Area Forecast Discussion
Issued by NWS St. Louis, MO

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144
FXUS63 KLSX 182340
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
640 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.

GKS

.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
solution.

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.

KD

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 559 PM CDT Sat Mar 18 2017

SPECIFICS FOR KUIN: VFR for at least the first 0-6 hours. Winds
will veer overnight with the passage of a surface ridge, then
increase to near 10 kts after 15z and remain so through the end of
the TAF period. Expect increasing VFR clouds after 09z ahead of a
developing warm front. Scattered rain showers between 09-15z
should remain south and west of KUIN.

SPECIFICS FOR KCOU: VFR for the first 0-6 hours. Winds will veer
overnight with the passage of a surface ridge, then increase to
near 10 kts after 15z and remain so through the end of the TAF
period. Expect increasing VFR clouds and scattered rain showers
after 09z ahead of a developing warm front.

SPECIFICS FOR KSTL, KSUS, KCPS: VFR for at least the first 0-6
hours. Winds will veer overnight with the passage of a surface
ridge. Expect increasing VFR clouds after 09z ahead of a
developing warm front. Scattered rain showers after 09z should
remain west of the St. Louis metro area terminals.

Kanofsky

&&

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

$$

WFO LSX



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