Hurricanes in the mid-Atlantic: How bad do they get?

August 8, 2013
Hurricane Isabel, seen from satellite, on the morning of September 18, 2003. NASA.
Hurricane Isabel, seen from satellite, on the morning of September 18, 2003. (NASA)

Hurricanes occasionally make landfall in the Mid-Atlantic states. Have you ever wondered what year experienced the greatest number of storms? What was the costliest or deadliest storm? How about the oddest storm track? In this piece we examine the threat of tropical tempests looking back at history.

The data

An extensive national dataset on hurricane tracks and intensities, compiled by the National Hurricane Center, was used to examine all occurrences of tropical cyclones crossing the mid-Atlantic region from 1851 to 2012. The term “tropical cyclone” includes all named systems, including tropical storms and those in the post-tropical phase, but excludes tropical depressions.

In this analysis, the mid-Atlantic region extends from North Carolina to Pennsylvania, from West Virginia to New Jersey, and includes all states in between.  It excludes storms in which the center passes offshore.

Mid-Atlantic hurricanes, by the numbers

Of the 1,470 named storms impacting the continental U.S. since 1851, the mid-Atlantic has been visited by 270 of these, or 18 percent of the total. Statistically speaking, that’s an average of 1.7 landfalling storms per year.

However, our region can experience long stretches of inactivity.  For instance, the seven-year period from 1917 to 1923 witnessed only two landfalling storms. Yet over a similar duration, from 2002 until 2009, we experienced 21 storms. The greatest number of storms in any year was seven, occurring in 2005.

Our region’s level of activity is probably best described as episodic, with frequencies in-line with the well-known Atlantic Multidecadal Oscillation (AMO). The AMO describes a cyclic rise and fall of Atlantic basin hurricane activity on time scales of 20-30 years, tied to intrinsic (natural) modes of climate variability.

Keeping track of the track

Hurricane track is influenced by several factors, but fundamentally on the strength and position of the mid-Atlantic Ridge – a sprawling, subtropical dome of high pressure across the central and western Atlantic. The ridge establishes a broad-scale, clockwise circulation that wheels embedded hurricanes westward across the Atlantic, then re-curves them toward the north.

If, in a given season, the ridge lies close to the U.S. mainland, storms are repeatedly steered into the Mid Atlantic. When displaced farther east, most storms will re-curve harmlessly over the ocean.

Historically, hurricanes or their remnants have approached our region from nearly every compass direction, except the north. By far, the most common trajectory (60%) is from the south, describing an arc along the mid-Atlantic coastline.  Nearly 11% of storms arrive from the south but deeper inland, along the Piedmont, and 11% have tracked along the spine of the Appalachians. Examples of each of these are shown below.

: Examples of Mid Atlantic hurricane tracks along the coastal, Piedmont and mountain regions. Image generated using Tom Rabenhorst’s Atlantic Hurricane Analyst Tool (UMBC).
Examples of mid-Atlantic hurricane tracks along the coastal, Piedmont and mountain regions. Image generated using Tom Rabenhorst’s Atlantic Hurricane Analyst Tool (UMBC).

Then there are the oddball tracks, which describe the remaining 18% of the track database. They run transverse to the physiographic “grain” of our region (the southwest to northeast trend of the Appalachians).   Examples include Fran (1996) and Isabel (2003), both of which were steered toward the northwest;  the remnants of Camille (1969), which approached from the Ohio Valley;  and recently Sandy (2012) which hooked due west off the Atlantic.

And let’s not forget the rare loop or “hook” patterns, shown below, for Abby (1968), Ginger (1971) and an unnamed “Hurricane 1” (1901).

 

Examples of looping and hooking hurricanes terminating in the Mid Atlantic region. Image generated using Tom Rabenhorst’s Atlantic Hurricane Analyst Tool, UMBC.
Examples of looping and hooking hurricanes terminating in the mid-Atlantic region. Image generated using Tom Rabenhorst’s Atlantic Hurricane Analyst Tool, UMBC.

