Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 191654
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1154 AM CDT Sun Mar 19 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 340 AM CDT Sun Mar 19 2017

High pressure over Illinois will continue moving east today and a
developing trof of low pressure over the Great Plains will move into
the Midwest.  Low level moisture convergence is producing isolated
showers over southwest missouri at this time...and expect this
isolated convection to spread east this morning as the 850mb ridge
axis moves east of the Mississippi River.  Short range guidance is
hinting that coverage could get as high as 30-40% this afternoon
over central Missouri when isentropic lift and moisture convergence
is greatest, and convection allowing models support this as well.

Should see convection weaken during the late afternoon to early
evening as the low level jet refocuses further north over Iowa.
Model guidance is mixed on placement and amount of precip tonight.
The GFS is the furthest west with the bulk of the precip in our area
falling along and west of the Mississippi...and it`s also the
fastest in developing precip this evening.  ECMWF puts almost all of
the QPF over Illinois and also has all of the precip after midnight.
The NAM is very similar to the ECMWF but does have some precip west
of the Mississippi overnight.  The NSSL and NCEP WRF have little or
no precip over our area all night.  So...without a good consensus,
have 30-50 PoPs across the area tonight with the higher end of the
range across Illinois.  Guidance is consistent in showing MUCAPE in
excess of 1500 J/Kg (albeit in different locations) tonight.  This
instability combined with 30-40kts of deep layer shear should be
sufficient for elevated severe thunderstorms with the primary threat
being large hail.

Carney

.LONG TERM...  (Monday through Saturday)
Issued at 340 AM CDT Sun Mar 19 2017

Scattered thunderstorms will likely be ongoing early on Monday
morning, particularly over the eastern half of the CWA.  This is
where both the NAM/GFS are showing decent low level moisture
convergence occurring under modest mid level ascent associated with
a weak shortwave trough moving across the Missouri and Illinois.
This storms will move east early in the day as the forcing moves off
to the east.  A cold front will move slowly southeast across the
area during the day, and forecast MLCAPES will increase above 1000
J/kg during the afternoon along and south of I-70.  However,
forecast soundings are showing a significant inversion in the low
levels which will limit just isolated showers and thunderstorms over
southeast Missouri.  Favor the warmer NAM MOS temperatures on Monday
for highs which has temperatures climbing into the 70s which is
supported by 850mb temperatures in the 10-15C range.

Then much like previous runs, the GFS and ECMWF has the front
dropping to the south of Missouri and Illinois on Tuesday and
Wednesday.  Even though the front will be south of the area on
Monday night into Tuesday night, will keep a chance of showers going
over the area.  Still looks like a low level jet will transport
enough low level moisture into the area as a series of weak vort
maxes move across the region in west northwesterly flow aloft. There
will be a brief break in the rain on Wednesday, chances will
increase again Wednesday night into next Saturday.  This will occur
as low level flow turn out of the southwest and transports more
moisture into the area in response to an upper trough approaching
from the west.  Temperatures will be near normal at midweek, but
then climb back above normal at the end of the week as a warm front
moves north in response with the approach of the upper trough.  By
Friday and Saturday, expect a chance of showers and thunderstorms
over the area with highs in the 60s and 70s.

Britt

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1123 AM CDT Sun Mar 19 2017

Scattered convection will likely impact COU early this afternoon,
but should remain west or southwest of the St Louis area taf
sites. Could not rule out sprinkles in the St Louis metro area
this afternoon from low-mid level clouds. These low-mid level
clouds will dissipate or shift east of the taf sites late this
afternoon and early this evening. Elevated convection is expected
to develop late tonight in the UIN area on the nose of a west-
southwesterly low level jet. This convection should weaken and
shift toward the St Louis metro area early Monday morning. Due to
uncertainty in coverage and timing may just include VCTS in the
UIN taf late tonight, and VCSH in the St Louis metro area tafs
early Monday morning. There will be some LLWS late tonight, but
will leave it out of the tafs for now as the magnitude does not
look quite strong enough for inclusion in the tafs. The prevailing
cloud ceilings will likely drop into the MVFR catagory at the
tafs sites, and possibly down to IFR in UIN late tonight and early
Monday morning with increasing low level moisture ahead of an
approaching cold front. Sely surface winds will veer around to a
southwesterly direction Monday morning.

SPECIFICS FOR KSTL: Scattered convection will impact central and
southeast MO this afternoon, but should remain west or southwest
of the STL area. Could not rule out sprinkles in STL this
afternoon from low-mid level clouds. These low-mid level clouds
will dissipate or shift east of the STL area this evening.
Elevated convection is expected to develop late tonight in
northeast MO and west central IL on the nose of a west-
southwesterly low level jet. This convection should weaken and
shift toward STL early Monday morning. Due to uncertainty in
coverage and timing may just include VCSH in the STL taf early
Monday morning. There will be some LLWS late tonight, but will
leave it out of the STL taf for now as the magnitude does not
look quite strong enough for inclusion in the taf. The prevailing
cloud ceiling will likely drop into the MVFR catagory early
Monday morning with increasing low level moisture ahead of an
approaching cold front. Sely surface winds will veer around to a
southwesterly direction by late Monday morning.

GKS

&&

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

$$

WFO LSX



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