Winter Weather Discussion – 2006-12-2

Winter Weather Discussion for November 29 – 30, 2006.
Valid: 11/27/2006.

Highlights:

– Much colder air on its way!

– Frozen precipitation is possible late Wednesday into Thursday.

Discussion:

12z models continue the prior trend of pushing the cold air through the region in advance of a large southern stream closed low. The result looks to be much colder temperatures area wide by Wednesday night, followed by frozen precipitation for Oklahoma and parts of north Texas.

12z NAM-WRF/GFS agree with prior runs and each other regarding the evolution of the western U.S. longwave trof and subsequent eastward movement. This is also inline with the 00z ECMWF and see little reason to depart from the present logic. While there are some inherent timing/placement differences between the models, discussion about such would be “splitting hairs” at 72 hours out. One issue of major importance is model handling of the Arctic air. Currently the southern extent of the Arctic air is analyzed from the Oklahoma panhandle into northwest Oklahoma on northeast to Kansas City. 18z surface analysis from the Oklahoma Mesonet shows temperatures holding the 40s in far NW Oklahoma with the temperature gradient about the same over the past hour. The warm/moist air over central Oklahoma has yet to have an impact on the frontal boundary. The gradient has weakened a little, but nothing substantial. There is likely to be some movement with the front during the next 24-36
hours; however the models are way too aggressive in lifting the boundary into Nebraska. This is a characteristic model bias relative to shallow Arctic airmasses.

Problems from this point forward are 1) when will the front head south and 2) precipitation behind the front. While the models are in decent agreement on the mid-level progs, our good fortune does not translate to the surface. Here, we have a myriad of progs. The NAM-WRF’s frontal movement is ignored, along with much of its surface progs. The GFS is several hours ahead of the NAM-WRF with frontal passage in Oklahoma City Wednesday afternoon. This is most likely also too slow, but much better than the NAM-WRF. The GFS’s QPF/temp fields are used for this forecast.

The front should move into Oklahoma Wednesday morning, near Oklahoma City around early afternoon, and to the southeast corner by late evening. Temperatures will rapidly drop after frontal passage into the low 40s with wind chill values in the teens. By late evening the southern stream shortwave trof will be approaching the region. GFS indicates RH values above 80% across the region at 850mb and 700mb. 500mb RH values are rather dry with the best stuff in north-central OK into Kansas. Either way, the GFS creates QPF across much of Oklahoma with the most likely area in central Oklahoma on east. Right now a mixture of frozen precipitation types are likely across the state. Snow should start out first in the northern sections with sleet in central sections and freezing rain farther south. A transition to all snow will occur by late in the day. Still tough to talk about accumulations given model issues at hand and plenty of time to get into that issue.


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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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