparent in the
guidance is a weak cold front associated with the shortwave which
will move through the Mississippi Valley Friday.  All guidance
washes this front out before it gets too deep into our forecast convective trends are unclear.  Probably the best chance
for precip will be Friday/Friday night as the wave moves through the
Mississippi Valley and any outflow boundaries from Thursday night`s
convection over The Plains provide a focus for convection.  Models
spit out intermittent QPF over the area for Saturday through
Tuesday...primarily in the afternoon with airmass convection.  The
GFS is drier for our area since the Gulf low is further east with
the upper level ridge building directly over the Midwest.  Can`t
rule out the ECMWF solution though...especially since it`s been so
consistent the past few nights, so have kept chance/slight chance
PoPs in the forecast for Saturday through Tuesday.  PoPs increase
for Wednesday as the low opens up and the upper ridge weakens...tho
confidence isn`t particularly high given the positional differences
between the GFS and ECMWF.



.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1243 PM CDT Thu May 24 2018

VFR conditions are expected through the next several hours with
light southerly winds through the period. Few/sct mid-to-high
clouds will be more probable over central Missouri with mainly
clear skies eastward into Illinois.

A bkn mid-level deck over central Missouri is co-located with a
boundary that is expected to slowly drift eastward Friday
morning. Near the boundary, a few thunderstorms are possible.
Expect KCOU to be impacted first, but chances are fairly slim, so
stuck with VCTS for now. Most sites will remain dry with slightly
better chances for scattered storms being Friday afternoon.
Therefore, held off on any mention of VCTS at other 24 hour TAF
sites until later updates.


VFR conditions will remain in place through much of the period.
A few mid-level cu will be diurnally driven this afternoon with
sct/bkn high clouds at times.

Scattered storms may develop along a boundary Friday afternoon.
The better support for storms remains south and west of KSTL, so
kept the mention to VCTS. Until then, will stick with light
southerly winds and limited cloud cover.





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