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July-August 2009

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The Climate and Weather of Delaware, Maryland, and Washington, D.C.

The mid-Atlantic region of the United States is generally defined as the area of the East Coast between New England and the South. Stretching over a long expanse of territory, the mid-Atlantic states are home to such a wide range of weather and climate that it would be impossible to classify them in any efficient way. However, the two states of Maryland and Delaware and the city of Washington, D.C. (which, for the purposes of this article, will henceforth be referred to as a state), share a uniform topography that make them almost an entity unto themselves in terms of climate and weather, although the area still provides a potpourri of meteorological conditions that delight sun-worshippers and storm enthusiasts alike.

Located almost precisely in the center of the East Coast of the United States, these three states, aligned in a more-or-less east-west band, play host to hurricanes, nor’easters, and cold and heat waves, among many other meteorological conditions. And while most of its weather treks in from the west, the region also gets socked by weather from the east.

The topography of the area, from the mountains of Maryland to the beaches of Delaware, also contributes to diverse weather patterns, including sea breezes, mountain-induced downslope warming patterns, mountain snows, and valley and river fogs. Within this meteorological potpourri, there is likely a living environment to suit almost any lifestyle.

From the Mountains to the Oceans
Maryland is bordered by Pennsylvania to the north, West Virginia to the west, Virginia to the south, and Delaware to the east. Maryland’s weather is by far the most diverse of the region due to the ridges and valleys of its mountainous western regions and to its sloping eastern coastal plains. The Chesapeake Bay, formed when water from melted glacial ice flooded the Susquehanna River Valley, divides the state on the east, while to the south lie the Bay and the Potomac River. The area east of the Chesapeake, known as Maryland’s Eastern Shore, shares the mild climatic characteristics of the coastal regions of Delaware and Virginia.

Washington D.C., once a part of Maryland, is surrounded by its parent state on all sides except the southwest. The Potomac River flows into the Chesapeake Bay and defines the boundary between Virginia, Maryland, and Washington, D.C. The Tidal Basin, adjacent to the Potomac and located right next to the monuments on the National Mall, is a manmade inlet that lies at sea level. Because of the modifying influence of the river and the lack of any significant terrain variations, our nation’s capital receives far less diverse weather than Maryland does.

Delaware sits on the eastern shelf of Maryland, and its entire east coast is adjacent to water. The Delaware River flows from the north into Delaware Bay, which in turn feeds into the Atlantic Ocean. The only other state closer to sea level than Delaware, whose highest point reaches 450 feet, is Florida.
Taking the Temperature
The Köppen climate classification system categorizes these three states as humid subtropical, with mild winters and hot, humid summers. However, the region experiences four defined seasons, providing residents with sun, snow, sand, and surf, all within short driving distances.

In western Maryland, the Appalachian Mountains filter much of the air entering the region and therefore determine temperatures west of the Chesapeake Bay. Covering almost half of the western part of the state, the Appalachians form a protective barrier from colder, continental airmasses (from Canada and even modified Pacific airmasses). As continental air moves eastward, it flows downhill, warming adiabatically—sometimes by 10°F or more—and evaporating clouds. Sometimes western Maryland is locked in clouds, while areas to the east are mostly sunny.

Cold air can also get stuck east of the mountains, preventing a warmup even after a continental high pressure system tries to move eastward. As warmer air flows in from the southwest, it can override the trapped colder air (known as “cold air damming”) and set the stage for the region’s occasional bouts with ice storms. Only where Interstate Highway 68 cuts through a manmade gap in the mountains, from northeast West Virginia through the panhandle of Maryland, does cold air have an easier route eastward.

During the winter months, temperatures in north-central and western Maryland average in the low 30s. As one moves away from the mountains toward the coast (and larger bodies of water), temperatures become warmer. Washington, D.C., and other cities along the Delaware border and Maryland’s shoreline average in the high 30s to low 40s. Delaware’s temperatures and the temperatures along Maryland’s Eastern Shore remain warmer during the winter months than elsewhere inland because of the modifying influence of the ocean and the coastal storms that sometimes bring air from offshore regions (along or near the Gulf Stream) onto land.

