Initial 2015 Hurricane Seasonal Outlook
Atlantic Hurricane Season Forecast (April 2015)
Colorado State University (CSU) Atlantic Hurricane Season Forecast
Dr. Philip Klotzbach and Dr. William Gray of Colorado State University (CSU) have issued their April forecast for the 2015 Atlantic Hurricane Season.
Their forecast calls for 7 named storms, 3 hurricanes and 1 major (Category 3+) hurricanes between the months of June and November. This is the lowest number of storms that CSU has projected in a seasonal forecast since they began publicly releasing in 1984.
With the release of their forecast, CSU is predicting below-average tropical cyclone activity in the Atlantic Basin during the upcoming 2015 season.
The report cites several factors as to why lessened activity is being forecast.
- One such main factor is that a weak El Niño phase of ENSO (El Niño-Southern Oscillation) is currently underway, and it is expected to continue intensifying into a moderate-strength event as we enter the August-October time frame.
– The primary reasoning for this is the expectation that strengthening westerly wind bursts will push a Kelvin wave into the eastern and central tropical Pacific. Kelvin waves transport warm water from the western tropical Pacific to the eastern and central tropical Pacific.
– In the ongoing scenario, it is expected to enhance the current El Niño. The statistical and dynamical forecast models maintain a fairly wide range of outcomes, but CSU is putting more credence into the historically well performing ECMWF model that suggests a more robust El Niño.
- A second factor revolves around current sea surface temperatures in the tropical Atlantic, North Atlantic and off the US East Coast.
– Temperatures in the tropical Atlantic significantly cooled during the winter of 2014/2015. Plus, the current sea surface temperature pattern looks more in line with a negative Atlantic Multi-Decadal Oscillation (AMO) pattern.
– Much of this can be attributed to a positive phase of the North Atlantic Oscillation (NAO) has led to anomalously strong trade winds across the tropical Atlantic, which promotes mixing and the upwelling of cold water.
The information below shows the CSU forecast, including probabilities of landfall on the United States mainland.
The full report is available at CSU’s Tropical Meteorology webpage (http://tropical.atmos.colostate.edu). The next forecast update is expected Monday June 1, 2015.
CSU Major Hurricane Landfall Probabilities (June 1 – November 30)
Forecast Parameter Average Year 2015 (April 2015)
Entire U.S. Coastline 52% 28%
U.S. East Coast (including FL Peninsula) 31% 15%
U.S. Gulf Coast (FL Panhandle to Brownsville, TX) 30% 15%
Tropical Storm Risk (TSR) Atlantic Hurricane Season Forecast
Tropical Storm Risk (TSR) has also issued its April forecast for the 2015 Atlantic Hurricane Season. TSR’s Professor Mark Saunders and Dr. Adam Lea are forecasting 11 named storms, 5 hurricanes and 2 major (Category 3+) hurricanes between the months of June and November in 2015. The projected activity is expected to be 45% below the longrange norm since 1950, but 50% below the more recent 2005-2014 norm.
The report specifies two primary factors as to why a below normal hurricane season has been forecast.
The main reason is that the sea surface temperatures across the tropical North Atlantic and Caribbean Sea during the peak development months of August and September are now expected to be cooler than normal. It is also cooler than previously expected.
Second, current projections indicate that trade winds over the same regions will be stronger than earlier anticipated. These stronger winds will potentially help suppress cyclogenesis.
- However, the TSR July-September trade wind prediction is based on an expectation that neutral ENSO conditions will be occurring in August-September, which differs from the current modeled consensus ENSO outlook that El Niño will be ongoing from the International Research Institute.
- TSR stresses that uncertainties in 2015 trade wind speeds at this extended lead time are large given unknowns
surrounding ENSO and future sea surface temperatures in the North Atlantic and Caribbean Sea.
The report indicates that should the 2015 forecast verify, it could signal that 2013-2015 was the lowest three-year ACE Index total since 1992-1994, and could imply that the active phase of Atlantic hurricane activity that began in 1995 has likely ended. Great uncertainty remains, however.
Drs. Saunders and Lea currently project that there is a 13% probability that the 2015 Atlantic Hurricane Season ACE Index will be above-average, a 27% likelihood it will be near-normal, and a 60% chance it will be below-normal. The Accumulated Cyclone Energy Index is equal to the sum of the squares of 6-hourly maximum sustained wind speeds (in knots) for all systems while they are at least tropical storm strength. The ACE Landfall Index is the sum of the squares of hourly maximum sustained wind speeds (in knots) for all systems while they are at least tropical storm strength and over the United States mainland (reduced by a factor of 6).
The information below shows the TSR forecast and the range of uncertainty that surrounds the forecast. The full report is available at TSR’s webpage (http://tropicalstormrisk.com). The next forecast update is expected in early June 2015.
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