Area Forecast Discussion
Issued by NWS St. Louis, MO

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025
FXUS63 KLSX 250528
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1228 AM CDT Tue Apr 25 2017

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 353 PM CDT Mon Apr 24 2017

Not a lot of changes from the prev forecast.

Winds will diminish somewhat this evening, but are expected to
remain sely with continued mixing thru the night. This shud help
keep temps on the warmer side despite mostly clear sky.

Mdl solns differ regarding a s/w ejecting from the upper low early
Tues morning that may generate TSRA impacting the wrn portions of
the CWA around sunrise into at least mid morning hours. Have kept
PoPs dry for now, but if mdls come into better agreement, precip may
need to be added with updates.

Tilly

.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 353 PM CDT Mon Apr 24 2017

Convection will move into parts of northeast and central MO and
southwest IL Tuesday night as a deepening upper level trough and
associated cold front approaches.  A strong southwesterly low level
jet over southwest MO will bring increasing low level temperature
and moisture advection into this area.  Upper level divergence will
also be increasing ahead of the approaching upper level trough.  The
pops will increase and shift east-southeastward through our forecast
area on Wednesday and Wednesday night as the upper level trough
becomes more negatively tilted and the front moves relatively slowly
eastward with at least weak surface waves developing on the front.
Prefer the GFS and ECMWF model solutions over the NAM which have
been more consistent.  The NAM model briefly develops an upper level
low in the base of the trough leading to a stronger surface low on
the front.  More confident in the model solutions of the GFS and
ECMWF.  In either case the models continue to be slower with the
passage of the upper level trough and associated cold front which
would keep the potential for convection going across the eastern
portion of the forecast area through late Wednesday night.  The
slower progression of the front will also increase the probability
of severe storms Wednesday afternoon and evening across southeast MO
and southwest IL.  The deep layer shear will be favorable due to a
strong mid level wind max ahead of the upper level trough.  The
amount of instability in our area is more in question and will
depend on the location and amount of morning convection, cloud
cover, and exact position of the front during the time of maximum
diurnal heating. Showers/storms should shift east of the forecast
area by Thursday morning with cooler temperatures Wednesday night
and Thursday after the passage of the upper level trough and cold
front. The model solutions begin to diverge by Friday, but there may
be another round of convection late Thursday night and Friday due to
strong low to mid level warm air advection ahead of a southwest flow
shortwave. More significant convection is expected to shift
northward through our area Friday night and Saturday as a deep upper
level trough moves into the Plains and a warm front lifts northward
ahead of an approaching surface low. There may be the potential for
heavy rain Friday night through Saturday night.  There may also be
the potential for severe storms, mainly Saturday night as the upper
level low/trough and associated surface low and trailing cold front
move through the forecast area. Post frontal showers and a few
storms will continue Sunday and possibly into Sunday night until the
upper level trough/low finally shifts east of the region. Cooler
temperatures are expected for Sunday night and Monday with the deep
upper level trough either over or just east of our area, along with
low level cold air advection behind the surface low and trailing
cold front.

GKS

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1224 AM CDT Tue Apr 25 2017

VFR for at least the first 0-12 hours. The exception may be at
KCOU where scattered showers may develop towards sunrise. Winds
will remain southeasterly and increase in speed due to the
tightening pressure gradient ahead of a developing low pressure
system. Expect wind gusts of 20-30 kts after 15z and especially
after 18z. Gusts should diminish after 26/00z but wind speeds will
likely remain AOA 10 kts. MVFR ceilings and SHRA/TSRA are
possible towards the end of the valid TAF period.

Kanofsky

&&

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

$$

WFO LSX



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