Wednesday, January 2, 2013

Forecast: Heavy snow in Tulsa... Maybe. Ok, nevermind!

Channel 2 viewers in far eastern Oklahoma enjoyed a White Christmas. Up to 8" of snow reported.

You have probably have questioned the sanity of your local weatherman if you've lived in Oklahoma during an alleged snow event!

Why do snow forecasts vary so much?

Why do we sometimes miss the snow storms?

Here are some excuses (uhm, reasons):

Most "winter storm" type snow events in Oklahoma are caused by smallish, compact, intense Low pressure areas. If you are reading a detailed meteorological forecast discussion, you might see a Low called a shortwave, lobe and vort max to name a few.

Due to the Low's relatively small size but sometimes intense strength, big snowfall differences can result over a short distance. Your scenery can grow from zero snow into a winter wonderland in 20-30 miles!

So-- where does the heaviest snow occur ?

The highest totals are usually 100 to 150 miles north of the Low's center, mainly depending on the temperature profile, size and strength of the Low.

Where does the Low need to go for Tulsa (or other Oklahoma town) to get snow?

For Tulsa to get a major snow, the center of the Low must travel through southern Oklahoma or extreme north Texas-- basically traveling from W to E or SW to NE near the Red River.
This 500mb vorticity map is an example of a computer
model that meteorologists use to gauge the
strength of an approaching Low (near Arizona).
One of a zillion graphics to analyze!    
A Low has a counterclockwise circulation toward itself.

Why is there a short distance between heavy snow and zero snow?

A Low has a counterclockwise circulation toward itself.

This means that as bands of snow form, the precip rotates in a tight circle around and toward the center-- you might picture the circulation around a hurricane. It's the same idea.

(Sometimes you will see heavy snow on the north and west side of the Low with strong thunderstorms in the warmer air southeast of the Low.)

Much drier air and north wind exist just north of the snow bands, and this drier air evaporates the northern fringe of the snow. The dry air combined with the Low's tight circulation creates a sharp precipitation cut off. Sometimes an area as small as one county can be the snow line.

Another issue: snowfall forecasts are most likely to be a "busted forecast" if the Low travels too far south instead of north-- that's what happened on Christmas Day of 2012 at Tulsa ended up without measurable snow.

Several days before when the Low was 2,000 miles away, its center was progged by several computer models to track near the Red River. Instead, the Low moved about 100 miles farther south. The resulting snow line cut-off occurred between Tulsa and Interstate 40 instead of Tulsa and Bartlesville.

How much snow fell?

Viewers south and east of Tulsa reported totals of 0-1" near Muskogee; quickly increasing to 6" in the Warner/Interstate 40 area just south of Muskogee. 6-8" totals covered the ground near the OK/AR line south of Fort Smith. Low visibility and strong wind created near blizzard-like conditions for drivers.

While some complained about how horribly meteorologists performed, the snow was correctly forecast the night before. Dan Threlkeld was on the 10pm news that night, and he nailed it.

He lowered the snowfall to Tulsa to 0-1" for Tulsa. Most folks were spending time with their families on Christmas Eve night, so many didn't see the new data.

In summary:

- snowfall total forecasts more than 24 hours out can have huge errors.
- do not believe a forecast see you on the Internet a week away of heavy snow!
- a compact, Winter Storm type Low is hard to forecast due to its compact strength and small size.
- if a Winter storm type Low travels too far south, you could completely miss out on the snow.
- snow, precipitation type and snowfall amounts are the most difficult weather to forecast

Also, I have never tried to "hype" a weather event. I try to call it as I see it. :)

Hope this helps! Thanks for reading, George

Channel 2 viewers in McAlester and Checotah emailed us these pictures of their White Christmas.