Get ready for another tough hurricane season in 2006. If the forecast team at Colorado State University (CSU)
founded by Dr. Bill Gray (now headed by Dr. Phil Klotzbach) is correct, we are in for another much more active than usual hurricane season, with 17 named storms and 9 hurricanes. The average Atlantic season has only 11 named storms and 6 hurricanes. In their forecast for the upcoming hurricane season released today, the CSU team also projects that the U.S. will be at high risk from strikes by intense hurricanes again, with an 81% chance of a strike by a Category 3, 4, or 5 hurricane. However, the forecast notes that the from a purely statistical point of view, the number of landfalling hurricanes and major hurricanes in the U.S. should decrease in 2006 from what we have seen during 2005 and 2004. They note:
It is rare to have two consecutive years with such a strong simultaneous combination of high amounts of major hurricane activity together with especially favorable steering flow currents. The historical records and the laws of statistics indicate that the probability of seeing another two consecutive hurricane season like 2004-2005 is very low. Even though we expect to see the current active period of Atlantic major hurricane activity to continue for another 15-20 years, it is statistically unlikely that the coming 2006 and 2007 hurricane seasons, or the seasons which follow, will have the number of major hurricane US landfall events as we have seen in 2004-2005.
The CSU team uses observations of monthly average atmospheric winds and pressures over six specific regions of the globe taken this Fall to determine whether favorable or unfavorable conditions will exist for the 2006 hurricane season. All six of these "predictors" are favorable for an active 2006 hurricane season. A condensed summary:
1) El Ni�o, which acts to suppress Atlantic hurricane activity, is not expected to be active during the 2006 hurricane season. It is more likely that neutral or even La Ni�a conditions will develop, which are favorable for Atlantic hurricane activity.
2) The stratospheric quasi-biennial oscillation (QBO), an oscillation in the stratosphere that creates alternating westerly and easterly winds there, is expected to be in it west phase during the hurricane season of 2006. The west phase of the QBO has been shown to provide favorable conditions for development of tropical cyclones in the deep tropics.
3) The observed pressure and wind patterns over the North Atlantic will allow stronger than normal southerly winds to keep sea surface temperatures over the entire North Atlantic well above normal during 2006. Sea surface temperatures during 2005 were the highest measured since at least 1950, and helped fuel 2005's record-breaking intense hurricanes. Sea surface temperatures during the 2006 hurricane season may be just as warm.
4) Perhaps most importantly, observed wind and pressure patterns this Fall indicate that upper-level winds in the upper atmosphere (200 mb) will blow from the east during the hurricane season of 2006, like they did during 2005. This results in low wind shear over hurricane formation regions of the tropical Atlantic. Low wind shear is the key ingredient needed for tropical storm formation and intensification.
How good are these long-range hurricane forecasts issued in December? Last year, the December forecast called for an average 2005 hurricane season with 11 named storms, six of them becoming hurricanes. Obviously, the forecast was a bust--we had 26 named storms and 14 hurricanes. The updated forecast issued on May 31 of 2005 performed much better--15 named storms were forecast, with eight hurricanes. However, over the past five years (not including the forecast for the 2005 season), the skill of the December hurricane forecasts by the CSU team has been quite good--in four out of five years, the predicted number of named storms was within three of the actual number. Tune in April 4, when the CSU team issues their updated forecast for the 2006 hurricane season. Maybe Hurricane Epsilon will be gone by then!
I'll talk more about Epsilon tomorrow.