Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 5:50 PM GMT on December 08, 2006
It's going to be a more active than usual Atlantic hurricane season in 2007, but not hyperactive, according to the latest seasonal forecast issued by Dr. Bill Gray and Phil Klotzbach of Colorado State University today. The Gray/Klotzbach team is calling for 14 named storms, 7 hurricanes, and 3 intense hurricanes. An average season has 10 named storms, 6 hurricanes, and 2 intense hurricanes. The forecast calls for an above normal chance of a major hurricane hitting the U.S., both along the East Coast (40% chance, 31% chance is normal) and the Gulf Coast (40% chance, 30% chance is average).
The forecasters predict that the current moderate El Nino event will dissipate by the time the active part of the 2007 hurricane season rolls around. The sudden development of El Nino this year significantly reduced the hurricane activity, and dissipation of this El Nino by August of 2007 would likely create more favorable conditions for hurricane development than in 2006. Gray and Klotzbach support this forecast by examining the active hurricane periods of 1950-1969, and 1995-2005, and note that seven out of the eight seasons following El Nino years during this period were active Atlantic hurricane seasons, and all of these years witnessed either neutral or La Nina conditions.
The forecasters examined the observed atmospheric conditions and ocean temperatures in October-November 2006, and came up with a list of four past years that had a similar combination of a moderate El Nino event, warmer than average tropical Atlantic SSTs, and a weaker-than-normal Azores-Bermuda High. We can expect 2007 to be similar to the average of these four analogue years, they say. The four years were 2003 (16 named storms, 7 hurricanes, 3 intense hurricanes), 1966 (11, 7, and 3 of the same), 1958 (10, 7 and 5), and 1952 (7, 6 and 3). Hurricane Isabel of 2003 (Category 2) was the strongest hurricane to hit the U.S. in these four analogue years, and Category 4 Hurricane Inez of 1966 caused the most death and destruction, killing over 1000 people in its rampage through the Caribbean.
2007 Atlantic hurricane season forecast from Tropical Storm Risk, Inc.
It's going to be a bit rougher year than the Gray/Klotzbach team is forecasting, according to the British private forecasting firm Tropical Storm Risk, Inc. (TSR), who issued their 2007 Atlantic hurricane season forecast yesterday. TSR is calling for a season with 60% above normal activity--16 named storms, 9 hurricanes, and 4 intense hurricanes. They project that five named storms will hit the U.S., with two of these being hurricanes. In the Lesser Antilles Islands of the Caribbean, TSR projects two named storms, one of them being a hurricane. TSR cites two main factors for their forecast of an active season: above normal Sea Surface Temperatures (SSTs) are expected in August-September 2007 across the tropical Atlantic, as well as slower than normal trade winds. Trade winds are forecast to be 0.7 meters per second (about 1.5 mph) slower than average, which would create greater spin for developing storms, and allow the oceans to heat up due to reduced evaporational cooling. SSTs are forecast to be about 0.34 degrees C above normal. TSR gives an 80% chance that the 2007 Atlantic hurricane season will rank in the top third of active seasons observed since 1957.
Figure 1. 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 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.
How good are these December hurricane season forecasts? Unfortunately, they're pretty much worthless. The skill of the December forecasts issued by Dr. Gray and TSR (Figure 1) have averaged near zero since 1992. Not surprisingly, the forecasts get better the closer they get to hurricane season. The June and August forecasts show some modest skill, and are valuable tools for insurance companies and emergency planners to help estimate their risks. The problem with the December forecasts is that the current statistical computer models used to forecast El Nino are not skillful beyond about six months. For example, none of these models foresaw the current El Nino event that began in September--until April. Until we can forecast the evolution of El Nino more than six months in advance, these December forecasts are not worth paying much attention to. I think it's important for these groups to keep trying, though.
I'll be back Monday with a new blog. Tune into the blog of Mike Theiss this weekend, he'll be covering the launch of the Space Shuttle. Mike is a top notch storm chaser and weather photographer we're excited to have on our wunderblogging team!
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