Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 202005
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
305 PM CDT Sat May 20 2017

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 304 PM CDT Sat May 20 2017

Cold front now moving through western Missouri will continue to
progress eastward tonight.  Hi res convection allowing models
continue to insist that another broken line of thunderstorms will
develop ahead of the front between 5-7 pm.  Have kept mid-high
chance PoPs to this evening, tapering off overnight as the front
moves east of the region. 1000-1500 J/Kg MUCAPE ahead of the front
combined with around 40kts of 0-6km shear could produce some strong
to marginally severe storms.

Cooler and drier air will filter into the area behind the front
tonight.  Normal lows for May 21 are in the mid 50s (upper 50s in
St. Louis).  Should see low temperatures Sunday near or a few degrees
below that in the upper 40s to mid 50s.  Daytime highs on Sunday
will be below normal in the mid 60s to low 70s.  Most of the area
should be partly to mostly sunny, but northeast Missouri and west
central Illinois may have more cold advection stratocumulus similar
to current conditions over Kansas.

Carney

.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 304 PM CDT Sat May 20 2017

The global models are in relatively good agreement that the upper
low that will be along the U.S./Canadian border on Sunday evening
will drop southward into the western Great Lakes early this week.
After being dry on Sunday night and early Monday, a shortwave trough
will swing around the south side of the upper low on Monday night
and Tuesday bringing a cold front through the area with a chance of
showers and isolated thunderstorms.  This chance of showers and
isolated thunderstorms will continue into Wednesday as the upper low
shifts slowly southeast into the Ohio Valley.  Dry and partly cloudy
weather is not expected until late in the week when the upper low
moves off to the east and an upper ridge begins to build into the
Midwest.  GFS/ECMWF are already hinting at another chance of rain by
next weekend as the ridge moves off to the east and another
shortwave trough approaches the area from the Plains.

Temperatures early to mid week will be below normal underneath the
upper low with the clouds and the rain showers.  Temperatures will
return to near normal by late in the period as 850mb temperatures
climb to around 15C in southwesterly low level flow on the back side
of the high.

Britt

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1247 PM CDT Sat May 20 2017

Morning IFR and MVFR ceilings are lifting from south to north at
this time...with another MVFR deck moving east from central
Missouri. Expect the lifting trend to continue this
afternoon...and daytime heating should also allow the central MO
MVFR deck to lift to VFR by the time it reaches east central
Missouri later this afternoon. Model guidance is pretty insistent
that another round of showers and thunderstorms will develop along
the cold front during the late afternoon/early evening hours as
it pushes through central into eastern Missouri. Think coverage
will be scattered...30-50%...so did not go with TEMPOs at this
time. Think the last few showers should exit the eastern portions
of the area (vicinity of KSLO) by 07-08Z. VFR flight conditions
are expected to prevail outside of showers and storms.

SPECIFICS FOR KSTL:

Last shreds of morning stratus should move out of the vicinity of
the terminal any minute and VFR conditions are expected to prevail
for the remainder of the afternoon. There is a stratocumulus deck
moving east toward the terminal from central Missouri, but I
expect the bases of this ceiling will lift to VFR by the time it
reaches Lambert. Model guidance is pretty insistent that another
round of showers and thunderstorms will develop along the cold
front during the early evening hours as it moves toward the
terminal. Think coverage will be scattered...30-50%...so did not
go with TEMPO at this time. Expect VFR conditions to prevail
outside of any shower or storm that hits the terminal.

Carney

&&

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

$$

WFO LSX



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