Winter Weather Discussion 2009-4-2

Winter Weather Discussion for March 27-29, 2009.
Valid: 03/26/2009.

*Significant winter storm to impact Oklahoma*
*This is a Life Threatening Situation*

Highlights:

– Winter Storm Warning is posted for the northwest quadrant of
Oklahoma, Blizzard Warning for western Oklahoma panhandle, Winter
Storm Watch for northeast Oklahoma (see counties below).

– This is a Particularly Dangerous Situation, with the likelihood of
significant snowfall across Oklahoma.

– Current projections –

– Blizzard Warning: 12+ inches of snow with 30-40mph winds, with gusts
to 50mph, with snow drifts of 10-20 feet.

– Winter Storm Warning: 8-12 inches likely

– Winter Storm Watch: 4-8 inches likely

– Heavy and wet snow is expected, which will cause excess load on flat
roofs, trees, and power lines.

– Impacts: This is a high-end event, with impacts now expected to be
on the moderate to high end. Primary impacts expected to be to
transportation and power facilities. Power outages are possible given
expected winds and snow loading.

– All precipitation is expected to end by early Sunday morning.

– Severe thunderstorms are likely tonight into early Friday across
central and southeast Oklahoma. Primary threat is large hail,
although in far southeast Oklahoma a tornado threat will exist.

Discussion:

Oklahoma City is not currently in a watch, however the watch is just
north and west of Oklahoma City and I expect this to change. 06z/12z
NAM runs indicate around 4-6 inches of snow for Oklahoma City.
However, the forecast track of the upper system along with arrival of
the sub-freezing air will be critical to at which point the I-44
corridor and points south switch over to snow. Current thinking is
that Oklahoma City will go from rain to freezing rain/sleet Friday
afternoon. I do have concern this may occur during the afternoon rush
hour. By Friday evening snow will be occurring in Oklahoma City. 12z
BUFKIT data shows 5-6 inches in Oklahoma City with the 06z about the
same. Models will flip more or less over the coming days, but the
general 4-6 inches is expected to hold.

Regarding the rest of Oklahoma, a significant and unprecedented
snowfall event looks to be almost certain. 12z NAM is very close to
the prior run and the 00z GFS/ECMWF are in remarkable agreement with
the incoming closed low. Said closed low will move into the region
tonight and slowly move across late Friday into Saturday. By late
Saturday the low is expected to be in Arkansas with rapidly improving
conditions across Oklahoma. Typically in late March forecasters talk
about warm ground temperatures limiting the overall snowfall
accumulations. However, in this case the wet and heavy snow, along
with the expected rapid snowfall rate will easily overwhelm the warm
ground temperatures.

At this point I'm not questioning the general precipitation progs from
the models. The primary reason for this approach is the copious
amounts of moisture in Texas (i.e. 65F dewpoints). This layer will be
lifted over the cold dome in Oklahoma providing impressive moisture in
the dendritic snow growth region. With the expected upward forcing
and very cold atmosphere profiles, snow growth will be quite large,
resulting in heavy wet snowflakes. As such, the above noted snowfall
amounts from the NWS look good and are the best guess at this point.
Snowfall is very difficult to forecast and amounts will vary greatly
across the threat area. Given the amount of moisture present in the
lower levels over the atmosphere, CSI banding (convective symmetric
instability) is likely. Any area experiencing CSI banding will
receive very high snowfall rates and rapid accumulation.

I will likely be issuing these twice a day now as new model data are
received and potentially more frequently as the event occurs.

Counties:

Blizzard Warning – Cimarron and Texas
Winter Storm Warning – Ellis, Harper, Rogers Mills, Woods, Woodward,
Alfalfa, Blaine, Custer, Dewey, Garfield, Grant, Major
Winter Storm Watch – Ottawa, Tulsa, Rogers, Mayes, Delaware, Creek,
Wagoner, Kay, Noble, Kingfisher, Logan, Payne, Beckham, Washita,
Caddo, Canadian, Harmon, Greer, Kiowa, Jackson

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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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