* Wind advisory from 4 a.m. to 6 p.m. Monday for gusts up to 55 mph | High wind warning for areas east of the District for gusts to 60 mph *

9:20 p.m. update: The National Weather Service earlier expanded the high wind warning for the Eastern Shore further west to include Prince George’s and Anne Arundel counties and southward through Southern Maryland, where winds could gust to 60 mph between 6 a.m. and 6 p.m. on Monday. Otherwise no significant changes to the forecast below based on a look at the latest model output this evening.

If you haven’t already, we recommend you secure or bring inside loose outdoor objects before you go to bed, and make sure to charge electronic devices in case of power outages.

Original post

Multiple modes of hazardous weather are likely in the Washington region Monday because of a powerful storm system sweeping toward the East Coast, the same one triggering an outbreak of severe thunderstorms in the South on Sunday. Strong winds and downpours are expected throughout the region while some areas could see severe thunderstorms capable of spawning tornadoes.

Widespread strong wind gusts, up to 40 to 55 mph, are a given, highest between the midmorning to midafternoon hours. Downpours preceding and during these gusty winds could loosen soil, increasing the possibility of downed trees and power outages. A wind advisory has been posted for the entire region, and winds may be particularly strong in the mountains, along the Chesapeake Bay and over the Eastern Shore, where gusts could top 60 mph.

Thunderstorms also are likely in the region, any time between early Monday morning and midafternoon. The storms, which could produce damaging winds, hail and tornadoes, may be hit-or-miss rather than widespread and will depend on several ingredients coming together.

Rain showers associated with the storm system could begin after sunset Sunday, with the potential for heavier showers after midnight when winds also will increase. Thunderstorms could affect the region as early as the predawn hours Monday, at the same time winds become gusty, even when it’s not raining.

Most areas should pick up about an inch of rain through Monday afternoon, with higher amounts of up to several inches near the Blue Ridge Mountains where pockets of flooding are possible.

To prepare for this potentially hazardous situation, consider securing or bringing inside loose outdoor objects (such as lawn furniture or toys) and charging electronic devices in case of power outages. Also, be sure to have multiple ways to receive storm warnings.

Storm risk at a glance

  • Storm timeline: Early morning to midafternoon. There may be multiple waves of storms, the first coming through early in the morning, a second mid- to late morning and the last midafternoon with pauses in between rounds. Everything should shut down by 3 p.m. or so. The storms between late morning and midafternoon have the highest chance of becoming severe but also may be hit-or-miss.
  • Storm hazards: Damaging winds with isolated tornado activity possible. Secondary risk of small hail and flooding rains.
  • Areas affected: Showers and storms will hit much of the region, but instances of severe weather, including damaging winds, hail and tornadoes are likely to be more scattered. The highest risk of severe storms will tend to be in our southern suburbs and toward southern Maryland.

Discussion

The forecast map for 8 a.m. Monday morning is shown below. It displays the intensifying storm system over the Great Lakes, its pressure dropping rapidly, reaching a value comparable to a Category 1 hurricane. By Monday afternoon and evening, it rapidly pulls north into Canada.

Let’s discuss how it will play out in terms of potentially hazardous weather in the Washington region.

Heavy rain

Significant moisture will begin overspreading the District Sunday evening, direct from the Gulf of Mexico. These moisture values will be unseasonably large, and by 8 to 9 p.m. showers will advance from the southwest. The combination of strong uplift and significant moisture will lead to bouts of moderate to heavy showers overnight.

The figure below illustrates the most likely rain accumulation amounts, based on a blend of model guidance. Up to an inch of rain seems like a good bet area-wide, with some locally higher amounts. This is probably not enough to cause flash-flooding concerns in the immediate D.C. area, but locations in the Blue Ridge southwest of the District may pick up three to four inches.

Strong winds

The pressure gradient (change in pressure with distance) will become quite large around the parent low-pressure system Monday morning. Additionally, the storm will be undergoing rapid intensification. These two effects will combine to whip up strong wind gusts from the southwest through much of the day Monday.

The figure at the top of this post shows the likely maximum wind gusts, based on the blend of model guidance, which you can see will be quite high area-wide. Gusts in the 30 to 40 mph range will develop after sunrise and peak around 1 to 2 p.m. as the storm’s central pressure bottoms out. For a few hours, we will experience peak gusts of 40 to 50-plus mph.

These gusts are a sufficient enough concern that the National Weather Service probably will issue a Wind Advisory for much of Monday, with a High Wind Warning for higher elevations to our west; there, winds may gust to 60 mph. Wind gusts well over 50 mph are expected for a time along the Eastern Shore.

Typically, peak wind gusts around 50 mph do not lead to widespread power outages in our region. However, with over an inch of rain saturating the upper layer of soil, a heightened potential for treefall exists, mainly during the late morning and afternoon Monday.

Severe thunderstorms

The risk of severe storms remains, and this will become a concern starting early Monday morning. As displayed above (in the at-a-glance section), the National Weather Service Storm Prediction Center has placed much of the East Coast under varying risks of severe thunderstorms.

The greater D.C. region is highlighted under a slight risk for severe storms, which is level 2 out of 5 escalating threat levels. Here, “slight” breaks down to a 5 percent risk of a tornado and 15 percent risk of damaging wind gusts and hail, within 25 miles of a point.

The threat escalates to the enhanced level (3 out of 5), from central Virginia southward through the Carolinas. In this zone, the threat of damaging winds is greater, as is the tornado threat.

While the entire Eastern Seaboard will experience strong wind shear (increase in winds with altitude), a more unstable air mass, which provides fuel for storms, is expected over Virginia and the Carolinas. In this region, the Storm Prediction Center says, there may be one or more strong, long-track tornadoes capable of doing significant damage and threatening life.

For the immediate D.C. region, the magnitude of our severe threat hinges on how unstable our regional air mass becomes. The most likely window for any severe storms is between 9 a.m. and 3 p.m., when temperatures rise into the 70s and humidity increases; however, some intense storms are possible in the early-morning hours.

The graphic below shows simulated radar at 10 a.m. using a high-resolution prediction model. Any convective line will be narrow and move rapidly from west to east, with individual cells tracking from southwest to northeast within the line.

Lines may acquire an undulating structure, called a quasi-linear convective system (QLCS), which contains bowing regions of strong to damaging wind gusts and indentations where small areas of rotation develop. Farther south, from central Virginia into the Carolinas, the dominant type of thunderstorm probably will be isolated supercells, which are rotating storms that can sometimes produce tornadoes.

Tornado watches are likely from Virginia southward, and it’s possible the Washington region will be included.

Winds in the lowest few thousand feet of atmosphere will be howling at 70 to 90 mph. Our local concern is that downdrafts in convective cells can mix down a hefty portion of this momentum as severe gusts. The strongest gusts will be hit-or-miss.

Those in dwellings with dead or damaged trees on their properties should take notice and move occupants to safer locations during any severe thunderstorm or tornado warnings.

If a tornado warning is issued, go to the lowest floor where you live, away from windows. Put as many walls between you and the outside as possible. If in an apartment building, go the lowest level possible and seek shelter away from windows, potentially sheltering in a stairwell while maintaining safe distance from others considering covid-19 concerns.

Motorists should avoid travel during any severe storm warnings.

We will be updating this situation as we move through the day on Monday.

Andrew Freedman and Dan Stillman contributed to this report.