Rainfall of 3 to 6 inches is predicted along and east of Interstate 95, with isolated totals up to 8 inches. Lesser amounts of 1 to 3 inches are forecast to the west. The heaviest rain is expected between predawn Tuesday and the midafternoon hours, although intermittent showers and storms began Monday afternoon.
As Isaias makes its closest approach between Tuesday morning and afternoon, very strong winds are also expected along and especially east of the Interstate 95 corridor, prompting a tropical storm warning for the Washington-to-Baltimore corridor, including all areas east of Loudoun and Frederick counties.
The I-95 corridor could see gusts up to 40 to 50 mph. Here, the combination of gusty winds and saturated ground from heavy rain could cause downed trees and isolated power outages.
But the strongest winds are expected to the east, closer to where the center of Isaias is predicted to track. In areas adjacent to the Chesapeake Bay, from Annapolis into Southern Maryland, gusts could reach 50 to 65 mph and will are likely to topple trees and cause power outages.
If you live in the tropical storm warning zone, you may wish to charge your electronic devices in case of outages and secure or bring inside loose outdoor objects.
The most severe wind impacts are forecast to occur along the Atlantic beaches in Maryland and Delaware, including Ocean City, Bethany and Rehoboth, where gusts may top 60 or even 70 mph Tuesday.
Details on the predicted rainfall
While the strongest winds are forecast to be focused in our eastern areas, heavy rain will swell over a much larger area. Scattered showers and storms, with heavy downpours and gusty winds, developed Monday afternoon in the Washington region, but the bulk of the rain is expected beginning during the predawn hours on Tuesday through Tuesday mid-afternoon when the rain will quickly shutoff.
The heaviest totals are predicted near the Chesapeake Bay, while very little may fall along Interstate 81 and to the west.
The predicted rain is likely to cause “significant” flash flooding of streams and creeks, according to the National Weather Service. Low-lying, poor-drainage areas also will be susceptible to flooding.
There is also the potential for significant river flooding, according to the Middle Atlantic River Forecast Center, starting Tuesday and continuing even after the rain ends Tuesday night into Wednesday, as the fresh rainfall from smaller streams and creeks spills into rivers such as the Potomac and Shenandoah.
Here are predicted rainfall amounts from computer models for the District:
- American: 3.2 inches
- Canadian: 3.9 inches | High-resolution version: 4.5 inches
- European: 3.5 inches
- HRRR: 2.1 inches
- NAM: 5.3 inches | High-resolution version: 2.7 inches
- UK Met: 2.5 inches
Receiving three or more inches of rain in a day is unusual in Washington. Here are the most recent dates of that amount of rain or more:
Coastal flooding and storm surge
In addition to flooding from rainfall, minor to moderate coastal flooding is also possible along the shores of the Tidal Potomac and Chesapeake Bay from a surge of water up these waterways as the storm pushes north. This could affect vulnerable zones such as Alexandria, the Southwest Waterfront, Georgetown and Annapolis, prompting coastal flood warning and advisories for water levels one to two feet above normal.
The intensity of the wind and the amount of rain will depend on Isaias’s exact track. The consensus of model forecasts tracks the storm from just east of Richmond over the Chesapeake Bay to near Philadelphia. The strongest winds are likely along and east of where the storm center tracks, which is why we have high confidence in strong winds over the Delmarva Peninsula. However, Washington’s far southeastern suburbs are very close to this track, especially into Southern Maryland.
It’s also important to note that Isaias’s wind field is likely to expand after it makes landfall and interacts with a weather front and dip in the jet stream over the Mid-Atlantic, which is why the tropical storm warning covers areas as far west as the I-95 corridor.
The storm’s core of heavy rainfall is also likely to expand as it interacts with the front. That said, models do tend to project the heaviest rainfall just east of the city closest to where Isaias’s core is projected to pass.
Small shifts in the storm track to the west would increase wind and rain impacts in the immediate Washington area, while shifts to the east would decrease them.
Inside the storm: Explaining the rain and wind forecast
As Isaias makes landfall along the coast of the Carolinas on Monday night, either as a weak hurricane or strong tropical storm, the effects of the storm will already be surging north, because the storm will be interacting with preexisting weather systems over the Mid-Atlantic.
The graphic below shows surface weather maps for 8 p.m. Monday night and 8 a.m. Tuesday morning. At 8 p.m., the storm is making landfall, and a stationary weather front extends from southwest to northeast east of the Appalachians. The dark green region represents tropical moisture, imported by the storm, that is falling in the form of heavy rain showers and thunderstorms.
The front will focus the convergence and uplift of moisture, north of Isaias, continuing into the day on Tuesday with the stripe of heavy rain extending into New England. Another factor contributing to these “advance” or predecessor rains is an approaching dip in the jet stream or “trough,” as shown below.
On the east side of the trough, winds over the Mid-Atlantic are blowing strongly from the southwest in upper levels. The concentrated pocket of strongest winds is labeled “jet streak” with a core of 90 mph flow. Isaias is shown at 2 a.m. Tuesday by the small red ring. The storm will move into the right entrance region of the jet streak, shown by the large red ellipse.
In this region, the jet streak induces air to rise rapidly. This zone of rising air, combined with the surface front north of Isaias, will team up to spread northward in advance of the storm, up the I-95 corridor, from the Mid-Atlantic into New England.
As Isaias becomes embedded in the dip in the jet stream, its forward speed will increase through the day on Tuesday. This will affect wind speeds near the ground, as shown below.
The winds will be strongest to the east of the storm track. This is because they circulate around the storm from the south, and the influence of storm motion — also from the south — gets added. Note the stripe of 70-mph-plus peak wind gusts in the diagram. You first see it over coastal North Carolina, as the storm comes inland, then weakens.
But there appears to be a resurgence of 70-mph-plus gusts over Delaware and New Jersey. This is partly because the storm is accelerating.
Another reason for the resurgence could be because of a rejuvenation of the storm, as it feels the influence of the jet stream and comes beneath that pocket of strongly rising air. This could help lower the surface pressure for a few hours, intensifying the system and increasing surface wind speeds for a time.
Unless there is a radical shift in the storm track to the west, we expect that the D.C. region will remain west of track — where the storm motion (toward north) and wind direction (from north) oppose each other, weakening the winds. Peak gusts may surge briefly into the 35-45 mph range for a few hours either side of noon on Tuesday.
Ian Livingston contributed to this report.