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Heavy rain, severe storm threat late Saturday from remnants of Tropical Storm Bill

3:15 p.m. update: The National Weather Service has issued a flash flood watch Saturday evening to Sunday morning for the entire metro region, excluding southern Maryland.

“Widespread rainfall amounts of one to three inches can be expected with locally higher amounts possible. Rainfall rates of one to two inches per hour could occur with the strongest thunderstorms,” the National Weather Service statement says.

As always, poor drainage areas and small streams are most vulnerable to flash flooding.  Never drive across a flooded road: turn around, don’t drown.  And don’t allow children to play near streams during heavy rain.

Original post from 2:30 p.m. Friday

As the remnants of Tropical Storm Bill zip through the Washington, D.C. region late Saturday afternoon into Sunday morning, some heavy rain and even some severe storms are possible.

  • Storm timing and coverage: 4 p.m. Saturday night to 9 a.m. Sunday morning, most likely window 10 p.m.-5 a.m. (this window is subject to change)
  • Storm progression: West to east
  • Most likely storm hazards: Heavy downpours; lightning; gusty winds
  • Possible storm hazards: Isolated tornadoes; damaging winds; localized flash flooding; small hail
  • Unlikely storm hazards: Large hail; widespread damaging winds; large, long-track tornadoes; widespread flash flooding
  • General forecast confidence: Medium – biggest uncertainties are whether severe storms develop and the zone of heaviest rainfall

Currently classified as a tropical depression and centered near the border of Arkansas and Missouri, Bill is expected to trek east and transition to a post-tropical low over the next 24 hours.

By Saturday afternoon, Bill’s remnants should be centered over northern Kentucky, riding along a front extending eastward into the Mid-Atlantic. It then crosses the Appalachians Saturday night, transiting the D.C. region early Sunday morning.

Bill’s remnants are expected to remain quite dynamic aloft, with a lot of spin and rising air. The air mass ahead of the system is expected to be quite moist, with southerly winds feeding Gulf moisture. There is thus the potential for a 12 hour or so period of significant rain across our region.

How much rain will we receive? The latest guidance from the Weather Prediction is shown below, for the three-day period valid Friday morning through Monday morning. A general 1-2” area-wide rain is expected, with the maximum closer to the Baltimore metro region. Note the pocket of 2-3” forecast over the Appalachians, as Bill’s moist inflow gets lifted along the hill slopes of the Appalachians.

Because the rain is expected to be convective in nature i.e. intermittent moderate-to-heavy downpours (likely with thunder), locally higher amounts are expected, perhaps 3-4” in spots. Since portions of our region have dealt with recent heavy rains i.e. the severe thunderstorms of June 18, and rain the night before that, local flash flooding is a possibility. However, the progressive nature of Bill’s remnants should preclude a more widespread flooding threat.

Most models (shown below) tend to favor the heaviest amounts just north of the District into southern Pennsylvania.

GFS: Around 0.75-1″ metro D.C., more to north and northeast

High resolution NAM: 1-2.5″

European: 1-2″

Canadian: 0.25-1″

Will Bill’s remnants bring a period of severe thunderstorms? Thus far, the remnant has not produced an outbreak of tornadoes. When this occurs, it typically unfolds to the right and just ahead of the low-pressure center. If Bill becomes embedded in the warm sector, it will potentially have a supply of unstable air to work with late Saturday. However, the arrival of Bill’s core circulation east of the Blue Ridge will likely occur during the overnight hours, when instability is typically at a minimum.

With modest wind shear expected, decent instability and moisture streaming ahead of Ex-Bill, we can’t rule some severe weather – including a low-end probability of weak tornadoes – late Saturday afternoon or, more likely, into Saturday evening. The National Weather Service Storm Prediction Center has placed the region under a slight risk of severe storms.

Among the significant weather threats, periods of locally heavy rain are, for the time being, our greatest concern.