During the summer, diurnal temperatures in the panhandle average about 10°F cooler than the central and coastal portions of Maryland and Delaware, with lows in the 50s and highs in the low to mid-80s. East of Baltimore, temperatures are much warmer, with lows in the 60s and highs in the 80s. The modifying effect of the ocean tends to keep summertime temperatures cooler on the eastern shore of Maryland and Delaware. The central portion of Maryland, including Washington, D.C., is out of the reach of cooling effects from both the mountains and the water. Consequently, it experiences hot and humid summers, with daytime temperatures often reaching the high 80s to low 90s. To escape the heat, residents of Washington, D.C., and Maryland often flee to the shores along the mid-Atlantic coast, from New Jersey south to North Carolina. Rehoboth Beach, Delaware, which is the closest shore to Washington, D.C., and its suburbs, has been fittingly nicknamed the nation’s “summer capital.”

Thunderstorms, Lightning, and Flooding
Thunderstorms occur throughout the year, although 75 percent occur from May-August. These storms frequently bring damaging winds, tornadoes, hail, and lightning. Most are associated with cold fronts that sweep in from the west or with daytime development over mountainous terrain. The most severe thunderstorms, which account for
about 20 percent of annual storms in the region, occur mostly during the spring and early summer when the atmosphere is more unstable.

Lightning, on average, kills about one person per year in Maryland and one every few years in Delaware and Washington, D.C. In 1998, lightning struck a group of people attending the Tibetan Freedom Concert at RFK Stadium in Washington, D.C., sending 1 woman into cardiac arrest and injuring several others. Scores of lightning-caused house fires, some resulting in hundreds of thousands of dollars in damage, occur each year.

The only documented lightning-related plane crash in the United States occurred 45 years ago in Delaware. Lightning struck an airplane near Elkton, igniting fuel tank vapors and causing it to crash. All 81 aboard were killed.

The region’s extensive coastline, mountainous terrain, and heavy urbanization make it extremely susceptible to localized river and flash flooding. Nowhere is this more pronounced than in the Baltimore-Washington corridor, particularly as a result of summertime thunderstorms.

The nation’s capital experienced its worst flash flooding since 1944 on August 11, 2001, when 6 inches of rain fell in less than 3 hours. Damage totaled $6 million dollars. In 1993, both the Potomac and Monocacy rivers flooded after 12 hours of nearly continuous heavy rainfall, causing $5 million in damage to homes and businesses and forcing the relocation of many families. Hurricanes and winter storms also bring floods to the region.

Overall, Maryland, Washington, D.C., and Delaware receive an average of 40-45 inches of precipitation annually. Maryland receives more than half of its precipitation from thunderstorms during the summer months from May-August. Maryland also holds the record for the heaviest 1-minute rainfall in the United States and possibly the world (1.23 inches). It occurred during a thunderstorm on July 4, 1956, in Unionville.

However, rainfall during summer is not consistent, and severe droughts have occurred throughout the region’s history. One of the worst in the region was during 2002. Much of the eastern seaboard was in drought that year, with extreme to exceptional drought conditions across the region from about March through September. Rainfall in some places totaled less than 50 percent of normal.

Tornadoes and Hail
Tornadic events in the Delaware, Maryland, and Washington, D.C. area appear to be increasing annually, but the higher numbers can be attributed mostly to increased population and better detection. There are simply more people and communities in the path of these storms because of population growth, and our technology has dramatically improved, allowing us to spot more tornadoes. This holds true across the nation.

Still, Maryland averages three to four tornadoes annually, while Delaware averages just one. Only 1 recorded tornado has ever touched down in Washington, D.C., an F0 twister on September 24, 2001.

Shortly after the Washington, D.C., twister, the same parent supercell thunderstorm spawned an F3 tornado that ripped through the University of Maryland campus in College Park, completely demolishing one building and killing two students. The tornado, with its estimated 200 mph winds, picked up a vehicle with 2 sisters and carried them as high as a nearby 8-story dormitory.

On April 28, 2002, La Plata, Maryland, experienced one of the most severe tornadoes in the state’s history. A supercell thunderstorm spawned an F4 tornado that caused 5 deaths and more than $100 million in damage.

Tornadoes often accompany the circulation of a tropical cyclone as it moves northeastward from the southeastern United States. Hurricane Ivan, the strongest of the 2004 season, was responsible for more than 45 tornadoes across the Maryland-Virginia region. Six twisters (and 2 deaths) occurred in Maryland on September 17-18.

