Winter Weather Discussion 2010-2-1

Winter Weather Discussion for March 20-21, 2010.
Valid: 2010-03-19

*Significant Winter Storm to Impact Oklahoma*
*Prepare now for this storm*

Highlights:

– Winter Storm Watch for the northern 2/3rds of Oklahoma for tonight
through Sunday. This will change and I'll send an update when the new
information arrives.

– Winter Weather Advisory for the panhandle from 7pm tonight through
7pm Saturday for 3-5 inches of snow.

– This is a particularly dangerous situation with the potential for
widespread snowfall accumulations greater than six inches.
Additionally, very strong north winds will create wind chill values in
the single digits/low teens and blizzard conditions.

– Power problems are possible, especially early in the event as a
little bit of ice will go a long ways. Currently, freezing rain and
sleet are not expected to be a problem, but as we saw in mid-January
it only takes 0.01 or so inches of freezing rain to create a problem.

– This will be a long duration event for Oklahoma as snow is likely
from tonight through Sunday night. High-end impacts are expected to
the traveling public and moderate impacts to power infrastructure.

– A short-fused blizzard warning may occur as the event starts.

– Showers and thunderstorms are likely along the cold front tonight
and early tomorrow. Primary threats are isolated small hail and
strong winds.

Discussion:

12z models continue to show the same general scenario of an upper
system rapidly intensifying as it moves over Oklahoma Saturday and
Sunday. There are some significant differences this morning and this
leads to some major forecast confidence problems, very similar to what
we experienced leading up to the Christmas Eve blizzard. Problem of
it is the models were right with the 12-15 inch snow fall totals and
so the recent NAM amounts have to be considered. However, it is a
major departure from the prior run.

12z NAM indicates an upper system will start to intensify just west of
Oklahoma later today with an 80kt 500mb jet on the upstream side of
the trof at 12z Saturday. Additionally, there is a 100kt jet streak
at 300kts in the same area with -30C 500mb temperatures. This lends
support to the model forecast of rapid strengthening of the upper
system and an eventual turn east on Saturday and closes off by
Saturday evening. The 70kt jet streak rotates around the east side of
the now closed low with an 80kt jet max at the base. The closed low
continues to move rather slowly east as it continues to intensify over
the ARLATEX. By early Sunday morning there is a 90kt 500mb jet max
over south Texas. By 12z Sunday this model shows a 100kt jet max over
the same area with three closed contours around the center located
southeast of Idabel. Upper system then continues to move slowly east
and be located over central Arkansas Sunday evening.

Looking a closer to the surface, a cold front is noted from Amarillo
to Russell, KS this morning, this front will remain nearly stationary
today until the Arctic high drops out of Canada. By this evening the
front will be moving southward and be located near I-44. Temperatures
start to go below freezing in the panhandle by 10pm tonight and this
shifts southward during the overnight hours. By Saturday morning much
of the area north of I-40/west of I-44 (north and west of OKC) is at
or below freezing. As such, snow should be occurring in this area.
There will be a transition from rain to freezing rain, sleet, etc, as
the transition occurs from around 6 am Saturday to 8am. The freezing
line continues to move southeast during the morning and covers much of
the threat area by noon Saturday. This gives confidence to much of
the precipitation type being snow Saturday afternoon and beyond in the
threat area. Indeed, 12z NAM Bufkit data supports this conclusion.

The current track of the 700mb low is quite a concern as it puts
Oklahoma City in the deformation zone for heavy snow. Given the very
dynamic nature of this system, I wouldn't be shocked to see bands of
slantwise convection setup, resulting in rapid snowfall. NAM shows
250J/Kg MAX CAPE over Oklahoma through the entire event, so would
think there is sufficient instability for convection. Additionally,
925-700mb RH values indicate a plethora of moisture in the dendritic
snow crystal growth region for large flakes. As such, a heavy wet
snow is likely during much of the event and tampering off late Sunday.
NAM surface wind speeds show 20-25mph during the day Saturday, with
a potential increase to 25-30mph late Saturday. If the latter does
occur, it may necessitate a blizzard warning. While ground
temperatures are warm, rapid snowfall easily compensates for it.
There are some differences between the 12z NAM and GFS, but for now
not enough to modify the forecast. The GFS is a little faster with
the upper system and farther north with the 700mb low. The NAM's
placement of the upper system seems more realistic and matches better
with what happened in December and again in January.

It is emphasized that this is a dangerous situation for Oklahoma,
people should prepare now for this winter storm.

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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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