Heavy Rain and Flooding

Precipitation in Maryland is the resultant from moisture transported from the Atlantic Ocean, the Gulf of Mexico, or from the Continental US West of Maryland. Some storms found in air masses that originate in the arctic may be enhanced as they pick up increased moisture crossing the Great Lakes. Annual precipitation in the region of Maryland and the averages about 59 inches. Floods can occur in any season and can affect large areas.

Three principal sources of moisture contribute to precipitation in Maryland. During summer and early fall, moisture from the Atlantic Ocean often originates from a tropical airmass. During winter, air moving inland from the ocean at low altitudes is considered to be polar maritime in origin, although often having tropical maritime air in the upper atmosphere. Air from the Gulf of Mexico and Caribbean Sea, moving northward both east and west of the Appalachian Mountains, is considered to be tropical maritime in origin; during winter, tropical air often will overrun colder air near the land surface between the mountains and the coast. In addition to the oceans, important moisture sources include local and upwind land surfaces, as well as lakes and reservoirs, from which moisture evaporates into the atmosphere. Typically, as a moisture-laden ocean airmass moves inland, it is modified to include some water that has been recycled one or more times through the land-vegetation-air interface. In addition, during fall and winter, substantial snowfall in extreme western Maryland is produced from initially dry, arctic airmasses that accumulate moisture as they cross the Great Lakes from Canada to Maryland.

Based on the 1981-2010 normal, statewide average annual precipitation is about 59 inches. The minimum recorded average annual precipitation is 37.4 inches at Cumberland; the maximum is 47.8 inches at Oakland. Monthly distribution of precipitation in Maryland is uniform and averages 3-4 inches, except for a maximum of about 4-5.5 inches during the summer.

Large storms that produce exceptionally large quantities of precipitation generally are associated with a strong, northward-displaced Bermuda High off the Atlantic Coast. When coupled with a deeper than normal trough of low pressure over the middle United States, this pattern produces enhanced moisture flow from the southeast, which, if it persists for more than a few days, can cause intense precipitation east of the Appalachian Mountains.

Most flash floods are caused by intense, localized, convective thunderstorms. These storms, which are most frequent in summer, occur in late afternoon and sometimes last well into the evening. A persistent, active frontal system lingering in the area also can gradually saturate the ground with slight to moderate rainfall for several days. Then, a single, intense storm moving along the frontal system can induce floods because of the saturated ground conditions.

Hurricanes and other convective tropical storms from the Gulf of Mexico and the Atlantic Ocean have inundated large areas with intense rainfall, commonly 6 inches or more in less than 24 hours. Flooding can be either local and sudden, or regional and prolonged.