Winter Weather Discussion – 2008-15-1

Winter Weather Discussion for December 15-16, 2008.
Valid: 12/11/2008.

Highlights:

– Winter Precipitation is possible late Sunday through Tuesday
afternoon across a large part of Oklahoma. Current thinking is that
a mixture of freezing rain/drizzle and sleet will be the primary
precipitation types throughout the event. Impacts: Low to Moderate
impacts are expected overall. Road conditions will be a high impacted
issue as ice coats bridges and overpasses. Additionally, some areas
will experience ice accumulations on surface roadways. Impacts beyond
the roadways is expected to remains limited with only isolated power
outages.

– Winds will be very strong from the north on Monday and will
exacerbate any roadway or power line icing. Additionally,
temperatures will be in the 20s across much of the state on Monday.

– High fire danger is likely on Sunday in the southwest part of Oklahoma.

Discussion:

Review of available model guidance points to average confidence
regarding the forecast at D5 and D6 (Monday/Tuesday). Sunday will be
a very warm day for much of the state as strong south and
southwesterly winds proceed an Arctic cold front. Said front should
move into northern Oklahoma during the afternoon and race across
Oklahoma by late Sunday night. Temperatures will rapidly fall and be
below freezing within a few hours of frontal passage. A northern
stream shortwave trof will be lifting across the central U.S. as the
front moves through. Isolated showers with rain and freezing rain may
occur across eastern Oklahoma during frontal passage.

Attention then turns west at the northern stream longwave trof over
the western U.S. This feature will result in southwest flow aloft
across Oklahoma. As such the Arctic airmass at the surface is only
expected to be of the shallow variety. This means precipitation will
be ice or sleet. The models are in agreement that precipitation is
likely across the area but amounts appear to be light. At this point
conditions Monday and Tuesday do not appear to be a disaster in the
making. However, it should be strongly noted that a little bit of ice
goes a long ways. As such, while the overall impacts will be low to
moderate, roadways will be highly impacted. Since the cold air has
not started southward yet and model sampling is never good with these
types of airmasses, it is way too early to talk about the ice/sleet
line during the days in question. It is likely that sleet will be the
main type across northern Oklahoma with a transition to freezing
rain/drizzle elsewhere in the state. However, strong WAA may result
in freezing rain for a larger part of the state than currently
anticipated. There are plenty of details that will have to be sorted
out in the coming days.

The bright side here is that both the GFS and ECMWF indicate that the
western U.S. system will lift out in pieces with no one big weather
maker occurring. Furthermore, the first piece of energy to eject
eastward on Sunday moves through the area before the cold air can take
hold. After that it will be up to smaller shortwave trofs and WAA to
cause precipitation. One such system is progged by the ECMWF to move
across Kansas on Tuesday.

Beyond Tuesday model agreement rapidly ends and there is little
confidence or forecast skill for D7+ time periods regarding winter
precipitation. I'm iffy that the cold air will leave as fast as the
GFS takes it away on Wednesday. As such, when the next system lifts
out of the longwave trof on Thursday, there could be additional
impacts. Once again, model agreement is quite low at the D8 time
period and so is forecast confidence.

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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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