9 Sep 2013

First Atlantic hurricane of season likely as tropics spring to life

The Atlantic hurricane season officially began on 1June and has been eerily quiet so far this year, with only seven tropical storms up to yesterday – none of which had gone on to become hurricanes.

However, it looks very likely that this will change by midweek, with Tropical Storm Humberto forecast to reach hurricane status.

Having started off as a tropical depression just to the south east of the Cape Verde islands, it has gathered strength – feeding off the energy from the warm tropical waters.


Tropical storm warnings have been issued for the southern Cape Verde islands, with 3-6 inches of rain falling widely in the next 24 hours, with mountainous areas having as much as 8 inches.

This heavy rain combined with strong winds of up to 70mph will bring a risk of disruption with flooding possible.

Subsequently, Humberto is set to move north westwards over open waters, reaching hurricane strength for around two days before cooler water cuts off its energy supply, causing it to weaken.

What was the forecast for this year’s hurricane season?

Earlier this year, NOAA produced its annual forecast for the number of hurricanes expected in the Atlantic ocean basin this season.

It predicted a 70 percent chance of an above-normal season which lasts from 1 June to 30 November.

However, the lack of storm activity so far – especially no hurricanes – has raised questions amongst those in the meteorological community as to whether this will end up being the case.

In an average season, there would have been three hurricanes by 9 September, with one of those a major hurricane at category three or above.

Why the lack of hurricanes so far this year?

The reason for the lack of activity so far has been down to two main factors – dry, dusty air feeding off Africa and strong wind shear over the mid-Atlantic.

Hurricanes need a depth of moisture and warmth through the atmosphere in order to flourish. Dry, dusty air inhibits the presence of moisture high up, making it harder for storms to develop.

Wind shear is the measure of how the speed and direction of the wind in the atmosphere changes with height.

If wind shear is strong, as has been the case so far this season, storms find it hard to form because air is being pulled in so many different directions. This acts like a tug of war, tearing any potential storms apart.

Is the level of activity set to increase?

Statistically, the peak of the hurricane season in the Atlantic is on 10 September, as shown in the graph below.

hurricane_peak_NOAA_g_wpAt the moment, the amount of dry air and wind shear in the tropical Atlantic has lessened, making conditions more favourable for hurricanes to form.

It’s also worth adding that just because it has been a quiet season so far, there is still plenty of time for things to turn lively – something that has proven the case in previous years.

I’ll be keeping an eye on the tropics in the coming weeks and bringing you updates here on my blog, as well as on Twitter – @liamdutton

Satellite image: CIMSS

Tweets by @liamdutton