Summertime Storms

The summer of 2014 has had several unique characteristics as we reach the official midpoint of meteorological summer – July 15^th (meteorological summer being June 1^st through August 31^st).  Many of the weekends during late spring and summer have been both sunny with relatively comfortable relative humidity.  That’s not to say there haven’t been some hot days as a record high of 97 degrees was set in the Nation’s Capital on June 17^th. 

July has gotten off to a fairly stormy start in the Mid-Atlantic Region.  In fact, according to National Weather Service records, today will be the 8^th day during the first half of July that severe weather has been observed in the Mid-Atlantic Region.  These summertime storms and bouts of extreme heat can be largely attributed to the position of the “Bermuda High”: a semi-permanent area of high pressure common during the summer off the East coast of the United States.  It helps pump ample amounts of warm, humid weather northward from the Deep South during the summer months.

Some may have heard the term “polar vortex” mentioned again during the last few days and thought back to how cold the winter was for much of the Eastern United States.  The truth is the “polar vortex” is also a semi-permanent feature that hovers above the North Pole and helps regulate cold air.  The jet stream will take a temporary plunge across the Central and Eastern United States the next few days, bringing cooler than average weather from the Midwest to the Mid-Atlantic and Northeast. 

The leading edge of the cooler air – the cold front that is passing through the Mid-Atlantic Region today – caused the heavy showers and thunderstorms that developed this afternoon.  The “Bermuda High” and “Polar Vortex,” while interesting, are not new weather phenomena and should not cause alarm once better understood.  So Washingtonians should enjoy the sunny, fall like weather tomorrow and Thursday with highs only in the low 80s – typical of early to mid September.  Hazy, hot and humid weather will be back soon.

Weather Quiz

“What makes a thunderstorm “severe”?   The presence of:  

A.  Hail of at least 1" in diameter or greater.

B.  Wind gusts of at least 58 mph.

C.  A tornado.

D.  Any of the above.

Please submit your answers as a comment below.  Come back next week to find out if you were correct and why.

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ANSWER TO QUIZ FROM JUNE 30^th:    The busiest month of the hurricane season is September because the Atlantic Ocean is its warmest and wind patterns are most favorable for tropical storm and hurricane development.

July 12: This Day in Hurricane History

The first named storm of the 2014 season, Hurricane Arthur, was a Category 2 hurricane with sustained winds of 100 mph.  It gave the Outer Banks a glancing blow on July 4 and became the earliest hurricane on record to make landfall in North Carolina.  According to the National Hurricane Center, damage from Arthur was minimal and there were no reported fatalities.

  

However, on this date in 1996, a more significant system struck North Carolina.  Hurricane Bertha made landfall as a Category 2 storm near Wilmington, North Carolina and went much further inland. At its strongest, Bertha had been a major, Category 3 hurricane with maximum sustained winds of 115 mph.  Bertha formed in the tropical Atlantic Ocean and was a rare July “Cape Verde” hurricane.

Hurricanes that originate in the tropical Atlantic are dubbed “Cape Verde” hurricanes since they form not far from the Cape Verde Islands off the coast of West Central Africa. These hurricanes are usually among the longer lasting storms since they have the entire tropical Atlantic Ocean to travel across before reaching the Caribbean Sea. However, Cape Verde hurricanes do not usually form until early to mid-August when the ocean has had a chance to warm sufficiently.

Bertha was an exception having been named a tropical storm July 5, then upgraded to a hurricane on July 7.  Damage in the Mid-Atlantic was extensive, totaling $250 million from North Carolina northward to New England. At Dulles International Airport, a total of 1.42” of rain fell from the storm, while a weak tornado formed in Dundalk, Maryland.

How Hot Can it Get ?

This is the hottest part of the year in the Nation’s Capital, with the average daily high temperature being 89 degrees from July 7 to the 22.  Fortunately, there are no extreme heat waves on the horizon as temperatures are expected to remain within a few degrees of average for the next several days. 

