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For hurricanes, September set record for ferocity

While tropical storm numbers were near average, last month was monstrous in Atlantic Basin based on an index of strength and duration.

Diary of a mad season: Tropical-storm tracks in Atlantic Basin
Diary of a mad season: Tropical-storm tracks in Atlantic BasinRead moreNational Hurricane Center

With both Irma and Maria reaching mega-Category 5 status, by one key measure September was the most-active for hurricanes on record, according to the National Hurricane Center.

Last month's Accumulated Cyclone Energy, which takes into account the strength and duration of tropical cyclones, well surpassed the old standard established in 2014.

Deadly and destructive Irma, which formed Aug. 30, gained Category 5 status in September with peak winds of 185 mph.

The second half of the month, of course, was dominated by Maria, which devastated Puerto Rico. Its top winds were measured at 175 mph.

Actually, in terms of raw named-storm numbers that formed in September, four, last month was near average.

However, three of those storms, which gain names when top winds reach 39 mph, became major hurricanes – Maria, Lee, and Jose.

Phil Klotzbach, the Colorado State University hurricane specialist, notes that as of Sept. 30, named storms had prowled the basin, which include the Caribbean and Gulf, for 38 consecutive days, the third-longest streak on record.

So far this season, with 14 named storms, this has been the third most-active season, behind 1933 and 2004, the hurricane center says.

Still to come is a measure much closer to home: Ultimately, this is going to rank among the costliest on record for U.S. taxpayers once the disaster-aid and flood-insurance payments are calculated.