A well above average Atlantic hurricane season is on tap for 2008, according to the latest seasonal forecast
issued today by Dr. Bill Gray and Phil Klotzbach of Colorado State University (CSU). The Gray/Klotzbach team is calling for 15 named storms, 8 hurricanes, and 4 intense hurricanes. An average season has 10-11 named storms, 6 hurricanes, and 2 intense hurricanes. The new forecast is a bump up from their December forecast, which called for 13 named storms, 7 hurricanes, and 3 intense hurricanes. The new forecast calls for an above average chance of a major hurricane hitting the U.S., both along the East Coast (45% chance, 31% chance is average) and the Gulf Coast (44% chance, 30% chance is average). The Caribbean is also forecast to have an above average risk of a major hurricane.
The forecasters cited several reasons for their forecast of an active season:
1) Above-average sea surface temperatures (SSTs) in the eastern subtropical Atlantic and cooler-than-normal SSTs in the South Atlantic have weakened the Bermuda-Azores High. This has resulted in lower surface wind speeds over the tropical Atlantic, and these weak trade winds are expected to persist into hurricane season. Weak trade winds reduce the amount of evaporative cooling of the ocean, resulting in warmer SSTs and lower surface pressures during hurricane season. Hurricanes like to form in an environment with low surface pressures and high SSTs.
2) Hurricane activity in the Atlantic is lowest during El Niño years and highest during La Niña or neutral years. The current strong La Niña event has begun weakening noticeably in the past few weeks. However, there is probably not time for a full-fledged El Niño event to replace it by hurricane season, and it is expected that we will have weak La Niña or neutral conditions this hurricane season. None of the computer models are forecasting a switch over to El Niño conditions this year (Figure 1). Keep in mind, though, that the accuracy of these long range models is poor, particularly for forecasts made in March and April.
3) We are in the midst of an active hurricane period that began in 1995.How accurate are the April forecasts?
Today's forecast includes the statement, "These real-time operational early April forecasts have not shown forecast skill over climatology" during the 13-year period 1995-2007. In other words, today's forecast has no skill, and should merely be viewed as an interesting experimental research product. I like the fact that they are trying to make useful seasonal hurricane forecasts, but we should wait until their June 3 forecast before putting faith in their 2008 hurricane season forecasts. The CSU team has posted an 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). You can see from their numbers that the December and April forecasts have near zero skill, but the early June forecasts have substantial skill. To rectify their poor April forecast skill, the CSU team is trying a new scheme for this year's April forecast. Hopefully the new scheme will show positive skill forecasting upcoming hurricanes seasons, and not just "hindcasting" the past ones. For now, you're best off just paying attention to their early June forecast, which has been quite skillful over the past ten years.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. Image credit: Columbia University's IRI
.2008 Atlantic hurricane season forecast from Tropical Storm Risk, Inc.
The British private forecasting firm Tropical Storm Risk, Inc.
(TSR), issued a 2008 Atlantic hurricane season forecast this week as well. TSR has almost the same forecast as the CSU team--14.8 named storms, 7.8 hurricanes, and 3.5 intense hurricanes. Unlike the CSU team, these numbers represent a decrease
from their December forecast numbers, which were 15.4 named storms, 8.3 hurricanes, and 3.7 intense hurricanes. I like how they put their skill level right next to their forecast numbers: 7% skill at forecasting the number of named storms, 11% skill for hurricanes, and 10% skill for intense hurricanes. That's not much better than flipping a coin, but it is better than the near-zero forecast skill of the Gray/Klotzbach April forecasts. However, TSR doesn't mention the fact that part of their skill may be due to the fact that they issue forecasts of fractional storms (we're not going to get 7.8 hurricanes this year!) If we round these numbers to whole storms, the TSR skill numbers may decrease.
TSR projects that four named storms will hit the U.S., with 1.7 of these being hurricanes. In the Lesser Antilles Islands of the Caribbean, TSR projects 1.5 named storms, 0.7 of these being hurricanes. TSR cites one main factor for their forecast of an active season: slower than normal trade winds July-September over the Caribbean. Trade winds are forecast to be 0.4 meters per second (about 1 mph) slower than average, 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 tropical Atlantic during hurricane season.Figure 2.
Accuracy of long-range forecasts of Atlantic hurricane season activity performed by Bill Gray and Phil Klotzbach of Colorado State University (colored squares) and TSR (colored lines). The CSU team's April forecast skill is not plotted, but is near 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