Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181800

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
100 PM CDT Sun Mar 18 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 405 AM CDT Sun Mar 18 2018

Fog will continue to develop within areas of clearing skies early
this morning. Visibility should improve areawide by mid-morning.

Quiet weather is expected for most of the morning and at least the
first half of the afternoon due to the combined influences of upper
ridging and a surface high pressure center over the Great Lakes.
Meanwhile, a PV anomaly noted over SoCal and southern NV per water
vapor imagery and RAP 1.5 PVU analysis will progress eastward and
induce surface cyclogenesis near the CO/KS/OK/NM border. The
resulting low pressure system will slide through OK tonight and is
expected to be near the OK/MO border by 12z Mon. Although the
surface warm and cold fronts associated with this feature will
remain south of the region, moisture convergence along the elevated
warm frontal boundary will produce areas of showers and isolated
thunderstorms from late afternoon through tonight. The rain will
move into the area from southwest to northeast as the warm front
lifts northward, and favorable LER jet dynamics aloft will provide
additional lift after midnight.

Temperatures will be 5-10 degrees warmer today across nearly all of
the CWA. The exception will be in the extreme southernmost areas
where high temperatures will be similar to yesterday. The presence
of clouds tonight, as well as precipitation across at least the
southwestern one-third to one-half of the CWA, will keep overnight
lows around 5-7 degrees warmer than last night.


.LONG TERM...  (Monday through Saturday)
Issued at 405 AM CDT Sun Mar 18 2018

System to continue sliding east across southern Missouri on Monday.
Will see showers and a few thunderstorms persist before tapering off
from west to east early Tuesday morning.

Beyond that, surface ridge to build in by mid week with northwest
flow aloft. Models show weak shortwaves sliding southeast through
region during this period. However, moisture will be limited, so
kept forecast area dry through Thursday morning for now.

Extended guidance still have major timing, placement and strength
differences with next system that will affect region for the last
part of the work week and into next weekend. For now kept things as
is using what NBM gave for pops, with best chances of precipitation
Thursday night through Friday night.

As for temperatures this week, below normal conditions expected
Monday through Wednesday, before moderating for the last half of the
extended period.



.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1240 PM CDT Sun Mar 18 2018

Expect band of MVFR stratus to continue to persist across much of
eastern Missouri and west central and southwest Illinois through
this afternoon and into tonight. There is erosion of the cloud
mass from the southwest and the northeast, it is very slow and
think the majority of the area will stay broken to overcast with
MVFR ceilings between 1000-2000 feet. Any locations that do
scatter out will likely fill back in after sunset. Low pressure is
expected to move from Colorado across the Plains and along the
Missouri/Arkansas border tonight through Monday. Showers and a few
thunderstorms are likely starting tonight and continuing into
Monday as the system moves through. The highest chance for rain
will be mainly across central, east central, and southeast
Missouri as well as southwest and south central Illinois. While
low MVFR ceilings are expected to prevail through Monday, heavier
rain showers and thunderstorms could bring ceilings/visibilities
down to IFR at times.


Expect MVFR stratus with bases between 1200-1800 feet to prevail
at Lambert through most of the forecast period. Isolated showers
Current thinking is that rain showers are possible after 09-12Z,
and more persistent rain will overspread the STL metro area during
the late morning or early afternoon. IFR conditions are possible
with this heavier rain, but there seems to be little consensus in
model guidance on this question.





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