Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 180808

Area Forecast Discussion
National Weather Service Saint Louis MO
308 AM CDT Sat Mar 18 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 306 AM CDT Sat Mar 18 2017

A short wave is fcst to track from near DLH this evng to NW PA by
Sat evng. A cold front assoc with the short wave currently extends
from N cntrl IL to the STL metro area to SW MO. Low clouds continue
to clear from NW to SE with the back edge roughly along I44 in MO
and I70 in IL. This is just in advance of the bndry which should
clear the sthrn zones this evng. Most guidance indicates FROPA will
be dry this aftn/evng but can`t rule out a couple light SHRAs across
the ern Ozarks...but the bulk of the activity should remain S of the
CWA overnight. 1030mb SFC ridge builds into the region tonight and
should keep conditions quiet thru Sat. CAA thru the day Sat will
keep temps in check. After some areas that cleared out by early aftn
today reached into the lower 70s...most locations will remain in the
50s tomorrow. The exceptions are portions of cntrl and SE MO which
could rebound into the lower 60s after a chilly start in the mid 30s
to near 40.


.LONG TERM...  (Sunday through Friday)
Issued at 306 AM CDT Sat Mar 18 2017

Main concern during the period deals with the potential for a few
severe thunderstorms on Sunday night.

There is good agreement between the GFS/NAM/ECMWF that the upper
pattern will become more zonal late in the weekend as a east coast
trough moves east and the upper ridge currently over the Rockies
dampens out as it moves east across the central CONUS.  This will
allow the upper flow to become more zonal by Sunday.  Winds will
turn out of the south by Sunday as the surface high moves off to the
east allowing for some moisture return from the Gulf.  Temperatures
will climb back above normal as both the GFS and ECMWF show 850
temperatures climbing into the 10-15C range.  There will be an
increasing chance for scattered showers and thunderstorms on Sunday
evening as the GFS shows low level moisture convergence and
instability increasing across Missouri and Illinois.  Will increase
chances to likely over the eastern half of CWA overnight as the
moisture convergence increases.  Given the expected steep lapse
rates and deep layer shear over 40kts, there could be a few elevated
severe thunderstorms Sunday night capable of producing large hail.
Showers and thunderstorms will shift off to the east on Monday
morning as the low level moisture convergence shifts to the east.
The upper flow will turn slightly out of the northwest on Monday
causing a cold front to move southeast through the CWA. There will
be some instability ahead of the cold front, so will keep a chance
of thunderstorms on Monday. However, forecast soundings are showing
a notable inversion ahead of the front which may limit shower and
thunderstorm coverage during the afternoon.

GFS and ECMWF are in good agreement that the front will slowly move
south through the Mid South into the Lower Mississippi Valley
Tuesday and Wednesday.  Rain chances will continue Monday night into
Tuesday night over much of the CWA as a series of weak shortwave
troughs moves across the area at the same time weak low level
moisture convergence continues.  While it appears most of the area
will be dry on Wednesday, rain chances will increase late next week
as a longwave trough approaches the Midwest from the western CONUS.
This will cause an increase in moisture transport and low level
forcing.  Temperatures will remain at or above normal during the



.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 955 PM CDT Fri Mar 17 2017

VFR conditions and dry wx will prevail at the TAF sites for much
of the valid period. The only item of concern continues to be an
area of MVFR CIGs to our north, now reaching the quad cities area.
Some new model guidance has backed off on low cloud extent, while
other model guidance has become more aggressive with it.
Considering its current position and flow continues to present a
threat to terminals near the MS river for early Saturday morning,
will continue the course of a period of MVFR conditions at UIN and
a SCT cloud layer around 2500ft for STL metro sites until a
clearer direction can be determined. See no strong reason to back
down from current forecast. Otherwise, NW surface winds will
persist for much of the valid period, only veering NE and
becoming light around sundown Saturday night.

SPECIFICS FOR KSTL: VFR conditions and dry wx to prevail.
Continuing to watch region of MVFR CIGs to the north as they drop
southward overnight and into early Saturday morning. Latest model
guidance continues to be split on how much impact it will have on
STL and with it still several hours out, will continue the SCT
layer between 14-18z with the understanding it may need to be
upgraded to a CIG later tonight. Otherwise NW surface winds
will persist until Saturday night when they become light and
variable as a RIDGE of high pressure builds in.





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