• Mon. Nov 23rd, 2020

Severe storms possible in D.C. area Friday, with remnants of Hurricane Laura bringing more Saturday – The Washington Post

The highest chance of severe storms with damaging winds will be north and northeast of the District, but storms with heavy downpours and lightning could affect much of the area.

A severe thunderstorm watch means conditions are favorable for severe storms but not guaranteed. Stay alert. If a severe thunderstorm warning is issued, it means a severe storm is imminent, and you should seek shelters indoors.

Original article from 12:15 p.m.

Thunderstorms, some of which may be severe, are predicted for parts of the Washington region both Friday and Saturday.

Friday’s storms, developing as soon as mid- to late afternoon in our northern areas, are the result of the combination of steamy air and a broad zone of low pressure over the region. The biggest threat of severe storms with damaging winds will be north and northeast of the District, but numerous storms with heavy downpours are possible throughout the region Friday evening.

On Saturday, the remnants of former hurricane Laura will zip through the region, bringing rain and storms. It will not rain all day but, between the morning and evening, there may be a period with heavy storms that could contain strong winds and, perhaps, an isolated tornado or two. Conditions will rapidly improve Saturday night and Sunday.

Friday’s storm threat

The D.C. region is in a slight risk zone for severe storms, level 2 out of 5, Friday afternoon and evening. Friday morning, a large complex of thunderstorms organized along Lake Erie, over northwest Pennsylvania, and is traveling toward the southeast. That complex is shown in the radar snapshot below.

The National Weather Service Storm Prediction Center is concerned that, with further destabilization of the atmosphere (warming of the surface from the afternoon sun), severe weather could be on the uptick with this storm complex as it slides our way. A severe thunderstorm watch may be in the offing, particularly for north-central Maryland and Baltimore.

The southern extent of this line is uncertain, but the timing seems to be in the window between 3 and 6 p.m. The storm complex has sufficient wind shear (increase in winds with altitude) and an air mass that will become more unstable through the afternoon, enabling it to maintain modest intensity.

There is not much in the upper atmosphere in terms of jet stream dynamics helping to organize this complex of storms. It appears to be more internally organized and propelled by a large, expanding outflow of cool downdraft air. A stationary front draped from northwest to southeast across Pennsylvania is helping to guide the storm complex toward the southeast, along which is concentrating a warm and humid air mass.

A couple of the high-resolution models, such as the one shown below, do show the complex expanding south and east, and affecting north-central Maryland. One model brings the leading edge of the storm complex down to the District, during mid- to late afternoon.

The potential exists for at least isolated to scattered strong to, perhaps, severe wind gusts, and locally heavy rain, associated with this complex. So far, the threat of widespread damaging wind, if any, increases north and east of the District.

The Baltimore region appears to be more squarely in the impact zone later Friday afternoon. At this point, we are not considering a potential derecho impact in the D.C. region, and it’s not a given that this storm complex will intensify to the level of a derecho across Pennsylvania and northern Maryland.

Friday evening, moist low-level flow from the southwest is on the uptick, with the approach of a cold front and the remnants of post-tropical Laura. Showers and thunderstorms, as shown by some models, will become more numerous and widespread through the evening. Humidity levels in the lower and middle atmosphere will increase, leading to the threat of locally heavy rain. Flood watches may be issued for a large part of our region, with the threat increasing Saturday.

Saturday’s severe weather threat

On Saturday, a cold front approaches from the west, and our region is deep within the warm sector of a low pressure system over the Great Lakes (shown below).

Also shown is Laura’s remnant low-pressure center, crossing into West Virginia in the early morning, embedded within the warm sector. The low is expected to traverse the mountains of Virginia during the afternoon and cross the Bay during the evening. Later Saturday, it begins interacting with the cold front, which eventually catches up and merges with the low pressure vortex. The timing of this interaction is uncertain.

The arrival of Laura’s concentrated zone of spin, still pronounced in the middle and upper atmosphere, will create strongly rising air and abundant wind shear. Since these dynamic winds will be embedded in an unstable and humid air mass, the likelihood of strong to severe storms, and locally heavy rains, will increase, particularly during the afternoon.

A higher flood potential exists along the mountainous terrain, where Laura’s moisture-laden winds and low-level energy interact with upslope regions along the western and eastern flanks. The rapid forward speed of the low-pressure zone, as it becomes embedded in fast jet stream flow, will limit rain accumulation.

We will be carefully monitoring Saturday’s severe weather threat. Upgrades in severe and/or flooding potential, for some areas, are possible.