Tropical storm Michael is sweeping toward southeast Virginia Thursday evening, unloading up to seven inches of rain in a short period, which is causing flash flooding.

Flash-flood watches are posted over a huge area that includes much of North Carolina, the southeastern half of Virginia, southern and eastern Maryland, and the entire Delmarva Peninsula. A watch is not in effect for Washington itself, but its southeast suburbs are included.

Thursday evening, Michael was unloading extremely heavy rain in north central North Carolina and and southeast, central and northern Virginia, where large areas were under flash flood warnings, including Greensboro and Raleigh in N.C., and Lynchburg and Charlottesville in Va.

The Weather Service issued a special alert declaring “severe flash flooding" was likely in eastern Virginia around Richmond, with rainfall rates of three inches per hour.

Thursday afternoon, the Roanoke area, which had received up to a half foot of rain, had been under a flash flood emergency, the most severe flood alert.

Extremely heavy rain was forecast for a swath from Richmond to Salisbury, Md. Thursday night, where four to seven inches are predicted. Higher amounts locally cannot be ruled out.


Additional rainfall predicted by National Weather Service. (PivotalWeather.com)

Outside this zone of heavier rainfall, one to three inches is most likely, although predicted amounts drop below one inch northwest of Washington. (Note that the rainfall projection shown above for the D.C. area is almost certainly underdone, and will be closer to an inch, if not more.)

Because of the spin in the atmosphere associated with Michael’s circulation, tornado watches also are in effect for southeast Virginia and northeastern North Carolina. Late Thursday afternoon, tornado warnings had been in effect south of Richmond.

Rain timing

Because Michael is moving very quickly, to the northeast at 24 mph, the rain will not last particularly long in any one area — generally about four to eight hours. But when it does fall, it will come down heavily. Rainfall rates associated with Michael’s core may reach up to four inches per hour.

Here’s a forecast radar animation showing the speed and intensity at which the rain will come through:


Simulated radar from noon Thursday to 8 a.m. Friday.

Expect moderate to heavy rainfall in the following cities at the following times:

  • Raleigh, N.C.: Through Thursday evening.
  • Roanoke: Through Thursday evening.
  • Richmond: Mid-afternoon Thursday through midnight Friday.
  • Washington: Late afternoon Thursday through 2 a.m. Friday.
  • Salisbury: Thursday evening through Friday predawn.

Flood risk

The National Weather Service has placed western North Carolina, southeast Virginia and the southern Delmarva in a moderate-to-high-risk zone for excessive rainfall that could lead to flash flooding.


Risk of excessive rainfall. (National Weather Service)

The zones in red and pink in the map above correspond to where the core of Michael’s circulation will pass, where rainfall is likely to be most intense — perhaps totaling four to seven inches — and where the risk of flooding is highest.

“Four to seven inches of rain falling quickly in a region that also saw Hurricane Florence this past month, as well as one of the wettest years on record thus far, is plenty of reason to introduce a High Risk of excessive rainfall,” the National Weather Service wrote in a discussion.

Wind and tornado risk

Straight-line winds

Michael’s maximum sustained winds have dropped to 50 mph and will continue to lower. The strongest winds are focused mostly near its center, which will track through the central Carolinas. These winds, by themselves, probably wouldn’t cause substantial problems.

However, winds exceeding 30 mph, buffeting areas where heavy rainfall is occurring or recently occurred and the ground is saturated, could result in trees coming down. Winds of this intensity could affect much of the Mid-Atlantic, even in zones some distance away from Michael’s core, in the storm’s wake early Friday.

“Gusty winds combined with saturated soil will bring down some trees, possibly resulting in a few power outages,” the Weather Service office serving the Washington and Baltimore region wrote.

Tornadoes

It is common for landfalling tropical weather systems to spawn tornadoes, and Michael is no exception.

A tornado watch covers northeastern North Carolina and southeast Virginia through 9 p.m. Thursday.

“Showers arcing across southern into central North Carolina should intensify and develop into embedded thunderstorms as they spread northeast in association with Michael,” the Weather Service wrote. “Low-level shear and pockets of downstream surface heating will be favor for at least a few tornadoes, mainly this afternoon into early evening.”

Richmond, which was hit by a tornado swarm when the remnants of Hurricane Florence passed through the region less than a month ago, is included in this watch.

Storm tracking map