BackgroundA tropical cyclone is the generic term for a low-pressure system over tropical or sub-tropical waters, with organised convection (i.e. thunderstorm activity) and winds at low levels, circulating either anti-clockwise in the northern hemisphere or clockwise in the southern hemisphere. A tropical storm is a tropical cyclone with mean wind speeds of at least 39 m.p.h. The terms hurricane and typhoon are regionally-specific names for a strong tropical cyclone with wind speeds of at least 74 m.p.h. The number of tropical storms forming over the North Atlantic is influenced by sea-surface temperatures (SST), over both the tropical Atlantic, and the tropical Pacific.
Method At the start of every North Atlantic season (which usually runs from June to November) the Met Office forecasts the number of tropical storms expected throughout the period. This is done using a dynamical seasonal prediction computer model of the global atmosphere-ocean system called GloSea. The forecast is based on GloSea representation of dynamical and physical processes characteristic of tropical storms. This is done by counting the frequency of tropical storms in the model forecasts. However, as the dynamical model grid does not fully resolve tropical storms, numbers are calibrated using tropical storm behaviour in past forecasts. The forecast process also includes predictions of the sea-surface temperature (SST) anomalies. This season, a cooling trend in SST is expected in the tropical North Atlantic, and this favours fewer tropical storms than seen in recent years prior to 2006 (see the table). |
Forecast for July to November 2007Issued 19 June 2007 10 tropical storms are predicted as the most likely number to occur in the North Atlantic during the July to November period, with a 70% chance that the number will be in the range 7 to 13. This represents below normal activity relative to the 1990-2005 long-term average of 12.4. Note: The forecast is for the five full months remaining in the June-November Atlantic tropical storm season. There have already been two named storms – Andrea and Barry.
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| Recent studies have shown that GloSea and other European models have considerable skill predicting the number of tropical storms, for example successfully predicting the change from the exceptionally active season of 2005 to the below-normal activity of the 2006 season. This marked difference between seasons was missed by a number of statistical prediction methods, which have traditionally formed the basis of most published forecasts.
The forecast has been produced following research collaborations with the European Centre for Medium-Range Weather Forecasts (ECMWF). Verification During the 2007 Atlantic season there was a total of 12 named storms in the July-November time frame which was the subject of the Met Office forecast issued in June. This was two more than the 10 predicted, but within the 70% probability range of 7-13. In total there were 15 named storms in the whole season (including months outside of July-November), which is above the long-term average. However, the majority of these storms were relatively weak and short-lived, which resulted in below-average activity for the season as a whole when using measures which take the strength and longevity of storms into consideration. The Met Office will issue a forecast for the July-November 2008 period on 18 June at 1000 BST.
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