Monday, September 27, 2010

Carolinas have 2 seasons ... at once

Today's Carolinas' weather is featuring a situation seen almost nowhere else in the country.

It's called a cold air wedge (or cold air damming), and we'll talk about it a lot in the coming months, as it's usually seen in our area from late September into April or May.

It's the reason why we sometimes have two or even three different seasons' weather at one time in the Carolinas, depending on where you are.

Today, it's summer at the coast and fall inland, and the dividing line between the seasons could produce some stormy weather later this afternoon as it moves closer to Charlotte.

Cold air damming, or a cold air wedge, is created when cooler air is trapped against the mountains. Typically, the clockwise flow from high pressure over the Northeast sends cooler air into the Carolinas. Cold air is more dense than warm air, and it sinks. The cold air piles up against the mountains and becomes trapped across the foothills, Piedmont and even the sandhills.

When low pressure systems move northward -- sometimes along the coast, sometimes across the Piedmont, and sometimes (like today) across eastern Georgia and eastern Tennessee -- they have a counter-clockwise air flow that tends to drag warmer air from the southeast inland. That often creates a warm front.

Places to the east (coastal side) of the front have temperatures much, much warmer than inland.

At 10 a.m. today, it was 81 degrees in Beaufort and 59 in Asheville. Charlotte's 66-degree reading was 7 degrees cooler than at Myrtle Beach.

I've seen episodes when it was 80 degrees in Columbia and Fayetteville and in the 50s in Charlotte. Sometimes you can drive down I-77 or east on U.S. 74 and encounter a change of 20 degrees' temperature in 10 miles.

Normally, the warm fronts never make it all the way inland past Charlotte. Eventually, the trapped cool air (which is damp and creates clouds and precipitation) is washed out to sea by a stronger cold front.

Today, as the warm front moves slowly inland, meteorologists say it could trigger heavy thunderstorms. The Storm Prediction Center in Norman, Okla., has put south-central North Carolina and much of central and eastern South Carolina in the "risk" area for severe thunderstorms and possibly even a weak tornado later today.

The warm front is a severe weather-producer, in part, because it creates a condition in which winds are spinning from different directions in the atmosphere.

It's something we'll be watching as the day progresses.


lkmi said...

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