A Look at the Science Behind This Year's Wild Weather

It might be easier to ask the question what weather event didn't happen in the Valley this year?

Now we'll take a look at some of the unique processes that caused this wild weather.
A strong El Nino pattern of 2010 turned into a strong La Nina year of 2011.

This strong swing between weather patterns is unusual but the abrupt change also led to many of the flooding events that happened the first half of this year.

What La Nina does to the U.S. is it pushes the jet stream a bit more to the north, making the clash between warm air and cold air stronger.

Thus making regular weather patterns more extreme.

At the end of April a stalled low pressure system was forming over the Gulf coast.

As this system gathered heat and moisture it slowly moved across deep south.

Coupled with a strong mid level jet stream to produce wind shear, this was basically a picture perfect setup for strong destructive tornadoes that were headed for the Valley.

The most destructive storms hit in the late afternoon at the time when daytime heating is strongest.

It moved up the Tennessee Valley into Virginia around midnight.

The tornadoes that hit our region were in the middle of the night but the lack of sunlight did not stop the wind shear, although things could have been much worse.

La Nina can also bring an above average hurricane season, and that's just what we saw in 2011.

Although the valley's impacts from Hurricane Irene were low.

La Nina weakened over the summer but deep into fall, an unusual weather pattern setup just days before Halloween.

High pressure over New England pumped in cooler temperatures coupled with a developing system that formed over the Mid-Atlantic.

There was enough cold air for snow to fall, blanketing the Valley with several inches of snow.

It was the second earliest snow storm on record for the Valley, but the earliest for many other parts of the East Coast.

La Nina is still expected to strengthen this winter.

Typically La Nina events last up to a year, but it's not unheard of for them to last two years as it is doing now.

These weather patterns also occur about every two to seven years.

So it may not be the norm but it's not completely unusual.

This year brought 12 natural disasters to the U.S. that caused a billion dollars or more of damage, that's a record high.

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