A satellite image shows a system that may become Tropical Storm Irma off the Southeast coast. (NOAA)

A tropical disturbance near the northeastern Florida coast could become the season’s ninth named storm late Monday or Tuesday: Irma. This system will have minuscule effects compared with Harvey in Texas but may prove briefly disruptive for Mid-Atlantic beaches.

Tropical storm watches span from the central South Carolina coast to the southern coast of North Carolina. A warning has been issued for the North Carolina Outer Banks, where the storm’s center will come closest.


(National Hurricane Center)

Heavy rain is expected to be the main hazard. Between Monday morning and Wednesday morning, up to nine inches of rain is possible in a swath from the South Carolina border up through the Outer Banks.


Five-day rainfall forecast. (NOAA/WPC)

The system, termed Potential Tropical Cyclone Ten, is poorly organized and already beginning to merge with a nearby stationary front. The window for intensification is small.

If it does strengthen to a well-defined low pressure, the threat of storm surge along the Carolinas will increase, but the coastal flood threat is minor and comparable to a typical nor’easter. Waters may rise up to one to two feet above normal.

Tropical-storm-force winds should reach southern South Carolina late Monday afternoon, southern North Carolina overnight Monday and the Outer Banks by Tuesday morning.


(National Hurricane Center)

From Virginia Beach to the Delmarva Peninsula, rain from the system is likely to begin Tuesday from south to north, quickly tapering off as the system races away to the northeast Tuesday night. Two to four inches of rain are possible in the southern Delmarva and, at the coast, winds may gust over tropical-storm force. Waves may reach eight feet or so and, with the likelihood of rip currents, swimmers should stay out of the water through Tuesday or Wednesday.

In the Washington area, minor effects from this system should be felt mainly east and southeast of Interstate 95. Rain and wind will tend to increase in intensity late Monday night into Tuesday morning as one heads toward Southern Maryland and across the Chesapeake Bay, before quickly tapering off later in the day.

After passing the Mid-Atlantic coast Tuesday, the system could clip Cape Cod on Wednesday.

Forecasters have monitored this system for 16 days since it left the African coast, and it has been stalled over Florida for six days. Although a minor impact compared with the more newsworthy Harvey, rainfall totals over South Florida have been fairly impressive, too, over a foot in spots.


Observed rainfall totals over the past week. (NOAA)

Much farther east, a new healthy tropical wave has just exited the African coast — model guidance indicates that there is a good chance it will become the next named storm later this week (Irma, if the coastal Mid-Atlantic system doesn’t get named, or Jose, if it does).

Over the past 50 years, the average date for formation of the ninth named storm is Sept. 30, and the 10th is Oct. 14. So this season is running about a month ahead of schedule. In terms of Accumulated Cyclone Energy, or ACE, the season is at 97 percent of average for this date.