Updated hurricane season predictions from CSU, NOAA, and TSR

By: Dr. Jeff Masters , 2:27 PM GMT on August 09, 2008

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The tropical Atlantic is quiet and there are no threat areas to discuss today. Three of the four reliable computer models predict that some development could occur between Wednesday and Saturday between Africa and the Lesser Antilles Islands.

The updated August 5 hurricane season forecast from Colorado State University
The forecast team at Colorado State University (CSU) led by Phil Klotzbach and Bill Gray issued their August forecast of Atlantic hurricane activity on Tuesday. The new CSU forecast calls for seasonal totals of 17 named storms, 9 hurricanes, 5 intense hurricanes, and an ACE index of 175. The ACE index is a more intelligent way to assess total activity for a season, and is computed by summing the squares of the estimated maximum sustained wind speed of each named storm (when its wind speed is 39 mph or higher), at six-hour intervals. The resulting number (divided by 10,000 to make it more manageable), is called the Accumulated Cyclone Energy (ACE). An average hurricane season has 10 named storms, 6 hurricanes, 2 intense hurricanes, and an ACE index of 96. So far this season, we have had 5 named storms, 2 hurricanes, 1 intense hurricane, and 39 ACE units, so the CSU team is expecting 13 more named storms, 7 hurricanes, 4 intense hurricanes, and 136 more ACE units. They give a 67% chance that an intense hurricane will hit the U.S. (52% is the historical average). They say that conditions this year are most similar to 1926, 1961, 1996, 1998, and 2000. These seasons had 6, 7, 6, 3, and 3 intense hurricanes, respectively. The CSU team's August numbers are an increase over what they predicted in their June 3 forecast, which called for 15 named storms, 8 hurricanes, and 4 intense hurricanes. The CSU team notes that is is pretty unlikely that we will see an El Niño event this hurricane season, as we are just two week from the beginning of the peak part of the season, and there is no indication that El Niño conditions are developing. El Niño events substantially reduce hurricane activity, due to the increased levels of wind shear they bring to the tropical Atlantic.

The updated August 7 hurricane season forecast from NOAA
NOAA released their updated Atlantic hurricane season forecast Thursday, and they also increased their numbers. NOAA is calling for an 85% probability of an above-normal hurricane season, a 10% chance of a near-normal season, and a 5% chance of a below-normal season. Their May forecast called for only a 65% chance of an above-normal hurricane season. The August forecast calls for a total of 14-18 named storms, 7-10 hurricanes, 3-6 major hurricanes, and an ACE index of 135-220 this season. Since we've already had 5 named storms, 2 hurricanes, and 1 intense hurricane, this means we can expect an additional 9-13 named storms, 5-8 hurricanes, and 2-5 major hurricanes between August 7th and November 30th.

The updated August 5 hurricane season forecast from Tropical Storm Risk, Inc.
Tropical Storm Risk, Inc. (TSR) released their August 5 update this week, and are calling for a 95% chance that 2008 will be in the top 1/3 of years historically. The latest TSR forecast calls for 18.2 named storms, 9.7 hurricanes, 4.5 intense hurricanes, and an ACE index of 191. This is an increase from their June numbers of 14.4 named storms, 7.7 hurricane, 3.4 intense hurricanes, and an ACE index of 131. TSR says that the expected very light trade winds over the tropical Atlantic for the remainder of hurricane season are the main reason for their boosted forecast. Light trade winds allow the ocean to heat up more, due to decreased evaporative cooling, and reduced mixing up of cold water from the deep ocean. Light trade winds also contribute to increased spin in the atmosphere at low levels, allow incipent tropical storms to spin up more readily.

Exceptional July activity portends an active main part of hurricane season
As seen in the plot of typical hurricane season activity (Figure 1), only about 10% of a typical season's activity occurs by August 8. However, this year has been unusually active. We've already had nearly 50% of the typical activity of an entire season. An average season has 10 named storms, 6 hurricanes, 2 intense hurricanes, and an ACE index of 96. We've had 5 named storms, 2 hurricanes, 1 intense hurricane, and an ACE index of 39 so far in 2008. This year's activity (prior to August 1) ranked 4th all-time (since 1851) for the amount of Accumulated Cyclone Energy (ACE). Only 2005, 1916, and 1933 had more. All three of these years had at least five major hurricanes and ACE values of at least 175.

A big part of the reason that CSU, NOAA, and TSR have bumped up their August forecast numbers is because of the extremely high levels of early-season tropical storm activity. High levels of activity in the deep tropics in June or July are a strong indication that the rest of hurricane season will be above average in activity, according to Goldenberg (2000). Goldenberg found that if one looks only at the June-July Atlantic tropical storms and hurricanes occurring south of 22°N and east of 77°W (the eastern portion of the Main Development Region (MDR) for Atlantic hurricanes), there is a strong association with activity for the remainder of the year. According to the data from 1944-1999, total overall Atlantic activity for years that had a tropical storm or hurricane form in this region during June and July were at least average, and often above average. Hurricane Bertha formed in this region in 2008, so according to Goldenberg's research, we are due for an above average hurricane season.


Figure 1. Average Atlantic hurricane and tropical storm activity through the year. Typically, only about 10% of a season's activity occurs by August 8.

Summary: expect a very active year
While I doubt we'll see anything like 2005's record hurricane season this year, I do think that the very active July we had portends that 2008 will be one of the twenty most active hurricane seasons on record. I expect four major hurricanes, with at least one of these hitting the U.S. The fact that the tropics are very quiet at present is the proverbial "calm before the storm". As seen in Figure 1, hurricane season activity takes a big jump around August 18, and I expect the Atlantic will start getting very active around that date.

References
Goldenberg, S.B., 2000: "Intraseasonal predictability of Atlantic basin hurricane activity" Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, pp.59-60

My next update will be Monday, unless there's some significant development Sunday.
Jeff Masters

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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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