Hail is often present during tornadic events and is more prevalent in the panhandle of Maryland due to orographic lifting and cooler ground temperatures in mountainous areas (which allow for less melt on descent). Hailstones in Maryland have reached 4.5 inches in diameter on a few occasions, in particular during the F4 La Plata tornado in 2002.
Hurricanes and Nor’easters
Since 1985, 4 major hurricanes and more than a dozen tropical storms and depressions have made their way up the East Coast to inundate the Northeast and mid-Atlantic with heavy rainfall and strong, damaging winds. One of the most devastating, Hurricane Floyd, dropped more than a foot of rain in several areas along Maryland’s Eastern Shore and Delaware. Property damage totaled $14.75 million on the Eastern Shore and $8.37 million in Delaware. Two children in Delaware drowned during this event.

In 2003, heavy rain and storm surge due to Hurricane Isabel’s high winds flooded coastal areas of Maryland and Delaware. The storm track (a southeast-to-northwest trajectory west of the Chesapeake Bay) allowed for a storm surge to move up the Bay, inundating parts of downtown Baltimore, which is far removed from coastal storm surge areas. Not only did Isabel cause more than 1 million residents of Maryland and Washington, D.C., to lose power, but her damage totals crept close to $1 billion.

Unique to the East Coast are mid-latitude cyclones called nor’easters, which are often associated with a coastal frontal boundary. These strong low pressure systems originate in the Gulf of Mexico or along and/or offshore of the southeast Atlantic Coast and trek toward New England. Nor’easters are known for their strong southerly offshore winds and strong northeasterly winds that pull cold air from Canada southward over land. As the storms continue to move northward and polar air masses mix with warmer gulf stream waters, they often increase in intensity.

While nor’easters might sometimes look like hurricanes on a satellite image, they are actually different in several ways. Nor’easters (which only occur in the cold season) feed off of cold and warm air interactions in middle latitudes, while hurricanes (occurring in the warm season) thrive on warm air and water in the tropics. Historically more devastating than hurricanes, stalled nor’easters can batter the coast for days, causing severe beach erosion and flooding from high storm surges.

One such storm, the Ash Wednesday storm of 1962, was said to be comparable in strength to some of the most intense hurricanes on record. It was also the strongest nor’easter of the century, according to the U.S. Geological Survey. Waves reaching 25-40 feet in height slammed into the coastlines of Maryland and Delaware, destroying millions of dollars of beachfront property, claiming 40 lives, and causing significant beach erosion throughout its 5-day visit.

Because they typically occur during the winter months, nor’easters can also become major snow-producers and have been the cause of some of the Northeast’s most memorable snowstorms and blizzards.

Blizzards, Ice Storms, and Floods
As noted earlier, the Appalachian Mountains can affect the type of precipitation that falls in the region. If cold air gets trapped east of the mountains, this can lead to ice storm conditions along and just east of the mountains. The damming effect can also contribute to new storm development off the mid-Atlantic shore. When coastal storms develop, the colder air remains over the region and significant snowfalls are more likely. Still, the location of the rain-snow line, or the position that decides the type and location of precipitation areas, often lies across the larger Washington, D.C., metro area. Sometimes the northwest suburbs will receive heavy snowfall while southeastern areas experience all rain or a snow-to-rain transition. If the coastal low pressure system is far to the southeast, the northwest suburbs might get no snow, while the southern suburbs are snowed in. This scenario, which often involves a shift in the snow band of 50-100 miles, makes a major difference in observed weather conditions anywhere in the region. Overall, snowfall amounts decrease from about 80 inches in the far western mountain areas to about a foot on Maryland’s eastern shore and across Delaware.

The region’s average annual snowfall (15-20 inches except for mountain locales) often accumulates in a few major storm events. One of the most memorable snowfalls in the region was part of the “Blizzard of ’96.” This massive storm system formed along the Gulf Coast and then moved northeastward, passing just southeast of Washington, D.C., during the January 6-7 weekend. While only reaching blizzard strength (winds of at least 35 mph and less than 1/4 mile visibility for at least 3 hours) at a few observing locations in this region, blizzard-like conditions occurred elsewhere along its path. Some two to three feet of snow fell across the region and caused the federal government, schools, and businesses to shut down for almost a week.

