Depending on the track of Category 4 Hurricane Florence, a storm surge could come up the tidal Potomac and Chesapeake like a bulldozer, pushing water multiple feet above normally dry land along their shores.
In addition, quick-hitting tornadoes could form in some of the storm bands that pass through the region.
"The potential for a disaster is high,” said Jim Lee, meteorologist in charge at the National Weather Service serving the Washington area. But he stressed uncertainties in the specific outcomes, noting that storm effects are still several days away. “Time will tell as the week unfolds."
We are very vulnerable
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, and it could become severe in a worst-case scenario.
"Certainly we're primed," Lee said of any flooding. “Even if we're on the northern fringe [of the storm], we're going to have flooding issues. We're sitting on a powder keg."
As of late August, Washington had already received a year's worth of rain, about 40 inches. Since then, we've picked up another half a foot. The 45.86 inches of rain so far this calendar year ranks it as the fourth-wettest in records that date to 1872.
Because our soils have been essentially transformed into mush, trees are no longer well rooted to the ground. Strong winds, possibly up to tropical-storm force, could bring them down in great numbers.
Lee said his office received eight reports of downed trees over the weekend from gusts of 30 to 35 mph. He expects tree damage would increase markedly if winds reach 40 mph or higher.
Scenario 1: Florence tracks into southwest and west-central Virginia. (40 percent chance)
In this scenario, the storm would make landfall between northern South Carolina and the North Carolina Outer Banks and move northwest through central North Carolina into southwest or south-central Virginia, where it would slow or stall.
In the Washington region, there would be a sharp gradient between more-moderate rains to our north and northwest and extremely heavy rainfall to our southwest and south that could continue for days — even into early next week.
Serious, possibly disastrous flooding would occur in central Virginia and could spill into the central and northern Shenandoah Valley and north-central Virginia piedmont.
The immediate Washington region would receive several inches of rain, certainly enough to cause flooding but perhaps short of widespread, severe flooding.
Wind gusts would probably reach tropical-storm force, causing downed trees and scattered power outages.
As the storm moved north, southerly winds would push enough water up the Chesapeake Bay and Potomac River to cause minor tidal flooding.
The European weather model supports this scenario.
Scenario 2: Florence slowly tracks northward just west of Interstate 95 (25 percent chance)
In this scenario, the storm would make landfall between northern South Carolina and the North Carolina Outer Banks, and progress north through eastern North Carolina and east-central Virginia along the Interstate 95 corridor. It would eventually pass through Northern Virginia west of I-95.
This is a worst-case scenario for the region. It would push a large storm surge up the Potomac and Chesapeake Bay, causing severe coastal flooding, perhaps not unlike Hurricane Isabel in 2003 or the 1933 Chesapeake Potomac hurricane.
Depending on how fast the storm was moving and whether it slowed or stalled along its track, significant-to-severe flooding from heavy rainfall would affect the region.
Sustained winds would probably reach tropical-storm force, with gusts over 60 mph causing widespread downed trees and power outages.
The UKMet model, run by Britain's Met Office, forecasts a scenario similar to this.
Scenario 3: Florence tracks well to our south, remaining in North Carolina (20 percent chance)
In this scenario, the Washington region generally dodges the bullet as the heaviest rain and strong winds remain well south of the region. Some rain and gusty winds could still skirt the area, especially on the south side, and cause pockets of flooding and a few downed trees, similar to the rain and wind this past weekend.
The American model (both the new and old) favors this kind of scenario.
Scenario 4: Other (15 percent)
As or just after Florence makes landfall, steering currents will become weak, which turns its future motion into a bit of a wild card. While we have outlined above what we think are the most likely scenarios, there's still the possibility it just meanders near the coastline or edges north or south of current expectations.
We're still four to five days, at least, from any direct effects from this storm, and we will try to reduce the number of scenarios and provide more specifics about what to expect as time wears on.
While there is a great deal of uncertainty with respect to the specific effects from the storm in our region, now is certainly the time to begin thinking about preparation for this storm. Prepare for flooding by decluttering drains and gutters. If you have a basement that floods, think about moving any valuables to higher ground.
Do you have trees that could fall on your house during a windstorm? Trim them now.