DC’s Largest Snowstorm

Although the Nation’s Capital avoided the prolific snowfall totals of 1 to 3 feet seen in parts of New York and New England yesterday, the city’s largest snowfall on record occurred on this date in 1922.  Known as the “Knickerbocker Storm,” the snow began on January 27 and didn’t end until January 29.  By the time it was over, a record of 28.0” had fallen in the Nation’s Capital. 

Unlike the “Alberta Clipper” that brought this season’s most appreciable snowfall to date (2.4” at National Airport on January 6), the “Knickerbocker Storm” was a coastal storm that had the ideal track to bring copious snowfall to the Mid-Atlantic Region.  A coastal storm, or “Nor’Easter” as they’re commonly referred to, has the capacity to draw prodigious amounts of moisture from the Atlantic Ocean.

The 1922 storm brought such a high volume of snow that tragedy occurred at D.C.’s crowded Knickerbocker Theater.  The tremendous weight of the snow caused the roof on the theater to collapse while it was full of people.  That roof collapse claimed almost 100 lives, while many more were injured.  That remains one of the worst weather-related disasters in Washington, D.C., history.

More recently, there have been a series of major snowfalls during the last two decades in the Mid-Atlantic Region.  The January 1996 storm brought 17.1” of snow to the Nation’s Capital, while a storm in February 2003 dumped 16.7”.  The December 2009 storm brought 16.4”, while the February 2010 blizzard over Superbowl weekend left 17.8” of snow.  These are all in the Top Ten for DC snowstorms, but nowhere near the amount that fell 93 years ago in the “Knickerbocker Storm.”

“Surprise” Snow: Could it Happen Again ?

Snowfall Accumulations 1-25-2000    Source: NWS

On this date in 2000, Washingtonians had one of their largest snowstorms in recent memory.  What made that storm memorable to meteorologists and weather enthusiasts is the fact that the forecast models at the time did not do a good job.  As recently as 24 hours before the snow began on January 25, the computer models used kept the majority of the precipitation east of the DC Metro Region. 

 

It wasn’t until Monday evening, January 24, 2000, that meteorologists realized the storm was going to be much more significant than the computer models originally suggested.  In fact, winter storm warnings weren’t issued until late evening on January 24.  However, some people had gone to bed before they saw these updated forecasts and expected only a light dusting by morning.  Those people woke up to a major surprise on January 25, 2000. 

The Mid-Atlantic Region saw a significant amount of snow from the Nor’Easter with daily snowfall records at all three area airports.  National Airport recorded 9.3”, Dulles Airport in Sterling, VA, recorded 10.3” while BWI Airport saw a storm total of 14.9” for their highest snow totals since the “Blizzard of 1996.”  I was a student forecaster at the time with my school’s Campus Weather Service in Pennsylvania and vividly remember watching the snow move farther north and west each hour on radar until it enveloped the entire Mid-Atlantic Region and Susquehanna Valley. 

The relatively poor job the forecast models did with this storm 15 years ago was the subject of several studies that helped lead to a series of improvements since then.  Forecasting winter storms has an inherent amount of uncertainty to it and that makes routinely checking the latest forecasts critically important. 

Not only has weather forecasting improved due to improvements in technology but there is another thing that’s evolved in the last decade and a half too.  The explosion of social media all but guarantees that the weather forecast is readily available to the general public through Twitter, Facebook, smartphone apps, as well as conventional means such as TV, radio and the WUSA9 website.  The likelihood of snow tonight and early this week will put these tools to good use.

Weather Quiz

True or False.

Washington, D.C. typically sees its highest snowfall totals from “Nor’Easter's.”

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Answer to Weather Quiz question from January 10.

According to statistics from the National Weather Service, Denver averages the most snow of the four cities with an annual average of 57.5”.  Washington, D.C., is third on the list behind Chicago with an annual average of 15.4”.

It’s All About Timing

January 6 Snowfall -- Bethesda, MD

Although they don’t usually produce prodigious amounts of snow, “Alberta Clippers” can produce a few inches in spots.  Such was the case earlier this month when 2” to 4” accumulations were common across the region.  An “Alberta Clipper” is a fast moving area of low pressure that originates in the Canadian province of Alberta and typically moves Southeast across the Northeastern United States.  Another “Alberta Clipper” will pass through the Mid-Atlantic Region late tonight and tomorrow.  Forecasting any accumulations are especially difficult with the typically fast moving and light bands of precipitation characteristic of such a system.

The major difference between the “Alberta Clipper” on January 6 (that brought 2.4” to National Airport and 4.2” to Dulles Airport in nearby Virginia) and tomorrow’s is the amount of cold air.  Part of what made the snowfall earlier this month so tricky for area schools and commuters was that there was ample cold air already in place so the snow began to accumulate with the first flakes.  The snow also began at roughly the same time as the morning rushour and road crews simply cannot treat the roads if they get stuck in rushour traffic.