Though Baltimore received 5.4 inches more snow than Washington, D.C., in that event, it wasn’t until 2003 that Baltimore experienced its heaviest snowstorm in recorded history. The President’s Day Storm (not to be confused with the same-named storm of 1979), which took place over the weekend of February 15-17, was responsible for shutting down Reagan National Airport in Washington, D.C., as well as Baltimore-Washington International Airport near Baltimore. Dulles International Airport remained operational with one runway. Some of the highest snowfall accumulations in the region occurred in Maryland, with totals reaching 28 inches in Baltimore and more than 40 inches in the mountains.

In the Maryland-Delaware-D.C. region, ice storms can actually prove to be a more devastating winter phenomenon than snowstorms. The most common scenario for these events involves cold air damming. With a shallow cold layer of air lying underneath rain-filled clouds, precipitation does not freeze until it comes in contact with objects on the ground. The frozen precipitation can weigh down tree limbs and power lines, causing widespread power outages.

Such was the case from January 14-15, 1999, when a low-pressure system moved into the mid-Atlantic from the Tennessee Valley. Snow fell in some locales, but much of the precipitation, even with sub-freezing temperatures, was in liquid form. In some places, up to half an inch of ice accumulated. The weight of the ice on trees and power lines, in combination with winds over 40 mph, caused $3.2 million in damage. The Potomac Electric Power Company (PEPCO) reported that the ice storm was the worst in its service history. However, slippery roads and sidewalks proved to be a larger threat; more than 500 people were treated at area hospitals after falling on the ice. In addition, 30 Montgomery County, Maryland, school buses slipped off the roads.

What made this storm unusual was the fact that Montgomery County and portions of northern Fairfax County, Virginia, were iced in, while Salisbury on Maryland’s Eastern Shore recorded temperatures in the mid-50s.

Once the worst of winter is over, the region can still fall victim to Mother Nature’s whims in the form of flooding. Winter and spring are peak times for larger-scale flooding events, when winter snow and ice melt away and heavy rainfall ensues. Washington, D.C.’s, location on the banks of the Potomac River places the city at high risk.

Following heavy snowfall during the “Blizzard of ’96,” temperatures in the latter half of January soared into the 50s and 60s. Warm, southerly winds circulating around a storm west of the Appalachian Mountains melted 12-18 inches of snowpack within 12 hours. The Potomac River rose up to 21 feet above flood stage over the course of the weekend, producing Washington’s fifth-largest flood in 100 years.

This and the flooding caused by Tropical Storm Fran later that year caused almost $65 million in damage to the Chesapeake and Ohio (C&O) Canal. These costs were just to restore the canal back to its former condition. In 1997, repairs began on the bridges and retaining wall to allow the 4 million visitors per year to continue to visit the historical site for hiking, boating, and various other outdoor activities.

Heat Waves
Washington, D.C., is popularly known for being one of the hotter cities on the East Coast in summer, and the reputation is well deserved. The entire region can experience severe heat waves. In July, 1995, temperatures peaked at 99°F in Washington, D.C., but heat indices, which incorporate dew point effects, remained above 120°F. This contributed to two deaths from hyperthermia and eight others due to heat-related exposure. Later that month, oppressive heat was responsible for four more deaths. Heat wave conditions were likely intensified by the urban heat island effect. Farther toward the coast, however, a strong high pressure system during the Independence Day weekend of 1999 caused a heat wave responsible for the deaths of 4 people as well as thousands of chickens along the Delmarva Peninsula. High temperatures and humidity caused heat indices to exceed 110°F and led to record utility demands.

The states of Maryland and Delaware, as well as the District of Columbia, provide residents and visitors with as much variety in terrain and climatic variations as one can find anywhere within a short driving distance. Those wishing to escape the blustery winter snows of Western Maryland’s mountains can often find sunshine and at least slightly warmer weather to the east. In the summer, it’s easy to escape the heat by visiting the beaches of nearby Delaware and Maryland or driving to the mountains. Such variations make the region an exciting place to live, climatologically speaking, and ensure that there is something for everyone.                

H. MICHAEL MOGIL is a Certified Consulting Meteorologist and science writer who lives in Naples, Florida. KRISTEN L. SEAMAN is a senior at Embry-Riddle Aeronautical University (Daytona Beach, Florida) planning a career in the media industry.    

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