Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 171202

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
702 AM CDT Sat Mar 17 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 319 AM CDT Sat Mar 17 2018

A surface low was located across central MO at 07z per MSAS MSLP
analysis, and the low pressure system`s primary upper PV anomaly was
located near the MO/IA border. A secondary PV anomaly was located
slightly farther to the west and north over NEB based on RAP 1.5 PVU
fields and water vapor imagery. The entire system will continue
moving eastward today while simultaneously weakening as an upper
ridge builds across the central CONUS.

As the upper vort centers move across the region during the day,
they may support sprinkles or light rain showers across the northern
half of the CWA, especially over northeastern MO and west central
IL. Meanwhile, the trailing cold front which was moving through the
area early this morning will briefly stall to the south of the area
until a surface high pressure center builds from the northern plains
into the Great Lakes tonight and pushes the boundary farther

The combination of a slightly cooler post-frontal air mass,
northwesterly winds and low-level CAA, and persistent cloudiness
across at least the northern half of the area today will yield high
temperatures which should be within a few degrees of yesterday`s
highs for most locations. The extreme southern and southeastern
edges of the CWA may get a bit warmer than currently forecast
because they will have a warm start to the morning ahead of the
frontal passage and could remain mostly clear for long enough to see
a quick warm-up ahead of the front. Overnight lows ranging from the
low 30s in NE MO to the upper 30s in SE MO look reasonable given
that surface winds will retain a northerly component through at
least 09z.


.LONG TERM...  (Sunday through Friday)
Issued at 319 AM CDT Sat Mar 17 2018

Surface ridge to move off to the east during the day on Sunday. So
will see winds become easterly. In the meantime, next system to
begin lifting out of the Central Plains so clouds and low level
moisture will be on the increase. As for high temps, they will be in
the 50s Sunday.

00z model runs continue to have differences in strength, timing and
placement with this system. NAM is the furthest north while ECMWF is
farthest south and fastest. For now will go with NBM. Will see
activity develop and spread northward Sunday night. Some weak
instability with this system, so added thunder mention for portions
of southern MO during this time. Better chances of showers and
storms as main system moves through region on Monday. Rain to taper
off by daybreak on Tuesday as system exits. Also, with some colder
air filtering in on back side of system, will see rain become mixed
with snow before ending.

Cooler and drier weather expected Tuesday with highs only in the
40s. Weak surface ridge to build in by midweek with temperatures
moderating. Could see some scattered showers/flurries Tuesday
through Thursday due to active northwest flow aloft, but moisture
will be limited and hard to pin down timing of these shortwaves, so
kept forecast dry for now.

Beyond that, next best chance for rain will be Thursday night
through Friday.



.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 628 AM CDT Sat Mar 17 2018

SPECIFICS FOR KCOU, KUIN: An area of IFR to MVFR clouds is
wrapping around an upper disturbance. Based on upstream
observations, short periods of IFR cig/vis are possible at KUIN
during the first 0-4 hours of the valid TAF period. A few
sprinkles or light rain showers will be possible during the first
0-6 hours. Conditions are expected to remain at or below MVFR for
at least the first 0-12 hours of the TAF period.

SPECIFICS FOR KSTL, KSUS, KCPS: A very brief period of VFR
conditions can be expected at the St. Louis area terminals between
1130-1230z, but satellite imagery shows that this improvement will
be very brief indeed. A solid deck of MVFR clouds is wrapping
around an upper disturbance and will reach the St. Louis metro
area shortly after 12z. MVFR ceilings are then expected to persist
for most of the valid TAF period. IFR conditions are possible
after 18/06z, but confidence was too low to include IFR during the
last 6 hours of the TAF period.





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