Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 211144

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
644 AM CDT Tue Mar 21 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 330 AM CDT Tue Mar 21 2017

Cold front had moved south into far northern Arkansas and Kentucky
early this morning.  Winds remain out of the north over the area.
There will be an increasing chance for showers and thunderstorms in
area along and south of I-70 today.  A weak shortwave trough
currently seen in the water vapor imagery over the Central Plains
will move east southeast across the area at the same time there will
be some modest low level moisture convergence over the area. Another
area of showers will be found across northeast Missouri and west
central Illinois from mid morning into mid afternoon.  Temperatures
today will not be nearly as warm as yesterday with cold air
advection and the clouds.  Highs today will be closer to the GFS MOS.

The cold front will move farther south of the area tonight and high
pressure will move into the area from the north.  This will cause
the scattered showers to move south of the area during the evening
hours.  Skies will become partly cloudy along and north of I-70
allowing for better cooling.  Stayed closer to GFS MOS guidance for


.LONG TERM...  (Wednesday through Monday)
Issued at 330 AM CDT Tue Mar 21 2017

High amplitude pattern will set up Wednesday with a longwave trof
over the eastern U.S. and a ridge building over the Great Plains.
Wednesday`s surface weather will be dominated by a large high
centered over the Upper Midwest.  The high will drift east-southeast
through the day.  Easterly flow during the day will keep
temperatures below normal in the 40s during the day.  The high will
continue moving east-southeast through Thursday as the upper ridge
over the Great Plains moves over the Mississippi Valley.  Low level
flow will turn back to the south allowing temperatures to return to
near seasonal normals.  Models are also showing fairly rapid
moisture return Thursday...and therefore are printing out light QPF
across much of the area.  While models sometimes over forecast QPF in
strong moisture return situations, there`s enough moisture
convergence and warm advection with a 20-35kt low level jet that I
don`t want to significantly undercut PoPs from guidance at this
time.  If clouds and precip don`t materialize on Thursday it will
likely be much warmer than currently forecast.

The upper level ridge will move east of the region on Friday and a
strong stacked low pressure system will develop over the Great
Plains.  This system will control the weather through the weekend.
Strong southerly flow ahead of the system on Friday will continue to
push temperatures upward.  Increasing deep moisture is producing a
lot of cloudiness in the guidance, and like Thursday, if these
clouds don`t materialize the going forecast in the mid 70s may be
too cool.  First wave of precipitation looks likely Friday night as
a shortwave rotates around the upper low and ejects into the
Mississippi Valley.  The system lifts northeast Saturday and
continues northeast through Sunday.  Current indications are that
there will be several waves of showers and thunderstorms through
Saturday night with diminishing chances Sunday.  Medium range models
show some dry time late Sunday and Sunday night with another chance
of rain developing early Monday as another storm system moves from
the Great Plains into Missouri.



.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 643 AM CDT Tue Mar 21 2017

There is the potential for areas of MVFR visibilities in fog to
affect the St. Louis terminals through 14Z. Most of the fog and
low ceilings will be found over southern Illinois and southeast
Missouri this morning. Scattered showers and thunderstorms are
expected to stay mainly south of KCOU and the St. Louis area
terminals. Otherwise, mainly dry VFR conditions are expected
during the period with north to northeasterly winds.


Dry and VFR conditions are expected through the period. Any fog
early in the period is expected to dissipate by 14Z. Scattered
showers and thunderstorms that do develop later this morning and
this afternoon are expected to stay south of the terminal.
Northeasterly winds will continue.





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