What Goes into Predicting the Weather in the Nation’s Capital?

A Clear Day in the Nation's Capital   Source: National Park Service

Meteorologists often enjoy the challenge of a tough forecast.  However, forecasting the weather in the Nation’s Capital can be especially challenging even for seasoned meteorologists. That can be attributed to some local factors that influence the weather in and around Washington, D.C.

Downtown temperatures are often warmer than suburban areas outside the city.  The combination of steel and concrete in buildings and heat of automobile traffic keep both daytime and nighttime temperatures a few degrees warmer than outlying areas.  A good example of this occurred on January 8.  The low temperature at National Airport was 12 degrees.  Meanwhile, it was almost ten degrees colder at the more rural Dulles Airport where the temperature fell to 3 degrees.  Known as the “Urban Heat Island Effect,” the difference of just a few degrees can be the difference between a cold rain, a wintry mix and an accumulating snowfall.

Another phenomena that can influence temperature can be found on the Chesapeake Bay – or any body of water.  Since water heats up more slowly than the air – and also cools off more slowly – it can have a large influence on the temperatures of the adjacent land.  Since air over land warms relatively quickly and warm air rises, cooler air from over the water rushes in to fill the void that’s created by the rising warm air over the adjacent shore.  This is known as the “Sea Breeze.”  The reverse happens at night when the wind blows offshore.  Since National Airport is right on the Potomac River, a breeze off the Potomac River can cause the temperature to be much different than nearby areas away from the water.

A third factor meteorologists wrangle with when making a forecast is when an area of high pressure over the Northeastern United States is slow to move out to sea.  Winds blow clockwise around high pressure systems in the Northern Hemisphere.  So an area of high pressure over New England would produce generally northeasterly winds across the Mid-Atlantic Region.  This kind of wind can produce chilly temperatures and low clouds. 

You may sometimes here the expression “cold air damming” and that refers to cold air literally being trapped or “dammed” up against the Appalachian Mountain chain.  A good example of this happened this past Sunday (February 8) when the wind changed direction at National Airport and that allowed the temperature to reach a record tying high of 68 degrees.  Meanwhile, Hagerstown, MD, remained entrenched under the northeasterly wind flow – or “wedge” of cooler air – and saw a high temperature of only 41 degrees.

Hopefully, this helps provide readers with a better sense of some of the local features that meteorologists, such as my colleagues on the WUSA9 weather team, analyze when preparing their weather forecasts.