Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181659
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1159 AM CDT Sat Mar 18 2017

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

...Corrected...

High pressure over the Great plains will drift southeast into the
Mississippi Valley today.  This high will bring cooler temperatures
to the region with highs mainly in the mid and upper 50s.  The ridge
will continue moving east tonight and low level return flow will
develop after midnight.  Short range guidance develops an area of
moisture convergence over western into central Missouri between 06-
12Z Sunday morning.  GFS and NAM print out precip over western
Missouri before 12Z Sunday.  Current indications are that the
precipitation should stay out of our area until after 12Z.  Lows
tonight will dip down to the low and mid 30s as the high drifts
overhead.  May actually see the lowest temperatures between 06-09Z
with steady temperatures or a little bump before sunrise.

Carney

.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
period.

Britt

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1141 AM CDT Sat Mar 18 2017

Surface ridge centered over eastern portions of South Dakota and
Nebraska will move east-southeastward. Low level cloudiness on the
backside of a surface low over the Ohio Valley region should
remain east of the taf sites. Just some mid-high level clouds
later this afternoon and tonight with the northwest surface wind
becoming light this evening. Increasing low level moisture over
western into central MO late tonight and Sunday morning will lead
to low level, VFR clouds in COU Sunday morning as well as
scattered showers. The surface wind will strengthen from a
southeast direction Sunday morning as the surface ridge shifts
east of our area.

SPECIFICS FOR KSTL: Surface ridge centered over eastern portions of
South Dakota and Nebraska will move east-southeastward. Low level
cloudiness on the backside of a surface low over the Ohio Valley
region should remain east of the STL area. Just some mid-high
level clouds later this afternoon and tonight with the northwest
surface wind becoming light this evening. Increasing low level
moisture spreading eastward into eastern MO on Sunday will lead to
VFR, low-mid level cloudiness by Sunday afternoon, but any
showers should remain west of STL. The surface wind will
strengthen from a southeast direction Sunday as the surface ridge
shifts east of our area.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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