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Area forecast discussion...updated aviation 
National Weather Service St Louis MO
1156 PM CST Wed Dec 17 2014

Update:
issued at 837 PM CST Wed Dec 17 2014

Radar echoes approaching the southwestern portion of our forecast
area this evening from southwestern MO. The weak reflectivities in
our forecast area are not likely making it to the ground due to
the dry layer around 900 mb in height. As the forcing increases
over our area ahead of the main shortwave now extending from
southeast Colorado southeast through the Texas Panhandle and upper level
divergence increases between the approaching upper level jet
streak over the Southern Plains and the stronger upper level jet
streak over the Ohio Valley region, the lower levels will saturate
allowing the precipitation to reach the ground. The precipitation
may initially be a mix of snow and sleet across southeast MO, but
should quickly transition to all snow due to evaporate and
nocturnal cooling. The latest NAM model is a little slower with
the onset of the precipitation with the precipitation not making
it to cou or familiar until shortly after midnight, and not getting into
the St Louis Metro area until almost 6 am.

Gks

&&

Short term: (through Thursday night)
issued at 231 PM CST Wed Dec 17 2014

Lift ahead of an approaching shortwave will bring light snow to
the area tonight and tomorrow morning. The lsx County Warning Area will be
positioned beneath favorable upper level divergence (ler of an
upper jet), and models also show frontogenesis developing between
900-850 hpa across central and southeastern MO. Forecast snowfall
amounts are less than 2 inches across the cwa, and some locations
may not receive very much snow at all due to the initially dry
levels between approximately 900-600 hpa (per ksgf/kilx 12z
raobs) which must be overcome before precipitation can reach the
ground. Surface temperatures are comparatively warm, and even
minor accumulations may be limited to grassy/elevated surfaces.
Any lingering snow should taper off after noon on Thursday,
perhaps transitioning to drizzle due to the loss of ice crystals
aloft.

Other than the Novelty of seeing snowflakes, few impacts are
expected. The Winter Weather Advisory will be maintained for
collaboration purposes.

Kanofsky

Long term: (friday through next wednesday)
issued at 231 PM CST Wed Dec 17 2014

Our area may be too far north to receive much precipitation from
the next system to move across the southern part of the US. The
upper trough is forecast to move through MO/AR/la on Friday night,
but the associated surface low will be hugging the Gulf Coast.
Trended a bit lower and farther south with pops for Friday night.

Nwp guidance points to additional systems which could bring rain
or snow to the area on Sunday (passage of an upper trough), Sunday
night into Monday (strengthening upstream vort max), and/or the
middle of next week (closed low migrating into northern MO or
southern ia), but there is too much variability attm for much more
than broad-brushed pops at or below 40% with ra/sn.

Kanofsky

&&

Aviation: (for the 06z tafs through 06z Thursday night)
issued at 1137 PM CST Wed Dec 17 2014

Snow will move kcou in the next 1-2 hours and into the St. Louis
Metro area tafs between 9-12z. Still appears that conditions will
drop to IFR or low MVFR within 1-3 hours after the onset of the
snow. KUIN will see the onset of snow around 12z. Accumulations of
snow is still expected to be around 1 inch at kcou and less at
KUIN and the St. Louis Metro taf sites. The snow will switch over
to drizzle during the afternoon before ending by late in the
afternoon. IFR or low MVFR ceilings are expected through the end
of the period.

Specifics for kstl: snow is expected to move into the terminal
after 09z with VFR conditions initially given dry air in the
atmosphere. IFR conditions and steady snow is expected by 14z with
an accumulation of less than 1 inch. The snow should switch over
to drizzle by 18z with IFR ceilings lasting through the rest of
the taf period.

Britt

&&

Lsx watches/warnings/advisories:
MO...Winter Weather Advisory until 10 am CST Thursday for Boone MO-
Callaway MO-Cole MO-Crawford MO-Gasconade MO-Iron MO-
Madison MO-Moniteau MO-Osage MO-Reynolds MO-St. Francois MO-
Washington MO.

Illinois...none.
&&

$$

Weather forecast office lsx

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