A stalled weather front is expected to keep heavy rain over the Jacksonville area through Saturday, Aug. 29, raising the threat of isolated flooding in low-lying neighborhoods along the St. Johns River, including Mandarin.

The National Weather Service Jacksonville office posted an alert Wednesday, Aug. 26, warning of a slight risk of flash flooding. The agency said increasing moisture and slower-moving thunderstorms would bring heavy rain over short durations during the late afternoon and early evening, with flooding concerns concentrated along the I-95 corridor and on roadways with poor drainage.

News4Jax meteorologist Chris Holtzman said isolated flooding is possible, particularly in areas hit by multiple rounds of storms. He attributed the prolonged storm threat to a front stalled across Georgia that will keep rain coverage widespread through the end of the week, according to a forecast published Tuesday, Aug. 25. Individual storms could become strong to severe, with gusty winds and frequent lightning.

Scattered showers and storms are forecast Wednesday through Saturday, with highs in the low to mid-90s each day. Sunday, Aug. 30, may bring isolated storms before conditions improve.

Why dry ground raises the risk

North Florida entered this week still carrying a rainfall deficit of 1.5 to 3.25 inches for the season, according to the Florida Public Radio Emergency Network. Meteorologists there noted that parched soil can trigger flash flooding more quickly than normal because water runs off rather than soaking in.

The latest drought monitor, as of Tuesday, Aug. 25, showed some improvement across Jacksonville, with several areas downgraded to moderate drought after rainfall the prior week. But the deficit remains.

What to watch along the river

The NOAA river gauge at Jacksonville (SJLF1) showed no active flood watches or warnings as of Tuesday evening. Minor flooding on the St. Johns begins at 1.4 feet above mean higher high water, at which point low-lying parks, flood-prone roadways and some boat ramps start to go under. Moderate flooding begins at 1.9 feet.

A century of deepening, widening and straightening the St. Johns has effectively doubled the tidal range in some spots, leaving Jacksonville more vulnerable to marine-sourced flooding, according to research cited in a 2021 News4Jax report. Jeff Edwards, a Mandarin resident who has lived on the river for more than three decades, described faster water rise during storms and the loss of seagrass and cypress trees along the bank in that same report.

Safety guidance

The city's Emergency Preparedness Division urges residents never to walk, swim or drive through flooded water. Six inches of moving water can knock a person down; 12 inches can sweep away a vehicle. Driving through standing water also creates a wake that can damage nearby homes and businesses, JaxReady noted.

As of Wednesday morning, JaxReady had not issued a specific advisory for this stalled-front event.

Residents can sign up for ALERTJAX weather and emergency notifications at JaxReady.com/ALERTJAX and monitor forecast updates at weather.gov/jax.