Winter Weather Discussion 2009-7-3

Winter Weather Discussion for December 23-24, 2009.
Valid: 12/22/2009.

Highlights:

– Showers and thunderstorms are likely starting tonight and continuing
into Wednesday for central and eastern Oklahoma. Severe thunderstorms
are also likely in far eastern Oklahoma with damaging winds the
primary threat on Wednesday.

– Freezing drizzle and snow are possible across parts of Oklahoma late
Wednesday into Thursday. Recent model data has greatly reduced the
threat to Oklahoma and now this appears to be a low-end event. Snow
is most likely north of a line from Elk City to Guthrie to Tulsa
during the forecast period. Snow amounts should be 2-4 inches with
maybe a 5 inch max somewhere in the panhandle. Areas immediately
south of this will likely receive just a dusting. Greatest impacts
will likely be the panhandle area.

– The main story from this event may be bitterly cold temperatures
Thursday through the weekend. Additionally, another system is
expected early next week and this one may have more frozen
precipitation potential.

Discussion:

12z models continue prior trends of developing a double barrel 500mb
closed low primarily across Kansas by late Thursday. This is an stark
contrast to the prior runs of moving a closed low through northern
Texas and eastern Oklahoma. This has fundamentally changed the
forecast for Wednesday into Thursday. All available model guidance
has amazingly converged on this new solution and there is run-run
consistency within intra-model guidance. As such, the potential for
winter weather has decreased significantly for much of Oklahoma. The
panhandle area will likely see the most snowfall and is currently
under a winter storm watch.

Upper system is progged to dig southward through Nevada into Arizona
through Wednesday morning. Models show a 80-100kt jet max on the
southeast side of the low, which should effectively eject the system
northeast into Kansas. However, a secondary vortmax hangs back over
Arizona and will shift eastward late Thursday. A large closed low
develops over the central U.S. by Friday and slowly moves east.

Impacts for Oklahoma are likely Wednesday, ending late Thursday.
Initially, showers and thunderstorms will be the primary threat as
warm/moist Gulf air moves northward into parts of the state.
Currently dewpoints in the mid-50s have moved into Oklahoma. The
Storm Prediction Center has a slight risk for eastern Oklahoma on
Wednesday and this appears to look good. The primary threat will be
damaging winds. As the lead upper system moves through the panhandles
late Wednesday, the associated surface low will shift northeast to
Joplin and allow an Arctic cold front to move into Oklahoma. Since
the 850mb low will still be west of the surface low, there will be a
layer of warm air aloft. As such, freezing drizzle and maybe a little
freezing rain will occur as surface temperatures cool to freezing and
below. Eventually, by Thursday morning the atmosphere should be cool
enough to support all snow across much of central and northern Oklahoma.

Since much of Oklahoma will be on the southeast side of the low,
moisture will likely be removed by the 700mb low resulting in limited
potential for snowfall. I'd expect flurries Thursday as the 850mb low
occludes with moisture wrapped all the way around it. Since crystal
growth won't be favored in this region, small flurries should be about
it. Additionally, there won't be a moist layer for dendritic snowfall
growth. So while snowfall is expected, northern Oklahoma stands the
best chance of accumulating snows. As noted above 2-4 inches seems a
good bet with a 5 inch total, most likely in the panhandle region.
Farther south into Oklahoma City, a dusting is the most likely
scenario. NAM BUFKIT data for OKC has only 0.093 inches water
equivalent through the 84 hour period. The GFS is similar, although
could be wrong if convection indeed develops in this area, eastern
Oklahoma being the favored area.

While impacts to travel are expected early Thursday (primarily in
northern OK), the event does not seem severe enough to continue
discussions. I'll monitor on Wednesday and issue an update should the
forecast radically change again. Attention will need to be focused on
the southwest U.S. heading towards the region next Monday/Tuesday.

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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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