High-resolution NAM model shows storms rolling through Washington region between 8 and 11 p.m., but it could be a bit earlier or later.

Hazardous Halloween weather is possible in the Washington area and many parts of the Mid-Atlantic, where a line of storms, some of which may be severe, will sweep through during the evening.

“A potentially dangerous weather event is unfolding for Thursday,” wrote the National Weather Service serving the Washington region.

Our general advice to parents and partygoers: Carefully monitor forecasts Thursday, but there could be a safe window for trick-or-treating before storms arrive. Generally, the earlier you can head out, the better. This especially holds true in the western parts of the D.C. region, where storms will arrive first.

However, given the possibility the timing of storms could shift earlier in the evening, it’s critical to follow forecast updates on Thursday and have a plan to quickly seek shelter if severe weather moves in.

Based on the latest model forecasts, here is when stormy weather is most likely to arrive:

  • Interstate 81: 5 to 9 p.m.
  • Washington’s western suburbs: 7 to 10 p.m.
  • Interstate 95: 8 to 11 p.m.
  • Chesapeake Bay: 9 p.m. to midnight

The storms are likely to arrive suddenly, unleashing a burst of very heavy rain while strong to isolated damaging winds are also likely. A brief, isolated tornado is not out of the question.

The National Weather Service Storm Prediction Center has placed the region in its “enhanced risk” zone for severe weather, level 3 out of 5, which is unusual for this late in the year.


Zones of risk for severe weather from the National Weather Service Storm Prediction Center. (NOAA)

A brief shower or two could form ahead of the main line of storms in the afternoon and early evening, but they would probably just present a nuisance, rather than serious disruption, for trick-or-treaters.

How the storms will evolve

The cold front — which is instigating this unwelcome, messy weather — looks to be draped across the eastern Tennessee Valley around lunchtime Thursday. Ahead of it, partial sunshine will help temperatures soar to 70 to 75 degrees, with gusty southerly winds and high humidity.

This is the same front responsible for snow and record cold in the Mountain West.


National Weather Service weather map shows cold front approach Appalachians on Thursday afternoon. (NOAA)

By dinnertime, the cold front should be crossing the Appalachians.

The main line of storms will cross the Interstate 81 corridor between about 5 and 9 p.m. It will stretch from South Carolina all the way up to New York, where it will become enveloped in a larger mass of rain.

In Washington, Philadelphia and Baltimore, the best shot of severe weather looks to increase around 8 to 11 p.m. Some of the higher-resolution models indicate the front may arrive toward the earlier end, while others suggest its progress slows some after it crosses the mountains.


In New York, the severe risk should hold off until after 10 p.m.

Behind the storms and attendant cold front, temperatures will drop sharply, as much as 10 to 15 degrees in an hour, for people out later Thursday night.

Impacts

Southerly winds ahead of the front will steadily increase through the day, gusting to around 30 mph late in the afternoon and early evening, even in the absence of rain.

Environmental conditions are conducive for pockets of severe weather, including damaging wind gusts and perhaps a brief, isolated tornado.

As the main line of storms moves through, brief wind gusts of 50 mph-plus are possible. Don’t wait to see/hear thunder or lightning, as this fine line of downpours may not produce much, if any, lightning.

There is also a non-zero risk of a tornado or two along any kinks that develop in the squall line, where locally rotating winds could foster a brief spin-up. The best risk for this appears to be in the western parts of the D.C. region as well as to the northeast in eastern Pennsylvania and New Jersey. Some models indicate a “mesolow,” a smaller rotating low-pressure system wrapped up in a larger storm, may develop and enhance the severe risk in these areas.

Otherwise, spotty wind damage is possible, especially with fully leafed trees. Strong high-altitude winds may also be mixed down from above toward the ground.

Technical discussion

The setup features a fairly potent wave of low pressure in the upper atmosphere, embedded in strong jet-stream flow. The wave will support rising air across the Mid-Atlantic during the evening.

The strong jet dynamics will also enhance “wind shear,” or a change of wind speed/direction with height. Storms that form will mainly be driven by wind shear and dynamics, rather than moist, unstable air. These types of cool-season events are termed “high shear low CAPE," with “CAPE” referring to Convective Available Potential Energy, the energy available for rising motion.

Mostly cloudy skies are likely to prevail during the afternoon and limit the amount of air mass destabilization, which is a good thing. However, abundant low-level moisture and warm air arriving on southerly winds will still generate enough instability to trigger storms. If we break into more sunshine, the risk of severe storms will increase.

However, the strong wind shear will tend to organize heavier storm elements and can promote small areas of rotation within them.


A line of strong to locally severe downpours with gusty winds and a small tornado risk will approach the Mid-Atlantic during Halloween trick-or-treating. This simulation is from the HRRR model, which predicts storms to reach the Interstate 95 corridor just after 8 p.m., on the earlier side of model predictions. (WeatherBell.com)

If the wind shear ends up being a tad stronger than forecast today, then the squall line could develop kinks or broken “S-shaped” segments, where small areas of rotation (mesocyclones) set up. Such a Quasi-Linear Convective System (QLCS) increases the odds of a few transient, weak tornadoes.

Uncertainty

Models are converging on timing, though uncertainty still remains. Models also often struggle to resolve how much rain occurs ahead of the main line. In this case, a few showers are possible, but a soaking is unlikely until the wavy line of storms moves through. We’ll continue to refine our forecast as need be.