Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231745

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1245 PM CDT Sun Sep 23 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 349 AM CDT Sun Sep 23 2018

A surface ridge extended from the Great Lakes region southwest
into northeastern MO with a stationary front extending from the
southern Plains northeast into the Tennessee Valley region.
Persistent cloudiness continued across southeast MO and southwest
IL, southeast of STL.  Steam fog was developing in the river
valleys, particularly at SUS and JEF where the sky was clear, the
surface wind calm, and the air temperature quite cool over the warm
river water.  It appears that most of the rain today will remain
southeast of our forecast area, although could not rule out patchy
light rain, mainly this afternoon across parts of southeast MO,
south of FAM into a small portion of southwest IL.  The cloud cover
will advect northwestward tonight with an increasing chance of
showers across southeast MO and southwest IL ahead of a weak
southern stream shortwave.  A modest south-southwesterly low level
jet over northeast AR into western KY will bring increasing low
level moisture into this area.  Will see slightly warmer
temperatures today and tonight with a gradual rise in the surface
dew points. Highs today will be close to seasonal normals for late
September, with lows tonight slightly above normal.


.LONG TERM...  (Monday through Saturday)
Issued at 349 AM CDT Sun Sep 23 2018

Broad, cyclonic upper level flow will dominate the work week, with a
few focused shortwaves passing through the central CONUS. The
first wave will eject out of the northern Rockies on Monday and
result in low level cyclogenesis across the Northern Plains while
a secondary surface cyclone develops across the TX/OK panhandles.
The secondary low will lift a stationary front north out of the
Mid-South and through the Mid Mississippi Valley, bringing above
normal temperatures back to the region on Monday and Tuesday.
Forecast guidance continues to show focused precip along the front
Monday morning, especially in areas east of the Mississippi, and
then lingering warm sector showers and storms.

The upper level shortwave will continue eastward Tuesday, kicking
both the northern and southern surface cyclones downstream. This
will result in a fairly potent cold front sweeping through the
region on later on Tuesday. Models still disagree on the exact
timing of the front, with the GFS/Euro putting the front on our NW
doorstep by 00z and the NAM/GEM a bit faster. Ahead of the front,
guidance is showing MLCAPE building to 1000-1500 J/kg and 0-6km
bulk shear around 40 kts. While the front will be the primary
source for low level forcing, all guidance shows continued warm
sector precip on Tuesday afternoon thanks to the cyclonic flow
aloft and weak warm air advection. With the above CAPE/shear
parameter space, there is a slight risk that any storms that do
develop ahead of or along the front become strong to severe.
Additionally, should the front move through the region a bit
earlier than expected, it would align well with peak instability,
and the threat for severe weather would increase. The severe
weather risk is highlighted by the latest SPC day 3 outlook, with
a slight risk across a large portion of the CWA.

We`ll remain beneath upper level cyclonic flow behind the frontal
passage, with another embedded shortwave passing the region on
Thursday. This wave will result in another round of surface
cyclogenesis across the northern CONUS, and will eventually drag the
tail end of another cold front through the region. With little
moisture at the surface, this will likely be a dry frontal passage
for us. The primary impact for the Mid-Mississippi Valley will be a
reinforcing shot of cooler air. Otherwise, surface high pressure
will dominate for the latter half of the work week. By the weekend,
forecast guidance begins to diverge quite a bit, but it does look
like an unsettled pattern will settle back in over the region,
with a deep surface cyclone moving into the Midwest



.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1213 PM CDT Sun Sep 23 2018

Dry and VFR conditions are expected at the terminals through 06Z,
then MVFR ceilings will move into the area from the south
affecting the St. Louis area terminals first and KCOU and KUIN
late tonight and Monday morning. Ceilings may be IFR at times, but
confidence is not high enough to include at this time. Scattered
showers and thunderstorms will also be possible over eastern
Missouri and southwest Illinois late tonight and on Monday, but
coverage is not expected to be widespread enough to warrant put
more than a VCSH in the KSUS and KCPS TAFs at this time.
Otherwise, winds will remain mainly light at this time.


Dry and VFR conditions are expected at the terminal through 06Z
before MVFR and possible IFR ceilings move in late tonight.  Scattered
showers and thunderstorms will also be possible over eastern
Missouri late tonight and on Monday, but coverage is not expected
to be widespread enough to warrant putting in the TAF at this
point. Otherwise, winds will remain mainly light at this time.





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