“The third winter storm to slam the Northeast coast last month reached its height on Friday, February 26, impacting nine states in the mid-Atlantic and New England through Saturday,” said Dr. Peter Dailey, director of atmospheric science at AIR Worldwide. “The storm, a powerful low pressure system, originated off the mid-Atlantic coast, intensifying as it came ashore. It moved slowly into New York Friday morning and gradually dissipated over New England over the weekend before moving out to sea.”
The storm combined heavy snowfall, flooding rain and impressive winds, some of which reached hurricane-strength. In Manhattan, where nearly 21 inches of snow were recorded in a 36-hour period, the storm set an all-time record for snow in the month of February. The storm also set a record there for accumulation in a single day. Elsewhere, snowfall totaled more than two feet over parts of eastern New York State and western Massachusetts. West Halifax, Vermont, received 38.5 inches of snow.
Dr. Dailey continued, “The repeated impact of winter storms in the Mid-Atlantic region in recent weeks has been influenced in part by the ongoing strong El Niño conditions. The presence of anomalously warm waters in the east Pacific during El Niño shifts the global atmospheric circulation in such a way as to displace the jet stream pattern over North America to the south. The storms that form in these conditions are more likely to impact states in the Southeastern and Mid-Atlantic areas. This effect of El Niño, combined with the ability of strong storms to pull in cold arctic air in their wake is responsible for unseasonably cold temperatures in states like Florida.”
“Over the past month, the jet stream has remained in generally the same location—aligned so that repeated storms have taken similar tracks passing over the Mid-Atlantic. Exacerbating the problem is the fact that storms in this region are often influenced by the nearby and warm Gulf Stream waters, which provide additional energy and humidity.”
When storms follow each other in such quick succession, as they have in recent weeks, there is little time for accumulated snow to melt. As a result, the potential for roof damage is increased. As with the two earlier storms, damage caused by snow accumulation is expected to be a major source of insured losses from this latest event.
Design loads for structures vary across the United States. As little as zero pounds per square foot is allowed in Florida, southern Louisiana, Texas, and parts of the Southwest, while as much as 100 pounds per square foot is required in Michigan’s Upper Peninsula and northernmost Maine. The design snow load for Washington, D.C., is about 30 pounds per square foot. With this tolerance, light metal and long-span roofs (such as on hangars or warehouses) are especially vulnerable to snow loads, as are flat or low-slope roofs. Ten to 20 inches of snow can produce loads of roughly 15 to 30 pounds per square foot on flat roofs.
Engineered structures—high-rise and government office buildings in New York—must conform to high load tolerances and damage to these structures is therefore expected to be minimal. But the roofs of marginally-engineered structures (such as convenience stores) can collapse under large accumulations of snow, particularly if their roofs have not been well maintained.
Dr. Dailey commented, “The late February winter storm was accompanied by higher wind speeds than the two earlier storms. Thus wind damage will also play a role in insured losses for this event. At the level of wind speeds observed, AIR would expect claims from damage to non-structural elements, such as roof coverings, cladding, awnings and signage. Damage to both structures and automobiles from fallen trees is also likely. While individual claims are not expected to be severe, the number of claims could be significant because the storm impacted a very wide area from New Jersey to Maine.”