Buckle your seat belts. Traffic is piling up in hurricane alley.
This week, the Atlantic got busy with Tropical Storm Harvey and two waves churning westward as Hurricane Gert, which formed last week, disappeared into the North Atlantic. Warnings and watches were up across the Windward Islands as parts braced for Harvey, the ninth tropical cyclone of the season.
While none of the current systems appears likely to intensify into major hurricanes, forecasters on Friday gave one wave that could approach South Florida early next week a strong chance of becoming a tropical depression or storm. And overall, the burst of activity signals the beginning of the season’s busiest time of year. And that’s on top of an already brisk pace.
“We’re on our seventh named storm and that is certainly above average,” National Hurricane Center senior forecaster Jack Beven said.
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Having three disturbances crossing the Atlantic at once — with a fourth in the rafters — is a little unusual, he said. But not unprecedented. In August of 1995, two tropical depressions appeared on a single day, eventually becoming hurricanes Humberto and Iris. Another depression appeared the next day, with a fourth forming the following day. That year also marked the start of a multi-decade long active period meteorologists believe is linked to a warm phase in the Atlantic.
The end of summer typically produces more hurricanes because normal weather patterns slide into place, Beven explained.
The Bermuda High, a high pressure system that generally keeps storms in check as sea surface temperatures rise in July, begins to weaken, he said. At the same time, upper level winds that can smother storms from above weaken and tropical waves rolling off the Africa’s west coast become stronger.
“You’ll see them happen every year,” he said. “Some years the wind shear never becomes favorable. If we have an El Niño the wind patterns in the tropical Atlantic do not [become] as favorable. And sometimes they can be quite hostile all through the season.”
Historically, the peak of the season hits Sept. 10, then begins tapering off. Hurricane Andrew, which marks its 25th anniversary this year, formed as a tropical wave on Aug. 16 and slammed South Florida on Aug. 24 as a fierce Cat 5 storm.
Forecasters named Harvey on Thursday evening after a hurricane hunter plane found sustained winds topping 40 mph. Earlier in the day, they designated it a potential tropical depression and began issuing their familiar cone of uncertainty map because it threatened to blow tropical storm force winds across the islands within 48 hours. It’s expected to continue to intensify as it crosses warm waters in the Caribbean Sea on Friday headed toward Central America. Tropical storm force winds extend outward about 35 miles.
A second wave, which potentially poses more of a risk to Florida, continued to become better formed in the central Atlantic. The storm is headed northwest at about 20 mph and while it’s too soon to tell where it will go, forecasters say it will likely gain strength. A third wave just west of the Cabo Verde Islands off Africa could also turn into a cyclone over the next five days.
Another tropical wave, which shows no signs of turning into a cyclone, is also moving over Puerto Rico and the Dominican Republic and will likely dump heavy rain on Florida over the weekend, said Miami National Weather Service meteorologist Larry Kelly. Forecasters called for about two inches of rain, but Kelly said amounts could be higher in places, leading to minor flooding.
So far this year, models have struggled to get the forecasts correct, said Colorado State meteorologist Phil Klotzbach, who issues a widely read preseason forecast based in part on historical trends. On Thursday, models split on whether Harvey would steadily strengthen, or fizzle over the central Caribbean.
“The models have the physical atmosphere as best we know,” he said. “But you have to make approximations and you don’t have observations everywhere and we don’t completely understand what makes tropical cyclones tick. There’s just challenges with them.”
When the season picks up, Klotzbach, begins issuing two-week mini forecasts to size up what’s ahead. In his last forecast posted Aug. 4, Klotzbach said the odds of Florida getting hit by a hurricane after July 31 are 61 percent. The chances of a Cat 3 hurricane or stronger are 27 percent. He also found that in past years with similar conditions — 1953, 1969, 1979, 2001, and 2004 — the number of named storms averaged a total of just over 14 named storms and more than eight hurricanes. So far this season, the Atlantic has produced seven named storms and two hurricanes.
Water in the tropical Atlantic has been warm, providing plenty of fuel for hurricanes, he said. But dry Saharan air has been higher than expected, keeping them from last very long or gaining much intensity. Storms typically needs lots of moisture to allow more instability in the atmosphere and more thunderstorms.
“The shear has been low but the moisture hasn’t been high enough,” he said. “Typically they go together, so we shall see.”
Klotzbach’s next forecast posted early Friday.