Following a record number of Atlantic Ocean storms over the past two months, including five that struck the United States, government scientists on Thursday updated their forecast for the remainder of the hurricane season, saying it was likely to be extremely active.
“It’s shaping up to be one of the most active seasons on record,” said Louis Uccellini, director of the National Weather Service.
Gerry Bell, the lead hurricane season forecaster with the climate prediction center of the National Oceanic and Atmospheric Administration, said there could be 19 to 25 named storms, those with sustained winds above 38 miles an hour, or 61 kilometers an hour, by the time the season ends on Nov. 30. Of these, seven to 11 could be hurricanes, with winds of 74 m.p.h. or higher, including three to six major ones.
“We’ve never forecast up to 25 named storms before,” Dr. Bell said. But he said that it was unlikely the season would be as active as 2005, when there were 28 named storms and the Weather Service had to resort to using the Greek alphabet for the last few.
And the forecast for major hurricanes, those with winds exceeding 110 m.p.h., was unchanged from the scientists’ preseason predictions, issued in May.
At that time, they said they expected an active season, with 12 to 19 named storms.
But the season, which officially began June 1, has already seen nine named storms, including the latest, Hurricane Isaias, which struck the Bahamas and the East Coast of the United States this week. That’s the most number of storms on record for the first two months.
Those months are usually relatively quiet; typically about 95 percent of storms occur between mid-August and the end of October, when ocean temperatures reach their peak and atmospheric conditions off the coast of Africa favor storm formation.
So far, five of the storms have struck the United States — three tropical storms and Hurricane Hanna as well as Isaias.
Dr. Bell said that, while it is more likely that storms would make landfall during an extremely busy season, forecasting the number that will do so is not possible, because landfall is affected by shorter-term weather conditions.
Dr. Bell also said that it was too early to tell whether climate change was contributing to the activity this season. Hurricane activity in the Atlantic is greatly affected by two elements of the planet’s climate system — natural variations, over decades, in sea surface temperatures in the North Atlantic, and shorter-term temperature variations in the equatorial Pacific Ocean.
The North Atlantic variability has led to increased overall hurricane activity since 1995. This year conditions in the equatorial Pacific — cooling sea-surface temperatures as the climate pattern known as La Niña starts to emerge — may be helping to increase activity as well by affecting wind patterns in the tropical Atlantic and Caribbean.
But Dr. Bell said that whatever the contribution of climate change to this season’s activity, global warming affects the impacts of storms. Rising sea levels increase the danger of storm surges, he said, and warmer air temperatures generally make storms bring more rainfall.
Both Hanna, which hit Northern Mexico and South Texas on July 25, and Isaias were Category 1 storms, with winds that did not exceed 95 m.p.h.
Isaias caused flooding and widespread power outages in the Southeast, Mid-Atlantic and Northeast. It also spawned tornadoes, including one in North Carolina that killed two people.
Hanna, which dumped more than a foot of rain in some areas, caused power outages and flooding as well.
But Hanna had another impact, as it coincided with an annual research cruise in the Gulf of Mexico to measure the so-called dead zone, an area of oxygen-poor water caused by agricultural runoff of fertilizers and other nutrients from farming in the Mississippi River basin. The low oxygen kills some marine organisms and forces others to move elsewhere in the gulf, which can harm the area’s fisheries and shrimp industry.
Based on runoff data and computer modeling, researchers had forecast that the zone would be large, about 6,700 square miles.
But this week scientists reported that winds and waves from the hurricane had stirred the waters, mixing the oxygen-rich and oxygen-poor areas together. Data from the research cruise, during which scientists take water samples at various depths, showed that the zone was only about 2,100 square miles. But researchers said that the zone could have begun to widen again after Hanna passed and the seas calmed.