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. 

"Snowmageddon"

Snow Totals February 5-6, 2010   Source: NWS

On this date five years ago, Washingtonians survived a crippling blizzard that will forever be remembered as one of the most significant winter storms ever. Dubbed “Snowmageddon” and the “Superbowl Storm,” that storm spanned two days ending on Superbowl Sunday.  Power outages were widespread and the Federal Government even closed for four consecutive days.

Back in 2010, it took days for roads to be cleared and power to be restored due to the magnitude of the 4^th largest snowstorm on record in the Nation’s Capital.  In fact, power outages lasted longer in many areas around the DC Metro Region following that storm than for Sandy in October 2012.  A whopping 17.8” fell at National Airport.

Having a second Top Ten snowfall in the Nation’s Capital during the 2009-2010 winter was unprecedented.  Earlier, the December 2009 storm saw 16.4” at National Airport, making it the 8^th largest snowfall on record.  Also, the 32.4” at Dulles Airport during “Snowmageddon” set a record for largest two-day snow total, surpassing the 23.2” that fell during a two-day snowstorm in January 1996.

The Superbowl Storm developed as an area of low pressure along the Gulf Coast that intensified rapidly. It continued to strengthen as it moved up the East Coast and became a “Nor’Easter.”  That enabled the storm to draw a substantial amount of cold air into the Mid-Atlantic and Northeast while also drawing in copious amounts of moisture from the Gulf of Mexico and Atlantic Ocean that fell as record snow. That was truly the ideal scenario for snow lovers across the Mid-Atlantic Region!

Danger on the Mountain?

Snow in the Colorado Rockies

Skiing is an excellent way to combine the beauty of nature with getting outdoor exercise.  However, it’s important to be aware of one of the greatest dangers that skiers face around the world: avalanches. The severity of an avalanche depends on a multitude of factors; including, depth of the snow, slope of the mountain, volume of traffic, temperatures, etc.

All these elements cause stress on the snowpack of a mountain that can lead to an avalanche. Similar to hurricanes and tornadoes, the severity of an avalanche is measured on a scale of 1 to 5. However, the avalanche scale is a logarithmic scale and each category increases by a factor of 10.

It is vital to ski only in properly designated areas with a partner in case you become separated. As is the case with rip currents at the beach, avalanches can and often do occur with little or no warning.  Avalanches move downhill extremely quickly following the path of least resistance so it’s virtually impossible to outrun – or ski fast enough – to escape an oncoming avalanche.

Avalanches are more common in the Rocky Mountains than they are in the Eastern United States.  That’s largely because the grade of the mountains are typically steeper in the larger mountain ranges out west.  The best safety is not skiing alone, in unmarked areas, or whenever there’s an announced avalanche danger. 

Weather Quiz

True or False.

If a winter is snowier than average (such as the 2009-2010 winter season in Washington, D.C.) then it must also be colder than average.

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Answer to Weather Quiz question from January 23.

True. 

Washington, D.C. does see its largest snowfall totals from “Nor’Easter's” such as the January 1996 or February 2010 snowfalls - both among the Top Ten largest on record in the Nation's Capital.