Forecast Pins Sept. 17 as Turning Point for Atlantic Tropical Activity

The U.S. Climate Prediction Center (CPC) and several private forecasting models on Wednesday flagged Sept. 17 as a likely pivot point for renewed tropical cyclone activity in the Atlantic, raising the prospect of storms forming in the central and western basin as hurricane season reaches its climatological peak.

The CPC's latest Global Tropics Hazards and Benefits Outlook assigns elevated probabilities for tropical cyclone formation beginning around that date, with model guidance indicating a 40 to 60 percent chance of development across the central Atlantic near the Cape Verde Islands and a 20 to 40 percent chance in the northwest Caribbean and the Gulf of America over the following two weeks. Forecasters also said the eastern Atlantic could see its odds rise if a particularly strong tropical wave emer-g- ing off Africa around Sept. 20 behaves as projected, potentially boosting formation chances there to 40 to 60 percent.

Meteorologists noted that the Atlantic basin has been unusually quiet as the season peaks. AccuWeather lead hurricane expert Alex DaSilva said Sept. 10 marked the first climatological peak in nearly a decade without a named storm in the basin and that "no tropical storms or hurricanes over the Atlantic basin on Sept. 10 has only happened three times over the last 30 years." He said surges of dry air earlier in the season helped produce a midseason lull that forecasters had anticipated.

DaSilva and other analysts cautioned that quiet conditions do not preclude a rapid uptick in activity once environmental ingredients — such as reduced dry air and favorable winds — align. "There is a low risk that a new tropical wave moving across the primary Atlantic development region could develop in the coming days," DaSilva said. He added that roughly 40 to 60 tropical waves typically move westward from the African coast during a season, and historically about one in five of those waves develops into a tropical storm or hurricane, with the probability rising around climatological peak periods.

Forecasters are tracking two tropical waves currently moving west off the coast of Africa. In addition, AccuWeather analysts pointed to a cold front located thousands of miles to the northwest and only a few dozen miles off the southern Atlantic coast of the United States that could help initiate tropical development this weekend or early next week.

The cautionary outlook follows a relatively sparse Atlantic season to date. The only major system to form so far was Hurricane Erin, which began as a tropical wave on Aug. 11, intensified to a Category 5 hurricane by Aug. 16 and maintained significant strength as it moved roughly parallel to the U.S. East Coast from Aug. 19 to 21. Erin turned eastward and lost tropical characteristics on Aug. 22, completing a transition to an extratropical system.

Outside the Atlantic, the National Oceanic and Atmospheric Administration (NOAA) this week reiterated its forecast for a below-normal eastern Pacific hurricane season, projecting 12 to 18 named storms, five to 10 hurricanes and up to five major hurricanes. The eastern Pacific season runs from May 15 through Nov. 30, two weeks longer than the Atlantic season.

Forecasters emphasized that probability guidance and model ensembles are tools to communicate changing risks but do not pinpoint exact storm tracks or impacts. The CPC outlook and private model ensembles aim to identify periods and regions of heightened tropical activity potential so emergency managers, mariners and coastal communities can monitor developments more closely as the season progresses.

With the seasonal peak now underway, operators of coastal infrastructure and residents in hurricane-prone areas are being advised to stay alert to updates from the National Hurricane Center and local emergency management agencies, and to review preparedness plans should tropical development materialize in the coming days and weeks.