Hurricanes pose the greatest risk for a widespread natural disaster in the Mid-Atlantic region from August to October.
The East Coast is in an active hurricane cycle, which began in 1995. Active cycles typically last 25 to 30 years. Past patterns along the East Coast suggest several notable storms strike during a four or five year period, in tandem with a less active time lasting several years. (The 2004 through 2010 seasons were generally tranquil, with a nearly total absence of hurricanes making landfall between Georgia and Maine.)
The Middle Atlantic states have well-defined cycles of increased hurricane activity. It’s as if someone turns on or off a faucet. Previous active cycles in the region occurred from 1876 until 1904 and from 1933 until 1961.The period from 1969 until 1979 brought the Mid-Atlantic several notable hurricane-related floods, although that time frame is considered less active in the North Atlantic. (Sometimes, the Mid-Atlantic region seems to march to its own drummer.)
The region’s climate is highly cyclical. Extremes are often followed by extremes. A drought, for example, may be followed by a hurricane-related flood. A period of years with little widespread severe storm activity is often followed by two or three years with several destructive storms affecting large sections of the Mid-Atlantic. These may include hurricanes and winter nor’easters.
Although August, September and October are often regarded as the peak of the hurricane season, the Middle Atlantic states have not experienced a significant October hurricane since Hazel in 1954. Before then, destructive October hurricanes tended to batter the region once or twice each decade and October’s monsters–hurricanes and nor’easters–were part of local lore.
Hurricanes that occur within a month or two of each other, in the same general geographic vicinity, often take roughly parallel tracks.
Coastal sections are due for a destructive hurricane. Hurricane Gloria in 1985 was the last to cause modest to significant losses to all Mid-Atlantic shore areas, from Virginia to Long Island, New York. The Great Atlantic Hurricane of 1944 was the last to cause severe damage along the shoreline from Virginia to New Jersey. The Mid-Atlantic coast has had a dearth of major coastal hurricanes since the early 1960s. Similar quiet periods occurred from the late 1820s until the late 1870s, and for about three decades during the early years of the 20th century. These less active periods were followed by many violent hurricanes, six or more within 25 years.
On rare occasions, intense hurricanes (Category 3 or stronger at landfall in North Carolina or while off the Mid-Atlantic coast) have produced a massive wave or series of waves described by those on shore as appearing like “tidal waves.” These caused immense damage to buildings and other structures along the shoreline. Generally, the storm waves quickly lost momentum and losses were limited to sections within a block or two of the beach. Such events have occurred along sections of the Mid-Atlantic coast in 1821, 1878, 1938, 1944 and, perhaps, at other times. The huge waves struck as the center of the storm made its closest passage, as easterly winds turned westerly. Interior sections are due for a highly destructive hurricane-related windstorm. Hurricane Hazel in 1954 was the last tropical cyclone to carry actual hurricane force (74 mph and greater) winds through a large section of the Mid-Atlantic interior, from Virginia to Pennsylvania. The region’s climatological history suggests that inland hurricanes such as Hazel occur about twice each century. Similar events occurred in 1667, 1724, 1769, 1775, 1821, 1878 and 1896. Highly destructive winds generally ranged roughly east-west in a 50 to 100 mile band in the storm’s northeastern sector, although a brief period of higher winds sometimes occurred after the center’s passage, when winds turned westerly.
The popular Saffir-Simpson hurricane intensity scale understates the potential for wind damage in the Mid-Atlantic region. A Category 1 hurricane (sustained one-minute winds of 74-95 mph) can be expected to do widespread Category 2 and 3-type damage. Isolated areas may see Category 4-type losses. Leafy trees and other vegetation, less wind-resistant structures and infrastructure, as well as, perhaps, other yet to be identified factors are responsible for the region’s wind susceptibility. Also, Saffir- Simpson estimates of tidal surge, particular in the Chesapeake Bay region, seem understated based on observations during Hurricane Isabel.
