Hurricane Ida made landfall Sunday as a high-end Category 4 hurricane, ravaging southeastern Louisiana, cutting power to all of New Orleans and bashing the coast with winds gusting above 150 mph. The system, with winds now relaxed, is now marching up the Appalachians as a tropical rainstorm, and it’s set to drench Washington and the Mid-Atlantic.
Parts of the Washington region have already seen more than nine inches of rainfall for the month of August, meaning the ground is saturated and can’t handle much more. Flash flood watches are up for most of the region, with the greatest potential for excessive rainfall north and west of the city.
The system’s leftover spin could also brew a string of rotating thunderstorms, with the potential to produce tornadoes in our area. The setup bears some shades of Hurricane Ivan, which unleashed a slew of tornadoes west of the nation’s capital in September 2004.
“Wednesday afternoon could be active in terms of tornado activity,” Jeff Halverson, Capital Weather Gang’s severe storms expert, wrote in an email. “We will really need to closely watch this. The combination of unstable air, strong wind shear and leftover spin energy from ex-Ida could come together to create numerous (but generally weak and short-lived) tornadoes.”
Heavy rain and flooding risk
Timing around the District: Rainfall will begin late Tuesday, with scattered showers and storms in the afternoon and evening, a few of which could be intense. A few bursts of heavy rain are possible as a warm front lifts north through Washington on Wednesday morning. The city may remain cloudy with little rainfall during much of the day before storms arrive from the midafternoon and evening, some severe.
- Areas to the west along Interstate 81 will see heavy rain all day Wednesday, because they will end up closer to the track of Ida’s remnants. Rainfall rates will top an inch per hour at times; the heaviest will fall between 10 a.m. and 4 p.m.
- Rain will end everywhere overnight Wednesday, with clearing skies and cooler, drier weather arriving Thursday.
Rainfall totals: In Washington, 1 to 3 inches of rain is likely; amounts vary by model and depend on the exact track of Ida’s remnants. While the heaviest rain will focus west and northwest of Washington, individual storm cells moving repeatedly over the same areas will bring the potential for flooding.
To the west and northwest near the Blue Ridge and the panhandle of Maryland, 3 to 5 inches is possible, with a few locales likely to exceed half a foot; this area includes Frederick, Winchester and Hagerstown. Average amounts will drop quickly south of Prince William County, generally under an inch (but heavier amounts could occur in localized downpours).
Here is how much rain is projected from different models for Washington:
- NAM and high-resolution NAM: 0.9 and 1.8 inches
- HRRR: 2.2 inches
- Canadian and high-resolution Canadian: 2.1 and 2.3 inches
- American (GFS): 0.8 inches
- UKMet: 2.3 inches
- European (ECMWF): About 1.5 inches
Flood risk: Flash flooding and urban flooding are possible, particularly because soil is saturated after a month of very heavy rainfall. Most counties around Washington are running 3 to 6 inches above normal for the month of August, and it won’t take much to spur renewed flooding.
- River flooding is possible too, particularly near and northwest of Washington, where most rivers are already near or at flood stage. The Potomac River near Georgetown is predicted to hit moderate flood stage on Thursday, for instance, and it’s one of many that will become swollen thanks to an influx of upstream heavy rainfall. The risk of river flooding will increase northwest of Washington and into the mountains; the Monocacy River in Frederick is forecast to reach major flood stage on Thursday.
Uncertainty: There is high confidence that the heavier amounts will be relegated to the west of Washington, with uncertainty in exactly how much D.C. proper gets. Subtle shifts of only 30 or 40 miles in the track of Ida’s remnants will have significant bearings on how much rain falls.
Severe thunderstorm and tornado threat
The National Weather Service’s Storm Prediction Center has placed the region in a Level 3 out of 5 “enhanced” risk for severe thunderstorm on Wednesday. “Several tornadoes and scattered damaging winds appear probable across parts of the Mid-Atlantic,” the center wrote. It assigns a 10 percent chance of a tornado within 25 miles of a point, which is unusually high for our region.
“The DC region is in the perfect 'sweet spot’ for the right ingredients coming together,” Halverson wrote.
Timing in D.C.: The greatest threat for tornadoes will exist from 11 a.m. to 5 p.m. in central and western Virginia and western Maryland, and 3 to 9 p.m. in the D.C. to Baltimore stretch.
Multiple rounds of storms are possible. Storms will fire quickly during the early afternoon, shifting east and perhaps redeveloping.
Hazards: Localized damaging winds from downbursts and brief tornadoes.
Discussion: A tropical air mass will overspread the Mid-Atlantic as a warm front lifts north during the morning hours. That warm front may stall near or just south of the Mason-Dixon Line, perhaps in the northern suburbs of Washington or near Baltimore. That warm front will impart a bit of extra twist in the lower atmosphere, which could help focus storms and bolster their proclivity to rotate.
- Pockets of strong to severe thunderstorms will abruptly develop during the afternoon hours and shift east with time, tapping into wind shear, or a change of wind speed and/or direction with height, brought about by Ida’s leftover spin. That will help storms rotate. Any tornadoes that form will be quick-hitting, erratic and may come with little warning. Cloud bases will also be low to the ground given the moisture-rich environment, obscuring any funnels and making them challenging to see.
- Tornado watches will likely be issued shortly after lunchtime for the area. That’s a sign to stay alert in the event warnings are issued. Warnings are an immediate call to action that necessitates seeking shelter.
