Winter Weather Discussion – 2009-1-1

Winter Weather Discussion for January 26 – 28, 2009.
Valid: 01/24/2009.

Highlights:

– Freezing rain is expected across parts of the state Monday
afternoon through Wednesday morning. Current amounts are expected to
be at least 0.25 inches, which would meet Ice Storm Warning criteria.
Location is generally north of a line from Ardmore to Poteau.

– Moderate impacts are possible across parts of the state. Impacts
to critical facilities also appear possible. There is potential for a
high-end event, however at this juncture I'm not willing to go that far.

– NWS Tulsa is hosting a conference call at 15:00 CST, OEM will participate.

Discussion:

00z/06z/12z model runs provide a variety of solutions for this
forecast period and confidence in any one solution is still rather
low. However, implications for the state are of great concern and
necessitate issuing discussions, if only to highlight the potential.
It should be clearly noted that forecast confidence is below average
for the forecast period (Monday – Wednesday), as if winter weather
forecasting isn't already tough enough.

Despite model variations there is decent agreement among the 12z
ECMWF/GFS model runs along with their prior runs. The NAM
(00z/06z/12z) continues to be the outlier at this point, but most be
considered in the overall scheme of things. NWS Norman and Tulsa are
responding to the the freezing rain threat and are partially the
impetus for this discussion.

For the purposes of this discussion I am siding with the GFS and ECMWF
models, but reducing their QPF for the time being. I'm going with
NWS's Norman and Tulsa regarding at least 0.25 inches of ice
accumulations over parts of the state. It must be emphasized that the
GFS produces 1.0 inches of ice for Oklahoma City, but in comparison to
the 00z run it had 0.3 inches. This should illustrate the forecast
difficulties and why caution is prudent right now.

Model specifics hold a shortwave trof digging southward along the
California coast on Monday. The longwave ridge that had been in this
area is gone with zonal flow east of the southern stream shortwave
trof. There is some ridging in western Canada in the northern stream
as a northern stream shortwave trof moves east late Monday into the
Great Lakes area. Models continue to forecast a reinforcing shot of
Arctic air into the region, which should augment the current Arctic
high already in-place. As such, shallow Arctic air will be located
across much of Oklahoma through the early to mid part of the week.
Models typically move the Arctic air out too fast but the 12z runs
actually hold it in-place. This gives increased confidence regarding
the temperature profile of the atmosphere as precipitation initiates
late Monday. It is seemingly clear that the precipitation will either
be a cold rain or freezing rain, there is nothing in-between at this
point.

As the southwest U.S. shortwave trof turns east towards the region
late Monday, a lee trof will develop in western Texas. Gulf moisture
is plentiful with current values of 60F across much of this region.
This area will be pulled north and lifted over the shallow cold dome,
isentropic lift. GFS and ECMWF models indicate at least 90% RH at
850mb, sufficient for precip to generate. 12z GFS BUFKIT data does
not show much lift until late Tuesday morning into early Wednesday
when there is decent Omega in the low/mid levels. This is when the
most significant precipitation occurs. Upper system then shears just
north of Oklahoma on Wednesday with the 850mb and surface lows
dissipating and drying out the region as we come under the more
dominate northern stream.

If current trends continue, enhanced wording will be needed in future
discussions. Another discussion will be issued on Sunday, but a brief
one may be required tonight to update model performance.

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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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