Rapidly advancing hurricanes, those with forward speeds greater than 30 mph, seem to have a greater ability to deflect higher winds aloft down to the surface, mainly in the northeastern sector (when the storm is tracking in a northerly direction). The strongest winds are location specific. As described by an observer in Washington, DC., after the great windstorm of September 1896, “In hundreds of instances a well constructed roof, rafters and all, was blown off, while close by very frail structures at the same height were uninjured.” Any Category 3 or stronger hurricane making landfall from the Carolinas on north and accelerating above 30 mph threatens destructive winds well inland.
‘Major’ hurricane Mid-Atlantic style: Meteorologists define a ‘major’ hurricane as possessing at least Category 3 strength (sustained winds of at least 111 mph). Because of the Middle Atlantic states’ susceptibility to wind damage, a Category 1 hurricane (sustained winds of 74-95 mph) tracking through interior sections will cause major destruction. Furthermore, a tropical cyclone that stalls off the Mid- Atlantic coast for more than a day may produce major shoreline damage even if it is something less than a Category 3.
Tropical cyclones bring great differences in rainfall and wind speeds over short distances. Tropical Storm Hanna (Sept 2008), for example, dumped less than two inches of rain to more than eight inches in the Washington, D.C., metropolitan area. Hurricane Isabel (Sept. 2003) lashed Washington and its suburbs with peak gusts ranging from less than 50 mph to nearly 80 mph. (Higher winds aloft reached the surface unevenly.)
When evaluating the severe weather risk associated with a hurricane, it is important to consider the possibility of violent storms in the air mass ahead of the tropical cyclone, in adjacent weather systems, and in the air mass behind. The worst weather may occur indirectly, outside the hurricane’s circulation, sometimes days before or after. Tropical Storm Nicole in 2010 is an example, as huge rainfall occurred in the tropical flow that was only partially part of Nicole. In September 2011, Tropical Storm Lee disintegrated as an organized weather system soon after landfall in September 2011. Its remnants merged with other weather features to produce catastrophic flooding in part of the Mid-Atlantic region.
The Mid-Atlantic region boasts some of the most intense short-term downpours on earth. A few are related to tropical cyclones. The intense rainfall does not necessarily correlate to elevation. Hurricane Camille, for example, dumped more than 27 inches of rain on sections of Nelson County, Virginia, within about five hours in August 1969. Thunderstorms stalled by a hurricane off the New Jersey coast bucketed Ewan, New Jersey, (just 20 miles south of Philadelphia) with more than 22 inches in about 10 hours during September 1940. (While Nelson County is mountainous, Ewan is nearly at sea level.)
On rare occasions, for unknown reasons, remnant hurricane systems have regenerated while their centers of circulation were over land. All such incidents have been heralded by a surge in rainfall intensity, resulting in generalized and flash flooding. Some examples include the Great Labor Day Hurricane of 1935 and hurricanes Cleo (1964), Camille (1969), Agnes (1972) and Gaston (2004).
A hurricane deluge is of particular concern in mountainous areas. Steep slopes in mountainous sections allow rapid runoff. An extreme event, such as occurred with Hurricane Camille, can liquefy the soil generating deadly mud flows. Meanwhile, Mid-Atlantic urban sections, often situated on hilly terrain, are at risk of flash flooding. Concrete, asphalt and other impervious surfaces reduce the absorption of rainwater, increasing runoff. The monsoonal downpours of a tropical cyclone can quickly turn destructive and deadly, as occurred with Hurricane Gaston in 2004 when it pounded the Richmond, Virginia., metro area with rainfall totals that topped 10 inches.
Remnant systems tracking through the Mid-Atlantic region may spawn tornadoes, particularly in the northeastern sector (if tracking in a northerly direction). They are generally short-lived, with intermittent tracks usually less than five miles. Pinpointing where a tornado will touch down is beyond our present knowledge. Although most have occurred during daylight, a considerable number have prowled at night.