Tomorrow’s system will pass through at a similar time with precipitation expected to begin around daybreak in areas west of town first.  The storm should be east of the region by tomorrow evening and any associated precipitation should be over by then too.  However, there is much less cold air in place for tomorrow’s system to work with.  And it’s been relatively mild the last two days too with high temperatures reaching 50 degrees in spots.  That makes the timing and location of any precipitation just as tough to forecast as type. 

If there’s a light accumulation on rooftops and grassy surfaces then that shouldn’t have much of an impact on road conditions.  However, if there is a more significant accumulation closer to the DC-Baltimore corridor, that could create travel trouble depending on both air and ground temperatures.  If this “Alberta Clipper” passed through on the weekend as opposed to a weekday school officials wouldn’t be faced with the difficult decision of opening schools amid adverse weather conditions.

There is a high degree of variability inherent with any forecast of winter precipitation.  Whenever temperatures are going to be near freezing, the precipitation forecast often becomes more challenging.  As we saw earlier this month, it doesn’t take much snow to have a significant impact on the daily routine for many in the Nation’s Capital.  That’s why monitoring the latest forecasts from your favorite meteorologists – such as those I work with at WUSA 9 – is so important.

The Threat of Post-Blizzard Flooding

Potomac River, Great Falls, MD

Common types of flooding in the Mid-Atlantic Region are from weakening tropical systems (such as from Irene and Lee in 2011) and slow moving thunderstorms, but there is occasionally a third cause of major flooding in this part of the country.  Rapid snowmelt in conjunction with mild temperatures and rainfall can cause significant flooding for the Nation’s Capital and surrounding suburbs.

Such was the case in January 1996.  Following one of the area’s largest snowstorms on record (with widespread snowfall totals of 15” to 25” across the DC-Baltimore corridor) there was a major shift in the weather pattern.  For three days starting January 17^th, 1996, high temperatures were significantly above average in Washington, D.C., and reached the 50s and 60s.  That led to rapid snowmelt of the record snowfall that fell the previous week. 

Complicating the rapid snowmelt brought on by the dramatically milder temperatures was significant rainfall of nearly an inch in spots.  The rainfall on top of the melting snow created the worst flooding much of the region had seen since June 1972, when the remnants of Hurricane Agnes caused severe flooding in the Mid-Atlantic Region and Northeastern United States.

Some other reasons for the flooding included storm drains covered by copious amounts of snowfall and the fact exposed ground was either frozen or previously saturated by melting snow.  That left nowhere for the additional runoff and rainfall to flow.  So streams and rivers – including the Potomac – experienced significant flooding.  According to the U.S. Geological Survey, the Potomac River reached a peak flood stage of 19.3 feet during the January 19-21, 1996, flooding.  That’s significantly higher than flood stage on the Potomac River in Georgetown which is six feet.

Following a major winter storm, the safest way for snowfall to melt is gradually so no flooding or associated dangers (such as washed out roads or power outages) occurs.

How does Snowpack Make it Colder?

Clear skies enhance "radiational cooling"

The coldest part of the day is typically just after sunrise.  That’s because all the heat from the previous day will have radiated upward from ground level.  This process, known as “radiational cooling,” occurs every night.  However, there are certain variables that can either help or hinder this process.

Cloud cover, for example, reduces the amount of heat that is able to radiate upward, and that keeps the ground temperature warmer than it otherwise would be.  Being in or near an urban area also hinders the process, as the heat from buildings and automobile traffic keeps urban area mornings several degrees cooler than in suburban areas.  Since water heats up more slowly than air and cools off more slowly, being near a body of water reduces radiational cooling as well.

Ideal conditions for radiational cooling include clear skies and calm winds.  During the winter months, snowpack enhances radiational cooling.  Since white is a reflective color, areas with snowpack tend to remain a few degrees cooler during the day as solar energy is reflected away.  Given the solar energy lost to reflection, temperatures tend to fall rapidly at night under clear skies with calm winds and a fresh snowpack.  That’s happened several times in the last week since the Mid-Atlantic Region saw it’s first appreciable snowfall of the winter last Tuesday, January 6. 

Last Wednesday night under clear skies, with calm winds and a fresh snowpack across the region, temperatures plummeted to their lowest levels since January 2014 in the Nation’s Capital.  By Thursday morning, January 8, the low temperature had plunged to 12 degree at National Airport.  That was significantly colder than the daily average low of 29 degrees.  It was even colder at Dulles Airport where it was 3 degrees.

Such cold temperatures during the last week have left ground temperatures near the freezing mark.  That meant the snowfall early this morning began sticking as soon as the first flakes started falling, creating treacherous roads and sidewalks.  Although accumulations were generally light, with a dusting around the immediate DC Metro area, St. Mary’s and Calvert Counties in southern Maryland saw 1” to 2”.  Since both the air and ground temperatures were sufficiently cold, the snow began accumulating almost immediately causing school delays and closures.  That demonstrates, too, why listening to the latest weather forecasts is so important.