Met Office predicts above average Atlantic hurricane season

The Met Office Atlantic tropical storm forecast for 2013 is for 14 tropical storms between June and November, with a 70% chance that the number will be in the range 10 to 18.

The long-term average over the period 1980–2010 is 12 tropical storms. The last three hurricane seasons have all recorded above average tropical storm activity.

The most likely Accumulated Cyclone Energy (ACE) index — a measure of the strength and duration of storms over the season — is 130, with a 70% chance that the index will be in the range 76 to 184; the 1980–2010 average ACE index is 104.

For the first time this year, the Met Office are also releasing a forecast of the number of hurricanes (storms with winds of at least 74 mph), following the success of experimental forecasts produced throughout the 2012 hurricane season.

Between June and November 2013 the best estimate is for 9 hurricanes, with a 70% chance that the number will be in the range 4 to 14; the 1980–2010 average is 6 hurricanes.

Overall, these indicators point to a preference for above-average activity this year.

The evolution of the El Niño/La Niña cycle over the next few months is likely to play a large part in the North Atlantic hurricane season.

Joanne Camp, climate scientist at the Met Office, said: “El Niño conditions in the Pacific can hinder the development of tropical storms in the Atlantic whereas La Niña conditions can enhance tropical storm activity, so how these conditions develop will be important for the storm season ahead.”

The tropical storm forecast is produced using the Met Office’s new seasonal prediction system GloSea5. The model has higher resolution than its predecessor, with better representation of the complex physical processes that cause tropical storms and hurricanes. The forecast also uses information from the seasonal prediction system of the European Centre for Medium-Range Weather Forecasts (ECMWF).

For regular updates on tropical cyclones worldwide follow @metofficestorms on Twitter.

The worst storm in years?

Various articles in the news today said that the weather over the weekend was the worst storm to hit the UK in years, and that there is more to come this week. There was indeed a very deep area of low pressure in the Atlantic over the weekend. At its deepest, on Saturday 26 January, the central pressure of the depression was 932 millibars and it was sitting some 1,800 nautical miles west of the UK. It came closest to the UK during the day yesterday with a central pressure of 950 millibars but was still around 600 nautical miles to the north west of Scotland.

Satellite image from 26 January 2013

To put this into context, the storm that affected the UK on 3 January 2012 had a central pressure of 953 millibars but was centred right on the west coast of Scotland and brought winds in excess of 80 mph to the Central Belt and a gust of over 100 mph in Edinburgh. Property was damaged, as well as trees, and there was disruption on the road network and with ferry crossings. Power supplies were also affected significantly.

The storm in January 2012 was therefore much more disruptive and severe than any wet and windy weather we have seen so far this year.

Much of the recent severe weather has been attributed to the phrase “Weather Bomb”, which is not a perfect meteorological term but is defined as an intense low pressure system with a central pressure that falls 24 millibars in a 24-hour period. This happened to the depression over the Atlantic during the weekend but as it was miles away from the UK its impacts were minimal. A better description can be more directly linked to the meteorological phenomena known as rapid cyclogenesis. This is where dry air from the stratosphere flows into an area of low pressure. This causes air within the depression to rise very quickly and increases its rotation, which in turn deepens the pressure and creates a more vigorous storm.

Does the jet stream affect hurricanes?

Many news and weather articles, including our own blog, have reported recently that the UK’s wet summer is caused by the jet stream being situated further south than usual for this time of year.

However, some have questioned whether there is a link between this and the hurricane activity seen so far in the Atlantic.

So how is the Atlantic hurricane season shaping up and has it been influenced by the weather at higher latitudes?

Atlantic hurricane season so far

The Atlantic hurricane season officially started on 1 June, but in 2012 it began early with two tropical storms, Alberto and Beryl, forming in May.

After a break of almost three weeks there were two more storms, Chris and Debby, in the latter part of June, with Chris becoming the first hurricane of the season.

