Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 171746

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1246 PM 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 18z TAFs through 18z Sunday Afternoon)
Issued at 1235 PM CDT Sat Mar 17 2018

MVFR and IFR ceilings will continue to prevail across central and
eastern Missouri and west central and southwest Illinois at least
through tonight and into Sunday morning. IFR conditions are mostly
limited to northeast Missouri and west central Illinois at this
time...and some marginal improvement to low end MVFR is possible
this afternoon...but any improvement will likely be lost this
evening after sunset. There`s some potential for IFR to push
further south into central and east central Missouri later this
evening and tonight...though I`d like to see more agreement in the
guidance before forecasting IFR at KCOU or the STL Metro TAF
sites. Expect improvement in ceilings late in the
morning/afternoon on Sunday.


Expect MVFR ceilings to prevail at Lambert through Sunday morning.
While ceilings below 2000 feet are most likely through the`s possible the ceiling could lift to 2000-2500 briefly
this afternoon. Think any improvement will likely be lost after
sunset this evening. There`s some potential for IFR to push
further south tonight into the STL Metro area though I`d like to
see more agreement in the guidance before forecasting IFR at KSTL.
Expect the ceilings to lift and scatter Sunday afternoon.





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