Winter Weather Discussion 2009-7-2

Winter Weather Discussion for December 23-24, 2009.
Valid: 12/20/2009.

Highlights:

– Snow is likely across parts of Oklahoma Wednesday evening into Thursday.

Discussion:

12z/18z models have developed decent consensus regarding the expected
impacts around midweek. Forecast confidence has subsequently
increased and a major winter storm is now expected for the central
U.S. Where Oklahoma ends up in this event remains a little tough to
forecast, but there is good agreement that parts of the state will get
snow late Wednesday into Thursday.

Cold air is lacking ahead of the southwest U.S. system for midweek as
temperatures will be in the 40s and 50s across much of Oklahoma. As
the upper system moves through a Canadian high from western Canada
will be dislodged southward into the central U.S. This will result in
cold air advection across Oklahoma as the upper system is moving
through the state.

18z NAM has slowed the upper system and 18z GFS has done the same,
although not as slow. Either way, rain is expected to develop across
much of the state Tuesday night. As the upper system approaches west
Texas, the surface low will shift east and allow cold air to sink
southward. There will likely be a transition from rain to freezing
rain or sleet as warm air advection continues at 925/850mb. As these
low shifts east all precipitation will change to snow. This is
expected to occur early Thursday morning. The issue becomes at what
point precipitation will change to snow and from that point how much
snow will fall across the state. Given that this is a holiday week
and Thursday is likely to be the max travel day, problems are likely
despite how much falls. Since freezing rain is possible, there will
also be a threat of ice problems in the state, but this is not
expected to impact anything other than travel.

GFS is low on QFP with the NAM on the other end of the spectrum.
Given Gulf dewpoints in the 60s, there is plenty of moisture for
isentropic lift. Models also show RH values above 90% through
Thursday evening. As such I find the GFS's low QPF values in-general
suspicious. NAM QPF is much better, but unfortunately the NAM only
gets me through Thursday midnight, just before the change over to snow
is expected. I should have the new run within the hour and Bufkit
data by 10pm.

Amarillo issued a special weather statement for 1-5 inches of snow in
the panhandles with Lubbock also noting the potential. In Oklahoma
things will be a little more complex as we must wait for isentropic
lift to end and cold air advection to change the rain to snow. Model
progs continue to argue that northern Oklahoma will see the most snow
fall due to being in the deformation zone/wrap around. Depending how
fast the cold air moves in, I still would not be shocked to see some
CSI banding across the state Thursday morning.

A moderate-end event continues to look possible this week and all
persons are urged to monitor the latest weather conditions. Farther
north and east things are going to get really ugly, making for a very
tough holiday travel week. I'd also expect some type of winter
weather highlights from NWS offices by Monday evening.

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About Putnam Reiter

Putnam has been storm chasing since 1990 and is a co-founder of Hook-Echo.com. For his day job, Putnam works in emergency management for information technology.
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