Since then there has been another lull of almost three weeks in Atlantic tropical storm activity.

Has the Atlantic hurricane season been unusual?

In mid-July it is far too soon to make judgements about how unusual a hurricane season is likely to be based on activity so far. However, it was certainly unusual to see four tropical storms before the end of June as this has never been observed before in over 150 years of records.

The recent quiet spell in Atlantic hurricane activity is by no means unusual. Atlantic hurricane seasons are often characterised by bursts of activity followed by quiet spells.

The peak of activity usually runs from the second half of August through to October. Even in some years which turned out to be very active, early season activity was low.

For example, by this time in 2010 and 2011 there had been just one tropical storm. Each of these seasons ended up yielding a total of 19 storms.

Is the jet stream involved?

The jet stream which affects UK weather is much further north than where the majority of tropical storms develop and hence has no direct impact on their formation.

However, once a tropical storm develops and starts to move to higher latitudes the jet stream can influence where it ends up. For example, in September 2011 as Hurricane Katia moved northwards in the Atlantic it met a powerful part of the jet stream and was swept eastwards as a strong ‘post-tropical’ storm which brought stormy conditions to the northern UK.

Will it be an active hurricane season?

The Met Office seasonal forecast for Atlantic tropical storm activity issued in May predicted a near-average season with the most likely number of storms in the June to November period being 10. Since June two tropical storms have occurred so far (the two May storms fall outside of this prediction period).

One of the major influences on the season’s activity includes the existence of La Niña or El Niño conditions (natural cycles which affect sea temperatures in the equatorial east Pacific).

Having just come out of an extended period of La Niña conditions, forecasts suggests an El Niño could develop before the end of the current hurricane season.

This would suppress hurricane activity in the Atlantic by disrupting the airflow over the regions where they usually develop.

However, it is worth remembering that it is 20 years since the quiet El Niño influenced Atlantic hurricane season of 1992. Despite being a quiet season overall, it still managed to spawn the deadly and powerful Hurricane Andrew which brought devastation to parts of Miami in Florida.

For more information on tropical cyclones worldwide visit our web pages or follow @metofficestorms on Twitter.

What’s behind the stormy weather so far this winter?

So far this winter the UK has seen some very strong winds associated with a series of Atlantic storms. This has included some near-record strength gusts of wind in places, with Scotland particularly badly affected.

On 8 December a gust of 165mph was recorded at Cairngorm Summit (1245m above sea level) during the first of the winter storms – that gust was just 8mph off the strongest ever recorded in the UK (set at the same spot in 1986). Even at low level there were strong winds, with 105mph at Tulloch Bridge in Highland.

Another powerful storm affected Scotland on 28 December, and today we saw another – this time notching up winds of around 100mph in parts of Scotland. The 102mph gust at Blackford Hill in Edinburgh is the third highest recorded at that station, and the strongest since 1998.

So has this winter been particularly stormy? We actually expect stormy weather in the UK at this time of year. In normal circumstances, low pressure systems develop over the Atlantic and track from west to east – eventually moving over the UK bringing strong winds and often heavy rain with them. This year we’ve seen nothing to disrupt that westerly flow, leading to the succession of low pressures systems we’ve seen so far.

While the general weather pattern is what we expect to see at this time of year, the strength of the storms and winds has been unusual. This is down to the jet stream – the high altitude winds which blow from west to east across the Atlantic and brings us our traditional changeable weather.

Over the past few weeks the jet stream has occasionally been particularly strong and some of the low pressure systems have interacted with that as they have tracked over the Atlantic, boosting their strength. This has led some of the lows to develop into powerful storms which have brought very strong winds to the UK.

The weather is set to remain changeable for the rest of this week, and beyond that it’s not possible to say whether we will see any more particularly strong storms this winter. However, we’ll continue to keep everyone up to date with the latest outlook in our forecasts and warnings.