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Colorado State Hurricane Forecast Team Maintains Prediction For Active 1999 Season; Landfall Probabilities Above Average

Date:
April 9, 1999
Source:
Colorado State University
Summary:
Colorado State University's hurricane forecaster William Gray is maintaining his prediction for a 1999 hurricane season similar to last year's in the Atlantic Basin. Today's update calls for 14 tropical storms, nine hurricanes and four intense hurricanes, the same figures issued in Gray's initial 1999 forecast released last December.

FORT COLLINS--Colorado State University's hurricane forecaster WilliamGray is maintaining his prediction for a 1999 hurricane season similar to lastyear's in the Atlantic Basin.

Today's update calls for 14 tropical storms, nine hurricanes and fourintense hurricanes, the same figures issued in Gray's initial 1999 forecastreleased last December.

Meanwhile, he anticipates the probability of the number of major stormlandfalls at about 200 percent of the long-term average for the East Coast andFlorida Peninsula, 146 percent of long-term average for the Gulf Coast andslightly less than twice the long-term average for the Caribbean.

A major storm falls into category 3-5 of the Saffir-Simpson scale andhas winds of at least 111 mph. The long-term average is based on landfalls byintense hurricanes along the Gulf Coast and Eastern Seaboard from data from thelast century.

(A more detailed description of these landfall probabilities for theU.S. Atlantic and Gulf coasts for 1999, based on a new model, is planned for thelate spring.)

"We expect the current La Niña below-average water conditions in theeastern equatorial Pacific to continue through the 1999 hurricane season," Graysaid. "This should be an enhancing influence on this season's hurricaneactivity."

Gray repeated the warning issued in his prediction team's initialreport: a key factor in the forecast is the call for four major hurricanes withminimum winds of 111 mph. Historically, when such storms have made landfall, hesaid, they cause a vast majority of hurricane-spawned destruction as determinedon a statistical basis.

The April update reiterated its prediction for more low-latitudestorms, between 10 and 23 degrees north, that will affect the Caribbean. Inactive seasons such as the one anticipated, there is a higher occurrence oflow-latitude major hurricanes.

The predicted 14 tropical storms, nine hurricanes and four intensehurricanes compare to 14, 10 and three that occurred in 1998. Long-termstatistical averages yield 9.3 tropical storms, 5.8 hurricanes and 2.2 intensehurricanes annually.

In addition to La Niña, other climatic factors expected to affect theJune 1-Nov. 30, 1999, official hurricane season include westerly stratosphericwinds, expected above-average Atlantic sea surface temperatures and anticipatedbelow-average Caribbean Basin sea-level pressure in August and September.

"All these factors are, in addition to La Niña, expected to enhance 1999hurricane activity," Gray said. "There are no strong suppressing influences for1999 hurricane activities that we can see at this time."

The period from 1995-98 was the most active, four consecutive years ofhurricane activity on record, yielding 53 named storms, 33 hurricanes and 15major hurricanes. This and certain other climate signals suggest to Gray and hisassociates that a period of more major hurricane activity and more intense-stormlandfalls along the East Coast and in the Caribbean Basin is now underway.

The periods 1900-25 and 1970-94 were relatively quiescent in terms ofmajor hurricane activity, Gray said, while seasons from the early 1930s throughthe late 1960s generally were more active, with more intense storms lashing theAtlantic coast. He attributes this to a phenomenon called the Atlantic Oceanthermohaline circulation system, or Atlantic conveyor belt, which moves watersnorth from the vicinity of the Caribbean to an area east of Greenland. There,the current sinks to deep levels, moves south and flows into the South AtlanticOcean and beyond.

Warm water and high salinity in the conveyor belt strengthen it,producing more active hurricane seasons and more major landfalling storms alongthe eastern seaboard, Gray said.

"This ocean circulation, a northbound current that sinks and then movessouthbound, tends to go through decades-long changes," Gray said. "Ourinterpretation of climate data suggests that the Atlantic conveyor belt becamestronger between 1994 and 1995, and this has led to more major storms since thattime."

The pattern is reminiscent of that of the 1930s, 1940s and 1950s, hesaid.

The actual number of "weaker" storms - tropical storms and those inSaffir-Simpson categories 1 and 2, with winds up to 110 mph - aren't likely toincrease dramatically, Gray said, but because of the conveyor belt phenomenon,he expects the number of intense storms to rise.

Gray and co-authors Chris Landsea, Paul Mielke, Kenneth Berry and otherproject colleagues use a variety of climatic factors in their forecasts. Thosepositive indications dominating the 1999 season appear to include:

  • Westerly stratospheric winds, called the Quasi-Biennial Oscillation,that exist high in the atmosphere over the earth's equatorial regions andreverse themselves approximately every two years. When these stratospheric windsblow from the west, as they are projected to do for 1999, an enhancing effect onhurricane activity, especially major hurricanes, occurs.
  • The continuation of cold-water conditions known as La Niña in thewestern equatorial Pacific, a situation likely to promote an active hurricaneseason in 1999.
  • High North Atlantic sea surface temperatures and increased salinitythat suggest the changes observed beginning in 1995 signal a continuance of astrong Atlantic conveyor belt circulation, bringing with it the chance for moreintense hurricanes along the Atlantic coast. There is a chance this enhancedperiod could continue for two or more decades, Gray believes.
Other factors and their expected effects are a ridge of barometric highpressure called the Azores High, which in March was below average and anenhancing influence on hurricane activity, and an April 1 prediction ofbelow-average Caribbean Basin sea-level pressure anomaly for August andSeptember 1999, another enhancing seasonal hurricane factor.

###

GRAY RESEARCH TEAM HURRICANE FORECAST FOR 1999 SEASON
  12/98 April
Named Storms (9.3)* 14 14
Named Storm Days (46.9) 65 65
Hurricanes (5.8) 9 9
Hurricane Days (23.7) 40 40
Intense Hurricanes (2.2) 4 4
Intense Hurricane Days (4.7) 10 10
Hurricane Destruction Potential (70.6) 130 130
Maximum Potential Destruction (61.7) 130 130
Net Tropical Cyclone Activity (100%) 160 160

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

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

Note to editors: The completehurricane forecast and related research and press releases are available on theWorld Wide Web at:
http://tropical.atmos.colostate.edu/forecasts/index.html
Taped comments from Gray will be available after 8 a.m. MDT at 970-491-1525.


Story Source:

The above story is based on materials provided by Colorado State University. Note: Materials may be edited for content and length.


Cite This Page:

Colorado State University. "Colorado State Hurricane Forecast Team Maintains Prediction For Active 1999 Season; Landfall Probabilities Above Average." ScienceDaily. ScienceDaily, 9 April 1999. <www.sciencedaily.com/releases/1999/04/990409072547.htm>.
Colorado State University. (1999, April 9). Colorado State Hurricane Forecast Team Maintains Prediction For Active 1999 Season; Landfall Probabilities Above Average. ScienceDaily. Retrieved August 1, 2014 from www.sciencedaily.com/releases/1999/04/990409072547.htm
Colorado State University. "Colorado State Hurricane Forecast Team Maintains Prediction For Active 1999 Season; Landfall Probabilities Above Average." ScienceDaily. www.sciencedaily.com/releases/1999/04/990409072547.htm (accessed August 1, 2014).

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