Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 191151

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
651 AM CDT WED OCT 19 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 406 AM CDT Wed Oct 19 2016

The surface cold front that moved thru our region yesterday has
pushed south of FAM and is near MDH, which means in our CWA, it is
over the far southern portions of it in Reynolds, Iron, and Madison
counties in MO.  Already overrunning moisture is surging northward
towards central MO and never really left for STL metro and areas to
the southeast.  This is also translating into cloudy skies returning
towards the I-70 corridor early this morning with clear skies to the
north.  Consequently, temps have dropped into the 50s in areas with
clear skies, while readings are lingering in the 60s further to the
south.  Rain is pretty spotty at the moment with isolated showers
near and north of the cold front to as far north as the I-44 and I-
64 corridors.

Rich moisture thru the column is expected to continue to push north
this morning and settle into much of central MO and the rest of the
I-70 corridor by noon, where PWATs of 1.5" or more are anticipated,
which is over 2 standard deviations above the mean for this time of
year.  At the same time, a strong shortwave disturbance over CO will
approach from the west, inducing increasing broadscale lift to go
along with the moisture transport and WAA already going on.  The
surface cold front will also play a role, sliding back to the north
later this morning as a wave develops along it.

This is expected to result in a gradual increase and shifting north
of showers this morning, with a more substantial increase of coverage
this afternoon for STL metro and areas to the southeast--peaking
during the evening hours--and a slow retreat to the east overnight
as the broadscale lift, and the surface front, pull away.

The surface frontal boundary shifting northward a bit today will
return the threat for surface-based storms mainly this afternoon
into a larger portion of southeast MO and far southern IL and will
justify a risk for severe storms in these areas when the strong
shear thru the column is also considered.

In addition, the storms that do form in this zone should be able to
take advantage of the rich moisture in place and could drop some
locally heavy rainfall amounts for areas southeast of STL metro.
Have added this wording to the forecast to account.  Not concerned
with the need for a FFA at this time, though, with rather high FFG
values (1hr of 2.3 to 2.6" for instance) and day shift can re-
evaluate this need based on realized radar trends.

Otherwise, prefered MET MOS values on temps, which are still above
average, with values in the upper 60s and lower 70s for most
locations, save in the far southern CWA where 80s is expected.


.LONG TERM...  (Thursday through Tuesday)
Issued at 406 AM CDT Wed Oct 19 2016

Showers will likely continue Thursday morning across southeast MO
and southwest IL behind the surface low and trailing cold front and
ahead of the deepening upper level trough moving eastward into the
central US.  Models are depicting upper level divergence over this
area ahead of the upper level trough axis and in the right entrance
region of an upper level jet streak moving through the Great Lakes
ahead of the trough.  The GFS model appears a little too quick
moving the precipitation out of our forecast area and prefer the
ECMWF model solution which does not shift the showers out of our
area until afternoon.  Little diurnal temperature variation is
expected on Thursday due to low level cold air advection along with
cloud cover and morning precipitation.  Much colder temperatures are
expected Thursday night as a strong surface builds southeastward
into the area behind the cold front and the sky clears out, the
northwest wind weakens and the surface dew points drop into the 30s.
Patchy light frost is possible late Thursday night/early Friday
morning in northeast MO.  The 850 mb temperature will drop to around
2-3 degrees C in STL by 12Z Friday.  Lows Thursday night will only
be about 5 degrees below normal, but will feel colder after the
current stretch of unseasonably warm weather.  Dry and relatively
cool conditions can be expected for the end of the work week due to
northwest upper level flow behind the upper level trough, and as the
surface ridge moves through the area.  A gradual warming trend can
be expected for the weekend as an upper level ridge moves eastward
into the region, and the surface wind becomes southerly as the
surface ridge shifts east of the area.  A weak front will sag
southward through our area Sunday night and Monday but precipitation
is not expected with this front, and most of the cold air behind the
front will remain northeast of our forecast area.



.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 651 AM CDT Wed Oct 19 2016

VFR ceilings are expected at the TAF sites for much of the valid
period. A brief period of MVFR CIGs will be possible during the
midday and afternoon hours at COU and the STL metro sites but it
looks like more likely for MVFR CIGs to occur at the STL metro
sites for late this evening and overnight. Showers should
initially be scattered for COU and STL metro sites but is expected
to congeal enough for prevailing at STL metro sites later today
and into this evening along with some thunder in vicinity.
Otherwise, easterly winds to back northerly tonight with some
higher gusts to 18-20kts expected late tonight.





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