 

When to watch out

Tropical cyclones have traversed the mid-Atlantic in every month of the year except (as expected) the dead of winter and early spring (e.g. January through April). Six storms have hit during May and six in November. The peak month is September, accounting for 37% of all storms, consistent with the Atlantic basin’s climatological peak of hurricane activity around September 15.

But June is not necessarily “too soon”; witness Agnes (1972) which devastated much of the mid-Atlantic with widespread river flooding.  Nor is October “all over,” as the horrors of Sandy (2012) remind us.

All about intensity

Fortunately, the mid-Atlantic has never been visited by a Category 5 storm. That would be extremely rare, since the very warm sea surface temperatures required to sustain such a storm aren’t typically found at our higher latitudes.  But our region has experienced the impact of four Category 4 storms, the most recent of which was Hugo (1989). Hugo made landfall near Charleston, S.C., then headed northward along the Appalachians generating Category 1-2 winds.

The most likely intensity category during a mid-Atlantic landfall is tropical storm  (sustained winds > 39 mph), accounting for 37% of all our storms. Higher intensity categories are uncommon (14% for Cat 1, 15% for Cat 2, 4% for Cat 3, < 1% for Cat 4).   This is a very typical distribution for many natural hazards, in which intensity and frequency are inversely related, in what is termed the “law of rare events.”

In terms of property loss, Hurricane Sandy gets top billing, particularly in the northern portion of the mid-Atlantic, accumulating $68 billion in losses (second only to Katrina, with losses exceeding $125 billion). While Sandy is better described as a large “hybrid” system (blending extratropical and tropical characteristics), it did maintain an intense warm core and an eye wall during landfall in New Jersey. But Sandy was an outlier event in many ways. In fact, the next most expensive hurricane (based on national rankings) to strike the mid-Atlantic is Floyd (1999), number 13 down the list, followed by Isabel (2003, #14), Fran (1996, #17) and Agnes (1972, #21).   These storms individually generated damages in the range of $1 to $5 billion.

Fatalities from Mid-Atlantic hurricanes

Finally we consider lives lost. At least in modern times, the greatest number of tropical cyclone fatalities in our region arises from the effects of freshwater flooding, and often deeply inland. Occasionally these floods unfold several days post-landfall, many hundreds of miles from the coastal point of entry. Devastating floods have arisen when post-tropical remnants undergo transition to an extratropical vortex, and/or interact with the Appalachians.

It is difficult to accurately assess mortality statistics from historic natural disasters, and I have limited my perspective to the past 100 years. In compiling fatality metrics, I also do not consider those arising from ships lost at sea.

Since 1900, the greatest number of mid-Atlantic hurricane fatalities has not exceeded that of Camille in Nelson County, Va.   As shown below, this exceptionally rare flash flood and ensuing mudslides claimed an estimated 150-160 lives in the space of just eight hours. The storm approached from the west, during the night, and the Weather Bureau forecast was simply for a “chance of showers, clearing in the morning.”

 


Rain accumulation map derived from remnants of Hurricane Camille, August 18-19, 1969. (NOAA)

Diane in 1955 was another unusually deadly storm, claiming 100 lives in Pennsylvania (and 77 in Connecticut) from extreme flooding in the Poconos and Delaware Water Gap region. Agnes (1972) created widespread flooding throughout Virginia, Maryland and Pennsylvania, with heaviest rains concentrated along the Appalachians. It is the flood of record in Pennsylvania. A total of 82 lives were lost among these three states.

While other forms of inland severe weather develop during a hurricane’s post-tropical phase (i.e. tornado outbreak – Ivan, 2004;  intense gradient winds – Hazel, 1954;  storm surge in Chesapeake Bay – Isabel, 2003), heavy rainfall is the most common end result.

In future CWG posts I’ll delve more deeply into the many destructive facets of hurricanes in the mid-Atlantic.

Jeffrey Halverson is a Professor in the Department of Geography and Environmental Systems at the University of Maryland, Baltimore County. He is the newest contributor to the Capital Weather Gang. More info.

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