Monday, January 2, 2012

Some trends in our 2011 weather

Weather is loaded with trends, and I came across several when I was looking at what happened last year in Charlotte and across the Carolinas.

Our 2011 weather can be summarized rather neatly ... cold and snowy; then unseasonably warm; then very stormy; then hot and humid; then very wet; and finally warmer than average.

And all the major meteorological events of 2011 fall into one of those trends, except for Hurricane Irene. That's typical, because hurricanes are, by nature, renegade storms that form by themselves and create their own pattern, to some degree.

Overall, the past year was a bit warmer and wetter than average. Charlotte's average temperature for 2011 was about 1 degree above norms, and the precipitation -- 44.52 inches of it -- was 1.51 inches more than average.

Averages can be deceiving, though. Much heavier rainfall fell in some parts of the Charlotte region. During a five-day stretch in late September, about 1.6 inches was measured at Charlotte/Douglas International Airport -- the city's official reporting station. But there were parts of the area that got 5 or 6 inches during that period.

And while we were staying wet, drought conditions were reported in parts of western South Carolina.

So here's a look back at Charlotte's weather in 2011:

Cold and snowy start: December 2010 was the second-coldest in local history, and January picked right up with the trend. The start of the month was cold, and 4.1 inches of snow fell Jan. 9. To make matters worse, a quarter-inch of ice fell atop the snow on Jan. 9 and 10, so the result was really bad road conditions for a few days.

Overall, the temperature was 4.3 degrees below average, but it warmed up by the end of the month (70 degrees on the 30th), and that was a sign of things to come.

February started cold again, too, but there were signs that the Greenland block, which was sending arctic air masses into the eastern United States, would relent by the middle of the month.

Valentine's Day warm-up: It was like someone flicked a switch. On Feb. 14, after seven weeks of bitter cold, the temperature in Charlotte soared to near 70 degrees. And for the most part, it stayed warm for a while. We reached 82 degrees on Feb. 27.

The overall temperature in March was a bit below average, but there were plenty of warm days, including an 85-degree reading March 18. And there was no snow or ice.

A turn to stormy -- very stormy: It could be argued that April and May were among the stormiest months in Carolinas history. And it started early.

A persistent storm track from April through June carried low pressure systems from Texas across Louisiana, western Tennessee, northern Alabama and Mississippi, northern Georgia and Kentucky. That put the Carolinas in the warm and unstable sector of the low pressure systems.

It started April 4-5, with an outbreak of ferocious thunderstorms and 90 mph winds. The storms forced the closure of Interstate 485 for half a day. But that was just the start. On April 9, repeated rounds of hail-producing thunderstorms crossed Gaston, Cherokee, Chester, York and Lancaster counties. The hail damaged thousands of vehicles and houses, and insurance companies say it was one of the worst hail storms in recent years across the country.

Then on April 16, strong low pressure and a powerful cold front crossed the Carolinas. Meteorologists said, 24 hours in advance, that an outbreak of big tornadoes was likely. Charlotte barely missed what turned out to be a swarm of killer twisters. The first tornado formed north of Monroe, and another reached the ground in Rowan County.

As the tornadoes crossed U.S. 1, they grew in intensity. More than 20 people died across the Carolinas, especially in an area from Raleigh and Sanford east to the coast.

Two weeks later, killer tornadoes struck the Deep South, and the last remnants of those storms reached the Carolinas. The second-strongest wind gust of the year at Charlotte's airport, 37 mph, was measured during that series of storms.

There were thunderstorms on eight days in May, although the severity didn't match that of April.

A hot, humid summer: There were only two 100-degree days, and Charlotte got off easy, compared to cities in Tennessee, Louisiana, Arkansas, Oklahoma and Texas. But June, July and early August were unrelentingly hot and humid. There were 20 90-degree days in June and 25 in July. In July, the high reached 95 to 99 degrees a dozen times.

A measurement of the high humidity comes from the morning lows. When lows are warm, it's because the atmosphere is humid. And on 14 days in July, the morning low was 72 degrees or warmer.

Hurricane Irene, and a wet late summer: Irene battered the East Coast and caused misery for millions of residents in the Mid-Atlantic and New England. But the worst of the only hurricane to hit the United States this year was felt in far eastern North Carolina. Severe flooding and damaging winds affected the Outer Banks, Morehead City, Jacksonville, New Bern and Elizabeth City.

Farther inland, it was a very wet period. Rainfall exceeded 5 inches in August and September, and the two-month precipitation total of 10.73 inches was almost 3 1/2 inches above average.

Warm and quiet finish -- with one awful exception: The end of the year was, for the most part, mild. Temperatures were a bit below average in October, but November was warmer than average (1.3 degrees) and December was downright balmy (5.5 degrees above average). Rainfall was a bit above average.

But for the most part, it was a quiet period. The exception came Nov. 16, when killer tornadoes hit York County in South Carolina and Davidson County in North Carolina. In all, five people were killed, and each tornado reached EF3 intensity. A smaller, weaker twister struck near Shelby.