Break in humidity as Hurricane Ernesto helps bring unusually dry air to South Florida this weekend

Hurricane Ernesto's sojourn in the open Atlantic is expected to help usher in a reprieve this weekend to the maddening soup of humidity clinging to South Florida.

Beginning late Friday, Aug. 16, northeast winds from Ernesto's counterclockwise churn in conjunction with a front passing through the state are forecast to pull down a whisper of drier air from higher latitudes and reduce moisture over the Peninsula.

Daytime highs won't budge much. They are forecast to hover around 90 degrees through Sunday, which is near normal for this time of year.

But it's the dew point temperature — a measure of air saturation — that will dive into the low 70s, which is more akin to early October than late summer.

“This is not a normal August event,” said Jennifer Simmons, a meteorologist with the National Weather Service in Miami. “It's what we tend to see when we head into fall and winter. We’re going to get a nice break from the summer stickiness.”

Ahead of the humidity break, a front will push through the state and linger in the Florida Straits, meaning higher rain chances on Friday with temperatures that will feel as high as 110 in some areas of South Florida.

That changes Saturday, Aug. 17, as drier air follows the front and is enhanced by Ernesto.

Heat index, or “feels like”, temperatures may not reach 100 on Saturday and Sunday. That means a likely respite from the unusually prolific 28 days of heat advisories that have been issued this summer for Palm Beach County, including Thursday when the heat index, or “feels like” temperatures were forecast to reach 109 in West Palm Beach.

In 2023, there were just 16 heat advisories for the year.

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“With dew points at 72 this weekend compared to 77 or 78, it won’t feel anywhere near as hot as it has,” said Robert Molleda, meteorologist in charge at the NWS in Miami. “It might not be a lot less cool, but it will be less humid.”

In Palm Beach County, a heat advisory is issued when heat-index temperatures are forecast to reach 108 degrees or higher for at least two hours. In Broward and Miami-Dade counties, a heat advisory is issued if heat-index temperatures of 105 or higher are expected for at least two hours.

A summer of southeasterly winds hauling humid air out of the tropics is partly to blame for the high number of heat advisories, including a rare May advisory that was the first in at least two decades.

Bob Oravec, a lead forecaster with the NWS’ Weather Prediction Center in College Park, Md.., said people tend to use relative humidity when talking about sticky air, but that dew points are a more absolute measurement.

The dew point is the temperature the air needs to be cooled to in order to achieve 100% saturation. So, dew point is a measurement of water vapor in the air.

Relative humidity is a percentage describing the amount of saturation in the air relative to the temperature, and it can be misleading.

For example, if the temperature is 30 degrees and the dew point is 30 degrees, the relatively humidity would be 100%. That's not weather that most people would consider muggy.

“It’s been a hot summer,” Oravec said. “Starting Friday, drier air is coming into Florida. Dew points will drop to the low 70s and upper 60s in some places.”

NOAA considers dew points of 65 to 70, “muggy and somewhat uncomfortable.”

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It's judged uncomfortable when dew points are 70 to 75 degrees, and anything above 75 is “oppressive”, according to NOAA.

Eric Blake, a senior hurricane specialist at the National Hurricane Center, said on social media this week that one “silver lining” with Ernesto is it will drag down historically dry air for mid-August into South Florida.

As of Thursday morning, Ernesto was a Category 1 hurricane with 85 mph winds heading north at 13 mph toward Bermuda. The storm is notable for its large size, including tropical storm-force winds that extend 230 miles from its center. Hurricane-force winds are forecast to extend from Thursday’s 35 miles from center to 115 miles by Friday.

“This large wind field will generate some big waves, causing dangerous swimming conditions at beaches late this week and early next week along most of the Atlantic shore of the U.S. and Canada,” wrote meteorologists Jeff Masters and Bob Henson in their column for Yale Climate Connections.

The Melbourne office of the NWS is predicting building long-period swells along the Treasure Coast on Friday with rough surf, hazardous boating conditions and rip currents.

Simmons said Palm Beach County will have a low chance of rip currents through the weekend.

Ernesto is the third hurricane of the season, and is early based on historical data that shows the average third hurricane doesn’t form until Sept. 7.

Colorado State University senior researcher and hurricane expert Phil Klotzbach said just four other years since the satellite era began in 1966 have had three or more hurricanes by Aug. 14. Those years were 1966, 1968, 1995 and the hyperactive 2005 season.

Ernesto is forecast to grow to a major Category 3 storm with 115 mph sustained winds on Friday.  It will approach Bermuda early Saturday with slightly lower 110 mph wind speeds, which would make it a Category 2 storm.

The lower dew points won’t last long for South Florida. Temperatures and humidity begin to rise on Monday with rain chances growing to 50%.

Kimberly Miller is a journalist for The Palm Beach Post, part of the USA Today Network of Florida. She covers real estate and how growth affects South Florida's environment. Subscribe to The Dirt for a weekly real estate roundup. If you have news tips, please send them to kmiller@pbpost.com. Help support our local journalism, subscribe today.

This article originally appeared on Palm Beach Post: Break in humidity as Hurricane Ernesto helps bring unusually dry air to South Florida this weekend