Winter Weather Discussion 2009-7-1

Winter Weather Discussion for December 23-24, 2009.
Valid: 12/19/2009.

Highlights:

– Winter precipitation is possible across parts of Oklahoma Wednesday
and Thursday. Current forecasts point to multiple precipitation types
and intensity across Oklahoma. Forecast confidence is very low and
bust (not the good kind) potential is high.

Discussion:

Low confidence and potentially high impact weather event evolving for
the middle to end of the upcoming holiday week. Models have been
quite disparate for the past several days and have finally developed
some degree of agreement. Despite the perceived agreement,
significant differences remain. Of course this hasn't stopped some
Oklahoma City media members from declaring a blizzard by midweek.
Sigh. The model differences exist in the 500mb field and translate to
precipitation, surface low placement, temperature profile, etc. To
say that the forecast will change is an understatement.

12z ECMWF continues prior trends of developing a southwest U.S. closed
low on Tuesday. System remains progressive and ejects eastward into
the region by early Thursday. This model then wraps it up into a
large closed low over northern Illinois by Saturday morning. Since
the ECMWF maps I have only space 24 hours, it is tough to infer how
the upper system will move across Oklahoma. Current thinking would
take the 500mb closed low from north of Lubbock across Oklahoma to
south of Tulsa by Thursday morning as it takes on a negative tilt.
The surface low races along the KS/OK border. This solution would
seemingly keep much of Oklahoma in a liquid precipitation (albeit cold
rain) pattern through late Wednesday as the surface-850mb lows move
east and allow cold Canadian air to surge south. A transition to snow
would occur statewide early Thursday as precipitation tapers off. It
appears with this forecast the majority of snow would be in Kansas
with a trace to 4 inches across parts of Oklahoma.

The 12z GFS is a little more interesting, but only on its face. Once
digging into the BUFKIT data (I don't have that for the ECMWF), things
are much less clear. The closed 500mb low develops by Tuesday evening
in the SW U.S. and moves east to the NM/TX border by Wednesday morning
and on east over Ardmore by Wednesday evening to Joplin Thursday
morning. It continues on northeast from that point forming a large
closed low over the northeast U.S. by early Saturday. GFS tracks the
surface low along the Red River and this allows a quicker arrival of
cold air across much of the state. GFS QPF map looks rather ominous
with a band of 0.4 to 0.8 inches liquid equivalent across
central/northern Oklahoma. This would be in the 700mb deformation
zone and potentially some CSI banding. 700mb/850mb RH progs support
the QPF forecast Wednesday-early Thursday. BUFKIT data is where some
of the real interesting details reside. Oklahoma City does not
transition to frozen precipitation until 10pm Wednesday after the
majority of the precip has occurred with only a dusting to 1 inch
possible. While, data for Ponca City shows a transition to freezing
rain Wednesday evening and then a change over to snow where 5-7 inches
might occur. Unfortunately the resolution of the GFS BUFKIT data is
only 3 hours and therefore tough to determine when the transition to
freezing rain occurs and how much may accumulate.

The end result of this is that while the models are similar on
developing a southwest U.S. closed low, they disagree on some very
important parameters regarding impacts on Oklahoma. To recap, the
ECMWF's solution would yield mainly a cold rain for Oklahoma, with a
switch over to snow on Thursday. The ECMWF is about 12 hours slower
than the GFS. The GFS solution would yield greater snow potential for
Oklahoma and potentially freezing rain. These solution will likely
change in the coming days and it is impossible to accurately pinpoint
conditions for Wednesday. The potential is certainly there for at
least a moderate-end event and hence I'm initiating discussions. Type
of weather system expected and holiday travel could make this a
high-end event if the correct parameters/timing emerge. Another item
of note is this system will have to bring the cold air with it as
Arctic/Canadian air will not be entrenched across the area. Surface
snow cover map valid 12z this morning indicates snow cover in northern
KS northward to the border. Therefore incoming cold air isn't likely
to modify. At this point, models do not show sufficient 500mb
amplitude to dislodge an Arctic airmass in advance of the incoming
system. This occurrence appears predicated on the expected system
Wed/Thu and the eventual large closed low over the northeast U.S.

Just for Beaker, a Tulsa view of the GFS BUFKIT (assuming you read
this far). About 0.552 inches of rain and then change over to
freezing rain around 11pm on Wednesday with possibly 0.35 inches of
it. Then, change over to snow by 4am and 0.50 inches of something
(snow or freezing rain). Big stuff ends by noon Thursday with 1.6
inches of water equivalent total. Once again, this is one run of the
GFS for Tulsa only.

I guess I should lay off the chocolate.

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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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