Winter Weather Discussion 2007-1-1

Winter Weather Discussion for January 12 – 15, 2007.
Valid: 01/07/2007.

Highlights:

– Winter weather is possible across parts of the region Saturday – Monday.

– Much colder air is on the way, current thinking is Friday.

Discussion:

12z NAM/GFS and 00z ECMWF continue current trend of building an Arctic airmass into the region late in the week into next weekend. There are the usual differences among the models, not a shocker this far out.

GFS has been very consistent in developing a winter storm for the region, until the most recent run. I'm inclined to ignore this run and wait for future runs, in case the GFS has stumbled onto a reality the other models haven't. The 12z run develops a strong western U.S. shortwave trof and then races it rapidly eastward late Saturday. I seriously doubt this to be true given that 1) El Nino is going strong and 2) the pattern so far this year has been for SW U.S. closed lows. The GFS has had a bias toward this kind of rapid progression the past few weeks as noted by the November 30th snow storm and the following event. Additionally, the ECMWF is in good company with the UKMET on holding the upper system back west. Comparison of the 12z NAM at 84 hours and the ECMWF at 4 days seems to yield a similar trend with the NAM. Long story short, the 12z GFS is odd model out with itself and the other models. AFDAMA is great discussion and is on track with my thinking.

One thing that seems to ring true, the upper system will be slower than any model progs at this point. My forecast is based on this potential.

A review of HPC's opinion yields what I would think about the frontal positions later in the week. HPC has the cold front in central Oklahoma Friday morning and through the state by late Friday. This agrees well with my opinion, 00z ECMWF, and 00z GFS. As noted above the 12z GFS remains the outlier of the models. Current longwave trof over the central U.S. will continue eastward and drag a cold front well into the Gulf, much farther than originally thought. However, said longwave trof will remain progressive and model progs indicate moisture return will occur by Wednesday morning with rapid return of moisture by late Thursday. 12z GFS indicates 55F-60F dewpoints in Oklahoma at this time. Obviously said moisture will get pushed south as the Arctic cold front moves through the state. However, rich deep layer (for this time of year) moisture will remain close to the region with an open Gulf.

Arctic air should be in place across all of Oklahoma and oozing through Texas by early Saturday. Powerful SW U.S. closed low begins track east across AZ into NM by this time. Previously noted moisture should be lifted over the Arctic dome across much of the region. The resulting warm temperatures aloft over sub-freezing temperatures at the surface seems to set the stage for a widespread freezing rain event. The most likely area on Saturday appears to be the southeast 2/3rds of Oklahoma into Texas. In NW OK 850mb temperatures should be cooler with sleet and snow the possible types. However, it is way too early to get into this kind of detail. I'll be lucky if we even see winter precip in Oklahoma on Saturday. So, I'm going to leave the precip forecast here for now.

Even if the winter precipitation does not occur as progged, temperatures will be quite cold; especially when compared to the rather mild readings of the past few weeks. GFS surface temps have consistently been in the single digits across parts of the region on Monday 01/15. Typically some airmass modification is expected but time of year (ground temps/sun angle at min), rapid onset, and snowpack to the northwest reduce the chance of much modification. Additionally, model surface pressure lines connect to the Arctic region, indicating an Arctic outbreak across the region, at the very least a pretty darn cold Canadian one.

At this point it is tough to not break the glass and push the panic button. However, years of forecast experience yields a little restraint and knowledge that winter weather events rarely occurs as progged. It should be noted that going model progs set the stage for a significant winter weather event across the region. As such, discussions will continue for the next few days. The Oklahoma Department of Emergency Management will likely issue a press release on Tuesday highlighting the expected cold temperatures and potential winter weather; along with urging residents to prepare.

For more information, weather news, weather blog, and chase summaries go to http://www.hook-echo.com.

Common Terms:

AOA = Around or above.
AOB = Around or below.
CAPE = Convective Available Potential Energy.
CDC = Climate Diagnostic Center.
CPC = Climate Prediction Center.
ECMWF = Numerical model run, European Model.
GFS = Numerical model run, Global Forecast System.
LI = Lifted Index
LLJ = Low Level Jet.
NAM-Eta = Numerical model run, about to become the NAM-WRF
NAM-WRF = Numerical model run, the Weather Research Forecast (WRF)
OUN = Norman Forecast Office.
SWODY = Severe Weather Outlook Day 1, 2, 3, and 4-8.
SPC = Storm Prediction Center.
WWA = Warm Air Advection.
00z/06z/12z/18z – Time based on the UTC time scale, z = zulu.

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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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