Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181758

1258 PM 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 18z TAFs through 18z Sunday Afternoon)
Issued at 1255 PM CDT Sat Apr 18 2015

Two areas of concern with regard to rain. The first is a broken
band of SHRA/TSRA that has been edging east very slowly. The
thinking with this is that it will edge into central and northeast
MO this afternoon before it then pulls away to the north. This
will result in rain in the vicinity for COU for much of the
afternoon and then edging UIN during the evening. Some isolated
TSRA possible for STL metro this afternoon but looks too small to
mention at this time until we actually see something develop. The
other area of concern is larger area of rain that will develop
late tonight and into Sunday morning as a new area of low pressure
develops to our south and tracks to our east on Sunday. This area
of rain should impact STL metro by late tonight and then UIN by
late Sunday morning. COU appears to be more on the edge but enough
guidance is showing that some of this will fill-in to include rain
mention for them as well. Otherwise, easterly flow will back
northerly on Sunday as the LOW pulls away.

Specifics for KSTL: Isolated TSRA possible late this afternoon but
probs too small to mention. Otherwise, larger area of rain to
impact the airport late tonight and should include lower non-VFR
CIGs on Sunday morning--but questionable how low to go. For now
have MVFR levels. Increasing evidence of some re-development of
scattered TSRA late Sunday afternoon and into the evening and have
transferred the thunder mention to this period which looks more
favorable for thunder, if any happens.





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