Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 180310
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
910 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 CO southeast through the TX 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 OH Valley region, the lower levels will saturate
allowing the precipitation to reach the ground. The precipitation
may intially be a mix of snow and sleet across southeast MO, but
should quickly transition to all snow due to evaporative 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 FAM 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 CWA 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 AOB 40% with RA/SN.

Kanofsky

&&

.AVIATION:  (For the 00z TAFs through 00z Thursday Evening)
Issued at 545 PM CST Wed Dec 17 2014

Expect dry and VFR conditions through most of the this evening
before precipitation that is currently over southwest Missouri
moves into KCOU around 06Z in the form of snow. Dry air will
prevent snow from moving into the St. Louis area metro TAF sites
until around 12Z and KUIN around 15Z Thursday morning. VFR
conditions will drop to IFR conditions with in 2-3 hours of onset
of snow. Snow accumulations of around 1 inch is possible at KCOU
with less than an inch at the KSTL TAF sites and KUIN.
Accumulating snow is expected to end between 16-19Z...with light
drizzle and IFR or low MVFR ceilings persisting through the
afternoon hours. Winds will generally be light through the period.

Specifics for KSTL: Expect dry and VFR through at least 06Z. Then
snow chances will increase 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 from Midnight tonight to 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.

IL...NONE.
&&

$$

WFO LSX





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