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Area forecast discussion 
National Weather Service St Louis MO
451 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 Colorado 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 Kansas
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 Kentucky late tonight and into southeast
Illinois and southwest in by 18z Sunday. The NAM model is stronger with
this surface low versus the GFS, while the European model (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 Illinois. 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 European model (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 06z tafs through 06z Saturday night)
issued at 1148 PM CDT Fri Apr 17 2015

Specifics for kcou, kuin: a few showers are possible at kcou after
12z. MVFR conditions are possible if one of the showers happens to
move directly over kcou. There is a much greater chance of
rain showers/thunderstorms and rain after 19/03z at both kcou and KUIN when a slow-moving
low pressure system approaches the region.

Specifics for kstl, ksus, kcps: VFR conditions are expected for at
least the first 0-6 hours except at ksus where light fog is
possible towards daybreak given the airport's proximity to the
river. Elsewhere, increasing cirrus overnight and the lack of a
moisture source adjacent to the Airport should limit any fog
formation. Rain showers/thunderstorms and rain chances increase greatly after 19/03z when a
slow-moving low pressure system approaches the region.



Lsx watches/warnings/advisories:


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