The climate researchers at Colorado State University have released their spring forecast for the 2009 Atlantic hurricane season, and it’s calling for a relatively normal number of storms compared to average. The outlook is for 12 named storms, six hurricanes and two intense (category 3 or greater) hurricanes. The average from 1950-2000 is 9.6 named storms, 509 hurricanes and 2.3 intense hurricanes per year. There is a 54% chance of a major hurricane striking the US coast this year.
To develop their April forecast, the CSU researchers use a combination of observed sea surface temperatures in the Atlantic and sea level pressure off the African coastline in early spring. The observed values tend predict how much wind shear there will be from August through October, the heart of the hurricane season. The forecasters also look at analog years, where weather conditions are similar to this year. The number and intensity of storms in those years should be similar to what we get this year. Analog years for this season are 1951, 1968, 1976, 1985 and 2001.
The researchers also look at the chances of there being an El Nino or La Nina during the late summer. In El Nino years, the amount of tropical activity tends to diminish. Right now, we are in a weakening La Nina (cold sea surface temperatures in the central Pacific). There is a 50% possibility that La Nina conditions will develop by summer, and this caused the team to moderate its December forecast, which had called for 14 storms and seven hurricanes.
You can read the full forecast (PDF) here.