Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 190451

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1151 PM CDT Sat Mar 18 2017

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 301 PM CDT Sat Mar 18 2017

A ridge of high pressure centered over western Minnesota and eastern
Nebraska will move east-southeastward tonight and Sunday.  The low
level cloud cover over parts of southwest IL will advect east of the
forecast area by early evening.  There will be good radiational
cooling across west central and southwest IL tonight near the
surface ridge axis with little cloud cover, light winds, and
relatively low surface dew points forecast.  Lows in this area will
be at least slightly below normal.  Mid level clouds will advect
into central MO late tonight due to increasing low-mid level warm
air advection.  Could not rule out a few sprinkles towards morning
in central MO, but it appears that any measurable precipitation will
hold of until after 12Z as the surface/low levels will be initially
quite dry.  There should be scattered showers across parts of
central and southeast MO Sunday morning, west-southwest of the St
Louis metro area as models depict 850 mb warm air advection, theta-e
advection, and moisture convergence on the nose of a modest westerly
low level jet.  Most of this precipitation should dissipate by
afternoon, but there may still be a few light showers in southeast
MO during the afternoon.  Despite the rising upper level heights,
increasing 850 mb temperatures, and surface winds becoming
southeasterly on the backside of the surface ridge shifting east of
the region, the highs should not be much warmer than Saturday due to
the cloud cover limiting the solar insolation.


.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 301 PM CDT Sat Mar 18 2017

The synoptic pattern at the start of the long term period will
feature an amplified trough pushing off the eastern seaboard, with
another trough approaching the west coast.  In between, zonal flow
initially will turn more northwesterly by midweek before an active
southwesterly flow regime evolves by the end of the upcoming week.

The main near-term focus of the period continues to be on the
potential for a few strong/severe thunderstorms late Sunday night.
The latest guidance is in general agreement that a subtle shortwave
trough will push through the Northern Plains late Sunday.  In
response, a low-level jet of 35-40 knots will develop Sunday evening
into Sunday night.  Isentropic ascent atop a surface frontal
boundary draped over northern MO will lead to the development of a
cluster of showers/storms, likely rooted somewhere near 850mb given
a very stable near-surface boundary layer.  Forecast soundings
suggest anywhere from 1500-2000 J/kg of MUCAPE with 30-35 knots of
effective deep-layer shear.  This CAPE/shear combination could be
enough to support strong multicell or transient supercell structures
capable of severe hail.  It appears the best 850mb moisture
convergence and thus most likely area of seeing storms will be
across far northeastern MO into western IL.

Storms should weaken/dissipate through Monday morning as the low-
level jet veers and weakens.  The upper-level flow will turn more
northwesterly, allowing the surface front to sink southward through
the region.  Despite the fropa, strong capping indicated on forecast
soundings suggests the day will be mostly dry.  On Monday night, the
boundary will settle into southern MO.  Yet another shortwave trough
sliding through the northwest flow will lead to additional elevated
convection developing Monday night into Tuesday.

The surface boundary will push far enough south by Wednesday morning
to allow for dry conditions.  However, a strong PV anomaly
associated with an amplified trough moving onto the west coast will
eject into the Southern Plains on Thursday.  In response, a warm-air
advection regime will develop over the Midwest leading to yet
another chance of showers/thunderstorms Wednesday night into
Thursday morning.  By Thursday afternoon into early Friday, a
surface warm front will have lifted north through the region,
briefly putting an end to the precipitation as the area becomes
entrenched within the warm sector of a strong surface low developing
to the west.  The latest guidance continues to differ on the timing
of the strong surface low and associated cold front through our
region, with the GFS being much faster than the ECMWF.  The GEFS
mean is considerably slower than the operational GFS and more in
line with the ECMWF, thus have leaned more toward the ECMWF

Temperatures through the period will be a bit of a roller-coaster
ride.  Monday will be very warm with highs in the low to mid 70s.
However, the surface front dropping southward through the area late
Monday will cool temperatures into the 50s Tuesday and Wednesday.
Temperatures will be on the rise by the end of next week ahead of
the next system, with highs on Friday back into the upper 60s and
low 70s.



.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1146 PM CDT Sat Mar 18 2017

SPECIFICS FOR KCOU, KUIN: VFR for the first 0-6 hours at KUIN, but
there is greater uncertainty at KCOU towards the end of the first
6 hours. Increasing VFR clouds are expected at both TAF sites
after 09z ahead of a developing warm front, but KCOU is far
enough west that scattered rain showers could briefly lower
conditions to MVFR if a shower happens to move directly over the
terminal. Southeasterly to southerly winds will increase to around
10 kts after 14z due to the tightening pressure gradient.

SPECIFICS FOR KSTL, KSUS, KCPS: VFR for the first 0-6 hours. Winds
will continue to veer overnight with the passage of a surface
ridge axis. Winds will increase after 12z due to a tightening
pressure gradient. Expect increasing VFR clouds after 09z ahead of
a developing warm front. Rain is likely just beyond the end of the
24-hr TAF period.





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