Average hurricane season foreseen by CSU, but TSR predicts an active season
A near-average Atlantic hurricane season is on tap for 2009, according to the latest seasonal forecast issued today by Dr. Phil Klotzbach and Dr. Bill Gray of Colorado State University (CSU). The Klotzbach/Gray team is calling for 12 named storms, 6 hurricanes, and 2 intense hurricanes. An average season has 10-11 named storms, 6 hurricanes, and 2 intense hurricanes. The new forecast is a step down from their December forecast, which called for 14 named storms, 7 hurricanes, and 3 intense hurricanes. The new forecast calls for a near-average chance of a major hurricane hitting the U.S., both along the East Coast (32% chance, 31% chance is average) and the Gulf Coast (31% chance, 30% chance is average). The Caribbean is also forecast to have an average risk of a major hurricane.
The forecasters cited several reasons for reducing their forecast from an active season to an average season:
1) Sea surface temperature (SST) anomalies in the tropical Atlantic have cooled considerably since December. In fact, these SST anomalies are at their coolest level since July 1994. Cooler-than-normal waters provide less heat energy for developing hurricanes. In addition, an anomalously cool tropical Atlantic is typically associated with higher sea level pressure values and stronger-than-normal trade winds, indicating a more stable atmosphere with increased levels of vertical wind shear detrimental for hurricanes. Part of the reason for the substantial cooling since December is because a stronger than average Bermuda-Azores High drove strong trade winds. These strong winds acted to evaporate more water from the ocean, cooling it. Higher winds also increase the mixing of cool waters to the surface from below. However, in March, the Bermuda-Azores High weakened. The resulting weaker trade winds may allow SSTs to warm to above average levels by the coming hurricane season, if this weaker Bermuda-Azores High persists.
2) Hurricane activity in the Atlantic is lowest during El Niño years and highest during La Niña or neutral years. The CSU team expects current weak La Niña conditions to transition to neutral and perhaps weak El Niño conditions (50% chance) by this year's hurricane season. April and May are typically the months when the atmosphere will swing between El Niño and La Niña, which makes any seasonal forecasts of hurricane activity during April low-skill. The current computer models used to predict El Niño (Figure 1) mostly favor neutral conditions for the coming hurricane season. These models are primarily based on statistical methods that observe how previous El Niño events have evolved. Three of the newer computer-intensive dynamical models (similar to the GFS model we use to make weather forecasts) do predict an El Niño event by hurricane season. The reliability of all of these models is poor.
Figure 1. Computer model forecasts of El Niño/La Niña made in March. The forecasts that go above the red line at +0.5°C denote El Niño conditions; -0.5°C to +0.5°C denote neutral conditions, and below-0.5°C denote La Niña conditions. Three computer models predict El Niño conditions for hurricane season (ASO, August-September-October). However, most of the models predict neutral conditions. Image credit: Columbia University's IRI.
The CSU team picked five previous years when atmospheric and oceanic conditions were similar in April to what we are seeing this year. Those five years were 2001, featuring Category 4 storms Michelle, which hit Cuba, and Iris, which hit Belize; 1985, which had Category 3 Gloria in New England and Category 3 Elena in the Gulf of Mexico; 1976, which had Category 1 Hurricane Belle in New England; 1968, which had Category 1 Hurricane Gladys north of Tampa; and 1951, which had only one landfalling hurricane, Category 3 Hurricane Charlie in Mexico. The mean activity for these five years was 11 named storms, 7 hurricanes, and 3 intense hurricanes.
How accurate are the April forecasts?
This year's April forecast uses the same formula as last year's April forecast, which did quite well predicting the 2008 hurricane season. Last year's forecast included the statement, "These real-time operational early April forecasts have not shown forecast skill over climatology" during the 13-year period 1995-2007. Unfortunately, this year's forecast neglects to mention this fact. In fact, when looking at Excel spreadsheet of their forecast errors (expressed as a mathematical correlation coefficient, where positive means a skilled forecast, and negative means they did worse than climatology) their April forecasts have had negative skill between 1995-2008. In other words, you would have been better off using climatology than believing their April forecasts.
2009 Atlantic hurricane season forecast from Tropical Storm Risk, Inc.
The British private forecasting firm Tropical Storm Risk, Inc. (TSR), issued their 2009 Atlantic hurricane season forecast yesterday, but they are calling for an active year: 15 named storms, 7.8 hurricanes, and 3.6 intense hurricanes. TSR predicts a 63% chance of an above-average hurricane season, 24% chance of a near-normal season, and only a 13% chance of a below normal season. They give a 63% chance that 2009 will rank in the top third of most active hurricane seasons on record. The April 2009 TSR forecast is virtually identical to their December 2008 forecast, and is also quite close to their April 2008 forecast made for the 2008 hurricane season.
I like how TSR puts their skill level right next to the forecast numbers: 11% skill above chance at forecasting the number of named storms, 9% skill for hurricanes, and 7% skill for intense hurricanes. That's not much better than flipping a coin, but is better than the negative forecast skill of the Klotzbach/Gray April forecasts.
TSR projects that 4.8 named storms will hit the U.S., with 2.1 of these being hurricanes. The averages from the 1950-2008 climatology are 3.2 named storms and 1.5 hurricanes. Their skill in making these April forecasts for U.S. landfalls is 10-15% above chance. In the Lesser Antilles Islands of the Caribbean, TSR projects 1.4 named storms, 0.6 of these being hurricanes. Climatology is 1.1 named storms and 0.5 hurricanes.
TSR cites one main factor for their forecast of an active season: slower than normal trade winds from July - September over the Main Development Region (MDR) for hurricanes over the Atlantic (the region between 10° - 20° N from Central America to Africa, including all of the Caribbean). Trade winds are forecast to be 0.4 meters per second (about 1 mph) slower than average in this region, which would create greater spin for developing storms, and allow the oceans to heat up due to reduced evaporational cooling. TSR forecasts that SSTs will be near average in the MDR during hurricane season, and will not have an enhancing or suppressing effect on hurricane activity.
Figure 2. Accuracy of long-range forecasts of Atlantic hurricane season activity performed by Phil Klotzbach and Bill Gray of Colorado State University (colored squares) and TSR (colored lines). The CSU team's April forecast skill is not plotted, but is less than zero. The skill is measured by the Mean Square Skill Score (MSSS), which looks at the error and squares it, then compares the percent improvement the forecast has over a climatological forecast of 10 named storms, 6 hurricanes, and 2 intense hurricanes. TS=Tropical Storms, H=Hurricanes, IH=Intense Hurricanes, ACE=Accumulated Cyclone Energy, NTC=Net Tropical Cyclone Activity. Image credit: TSR.