Heavy rainfall moving into South Florida ahead of Potential Tropical Cyclone One, bringing significant flood threat Friday, Saturday


Though Hurricane Hunters investigating now-designated Potential Tropical Cyclone One Friday morning continue to find multiple swirls near the area of low pressure centered north of Mexico’s Yucatan Peninsula, the plane was able to “fix” the southernmost of these as a weak center.

This may be transient, however, as the blowup of showers and thunderstorms continues to the east of still-competing centers, indicating the continued presence of strong southwesterly wind shear, a persistent nemesis to the fledgling system over the past several days.

Though the development window is quickly closing, it would only take a slight tick up in organization of the center to designate the system Tropical Storm Alex as forecast by the National Hurricane Center.

At this stage, the designation of a tropical storm has no bearing on the anticipated impacts to South Florida.

The first round of showers and storms associated with Potential Tropical Cyclone One have already begun to move through South Florida and the Keys Friday morning.

Expect waves of weather Friday, culminating in the heaviest bands and squalls pushing through later Friday night into Saturday morning as the low pressure nears the southwestern Florida coast.

In addition to the primary heavy rainfall and flood threat – up to 10 to 15 inches locally – we’ll see a threat for isolated tornadoes Friday evening and early Saturday, primarily in far south Miami-Dade and Monroe counties.

Historically, tornadoes associated with tropical cyclones are not terribly strong (about 90 percent historically are rated EF0 or EF1), but the fast movement of these storms occurring during darkness and overnight will heighten the threat.

Some minor coastal flooding is possible along the southwest Florida coast later Friday evening and early Saturday as the storm system moves ashore.

As discussed in Thursday’s newsletter, though we often associate tropical systems with their strong winds, the weaker-winded storms can often bring impactful weather, usually in the form of heavy rainfall and flooding.

David Roth, senior forecaster at the government’s Weather Prediction Center, and his colleagues at the National Weather Service have curated an extensive tropical cyclone rainfall dataset for the continental U.S. going back to the turn of the 20th century.


Looking back at the thousands of observations for past storms across the state of Florida shows just how impactful the less blustery storms can be.

Back in June of 1992, a tropical depression, the first of the season, brought a deluge of flooding rains to the Sarasota area that lasted for several days.

More recently, Tropical Storm Fay in 2008 and Debby in June of 2018 each dropped over 27 inches of rain, causing widespread flooding and damage to numerous homes.

These of course are the extremes, but there’s a lesson to be learned from the recent past about not discounting low-grade storms early in the season.

Even a few inches of heavy rain in a short enough time can cause problems.

For us here in South Florida, we should return to a more typical summertime pattern of afternoon showers and thunderstorms as the weather pulls quickly away from the state on Sunday.

About the Author:

Michael Lowry is Local 10's Hurricane Specialist and Storm Surge Expert.