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.