Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 171825
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1225 PM 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.

BUFKIT soundings from the RAP and SREF support the idea that
evaporational cooling will produce light snow at precipitation
onset. As temperatures warm throughout the column during the
morning, the precipitation type will transition from snow to
rain. Most locations will only see up to a dusting of snow or
less, especially given the warm ground temperatures. However, as
noted in the previous discussion, some parts of the eastern Ozarks
could see around an inch of snow.

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.

Kanofsky

.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.

Tilly

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1225 PM CST Sat Feb 17 2018

Pcpn has ended at COU and will end at STL metro sites and UIN by
20z. IFR CIGs currently in place at the TAF sites will also
gradually edge eastward this afternoon, with clearing expected at
all sites by nightfall or shortly after. With wet ground, light
winds from high pressure building in. and clear skies, look for
some VSBY reductions in fog, especially for STL metro sites
because of the delayed onset of SE winds on the backside of the
high pressure. These VSBY reductions should all rapidly improve by
mid-morning, but depending on how this all plays out, could see
some potential for IFR VSBYs overnight. For now bottomed out VSBYs
at 3-4SM. As the area of high pressure moves away on Sunday, look
for SE-S surface winds to become gusty.

SPECIFICS FOR KSTL: IFR conditions to improve during the mid-late
afternoon to MVFR with clearing at or shortly after nightfall.
Some VSBY reductions in fog late tonight with potential for IFR.
Winds becoming gusty with fog dissipation on Sunday morning.

TES

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...Winter Weather Advisory until 1 PM CST this afternoon for Bond
     IL-Clinton IL-Fayette IL-Greene IL-Macoupin IL-Marion IL-
     Montgomery IL-Randolph IL-Washington IL.

&&

$$

WFO LSX



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