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Friday, December 12, 2008

Snow across the Southland

A surprise snowstorm blanketed the south from Houston to New Orleans with 1 - 5 inches of powder two days ago. This storm tied or broke records for the earliest measurable snow in the season. (Canal Street, New Orleans Courtesy NOLA.com).

This is the first snow along the Gulf Coast since the Christmas Eve snowstorm in 2004 that dropped 8 inches in Galveston and 4 inches or more further inland. That year we got a white Christmas and the kids were able to play outside at 6 am. There was even enough snow to make a small snowman.

That Christmas Eve, one of our boys even asked "What is that white stuff falling from the sky?... Oh THAT"s SNOW?!?!?" It was the first time he saw snow and was able to enjoy it.

Snow in New Orleans is a rarity. The last time it snowed was Christmas 2004; before that, the last snow recorded was in 1989, according to Jim Vasilj, a forecaster with the National Weather Service. Since 1850, snow had fallen in "measurable amounts" rather than traces in the city just 17 times, Vasilj said. Of the 17, today's snowfall was the earliest in the season recorded. (NOLA.com)

The first year I moved to Texas a light dusting gave us about 1/2 inch of snow to deal with. At that time I still remembered my winter driving and could slip past all the cars stopped on the side of the road. Back then the local TV stations would tell people to slow down when they came to a bridge. So people would drive as fast as normal and then put the brakes on when they came to an overpass. Many people were surprised when they suddenly spun into a ditch after loosing traction.

In Houston (Houston Chronicle)

Snowfall in the metro Houston area Wednesday caught forecasters somewhat by surprise. A significant chance for snowfall didn't show up in computer models until about 9 p.m. Tuesday."The midnight crew adjusted the forecast at that
time," Lewis said.

Because the ground in the Houston area was relatively warm — 77 degrees as late as Tuesday afternoon — neither snow nor ice was expected to stick and cause major transportation problems for long.

Still, because a freeze was expected to overnight, a winter weather advisory was issued Wednesday evening for much of Southeast Texas, including Harris County, because of the potential for slick, icy conditions north of Interstate 10 on bridges and other exposed areas.

By 9 p.m. Wednesday, the overpass on the Eastex Freeway at the
Beltway had frozen, authorities said. We are asking drivers to be careful on the roadway, mindful of the danger," said Harris County Sheriff's Office spokesman Lt. John Legg.

Daytime highs should return to the 70s by Sunday. Early next week, possibly Monday night, another cold front is expected to reach Houston, but it isn't expected to pack the punch of this week's chilly frontal system.

A good explanation of how this happened is in the forecast from Gene Norman of KHOU in Houston last Tuesday night, "Did I say sleet?"


That was the reaction around here when I uttered those words during tonight's newscasts. Computer models are showing that it will get very cold here after the front that brings Tuesday's rain moves through. Okay, we've seen a few bouts of chilly air move in, but this time, there's something else going on. In the upper levels of the atmosphere, a pocket of very cold, dense air will slide all the way down to south Texas late Tuesday and then slide up through East Texas early Wednesday. This means that the air aloft, around 5,000 feet could be well below zero even though surface temperatures will hover in the upper 30s/low 40s.

At the same time, a surface low will develop along front in deep South Texas late Tuesday and as that low moves up the coast early Wednesday, it will spread low clouds and rain inland. Depending on how close to the ground the cold air gets, clouds could have enough moisture in them to freeze. As that frozen precipitation heads toward the ground, it will melt in the somewhat warmer air near the surface. However, if it all doesn't melt, then we have the perfect scenario for sleet, basically, ice that hits the ground. Since the ground will be above freezing, the sleet will melt on contact. There won't be enough of it to accumulate, but it could be visually exciting.


As it was, the cold air was sufficient that the ice never melted and instead remained as snow as this graphic shows.

Overall, the snow reached all the way east to the the Mississippi River. Just as the forecast here in Georgia was considering the potential for the rain to switch over, the jet stream pushed warm air into our area and drove the ice and snow north. Today New York and New England are getting their fill of the wintery weather.

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