26 Nov 2013

2013 Atlantic hurricane season yields fewest hurricanes in 30 years

The Atlantic hurricane season officially draws to a close on Saturday 30 November, in what has been a memorable year, but for the opposite reason to recent years.

NOAA, the US national weather service, says that this year’s Atlantic hurricane season is expected to rank as the sixth least-active since 1950, in terms of the collective strength and duration of named storms and hurricanes.

Another startling statistic is that this year’s season has yielded the fewest number of hurricanes since 1982.


NOAA prediction versus what happened

This is in stark contrast to what NOAA predicted earlier this year in its seasonal Atlantic hurricane forecast, where it hinted at the following;

– 13 to 19 named storms (top winds of 39 mph or higher), including

– six to nine hurricanes (top winds of 74 mph or higher), of which

– three to five could be major hurricanes (Category 3, 4 or 5; winds of at least 111 mph)

However, in reality, there were 13 named storms, two of which became hurricanes and none of which became major hurricanes.

The lack of major hurricanes is significant, as this is the first time that this has happened in the Atlantic basin since 1994

Why has it been abnormally quiet?

Two major factors have been highly influential in limiting hurricane activity in the Atlantic basin this year – strong wind shear and the presence of a lot of dry air.

Wind shear is a measure of how wind speed and direction vary with height above the earth’s surface.

If this variation is great, as it has been this year, then it has a tug of war effect on storm clouds, pulling them in different directions and tearing them a part – lowering the chance of hurricanes forming.

duststorm_africa_NASA_wpThe presence of dry air over the Atlantic this year has been influenced by dominant areas of high pressure, as well as strong outbreaks of dry and stable air being blown across the Atlantic ocean from Africa.

As hurricanes are driven by tropical moisture rising up into the atmosphere, the presence of dry air caps thunderstorms and stops them from developing further into tropical storms and hurricanes.

The need for caution with seasonal forecasts

Whilst seasonal forecasts can be important for high-impact events such as the Atlantic hurricane season, this year demonstrates that such forecasts don’t always go to plan.

Even though scientists have a good understanding of how the atmosphere works, there are always unknowns that can throw a spanner in the works – resulting in predictions being some way off what ends up happening.

The level of uncertainty involved in long-range forecasting becomes even greater at mid-latitudes where the UK is located.

It is for this reason that weather forecasts beyond 5-10 days are difficult, with only general trends able to be picked out for a month ahead – something to bear in mind when reading claims that someone knows what is going to happen in a few months time!

Don’t forget, you can get the latest forecast on the Channel 4 Weather website. I also post regular updates on Twitter – @liamdutton

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