A storm will rapidly strengthen as it moves northward from the United States to Canada. The budding bomb cyclone will unleash “a firehose of moisture” – torrential rain and powerful winds from the Appalachians to the Atlantic coast – that can snarl travel, trigger flooding and cause power outages at midweek.
According to AccuWeather meteorologists, the central pressure of the storm will plunge, creating a giant vacuum in the atmosphere that will cause winds to rush in. When the central pressure crashes 0.71 of an inch (24 millibars) in 24 hours or less, like this storm is projected to do, bombogenesis will occur and a bomb cyclone will be born. (Bomb cyclones occur when a cold air mass collides with a warm air mass, which can happen over ocean waters. The rapid drop in pressure causes new air to rush in, creating wind.)
The beginnings of the storm were already producing torrential downpours and severe thunderstorms from Louisiana to Georgia Tuesday morning. Tornado warnings have been issued for some of the thunderstorms. Damaging winds and flash urban flooding will be the main threats from the storms in the Southeast into Tuesday night. Some of the major southern U.S. cities at risk for urban flooding into Wednesday morning include Atlanta, Charlotte and Mobile, Alabama.
As downpours and thunderstorms ramp up in the Southeast into Tuesday night, rain will expand northward and begin to soak the central Appalachians, mid-Atlantic and southern New England regions. A wintry mix will occur in northern New England where chilly air remains for a time. The storm will quickly reorganize along the upper mid-Atlantic coast Wednesday. This new storm center will become the bomb cyclone, amplifying the rain from the Appalachians to the Atlantic beaches and the wind along the Atlantic coast as it moves northward through Wednesday night.
Enough rain will fall–a general 1 to 4 inches– to boost stream, river, lake and reservoir levels and trigger urban flooding. The AccuWeather Local StormMax™ rainfall for this storm is 6 inches in the Southeast and 8 inches in the Northeast. “Some rain will act like a giant firehose and organize into an intense north-to-south, west-to-east crawling squall with gusty winds,” AccuWeather Chief On-Air Meteorologist Bernie Rayno said.
A plume of moisture will likely be traced for more than 2,000 miles–all the way from the northeastern U.S. to the Caribbean Sea and will “fit the definition of an atmospheric river,” Rayno added. An atmospheric river is a long, narrow region in the atmosphere that carries a large amount of water vapor. They can vary in size, even over 1,000 miles long and 250-375 miles wide. The average AR carries the same amount of water vapor as the mouth of the Mississippi River, but strong ones can carry up to 15 times that amount. When atmospheric rivers reach land, they release their water vapor as rain or snow, which can cause heavy precipitation.
This advancing squall will bring the greatest risk of accidents on the roads, and ground stops at some airports. Ponding, greatly reduced visibility and even thunder and lightning can accompany the squall along the Interstate 95 corridor. Fog alone has already been causing delays in some locations and will resume or continue until the storm departs Wednesday night. “Locally severe thunderstorms can occur on Wednesday in the coastal Northeast with the possibility of a few tornadoes in part of the mid-Atlantic,” Rayno said.
While much of the heavy rain will tend to fall east of areas that received multiple feet of snow in recent weeks around the Great Lakes, some downpours will reach Vermont, New Hampshire and Maine, where there is a risk of rapid and significant small stream flooding. Much of the snow cover in northern New England has the potential to liquefy as several inches of rain pour down with mild, moist air. The combination of rain and mild air will increase the rate of melting snow, which can trigger urban flooding and add significant weight to the load on roofs that have not been cleared of snow. This added weight could lead to roof failure on flat or shallow-angle gables.
Even without snow on the ground, the firehose effect of rain alone can lead to significant urban and small stream flooding from Delmarva to New England, including the major metro areas of Philadelphia, New York City, Boston, Providence, Rhode Island, and Hartford, Connecticut. Driving through flooded roads may not only ruin vehicles, but could be serious enough to be a life-threatening situation.
Powerful wind gusts can occur without any thunder and lightning from this storm. The strongest winds will tend to be from the south along the Atlantic coast from Wednesday afternoon to Wednesday evening. In this zone from eastern North Carolina to southeastern Maine and the Maritime Provinces of Canada, gusts will frequently range between 50 and 70 mph with an AccuWeather Local StormMax™ gust of 85 mph.
Farther west, winds may be light most of the time over the central Appalachians Wednesday. However, as the storm center moves toward Canada and bombs out, strong winds from the west and northwest will range from 30-50 mph with locally higher gusts. These same sort of westerly gusts will break through to the Atlantic Seaboard later Wednesday night and Thursday.
While sporadic power outages can occur as the storm strengthens in the Midwest and the Northeast, the greatest risk of regional problems, including damage to property and holiday decorations, will be in coastal areas of the Northeast and in eastern New England in particular. The impactful nature of the storm in coastal areas of the Northeast will be like a landfilling strong tropical storm or hurricane.
People are urged to secure decorations as well as trash cans, recycling bins, tarps and trampolines so they do not become airborne projectiles that can injure people or damage property. In the wake of the storm, Arctic air will rush in from the Midwest and can catch up to the back side of the storm’s moisture.
Aside from a period of heavy lake-effect snow from Wednesday night to Friday, a general burst of snow can quickly cover roads and make for slippery driving from the eastern part of the Ohio and Tennessee valleys to the Appalachians from Wednesday to early Thursday. Those with travel plans on the ground or in the air can expect significant delays from Wednesday to early Thursday as a storm rockets northward from the Gulf of Mexico to Quebec, Canada.
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