A heat advisory is in effect for Palm Beach County with temps that could feel as high as 109

A middling overnight cooldown and unlikely afternoon showers triggered another heat advisory for Palm Beach County on Wednesday, July 17 with temperatures that could feel as high as 109 degrees.

Sammy Hadi, a meteorologist with the National Weather Service in Miami, said the heat index or "feels like" temperatures are borderline for an advisory in Palm Beach County but that it was warranted considering the low chances of cooling rains.

"Really, primarily it's because we are in this persistent easterly flow, so we won't have shower and thunderstorm activity," Hadi said. "There is certainly a chance we will hit the criteria."

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. Both Broward and Miami-Dade counties petitioned the National Weather Service for the lower thresholds, which will be evaluated before being made permanent.

Wednesday is the ninth day this month that a heat advisory has been issued for Palm Beach County, meaning about half of July so far has been warm enough to meet advisory criteria.

Last week, heat index temperatures reached 114 degrees in West Palm Beach for a short period on Tuesday and stayed consistently above 105 for eight hours with only a short reprieve when a light shower moved over the National Weather Service gauge at Palm Beach International Airport.

While the heat index has been overwhelming, the daytime highs have been near normal for mid-July, hovering around 90 degrees.

Five heat records have been broken or tied this month, but all of them were for unusually warm overnight temperatures that didn't dip below 80 degrees. On July 9, the 81-degree morning low tied the record set in 1888.

The normal overnight low for this time of year is 76 degrees.

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The weather pattern pushing the triple-digit heat index temperatures is forecast to stick around the rest of the week as South Florida remains in the western end of the Bermuda High. That means light to moderate southeasterly winds will continue to push what showers do form more inland or over the Gulf Coast.

Another factor is an incoming belch of Saharan dust that will reach the area Friday into Saturday, further reducing rain chances by drying out the upper reaches of the atmosphere.

"Toward the end of the weekend and into early next week, the Saharan air layer will still have an impact on rain chances," Miami-based meteorologists noted in their Wednesday forecast.

Thursday and Friday are expected to reach daytime high temperatures near 90, with rain chances increasing to 70% Friday.

What is a heat advisory?

A heat advisory means people sensitive to warm temperatures should take precautions to avoid heat illness, including staying hydrated and seeking breaks if working or playing outside. Signs of heat exhaustion or heat stroke include symptoms such as muscle cramps, excessive sweating, a throbbing headache, passing out, confusion and seizures.

An excessive heat warning, the highest alert level on a four-tier scale, means everyone needs to take precautions to stay safe in the heat because "you may become seriously ill or even die," the NWS says.

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.