US, Canada: Severe weather, flooding across eastern US and Canada through early Dec. 26 /update 1

Event

Thunderstorms with rounds of heavy rainfall, strong winds, and flooding associated with a low-pressure system and a cold front are forecast to occur across portions of the eastern and southern US through at least early Dec. 26. The affected area includes the Florida Panhandle, southeastern Alabama, Georgia, South Carolina, North Carolina, eastern Tennessee, Virginia, eastern West Virginia, Maryland, the District of Columbia, central and eastern Pennsylvania, Delaware, New Jersey, New York, Maine, Rhode Island, Connecticut, Massachusetts, Vermont, and Hampshire in the US, as well as southeastern Ontario, southern Quebec, New Brunswick, and western Nova Scotia in Canada. The storm system is likely to bring heavy rainfall, flooding, damaging winds, and even isolated tornadoes.

Government Advisories
In advance of the storm system, a tornado watch has been issued for northeastern South Carolina and eastern North Carolina. Flood watches are in effect for central Virginia, western and central Maryland, central Pennsylvania, Upstate New York, central Vermont, southern and central New Hampshire, and southwestern Maine. Additionally, Environment Canada has issued red warnings and special weather statements for heavy rainfall in southeastern Ontario, southern Quebec, New Brunswick, and western Nova Scotia. Officials are likely to update and possibly extend the coverage of the relevant weather alerts over the coming hours.

As of the afternoon of Dec. 24, moderate rain is falling across Upstate New York, central and eastern Pennsylvania, central and western Maryland, West Virginia, Virginia, and eastern Tennessee. Meanwhile, an intense line of storms has developed across western North Carolina, western South Carolina, central Georgia, southeastern Alabama, and the Florida Panhandle. Forecast models indicate this area of intense rain will quickly move eastward over the coming hours, reaching eastern Pennsylvania, Delaware, eastern Maryland, the District of Columbia, eastern Virginia, and eastern Virginia by the evening. The heavy rain and storms will arrive in New Jersey, eastern New York, Ontario, and Quebec overnight Dec. 24 into Dec. 25, and the New England region of the US and Atlantic Canada by early Dec. 25. The rain may eventually change over to a period of snow or freezing rain across West Virginia, central Pennsylvania, western New York, Ontario, Quebec, New Brunswick, and Nova Scotia.

Hazardous Conditions
Forecast models indicate widespread rainfall totals of 5-10 cm (2-4 inches) are expected to occur in the hardest-hit areas, including southern Quebec, New Hampshire, southern Vermont, western Massachusetts, Connecticut, Upstate New York, central and eastern Pennsylvania, northern Delaware, New Jersey, central Maryland, and central Virginia. Many of these locations still have an extensive snowpack from the major snowstorm that occurred Dec. 16-17; the heavy rainfall is likely to bring rapid snowmelt, likely leading to flooding in this region. Rainfall totals of 2.5-5 cm (1-2 inches) are expected across the rest of the affected area. Locally higher amounts are possible where the most intense storms develop. In addition to heavy rainfall, the storms will be capable of producing damaging winds and tornadoes across the affected area through at least early Dec. 26. The highest risk of tornadoes is in the Florida Panhandle, eastern South Carolina, eastern North Carolina, and eastern Virginia. Damaging winds of up to 110 kph (110 mph) are most likely to occur near the Atlantic coast, especially on Long Island and in southern and eastern New England. This could lead to widespread power outages.

Should sustained heavy rainfall occur, it could trigger flooding in low-lying communities near rivers, streams, and creeks. Urban flooding is also possible in developed areas with easily overwhelmed or a lack of stormwater drainage systems. Sites located downstream from large reservoirs or rivers may be subject to flash flooding after relatively short periods of intense rainfall. Landslides are possible in hilly or mountainous areas, especially where the soil has become saturated by heavy rainfall and where there is an existing snowpack.

Transport
Floodwaters and debris flows could render some bridges, rail networks, or roadways impassable, impacting overland travel around affected areas. Ponding on road surfaces could cause hazardous driving conditions on regional highways including the I-10, I-14, I-20, I-24, I-26, I-40, I-70, I-74, I-75, I-76, I-77, I-78, I-80, I-81, I-83, I-84, I-85, I-86, I-87, I-88, I-89, I-90, I-91, I-93, I-95, I-97, and I-99 corridors in the US, as well as the Trans-Canada Highway, the Macdonald–Cartier Freeway, and
Autoroute de l'Outaouais in Canada. Authorities could temporarily close some low-lying routes that become inundated by floodwaters.

Severe weather could also trigger intermittent flight delays and cancellations at regional airports including, but not limited to, those serving Raleigh (RDU), Norfolk (ORF), Washington (IAD, DCA), Baltimore (BWI), Philadelphia (PHL), Newark (EWR), New York City (JFK, LGA), Boston (BOS), Montreal (YUL), Ottawa (YOW), and Quebec City (YQB), though these are unlikely to be severe or prolonged. Flooding could block regional rail lines; freight and passenger train delays and cancellations are likely in areas that see heavy rainfall and potential track inundation. Localized business disruptions may occur in low-lying areas.

Advice

Monitor local media for weather updates and related advisories. Out of an abundance of caution, confirm all transport reservations and business appointments before travel. Make allowances for localized travel delays and potential supply chain disruptions where flooding has been forecast. Do not drive on flooded roads. Charge battery-powered devices in the case of prolonged electricity outages.

Resources

US National Weather Service
US Road Conditions
Environment Canada
Canada Road Conditions