CSU team increases forecast, now calls for near-average 2017 Atlantic hurricane season


Colorado State University hurricane researchers have increased their forecast from their early April prediction and now call for a near-average Atlantic hurricane season. The primary reasons for this increase are a reduced likelihood of a moderate El Niño and recent anomalous warming in the tropical Atlantic.

The weak La Niña that occurred this past winter has dissipated, and there is the potential that a weak El Niño could develop by the peak of the Atlantic hurricane season. The odds of a stronger El Niño event, however, appear to have diminished since the early April outlook.

El Niño tends to increase upper-level westerly winds across the Caribbean into the tropical Atlantic, tearing apart hurricanes as they try to form. The tropical Atlantic has anomalously warmed over the past two months and is now warmer than normal.  In addition to providing more fuel for tropical cyclone formation and intensification, warmer tropical Atlantic sea surface temperatures are associated with a less stable atmosphere as well as moister air, both of which enhance organized thunderstorm activity necessary for hurricane development. The far North Atlantic, however, remains colder than normal, potentially indicate of a negative phase of the Atlantic Multi-decadal Oscillation.

Six hurricanes

The CSU Tropical Meteorology Project team is predicting 13 additional named storms during the Atlantic hurricane season, which runs from June 1 to November 30. Of those, researchers expect six to become hurricanes and two to reach major hurricane strength (Saffir/Simpson category 3-4-5) with sustained winds of 111 miles per hour or greater. Tropical Storm Arlene formed in April, so the forecast is for 14 total named storms in 2017.

The team bases its forecasts on over 60 years of historical data that include Atlantic sea surface temperatures, sea level pressures, vertical wind shear levels (the change in wind direction and speed with height in the atmosphere), El Niño (warming of waters in the central and eastern tropical Pacific), and other factors.

So far, the 2017 hurricane season is exhibiting characteristics similar to 1957, 1969, 1979, and 2006. “1957, 1979 and 2006 had slightly below-average hurricane activity, while 1969 was a very active season,” said Phil Klotzbach, research scientist in the Department of Atmospheric Science and lead author of the report.

The team predicts that 2017 hurricane activity will be about 105 percent of the average season. By comparison, 2016’s hurricane activity was about 135 percent of the average season.

The CSU team will issue forecast updates on July 3 and August 4.

This is the 34th year that the CSU hurricane research team has issued the Atlantic basin seasonal hurricane forecast. Recently, the Tropical Meteorology Project team has expanded to include Michael Bell, Associate Professor in the Department of Atmospheric Science. William Gray launched the report in 1984 and continued to be an author on them until his death last year.

The CSU forecast is intended to provide a best estimate of activity to be experienced during the upcoming season – not an exact measure.

Michael Bell cautioned coastal residents to take proper precautions.

“It takes only one storm near you to make this an active season,” Bell said.

The report also includes the probability of major hurricanes making landfall:

  • 55 percent for the entire U.S. coastline (average for the last century is 52 percent)
  • 33 percent for the U.S. East Coast including the Florida peninsula (average for the last century is 31 percent)
  • 32 percent for the Gulf Coast from the Florida panhandle westward to Brownsville (average for the last century is 30 percent)
  • 44 percent for the Caribbean (average for the last century is 42 percent)

Landfall probability

The forecast team also tracks the likelihood of tropical storm-force, hurricane-force and major hurricane-force winds occurring at specific locations along the coastal United States, the Caribbean and Central America through its Landfall Probability website.

The site provides information for all coastal states as well as 11 regions and 205 individual counties along the U.S. coastline from Brownsville, Texas, to Eastport, Maine.  Landfall probabilities for regions and counties are adjusted based on the current climate and its projected effects on the upcoming hurricane season.

Klotzbach and Bell update the site regularly with assistance from the GeoGraphics Laboratory at Bridgewater State University in Massachusetts.

Funding for this year’s report has been provided by Interstate Restoration, Ironshore Insurance and a grant from the G. Unger Vetlesen Foundation.

Released June 1, 2017
Tropical Cyclone Parameters Extended Range
(1981-2010 Climatological Median Forecast for 2017
in parentheses)
Named Storms (12)* 14**
Named Storm Days (60.1) 60
Hurricanes (6.5) 6
Hurricane Days (21.3) 25
Major Hurricanes (2.0) 2
Major Hurricane Days (3.9) 5
Accumulated Cyclone Energy (92) 100
Net Tropical Cyclone Activity (103%) 110
* Numbers in ( ) represent medians based on 1981-2010 data.

** Numbers include Tropical Storm Arlene which formed in April.