Hurricane

Blast of dry air may save South Florida from tropical cyclone. But it won’t stop the rain

By Jenny Staletovich

jstaletovich@miamiherald.com

August 21, 2017 01:34 PM

National Hurricane Center
National Hurricane Center

An unseasonable blast of dry Saharan air will help South Florida dodge a tropical cyclone this week, but won’t keep it from getting hammered by heavy rain, forecasters say.

A tropical wave, which last week threatened to intensify into a depression or worse, has so far been kept in check by the dry air, which typically doesn’t blow so far west so late in the summer, said University of Miami hurricane researcher Brian McNoldy. While there’s a chance the system could intensify as it rolls across warm waters today, it’s unlikely to have enough time to build much steam, he said.

“This is certainly one to advertise as we really dodged a bullet,” he said. “It would be a whole different story for us if that air wasn’t there.”

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A National Oceanic and Atmospheric Administration map shows the westward flow of dry Saharah air that has so far helped keep tropical waves in check.
Courtesy University of Miami Rosenstiel School of Marine and Atmospheric Sciences hurricane researcher Brian McNoldy

In their Monday afternoon advisory, National Hurricane Center forecasters gave the wave a 20 percent chance of becoming a tropical depression or storm over the next two days as it stalls over Florida. They upped the odds to 40 percent over five days because the system could get a boost when it collides with a front rolling in from the north as it moves up the coast, McNoldy said.

“That’s why you see that orange blob,” he said of the track map. “Over the next two days or so, it heads west, northwest and then stalls and then it heads to the northeast, so that’s how you get that bean-shaped thing.”

As storm bands start rolling ashore Tuesday, South Florida can expect to see widespread rain, meteorologists in the Miami office of the National Weather Service said. On average, one to two inches of rain will fall, but three inches or more are possible in areas, said meteorologist Andrew Hagen.

“Not everybody is going to see those types of rainfall totals. I think a lot of people will see one to two inches and a few people could see three inches on any given day,” he said.

Once the wave passes, rainy conditions are expected to last through Friday as southerly winds continue sucking moisture into the region, he said.

Flood warnings are possible, depending on amounts. Meteorologists put the odds of more than three inches at 60 percent for the greater metropolitan area.

In advance of the storm, the South Florida Water Management District began lowering canals across South Florida to make room for more stormwater. With so much rain in recent weeks, storage in the system is nearly topped out. Water conservation areas west of coastal urban areas that hold water have been full for much of the month, and on Monday, Lake Okeechobee stood at 13.39 feet deep, nearing the 15.5 foot level considered safe for the lake’s aging dike.

This time of year, tropical waves roll off the west coast of Africa with alarming frequency — one every four to six days. Hurricane season typically begins to pick up from mid August to mid September, when conditions become more favorable. This week, three systems crossed the Atlantic, including a second wave located about 800 miles east of the Leeward Islands and Tropical Storm Harvey, which broke up Saturday. The wave is not expected to become a cyclone, but forecasters warn there’s an 80 percent chance that Harvey’s leftovers reform after it enters the Bay of Campeche later in the week.

This year, dry air has continued blowing west later than usual, helping keep storms weak. What’s driving it is still not entirely clear, McNoldy said. A season high pressure system, called the Bermuda High, appears to be a little stronger than usual, which could be a factor, he said.

“Maybe that’s sucking up more winds that’s allowing air from the far east Atlantic to make it across and mix in. But it’s not totally clear right now why it’s doing that,” he said. “It’s one of those things that in a week or week and a half, it’s not there. We don’t expect it to [continue] through the rest of the season. But it is amazingly lucky for us now.”

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