Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181223

723 AM CDT Sat Apr 18 2015

.SHORT TERM:  (Through Late This Afternoon)
Issued at 406 AM CDT Sat Apr 18 2015

The upper level low over CO will move slowly eastward today.  As
southwest flow shortwaves move into our area, along with increasing
upper level divergence ahead of the the upper level low/trough,
scattered showers and storms moving north northeastward through
western MO should eventually translate eastward into the western
portion of our forecast area, west of the Mississippi River this
afternoon.  Followed the convective forecast of the 4 km NCEP WRF
for today which appears to have a pretty good handle on the current
convection.  Above normal temperatures can be expected again today,
slightly cooler across the western portion of the forecast area due
to increased cloud cover along with the potential for at least
scattered showers/storms this afternoon.


.LONG TERM:  (Tonight through Friday)
Issued at 450 AM CDT Sat Apr 18 2015

A much better potential for more widespread convection will occur
tonight and Sunday as the upper level low moves eastward through KS
tonight and Sunday morning, and through MO Sunday afternoon with
shortwaves continuing to be ejected out ahead of it.  The NAM and
GFS models develop a surface low which will move northeastward
through southeast MO and western KY late tonight and into southeast
IL and southwest IN by 18Z Sunday.  The NAM model is stronger with
this surface low versus the GFS, while the ECMWF is even weaker than
the GFS.  Will continue to follow the NCEP 4 KM WRF model which
develops a large area of convection associated with this surface low
which spreads northward into much of the forecast area late
tonight.  Will put the highest pops tonight across southeast MO and
southwest IL.  Convection will continue Sunday into Sunday evening
as the upper level low/trough opens up and moves through our area
with a cold front moving southeastward through the forecast region
Sunday night.  There is still a risk for severe storms on Sunday,
but it appears that this risk will be further south, mainly across
southeast MO where more instability is forecast.  Cooler
temperatures can be expected on Sunday due to the cloud cover and
precipitation, then cooler Sunday night and Monday due to low level
cold air advection behind the 850 mb trough/surface cold front.  A
cooler period is expected for the next work week as an upper level
trough/low moves into the Great Lakes region and we get into
persistent northwest flow.  The ECMWF model is a little colder than
the GFS, particularly for Wednesday through Friday as it brings a
cold front southward through our area on Wednesday with the zero
degree C 850 mb isotherm dropping southward to STL by Thursday
morning.  With model differences, especially later in the extended
forecast period, will forecast temperatures at least slightly below
normal for the work week.



.AVIATION:  (For the 12z TAFs through 12z Sunday Morning)
Issued at 633 AM CDT Sat Apr 18 2015

A tricky fcst with closed stacked low spinning over CO. This
feature is not fcst to move very far during the TAF valid
prd...only mvng to SW KS by 12Z Sun. Lobes of energy rotating
around the upper level disturbance will modulate the convective
threat. A band of SHRA/TSTMs has dvlpd this mrng across ern
OK/KS...though addtnl isld activity has dvlpd ahead of this area
with all precip mvng almost due north as it rotates around the
parent system across CO. Due to the slow mvmnt of this feature and
the fact that the individual convective elements are mvng north,
precip will be slow to progress across MO today/tonight. KCOU is
the lowest confidence site as it wil be closest to the activity
across wrn MO thru the day. It is also possible that addtnl SHRA/TSTMs
may dvlp in an unstable air mass across ern MO this aftn/evng but
not confident enough in coverage to include mention attm. Expect a
VFR fcst except in precip. Once precip moves into a terminal, it
should last several hours before ending.

Specifics for KSTL:

VFR fcst today with widespread convection mvng north across ern
KS/OK spreading into wrn MO. The may be some addtnl convection
that dvlps today east of this band of precip but not confident in
coverage to include attm. Due to the slow mvmnt of the system, the
precip will be slow to advance east. There could be a several hour
window this evng/tonight where precip will be near but not yet affecting
the terminal. Once precip moves in, it should last several hours.





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