Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 300419

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1119 PM CDT FRI APR 29 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 315 PM CDT Fri Apr 29 2016

Will show an increase in showers and thunderstorms this afternoon
this evening as low level moisture convergence begins to increase
over the area.  Will keep categorical PoPs over the entire area
overnight as low level moisture increases on the nose of a 40kt low
level jet.  This will occur under increasing mid-upper ascent of
large upper low now entering southern Plains. Severe weather
threat tonight will be limited by the lack of instability, but
moisture will be plentiful with increasing PWATs around 1.5 by
12Z. Widespread lows in the 50s still look good and are close to
MOS guidance.


.LONG TERM...  (Saturday through Next Friday)
Issued at 315 PM CDT Fri Apr 29 2016

(Saturday through Monday)

Main concern will continue to be the potential for severe storms on
Saturday and Saturday evening.  Widespread showers and thunderstorms
should be ongoing during the early morning hours as there will still
be some strong low level moisture convergence underlying the mid
level ascent.  This will allow for the widespread showers and
thunderstorms to end from southwest to northeast during the late
morning and early afternoon hours.  Still have some concern about
redevelopment of thunderstorms along and south of I-70 during the
afternoon and evening hours.  Latest run of the GFS is farther north
of the surface low which matches better with the more consistent
ECMWF and NAM.  Will continue to show chance PoPS for thunderstorms
during the afternoon and evening.  Model MUCAPES are in the 1000-
2000 J/kg range with deep layer that supports supercells.
Hodographs in the warm sector suggests a sheared environment that
would favor multicells with a damaging wind potential.  However,
could not rule out a brief tornado potential along the warm front.
Severe potential should end toward late evening once the cold front
moves east of the CWA.

Shower chances will continue into Sunday as the upper low passes
through the area.

(Tuesday through Friday)

GFS and ECMWF continues to show highly amplified upper flow over
North America next week with a deep trough over the eastern CONUS
and a ridge over the Rocky Mountains.  This will result in northerly
upper flow aloft through the period that is cutoff from a supply of
Gulf Moisture, so will keep forecast mainly dry for now.  GFS and
ECMWF does show a cold front that moves across the area from the
north on Wednesday night.  850mb temperatures look cooler today than
previous runs, which support temperatures closer to normal as we go
into early May.



.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1102 PM CDT Fri Apr 29 2016

Cloud ceilings will continue to gradually drop into the MVFR
catagory at the taf sites overnight as an intensifying southerly
low level jet brings increasing low level moisture into the area.
Showers and thunderstorms have already moved into COU and the St
Louis metro area late this evening and this activity will
continue overnight and into the early morning hours as a large
area of showers and storms over southwest MO moves northeastward
into our area. Ceilings and visibilities may at least briefly drop
into the IFR catagory late tonight and early Saturday morning with
the rain or showers and embedded elevated storms. Most of the
showers and storms should shift east-northeast of the taf sites by
late Saturday morning as a warm front shifts northeast of the
area. Easterly surface wind will veer around to a south-
southwesterly direction Saturday afternoon after passage of the
warm front. There may be scattered storms redeveloping late
Saturday afternoon or early evening mainly in the St Louis metro

Specifics for KSTL: Showers and a few storms will continue
overnight and into Saturday morning with the cloud ceiling
dropping into the MVFR catagory around 06Z Saturday. The ceilings
and visibilities will drop down into the IFR catagory at times
late tonight and early Saturday morning. Most of the showers and
storms should shift east-northeast of STL by late morning with
the potential of scattered storms redeveloping late Saturday
afternoon or early evening. The prevailing ceiling will become VFR
Saturday afternoon. Easterly surface wind will continue late
tonight, then veer around to a southeasterly direction Saturday
morning, and a southwesterly direction Saturday afternoon.





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