Colorado State Hurricane Forecasters Continue Prediction of Active Atlantic Season; Increased Possibility of U. S. Landfall

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William Gray and his forecast team at Colorado State University are continuing their earlier prediction of a slightly above-average hurricane season for 2001.

The forecast continues to call for 12 named storms, seven hurricanes and three major or intense hurricanes (those registering Category 3 or higher on the Saffir-Simpson scale).

Gray, professor of atmospheric sciences, and his colleagues foresaw an average- or slightly below-average season (June 1-Nov. 30, 2001) in their initial forecast issued in December 2000. These numbers increased slightly in the April 2001 update and reached their present level with the June 7 update.

By then the team was convinced that a significant El Niño event, which has an inhibiting effect on Atlantic Basin hurricanes, would not occur during this year’s hurricane season.

"We think this year will be somewhat in keeping with five of the last six seasons, 1997 being the exception, though it will be less active than any of the others, " Gray said. "We will see above-average activity, especially compared with the long-term downturn in activity experienced during the quarter-century period of 1970-94." The long-term (1950-90) average for seasonal activity includes 9.3 named storms, 5.8 hurricanes and 2.2 intense hurricanes.

"These numbers aren’t extreme, but they continue the trend in which hurricane activity appears to be on a multi-decadal upswing," Gray said.

The Colorado State team points to a number of factors as contributing to this year’s slightly above-average forecast. Chief among these are warmer-than-normal Atlantic sea surface temperatures, expected lower-than-average tropical Atlantic surface pressure, lack of a significant El Niño and, presently, evidence of an active multi-decadal era of increased tropical storm-hurricane activity.

To date (August 6), only two of the 12 predicted storms have appeared: Tropical Storm Allison, which brought heavy rains to Texas in June, and Tropical Storm Barry, which crossed the western Florida Panhandle early Monday.

"A slow or fast start to the season isn’t unusual," Gray said. The peak time for storm formation typically "ramps up" from about Aug. 20 to a peak around September 10th and then "ramps down" sharply to infrequent formation after Oct. 20, he said. Exceptions include Hurricane Mitch, which developed in late October 1999, and Hurricane Lenny, which appeared in late November of that same year.

"We’ve called for 12 storms and have seen two thus far, but in our experience that doesn’t suggest anything about the overall season," Gray said. "No correlation has ever been established between early season storms and what might occur later in the season.

"The primary factors for the increased values in the June and August forecast updates are more recent evidence that a significant El Niño event is unlikely this year and continuance of warm Atlantic surface temperatures."

A new aspect to the team’s research is the issuing of landfall predictions for a specific month. Team member Eric Blake devised an "August-only" prediction scheme that has shown considerable accuracy in "hindcasting" studies (i.e., matching observed past events given known data) over the last 50 years. This forecast uses data through July and has been shown to add improvement to full-season prediction. It successfully forecast last year’s August activity and for this month – August 2001 – calls for three named storms, one hurricane and one major hurricane.

Chances for U.S. landfall of at least one major storm are higher than average this year.

Landfall figures for this forecast estimate the probability of one or more Saffir-Simpson category 3-5 hurricanes making landfall somewhere on the U.S. coast during 2001 as 69 percent. Average for the past century is 52 percent. For the U.S. East Coast and Florida peninsula, the landfall probability is 50 percent (vs. 31 percent for the past century). The Gulf Coast, from the Florida Panhandle westward to Brownsville, has a major hurricane landfall probability of 39 percent compared with an annual average value of 30 percent.

Because of the difficulty of estimating landfall probabilities for small and scattered Caribbean islands, Gray offered no probabilities but said the chances of a major storm coming ashore in 2001 in this region are about average.

Despite the very active hurricane seasons since 1995, the United States has yet to feel the full effects of this increased activity. While 23 major (category 3-4-5) hurricanes have appeared in the Atlantic Basin (the North Atlantic, Caribbean Sea and Gulf of Mexico) between 1995-2000, only three made U.S. landfall and one of these, Bret in 1999, crossed nearly empty shoreline. By comparison, one in three Atlantic Basin major hurricanes normally makes landfall. By this ratio, seven or eight major hurricanes should have come ashore during the last six seasons. This difference was due to the atypical positioning of upper-level troughs along the East Coast during the height of most of the recent hurricane seasons, Gray said, causing most of the major hurricanes to be pushed away from the United States.

During the next few decades, the country is likely to face a great increase in hurricane threat from multi-decadal climate change. Two of the forecast’s co-authors, Gray and Christopher Landsea, co-authored an article that appeared in the July 21 issue of the journal Science. They and two other scientists examined this anticipated multi-decadal upsurge in storm activity, the result of stronger ocean currents redistributing heat energy across a wider span of the far North Atlantic.

Gray and his Colorado State team have long championed such a physical linkage between major Atlantic hurricanes and the Atlantic thermohaline circulation system. North-moving Atlantic Ocean currents bring warm, relatively salty water to the far North Atlantic, where larger quantities of this heavier, salt-laden water chills and sinks. One effect of the resulting redistribution of heat is more frequent storms with the potential to intensify to a major hurricane, perhaps hitting the eastern United States.

These changes have been in evidence since 1995, the Colorado State team has observed. After a quiet period from 1970-94, currents carrying warmer, saltier water northward reintensified. The result was that despite low storm activity in 1997 caused by the strongest El Niño ever recorded, the period from 1995 to the present has been the most active six consecutive years of hurricane activity on record. During this period, 79 named storms, 49 hurricanes and 23 major hurricanes have been observed in the Atlantic Basin.

The Colorado State hurricane forecast team consists of Gray, Landsea, Blake, Paul Mielke Jr. and Kenneth Berry. John Sheaffer and Philip Klotzbach assisted with important inputs to the team’s forecast, Gray said.

The forecasters use statistical analysis of climate signals plus "analog years" (years in which conditions strongly resemble those of the current year) to determine annual and individual monthly forecasts. In the case of 2001, the best "analog years" are judged to be 1951, 1952, 1960, 1996 and 2000. Most of these years had average or above-average activity.

"Should any of these storms, especially major ones, make landfall along populated areas of the recently greatly built-up southeast U.S. coast or peninsular Florida, I’m afraid we’ll see unprecedented damage, far exceeding that caused by other natural disasters," Gray said. "I suspect that most coastal residents do not realize the full magnitude of the hurricane threat to our country now and in coming years."


7 Dec 2000 7 Apr 2001 7 Jun 2001 7 Aug 2001
(NS) (9.3) 9 10 12 12
(NSD) (46.9) 45 50 60 60
(H) (5.8) 5 6 7 7
(HD)(23.7) 20 25 30 30
(IH) (2.2) 2 2 3 3
(IHD)(4.7) 4 4 5 5
(HDP) (70.6) 65 65 75 75
(MPD) (61.7) 60 60 70 70
(NTC)(100%) 90 100 120 120

* Number in ( ) represents average year totals based on 1950-1990 data.

** Hurricane Destruction Potential measures a hurricane’s potential for wind- and ocean-surge damage. Tropical Storm, Hurricane and Intense Hurricane Days are four, six-hour periods where storms attain wind speeds appropriate to their category on the Saffir-Simpson scale.