Area Forecast Discussion
Issued by NWS St. Louis, MO

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413
FXUS63 KLSX 092145
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
345 PM CST TUE FEB 9 2016

.SHORT TERM:  (Through Late Tonight)
Issued at 345 PM CST Tue Feb 9 2016

Scattered flurries will continue for the rest of the afternoon under
the strato-cu deck.  There is a clearing slot running down the west
side of the Mississippi River down almost to the Missouri River. The
clouds should continue to break up...although RAP and NAM soundings
show plenty of low level moisture trapped under an inversion into
the evening.  Think most of the area should be clear of the low
clouds by 06Z...though south central Illinois counties will likely
stay under the clouds into Wednesday morning. Cold advection on
northwest flow will continue tonight into early Wednesday.  Think
guidance temperatures in the single digits to mid teens look good
for lows tonight.

Focus turns to storm system developing on Wednesday.  This look to
be a purely warm advection/frontogenesis snow. GFS, ECMWF and NAM
are showing some pretty high values of warm advection...between 30
to 50 degrees C/12hr.  Forecast soundings show the moist layer
extending up into the dendritic growth zone with a decent amount of
lift through that layer as well.  Looks like the snow will hold off
until mid-late afternoon.  Due to the cold atmosphere and lift in
the best snow-growth zone, expect this will be a fluffy snow which
will accumulate pretty quickly...especially since highs will stay
well below freezing.

Carney

.LONG TERM:  (Wednesday through Next Tuesday)
Issued at 345 PM CST Tue Feb 9 2016

Model guidance continues to improve its consensus for the track and
impact on the storm system expected to be ongoing early Wednesday
evening.  The consensus track for max QPF stretches from north-
central MO thru the southwestern half of the STL metro area and
towards Cape Girardeau, MO.  The WAA associated with this event is
expected to be intense and will probably result in rapid onset and
development which, again, should be ongoing by sundown Wednesday
evening.  WAA remains intense thru much of Wednesday evening with
lift continuing well into the overnight hours, and for this reason,
will keep a mention of pcpn thru the nighttime hours with this
package.  A fairly broad stripe of up to 1" of new snow accum is now
looking possible with this system, with near 2" potential in its
core/peak QPF track.  If the WAA continues to be advertised as
intense with this system, these snow amounts, especially in the core
track, will probably need to be bumped up further due to high
anticipated snow to liquid ratios, when light winds are taken into
account from the surface RIDGE being over our region and a cold low-
mid level column.

Another system, of a more classic clipper variety, may impact our
region on late Thursday night into Friday, but the track for this
continues to be more to the northeast thru southern IA into central
IL, with only a modest mention of flurries or low PoPs justified at
this time.

Otherwise, a prolonged period of below average temps is forecast
from now thru the upcoming weekend, with very little moderation
expected in temps on Friday before the next shot of Arctic air
descends on our region for the weekend.  Only by early next week
does the persistent RIDGE over the western CONUS begin to translate
eastward and could mean a return to average to even above average
temps at some point next week.

It is near this transition period Sunday-Monday that another system
is expected to impact our region.  The GFS has finally come onboard
with the more consistent EC and GEM solutions but the nature of this
system is far from settled, with its initial onset possibly as early
as Saturday night and it may be completely done by Monday now.  This
earlier timing would certainly favor an all snow event, with more
average temps for mid-February set to return on the heels of this
event.  With the main storm still well over the north Pacific, there
is much that can go wrong or change and something to stay tuned for.
But as it stands now, could see widespread advisory criteria
snowfall.

TES

&&

.AVIATION:  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1123 AM CST Tue Feb 9 2016

MVFR ceilings will prevail along and east of the Mississippi river
this afternoon and into the evening. Light snow showers or
flurries will occasionally lower the visibility to 3-5SM where
clouds persist this afternoon as well. Expect ceilings over much
of central Missouri and some of northeast Missouri to scatter out
over the next few hours. Guidance suggests that most of the area
will go VFR between 06Z and 10Z tonight, but low clouds will
probably hang around thorugh most of Wednesday morning over south
central Illinois. Northwest wind will continue...but should
diminish Wednesday morning.

Specifics for KSTL:

Surface obs are showing MVFR ceilings upstream all the way through
eastern Iowa. Should see the ceilings clear out tonight...though
timing is uncertain at this time. Given the time of year and
continued lift in the atmosphere due to low pressure aloft, I
leaned more pessimistic on the ceiling forecast. Northwest flow
will continue, but should diminish Wednesday morning. Current
thinking on Wedenesday`s potential snow is that the majority of it
will be southeast of the terminal...and not until late afternoon
or early evening so have left mention of additional snow out of
the terminal forecast for this issuance.

Carney

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX



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