A drift of Saharan air will dry South Florida out over the next few days with rain chances as low as 10 percent mid week.
The National Weather Service in Miami is forecasting near record low levels of precipitable water, which is the measure of rain that would accumulate at the surface if all the water vapor in the air fell as rain.
Meteorologist Larry Kelly said normal precipitable water values are 1.8 to 1.9 this time of year, but they will fall to 1 to 1.3 through Friday.
“It will definitely be less moist and our dew points will come down to the low to mid-70s,” Kelly said. “The main thing with the drier air is the rain chances drop quite a bit.”
In West Palm Beach on Tuesday there is less than a 20 percent chance of rain. That drops to 10 percent Thursday. On Friday, rain chances are also 10 percent.
But even that is far below what’s normal for this time of year, Kelly said.
Typical daily rain chances are between 40 and 50 percent mid August, he said.
“The satellites are showing a large dusty air mass moving up through the Caribbean,” said Joseph Prospero, professor emeritus at the University of Miami’s Rosenstiel School of Marine and Atmospheric Sciences. “We’ve had some very intense outbreaks and we can still get spotty rain.”
Prospero and Jason Dunion, a meteorologist with the University of Miami who tracks Saharan dust, said this summer has been dustier than normal.
Neither could say for sure what would cause more Saharan air this year.
Dunion said they will be looking at,the strength and position of the subtropical ridge over the Atlantic and recent drought trends over Africa.
While a mat of moist air flows stubbornly high in the atmosphere, NWS meteorologist Andrew Hagen said the Saharan dust is lower down, existing between 5,000 and 15,000 feet, which will help decrease rain chances Friday and Saturday.
“On average per year we have about two pretty good SAL (Saharan air layer) events, but this looks like only a little bit of the SAL will make it over South Florida on Friday,” Hagen said. “When it’s strong, there’s a milkiness to the sky and sometimes you even see particles on cars.”
While this may not be a strong Saharan dust event, it will keep rain chances Friday and Saturday below 50 percent in the metro areas of Palm Beach County, Hagen said.
Since June 1, coastal areas of Palm Beach County have averaged 4.3 inches of rain, according to district gauges. That’s 1.1 inches above what’s normal for the second week in June and adds to the whopping 15.3 inches of rain the area got in May.
For the 16-county region managed by the district, May rainfall totaled 11.45 inches, about 7 inches above normal.
“This is the third year in a row Mother Nature has dealt us a bad hand and brought us an extreme beginning to the wet season,” said district Chief Engineer John Mitnik during a press conference last week on how the district is handling all the water. “May brought more than 300 percent of above normal rainfall.”
Jason Dunion, a meteorologist with the University of Miami who tracks Saharan dust, said weather balloons launched from Puerto Rico on Wednesday showed the air 1 to 3 miles into the atmosphere was up to 50 percent dryer than what would be expected because of the Saharan air layer, or SAL.
A Saharan air layer is made up of sand and mineral particles that are swept up from 3.5 million square miles of desert and carried by air currents 4,000 miles west across the Atlantic. The largest plumes can be the size of the continental U.S., and while June is the beginning of the Saharan dust season, the most potent plumes appear in July and August.
That’s when large thunderstorms start a march across Africa, south of the Saharan desert, and sweep dust high into the atmosphere before spinning it out over the Atlantic with easterly waves that can form hurricanes.
“That tropical wave season starts to ramp up in July and August and we think there is a link between it and when we see the Saharan air layer get bigger and much more far reaching,” said Dunion, who is also a research scientist with NOAA’s Hurricane Research Division.
There still could be soupy humid air at the surface, but clouds typically collapse once they hit the Saharan air layer.
“It tends to stifle thunderstorms, and could be a little warmer because it will clear out the clouds,” Dunion said.
This year, Dunion is tracking the Saharan dust with the supercharged GOES-East weather satellite that was launched in November 2016.
It carries the Advanced Baseline Imager — a 16-channel camera built by the Melbourne-based Harris Corp. The previous satellite had just five channels. Combining two infrared channels from GOES-East gives researchers a better image of the Saharan dust, and one that crosses the Atlantic from Africa into the Gulf of Mexico.
It also tends to dry out the upper atmosphere, meaning high temperatures and low chances of rain through the weekend.
At noon today, the heat index at Palm Beach International Airport was 103 degrees, while the real temperature was 89 degrees. Rain isn’t expected to make any kind of showing until Sunday when there is up to a 30 percent chance.
“The dust really suppresses thunderstorm development,” Fisher said. “Even though rain chances aren’t zero, they are really limited.”
The coverage of storms today will depend on how much Saharan air makes it into the area. With enough surface moisture present, meteorologists said there will be some locally heavy rainfall and gusty winds up to 40 mph.
“The threat for strong storms today will not be as high as recent days,” Miami forecasters wrote in a morning discussion. “The wild card will be how much dry air intrudes during the afternoon which may enhance dry air entrainment in updrafts for some stronger gusy winds in the 40 to 55 mph range.”
The biggest concern with today’s storms is lightning with the strongest rains expected in the interior and west coast of the state.
High temperatures in Palm Beach County are expected to reach 91, which is about normal for this time of year. The heat index, however, could hit 105 near Lake Okeechobee.
Sunday was the first day in 30 days where the high did not hit 90 or above. Sunday’s high reached only 87 degrees in West Palm Beach, which is 3 degrees below normal.
Tuesday is expected to be even drier as the Saharan dust makes it further into the Peninsula.
With plenty of sunshine Tuesday, temperatures are expected to be warmer, ranging in the low 90s on the coast to mid-90s inland.
Heat advisories are possible Tuesday with 105-plus heat index temps across the interior and Gulf coast.
The heavy Saharan dust is being blamed for the lack of tropical activity in the Atlantic basin this hurricane season. But, AccuWeather hurricane expert said there may be a slight chance of something spinning up off the coast of Africa late this week.
The National Hurricane Center said in its most recent forecast that tropical development is not expected during the next five days.
Kottlowski said the chance of a storm is a long shot.
“Any system that tries to get going over the western Atlantic late in the month and into early August will likely struggle with a vast amount of dry air and disruptive winds,” he said.