Three to six inches of rain, with locally heavier amounts, are predicted. “This amount of rain is likely to result in significant flash flooding of small streams and creeks Monday night into Tuesday evening,” the National Weather Service office serving Washington and Baltimore wrote Sunday afternoon.
Low-lying, poor drainage areas will also be susceptible to flooding.
In addition to the rain, areas east of Interstate 95 — especially close to the Chesapeake Bay — may contend with tropical-storm-force wind gusts on Tuesday exceeding 40 mph for a time. The combination of wind and rain could cause downed trees and power outages. The immediate Washington region is under a tropical storm watch for the possibility of winds this strong, but they are unlikely unless the storm track shifts to the west.
There is higher confidence in heavy rainfall.
Moderate coastal flooding is also possible along the shores of the Tidal Potomac and Chesapeake from a surge of water up these waterways as the storm pushes north. This could affect vulnerable zones like Alexandria, the Southwest Waterfront, Georgetown, and Annapolis, prompting a coastal flood watch for water levels one to three feet above normal.
How the rain and wind will evolve
Moisture surging ahead of the tropical storm could fuel scattered showers and thunderstorms Monday afternoon and night, which could be locally heavy and produce gusty winds. But Monday will not be a washout, and some areas may not see much.
The main slug of rain ahead of Isaias is likely to arrive between late Monday night and dawn Tuesday and continue into Tuesday evening. This rain could be extremely heavy at times before tapering off. Especially east of Interstate 95, some of the storm’s very heavy rain bands may unleash tropical-storm-force wind gusts.
While shifts in where the heaviest rain and strongest winds are expected are possible, computer models are consistent in predicting widespread amounts of 2 to 5 inches, with some show small pockets up to 8 inches. Here is how much the different models project for D.C. through Tuesday evening:
- European: 2.5 inches
- UKMet: 2.7 inches
- American: 3.5 inches
- NAM: 7.8 inches
- High-resolution NAM: 4.5 inches
- Canadian: 5.1 inches
It’s not just Isaias by itself that is projected to unload all of this rain.
The core of the tropical storm moving north-northeastward along the East Coast will encounter a preexisting, mid-latitude system that will prolong the period of rainfall and increase amounts.
As Isaias moves into the Carolinas, it will begin its process of “extratropical transition” — meaning it will begin to lose some of its purely tropical characteristics and increasingly take on the characteristics of a mid-latitude storm.
The figure below shows the forecast surface weather map for Monday night. A frontal system with weak low pressure over Ohio is sprawled across the eastern portion of the country. East of the cold front, the large Bermuda High over the western Atlantic is pumping hot, humid air northward across the Mid-Atlantic. The core of Isaias will advance northward into this soupy air mass.
A second frontal boundary is expected to stall across the Carolina-Virginia-Maryland Piedmont, as shown in the diagram above. Meanwhile, a deep plume of rich tropical moisture will overspread the Mid-Atlantic from the south (dark green shade).
In the upper atmosphere, a large, closed low-pressure system in the jet stream advances toward the East Coast with strong winds from the south ahead of it. These winds will draw in the tropical storm and begin to accelerate it northward over the Carolinas and Virginia.
The storm will also feel the influence of a “jet streak,” which is a core of fast winds that will induce vigorously rising air.
As the tropical storm becomes embedded in these mid-latitude weather systems, there are two important impacts on the weather.
First, the rich tropical moisture being imported by Isaias will be lifted and condensed within the jet streak. This will create an advance region of heavy rain showers and thunderstorms, called “predecessor rains,” over the Mid-Atlantic, starting Monday.
The core of heavy rains associated directly with the converging, spiral winds of the vortex won’t move through until Tuesday. But the result of predecessor rains, and direct rains of the storm, will be to add up impressive — if not dangerous — totals for parts of the Mid-Atlantic, over a 36- to 48-hour period.
Second, because the tropical storm will come under the influence of jet stream dynamics, the additional source of energy may keep Isaias more intense even as it moves inland. In other words, the storm may not decay as rapidly as it moves over the Piedmont. This will translate into strong sustained winds and even more powerful gusts.
Winds to the west of the track across the Mid-Atlantic will be weaker than those on the east side. Because winds blow counterclockwise around the storm, the southerly winds to the east of the track will add with the storm motion, boosting wind speeds there. To the west, rapid motion toward the north will partially cancel strong winds blowing from the north.
As long as the storm’s track remains to the east of D.C., the immediate metro area should be spared from any significant winds…save for a gusty few hours as the storm passes by. The wind-gust forecast map below reveals the strong gradient of winds, with winds howling along the Chesapeake Bay. Along both shores and over the core of the bay, winds can easily gust into the 40-50-mph range.
With several inches of rain, very saturated soils may promote isolated tree falls, as well as outages in the eastern part of the Washington region.
If the storm track shifts to the west, some of the stronger winds could move over the Interstate 95 corridor. If the storm shifts to the east, stronger winds would focus over the Delmarva.