September 11, 2018 at 3:41 PM
Hurricane Florence may unleash “catastrophic flash flooding” in the Carolinas, and the excessive rainfall could easily creep into Virginia. But, based on the best available computer model forecasts, the storm’s extreme rainfall is likely to remain south of Washington.
However, even as the storm’s effects on the region may be diminished, they may not be entirely denied. There is still some chance that the region will endure disruptive rain and wind from the storm. This risk increases as you head south into central Virginia toward Charlottesville and Richmond, and especially southern and southwestern Virginia.
In addition, even if the storm’s center remains well south of the Washington region, there may still be a push of seawater up the Potomac River and Chesapeake Bay sufficient to cause some flooding along its shores.
While computer model simulations have shifted storm effects south since Monday, they could edge north again, placing our region at greater risk. On the other hand, if the storm track edges farther south, the region could be largely spared from any meaningful effects.
It’s worth stressing that our region remains very vulnerable to rainfall. Our soils are saturated, and our rivers, creeks and streams are full, if not already overflowing. If we receive several inches of rain or more, flooding will be inevitable. Washington has received about 46 inches of rain this year, which ranks the fourth-most on record, year to date, since 1872. Six inches have fallen since late August alone.
Because our soils have been essentially transformed into mush, trees are no longer well rooted in the ground. If strong winds materialize, they could come down in great numbers.
We've reduced the number of scenarios for the region from four to three compared to Monday's update. The most likely scenario would result in modest impacts in the immediate area.
Scenario 1: Storms stays well south of Washington, with few or very minor effects (50 percent chance)
The storm would make landfall between northern South Carolina and southeast North Carolina and stall, possibly meandering west or even to the south.
Much of the Washington region might be only grazed by some breezy showers Friday, possibly lingering into the weekend. These would become more numerous and heavier from Fredericksburg and to the south.
The European and new version of the American (FV3) model support this sort of scenario.
Rainfall potential: Less than one inch.
Scenario 2: Storm stays south of Washington, with some rain and gusty winds — especially south (40 percent chance)
In this scenario, the storm makes landfall in southeast North Carolina and meanders inland, making little headway to the north. We would get brushed by occasional showers Friday into the weekend, which may produce some gusty winds, but total rainfall probably wouldn’t be enough to cause big flooding problems.
Areas from Fredericksburg and to the south and southwest would see more rain and wind. Flooding and power outages could become more widespread south of Charlottesville and Richmond.
Minor coastal flooding would be possible along the shores of the tidal Potomac and Chesapeake Bay.
The American (GFS) weather model supports this scenario.
Rainfall potential: One to three inches, heaviest south.
Scenario 3: Storm approaches Washington region with heavy rain and strong winds — especially south (10 percent chance)
In this scenario, the storm tracks through eastern North Carolina and into Virginia, where it stalls. Heavy rain and strong winds would affect the area starting Friday, causing flooding and power outages. The strongest winds — perhaps reaching tropical-storm force — and heaviest rain would most probably occur Saturday, and rain could linger into early next week as winds gradually subside.
The most severe effects would be in our southern areas. The Virginia Tidewater and Richmond would be particularly hard-hit, but rains would also be intensified over the higher terrain in central and western Virginia.
This track would push a significant storm surge up the Potomac and Chesapeake Bay, resulting in coastal flooding.
The Canadian model supports this kind of scenario.
Rainfall potential: Three to six inches or more, heaviest south.
On Wednesday, we’ll update these scenarios again and hopefully be able to narrow them down further and with more specificity. At the moment, there is still a lot of uncertainty in the storm’s course once it nears the coast and ventures inland given very weak steering currents.