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January 7, 2011 / Dave Gorham

Birth of a Nor'easter?

For the second time this week, it’s a beautiful day in Houston. The Upper Texas Coast is bathed in sunlight filtered through cirrus clouds and blue sky; we’re on our way to a high near 70F. Ideal for any time of the year, but it’s the calling card of the Gulf Coast in the middle of winter.

However, all is not as it seems.

Is that as ominous as, “It was a dark and stormy night”? Perhaps. An upper-level low pressure area is already tracking this way from California. Once it moves across Mexico and reaches the Gulf of Mexico, it’s expected that a vigorous surface low pressure will develop in the northern Gulf. In fact, the Houston National Weather Service office has already notified its SkyWarn group of trained volunteer spotters to be ready for possible severe weather tomorrow night and Sunday morning. This, in anticipation of the arrival of the upper low as it combines with abundant moisture. Heavy, flooding rain (2-3 inches) is likely while tornadoes have not been ruled out. Offshore, winds of 35-50mph are possible.

The GFS shows a nicely developed upper trough over western Texas. It's this trough that will kick off the low in the Gulf of Mexico, bring ice and snow to the South then move into the North Atlantic where "something" will happen.

What next? A couple of scenarios are possible. One scenario is, “not much.” As the low brings stormy weather to the Upper Texas Coast it then gets caught up in the strong upper flow and races eastward — without significant development — bringing rain and thunderstorms to the Southeast and then quickly into the Atlantic where it continues more east than north.

Another scenario allows the low to strengthen while still tracking across the Deep South. This stronger low will tap into the unusually cold air pushing south from the Plains and the Great Lakes (YWB blogged about this yesterday and earlier this week). The influx of this cold air could spell an ice storm for this region, from north-central Louisiana eastward through central Georgia and coastal South Carolina. Meanwhile, just to the north of the ice, heavy snow may accumulate from southern Arkansas (midday Sunday) through North Carolina (Tuesday). A noted side benefit of this system: drought relief. The projected path of precipitation will fall precisely where it’s needed most right now.

ImpactWeather's StormWatch team has identified areas of ice and snow as the low tracks across the South.

According to several current computer models, severe weather along the Upper Texas Coast, combined with an ice/snow event across the south makes the most sense.

What becomes of the low as it reaches the North Atlantic? Here the computer models split. The GFS takes it out to sea without developing a nor’easter. The Canadian model develops a nor’easter as it moves the low into the warm Gulf Stream and drives it northward along the U.S. East Coast. Other models combine elements of both possibilities.

Looking at the Friday morning computer models, it’s looking less likely for a nor’easter to move up the East Coast early next week (Tuesday/Wednesday). However, the atmospheric situation is very similar to the Christmas nor’easter two weeks ago. How did the models handle the low then? Days prior, the GFS took the low harmlessly out to sea. Then 24 hours prior it changed its mind, turning the low toward the coast as a full-on nor’easter. Forecasters relying solely on the GFS were caught with an unexpected and significant snow storm from the Mid-Atlantic to New England. The Canadian model kept the low mainly offshore, but it was much closer to the coast than the GFS low. The European model handled the nor’easter very well, lending credence to its current nor’easter development away from the coast.

The European model strengthens the low off the North Carolina coast Tuesday evening but has it well offshore by Wednesday evening. Image: Unisys.

Either way, the risk is certainly there. More so, it’s Friday afternoon and the weekend begins in just a few hours. Northern regions of the Gulf Coast states should be prepared for the likelihood of a significant snow and ice event this weekend, while those with East Coast concerns should be aware of the nor’easter development potential and stay apprised of the latest weather information over the weekend.

Meanwhile, we Houstonians have another 24-36 hours of pleasant conditions before “wet and soggy then cold and cloudy” conditions befall us. Definitely not the type of weather we like here on the Gulf Coast.

Despite abundant rain in 2010, many regions of the South and Southeast are in need of significant rainfall. Image: NOAA/NWS/NCEP/CPC.

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