Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 170947

Area Forecast Discussion
National Weather Service Saint Louis MO
347 AM CST Sat Feb 17 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 347 AM CST Sat Feb 17 2018

An area of precipitation has developed across parts of OK/KS/AR/MO
early this morning due to isentropic ascent ahead of an approaching
PV anomaly over the plains. Model forecasts of isentropic motion and
moisture show that the northern extent of the precipitation will
continue lifting northward through the morning hours while
simultaneously shifting eastward ahead of the trough axis. By 21-
00z, precipitation will end across most of the CWA due to a
combination of isentropic downglide and the passage of the trough
axis. A surface high pressure center will quickly move across the
area tonight.

Highs today will be several degrees warmer compared to yesterday,
and most location should reach the mid-40s to low 50s. Overnight
lows will be similar to last night albeit a few degrees warmer
across northeast MO and west central IL.


.LONG TERM...  (Sunday through Friday)
Issued at 347 AM CST Sat Feb 17 2018

Focus thru this forecast period will be precip chances, amounts and
p-type for Sun night thru Wed. Overall, not a lot of change
regarding this event with waves of precip thru the CWA Sun night
thru at least Tues night, likely into Wed as well.

Period begins Sun with a sfc ridge centered over the OH Valley
building ewd with zonal flow aloft. Still expect strong WAA above a
shallow cold layer. Mdls suggest a more esely to sly sfc wind for
Sun and have trended slightly cooler.

A leading s/w and sfc low across the Northern Plains will drop a
front into IA Sun night. A weak sfc wave shud develop along this
front and slightly deeper mixing, winds shud become more swly on
Mon. If deeper mixing can occur, the area has the potential for
seeing advisory level winds and gusts. There is still some question
as to how much cloud cover and precip will be around on Mon and have
therefore not trended winds that high yet. The front is expected to
slowly move south thru the CWA and shud be southeast of the CWA
sometime Tues night.

This is where mdls diverge even more. The GFS is a faster soln
moving the front thru by 00z Wed, with the ECMWF/GEM 6 to 12 hours
later. The ECMWF/GEM also suggest much more precip back into the
cold air, while the GFS has the precip ending as the cold air
arrives. At this point, confidence is too low to rule either soln
out and have kept some PoPs further north to account for the
ECMWF/GEM solns, but tried to focus higher PoPs where mdls are in
agreement. If the ECMWF/GEM solns verify, would expect a transition
to sleet and freezing rain as the cold air moves in with the front.
This is also possible according to the GFS as well, but for a very
brief period.

As for precip amounts, the GFS/GEM continue to suggest much higher
amounts thru Wed night, anywhere from one-half to over 5 inches of
rain. The ECMWF ranges from generally 1.50 to 3.50, but has more
widespread higher amounts. The NAM, thru 12z Tues, is once again
much drier. Namely, approx half that of the other mdls. Believe the
NAM soln is too dry, but remain cautiously optimistic.

Much cooler temps expected behind the front Wed thru the end of the
forecast period, tho still remaining near seasonal average.



.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1107 PM CST Fri Feb 16 2018

MVFR ceiling and visibilities will accompany an area of
precipitaiton that will move across the region Saturday morning. A
light east wind will become southerly tomorrow. Precipitation will
likely begin as snow before changing over to rain. Little if any
accumulation is expected given warm ground and air temperatures.
Depending on how much precipitation falls and how fast the clouds
can clear to the east tomorrow, the region could see areas of fog
develop into Sunday morning.


Expect snow to begin around 10 am and transition to rain before
ending. A period of MVFR conditions is expected. Ceiling will lift
and clear tomorrow afternoon and evening. Could see some fog
develop Sunday morning, but confidence is not high enough at this
time to include in the terminal forecast.



Saint Louis     47  31  56  48 /  80   0   0  80
Quincy          47  26  55  45 /  70   0   0  70
Columbia        51  29  58  48 /  80   0   0  60
Jefferson City  50  28  59  49 /  80   0   0  60
Salem           45  30  54  48 /  60   0   0  90
Farmington      45  30  55  48 /  90   0   5  70




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