However, just two years ago Washingtonians endured a record heat wave that began on June 28, 2012.  That was the first day of an 11 day stretch of high temperatures of at least 95 degrees at National Airport.  During this period, an all-time June record high of 104 degrees was set on June 29. 

That was also the day that the damaging complex of thunderstorms known as a “derecho” caused extensive damage and power outages around the Nation’s Capital.  It took the better portion of a week for power to be restored across most of the Mid-Atlantic Region, since the “derecho” took down numerous trees and power lines.  Winds gusted as high as 90 mph in places during these storms – equivalent to a Category 1 hurricane. 

During the record setting 11 day heat wave of 2012, another impressive weather feat occurred with 4 consecutive days of triple digit heat at National Airport from July 5 through July 8, 2012.  In fact, July 2012 was the second hottest July on record, narrowly behind July 2011.  That’s not to say comfortable weather doesn’t occur in July.  For example, on this date in 2009, the high temperature was a very comfortable 81 degrees at National Airport.    

Flooding in the Nation's Capital?

Flooding is one of the world’s most dangerous natural occurrences. The Population Reference Bureau reports that flooding claims more lives in the United States every year than either tornadoes or lightning. The Nation’s Capital has three major types of floods: (1) those caused by rapid snowmelt after a large storm; (2) the heavy rain from the remnants of a tropical storm or hurricane; and (3) a prolonged period of rainy weather that saturates the ground.

In January 1996, Washington, D.C. had its 5^th largest snowstorm on record. According to the National Weather Service, 17.3” fell at National Airport, with amounts nearly double that in the suburbs. By mid-month, however, there was a major shift in the weather pattern that led to much milder weather with temperatures in the 50s combined with heavy rain. The warmer weather caused rapid snowmelt and the heavy rain on top of the melting snow simply didn’t have anywhere to flow. This was due to a combination of snow clogged storm drains and ice jams on local waterways. Many low lying areas saw extensive flooding and the District of Columbia reported $10 million in damages.

The second major type of flooding across the Mid-Atlantic Region is caused by decaying tropical storm systems. A good example of this occurred in 1999 after a hot and dry summer. Water restrictions were issued before the Labor Day weekend of 1999. That’s when the remnants of Hurricane Dennis brought significant rainfall to the region and helped alleviate the drought conditions.

But the ground was not ready to absorb additional rainfall when, less than two weeks later, the remnants of Hurricane Floyd inundated the region. Rainfall totals up to 5” were common along I-95, while rainfall two to three times that occurred east of the I-95 corridor. Floyd’s remnants not only brought an abrupt end to the water restrictions, but conditions went to the opposite extreme, with significant flooding to local water ways and low lying areas.

Another infamous flood in Washington history occurred in June 1972 with the remnants of Hurricane Agnes. On June 21, 1972, not only were daily rainfall records set at National and Dulles Airports, but rainfall totals for the storm were 7.19” and nearly 11”, respectively. Rivers, streams and creeks overflowed their banks resulting in property damage.  It was also disastrous for the Chesapeake Bay with all the agricultural runoff that accumulated.

The flooding that resulted from such copious rainfall remains the benchmark for how devastating flooding can be across the Mid-Atlantic Region.  The Potomac River peak stream flow measured in cubic feet per second (CFS) in the aftermath of Agnes set a record of 359,000 CFS.  That’s compared to the January 1996 flooding flow of 347,000 CFS.  To put that in perspective, the average stream flow for the month of January along the Potomac is only 13,000 CFS.

The third significant type of flooding occurs more gradually since it doesn’t revolve around a singular event as the first two kinds do.  Many local residents will remember how wet late August and early September 2011 was.  The Mid-Atlantic Region saw appreciable rainfall from the remnants of two tropical systems as well as a few other weather systems.  In less than three weeks, from August 25 through September 11, 2011, National Airport observed 11.3” of rain.  That’s 4.65” more than the Nation’s Capital typical sees during all of August and September combined according to the National Weather Service.  This gradually led to significant flooding of local rivers, streams and creeks as the rain occurred over a two and a half week period.

It’s important for the public to be vigilant along bodies of water whenever flooding is a possibility. