Weekly Severe Weather Discussion

Severe Weather Discussion for December 22 – 28, 2008.
Valid: 12/21/2008.

Highlights:

– Continued cold this week with Oklahoma temperatures Monday upper 20s
to upper 30s. Tuesday and Wednesday will be in the upper 30s to upper
40s. Thursday and Friday may see a warming trend into the upper 40s
to upper 50s. Saturday and Sunday will be cooler with highs in the 40s.

– There is a chance of drizzle and freezing drizzle Tuesday morning
and showers in eastern Oklahoma in the afternoon. Another chance of
rain looks possible on Saturday mainly for eastern Oklahoma.

– Drought conditions continue to worsen in the state with 65% in at
least a D0 or worse. A small D2 area covers southeast Oklahoma.
Forecast precipitation chances are adjusted downward from normal due
to this.

– Merry Christmas to all!

Probabilities:

– Severe Thunderstorms in Oklahoma: < 5%
– Storm Chase: < 5%
– Winter Precip: 10%

Discussion:

Fast mid-level flow will continue this week with at least two
shortwave trofs to impact the region. Unfortunately neither of these
look to deliver much needed rainfall for the state. For the month of
December Oklahoma City is one inch below normal and looks like we'll
get every little the next ten days. Will Rogers has measured 0.05
inches the past 21 days. Coordination with Andy and review of the
Drought Monitor points to continued low precipitation chances through
the remainder of 2008. NWS 30 day plots show January being above
average precipitation, we'll see how that goes. I'd certainly be
happy with it.

Another consideration for this forecast week is forecast temperatures.
Models continue to be too aggressive with warming the Arctic
airmasses and this was noted this morning with a low of 10F at
Oklahoma City. Forecast highs today were 32F and we'll be lucky to
make 25F. As such, I have gone below MOS the entire week. The NAM
seems to have a decent handle on these issues and I have followed it
closely through 84 hours and then weighted towards the ECMWF after
that time period. Another thought is the snow pack across much of the
northern U.S. Even a Pacific high will chill some as it moves across
the snow pack.

Models are not making this an easy forecast and confidence is low.
ECMWF is the preferred model due to consistency and recent
performance. I'm not attempting to forecast some of the micro-scale
precipitation events noted in the GFS this week. Such events will be
low impacts and short time periods if they occur at all.

Shortwave trof #1 is due in Tuesday night into Wednesday. A cold
front will move through the area Wednesday morning as the upper system
passes just to the north of Oklahoma. Best chance of rain is Tuesday
as WAA occurs in advance of the 500mb low and a few micro-scale
shortwave trof ahead of the parent system Tuesday night. Cold front
is of Pacific origin so cooling will not be as dramatic as the past
few cold fronts. However, the surface high will travel over the snow
pack to our north and little modification is expected. Temperatures
will be in the 30s on Monday and warm slightly on Tuesday into the
40s. Wednesday will cool back down into the upper 30s to the mid 40s.

Oklahoma will be in between systems Thursday and Friday with
temperatures warming into the upper 40s to upper 50s. I've cut 12z
GFS MOS numbers way back as none of the ensembles support it and
recent trends would argue against it. While winds will be
southwesterly and the 12z GFS temperature plots really want to warm
things up, I'm holding back for now. I'll take 65F if it occurs on
Friday, but I'm betting against it.

Shortwave trof approaches the region on Friday and Saturday. ECMWF
and GFS disagree on the timing with the GFS about 12 hours slower.
This has impacts on the precipitation forecast for next Saturday.
I'll go with a blend for now. Associated surface high is of Pacific
origin and will be another cool down. Models are not showing Arctic
air at this time so don't see teens or single digits beyond tomorrow.
Temperatures Thursday and Friday will be in the upper 40s and 50's
with a cool down to the 40s for Saturday and Sunday.

The GFS does appear to load cold air in western Canada, so could see a
return to the cold pattern towards New Years.

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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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