CSU and TSR predict an extremely active 2011 Atlantic hurricane season

A continuation of the pattern of much above-average Atlantic hurricane activity we've seen since 1995 is on tap for 2011, according to the latest seasonal forecast issued Wednesday by Dr. Phil Klotzbach and Dr. Bill Gray of Colorado State University (CSU). They are calling for 17 named storms, 9 hurricanes, and 5 intense hurricanes. An average season has 10 - 11 named storms, 6 hurricanes, and 2 intense hurricanes. The new forecast is a very aggressive one, since only six seasons since 1851 have had as many as 17 named storms; 19 seasons have had 9 or more hurricanes. The 2011 forecast calls for a much above-average chance of a major hurricane hitting the U.S., both along the East Coast (49% chance, 31% chance is average) and the Gulf Coast (48% chance, 30% chance is average). The Caribbean is forecast to have a 62% chance of seeing at least one major hurricane (42% is average.) Five years with similar pre-season November atmospheric and oceanic conditions were selected as "analogue" years that the 2011 hurricane season may resemble: 2008, 1999, 1989, 1961, and 1956. The average activity for these years was 12 named storms, 7 hurricanes, and 4 major hurricanes.

The forecasters cited several reasons for their forecast of a much above-average season:

1) Unusually warm sea surface temperatures continue in the tropical Atlantic this fall, an indication that the active hurricane period we have been in since 1995 will continue (in technical terms, the positive phase of the Atlantic Multidecadal Oscillation, AMO, will continue.) SSTs in Atlantic Main Development Region (MDR) for hurricanes, from the Caribbean eastwards to the coast of Africa, were at a record high in October (November data are not yet available.)

2) Hurricane activity in the Atlantic is lowest during El Niño years and highest during La Niña or neutral years. This occurs because El Niño events tend to increase westerly upper-level winds over the tropical Atlantic, bringing high wind shear harmful for hurricanes. The CSU team notes that we are currently experiencing moderate to strong La Niña conditions, with an unusual amount of cool water present in the top 300 meters of the Equatorial Pacific Ocean off the coast of South America. Since 1979, only eight years have had similar amounts of cool water in November. The hurricane seasons that followed each of those eight years were unable to transition to El Niño conditions. Thus, the CSU team expects that we will have neutral or La Niña conditions in place for the Atlantic hurricane season of 2011, which should act to keep wind shear values average to below average, enhancing hurricane activity. Of the 16 El Niño/La Niña computer models that made November predictions for the July-August-September 2011 portion of hurricane season (Figure 1), only 4 (25%) predicted that El Niño would arrive.

How accurate are the December forecasts?
The CSU real-time December forecasts did not shown any skill over the period 1992 - 2007, so the forecast scheme was overhauled and a new scheme implemented for the forecasts made for the 2008 - 2010 hurricane seasons. This new scheme showed some decent skill in those three years, with skill levels 18%, 10%, and 30% above chance for predicting the number of named storms, hurricanes, and intense hurricanes, respectively. Still, three years is too short of a time period to evaluate the skill of these December forecasts, and we should view the latest forecast as an experimental research product. Last year's December forecast for the 2010 hurricane season predicted 13.5 named storms, 7 hurricanes, and 4 intense hurricanes. The actual numbers were 19 named storms, 12 hurricanes, and 5 intense hurricanes.


Figure 1. Forecasts of El Niño conditions by 20 computer models, made in November 2010. The longest range forecasts for July-August-September (JAS) at the right side of the image show that 4 models predict weak El Niño conditions, 7 predict neutral conditions, and 5 predict a weak to moderate La Niña. El Niño conditions are defined as occurring when sea surface temperatures in the Equatorial Pacific off the coast of South America (the "Niño 3.4 region) rise to 0.5°C above average (top red line). La Niña conditions occur when SSTs in this region fall to 0.5°C below average. Image credit: Columbia University.

2011 Atlantic hurricane season forecast from Tropical Storm Risk, Inc.
The British private forecasting firm Tropical Storm Risk, Inc. (TSR), issued their 2011 Atlantic hurricane season forecast on Monday. They are also calling for a very active year: 15.6 named storms, 8.4 hurricanes, and 4 intense hurricanes. TSR predicts a 66% chance of an above-average hurricane season, 22% chance of a near-normal season, and only a 12% chance of a below normal season. TSR bases their December forecast on predictions that sea surface temperatures next fall in the tropical Atlantic will be above about 0.3°C above average, and trade wind speeds will be about 0.7 m/s slower than average. The trade wind speed prediction is based on a forecast for a weak La Niña in August-September 2011.

I like how TSR puts their skill level right next to the forecast numbers: 2% skill above chance at forecasting the number of named storms, 1% skill for hurricanes, and 7% skill for intense hurricanes. That's not much skill, and really, we have to wait until the June 1 forecasts by CSU, NOAA, and TSR to get a forecast with reasonable skill.

Comparing 2005 and 2010 steering currents
The U.S. got extremely fortunate during the 2010 hurricane season that the steering currents carried most of the storms out to sea, or forced them to the south. A very graphic way of looking at this is to compare the amount of rain that fell due to tropical cyclones in the Atlantic in 2005 versus 2010 (Figure 2.) The U.S. received a direct hit only from Tropical Storm Bonnie, which hit South Florida as a minimal 40 mph tropical storm and caused no significant damage. TSR is predicting that this luck will not hold in 2010; they project that five named storms will hit the U.S., with two of these being hurricanes.


Figure 2. Rainfall amounts due to all Atlantic tropical cyclones in 2005 and 2010, as measured by NASA's TRMM satellite. Steering currents in 2010 tended to recurve many storms out to sea between the Caribbean and Bermuda, due to a large number of unusually strong troughs of low pressure moving off the U.S. East Coast.

Weekend winter storm for Eastern U.S.
The latest 06Z (1am EST) set of computer model forecasts for this weekend's winter storm over the Midwest and Eastern U.S. point towards a more northerly path for the storm, bringing the axis of heaviest snow through Wisconsin and Michigan. There is still a great deal of uncertainty in the predicted timing and strength of the storm, but a renewed blast of cold air Monday and Tuesday over the Southeast U.S. is still highly likely in the wake of the storm.

Jeff Masters