Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 131818
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1218 PM CST SAT FEB 13 2016

.UPDATE:
Issued at 1205 PM CST Sat Feb 13 2016

A lot of dry air in the low levels so it will take a lot for any
precipitation to make it to the ground. So backed off on pops for
northern portions of forecast area this afternoon. Still have a
sliver of slight chance pops far north for a few hours late this
afternoon. Otherwise, mid and high clouds continue to stream
southeast into forecast area. With such a cold start, will only
see highs in the upper teens to upper 20s.

Byrd
&&

.SHORT TERM:  (Through Late This Afternoon)
Issued at 358 AM CST Sat Feb 13 2016

Focus will be onset of SN event this afternoon.

The leading s/w, ahead of the main trof to impact the area tonight,
is currently just lee of the Rockies this morning. This s/w will
move twd the area along with additional moisture to gradually
saturate lower levels. Mdls still suggests that measurable snowfall
will reach the area after 00z this evening. However, have increased
PoPs for late this afternoon to speed up timing slightly as mdls
generally do not saturate quickly enuf. Regardless, little
accumulation is expected this afternoon as the main trof approaches
and pushes precip ewd.

As for temps, expect current clouds to spread ewd across the region
today. Going forecast looked reasonable and only minor changes were
made.

Tilly

.LONG TERM:  (Tonight through Friday)
Issued at 358 AM CST Sat Feb 13 2016

A longwave TROF will continue to prevail over eastern North America
thru the middle of next week.  This will continue the train of
disturbances, about one every day or two.

The next disturbance is currently just coming ashore on the Pacific
Northwest.  A good model consensus has developed with regards to the
track of it, thru far northern MO and central IL late tonight and
Sunday morning.  This track should become more definitive with the
12z model runs this morning once full sampling can be attained.  The
beginnings of this event still look to be the same, with an initial
narrow band of snow that should be ongoing in parts of northeast MO
early in the evening, with this migrating eastward into west-central
and parts of southwest IL towards midnight.  Snow from this will be
short-lived, but may lead to a quick half-inch or so of accum for
areas that get it.

WAA and broadscale lift will increase and become widespread with the
approach of the disturbance itself after midnight tonight and
continue into Sunday morning.  PoPs have been boosted to high-end
likely and categorical for what should be a light snow event for
most areas.  Already by late Sunday morning over central MO, ice
crystals are lost in clouds with mid-level drying, and this will
expand eastward into the rest of the forecast area during Sunday
afternoon.  This will effectively transition the event from
persistent light snow to a mix of flurries and drizzle, and where
surface temps are at or below 32F, freezing drizzle.

The most prolonged snow event should be in the northern CWA, such as
west-central and central IL, where near 3" of snow is expected, and
it is there where a Winter Wx Advisory will be issued.  Lesser
amounts expected elsewhere, with most locales set to receive only
around an inch.

What was originally progged to be a second part of this event, for
Sunday night into Monday, is still up in the air.  The actual
disturbance is still well offshore and the models have clearly
struggled with how the first disturbance will influence and set the
track of this one, with shifts in the track and timing with each
model iteration.  The GFS is much "wetter" with this than other
models for Monday, but for now have gone with the main consensus
which is dry.  Something that might not come into better focus for a
couple more model iterations.  Stay tuned, as this could lead to
additional snow for some areas, although with the progged warmer
temps, it may limit them to strictly areas to the north of STL
metro.  Speaking of temps, preferred leaning cool for maxes Monday
with these disturbances expected to slow down any warming trend.

Another disturbance is still progged for Tuesday as well, with the
GFS having come much more in line with the southerly EC/GEM track
and justifies higher PoPs.  Temps will again try to trend warmer
with the cold air eroding each day, but it still looks like some
areas will still be cold enough for pcpn from this event to be snow--
mainly north and east of STL metro, where elsewhere it will tend to
be rain.  A more stubborn retreat of the cold air could mean a few
inches of snow from this system, so again, stay tuned as these
disturbances finally track over land and can be sampled better.

Otherwise, look for a mainly dry forecast from Wednesday on, with
steadily rising temps with well above normal temps possible for next
weekend.

TES
&&

.AVIATION:  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1205 PM CST Sat Feb 13 2016

Mid and high clouds moving in ahead of next weather system. North
to northeast winds to continue veering and become southeasterly
and increase by this evening. As for precipitation chances, a lot
of dry air to overcome, so backed off on start of light snow. Have
it moving into KUIN by 07z Sunday, KCOU and STL metro area by 10z
Sunday. Cigs to lower to at least MVFR with onset of snow. Added
mention of scattered IFR clouds for now.

Specifics for KSTL:
Mid and high clouds moving in ahead of next weather system. North
winds to continue veering and become southeasterly and increase
by 03z Sunday. As for precipitation chances, a lot of dry air to
overcome, so backed off on start of light snow. Have it moving
into STL metro area by 10z Sunday. Cigs to lower to at least MVFR
with onset of heavier snow, around 12z Sunday. Snow to taper off
to flurries with winds becoming southerly, so some warm air aloft
with freezing drizzle possible by 18z Sunday. Confidence is low on
this so only have mention of flurries in taf at this time for
Sunday afternoon.

Byrd
&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...Winter Weather Advisory from 6 PM this evening to 6 PM CST
     Sunday FOR Lewis MO.

IL...Winter Weather Advisory from midnight tonight to 6 PM CST Sunday
     FOR Fayette IL-Greene IL-Macoupin IL-Montgomery IL.

     Winter Weather Advisory from 6 PM this evening to 6 PM CST
     Sunday FOR Adams IL-Brown IL-Pike IL.

&&

$$
WFO LSX


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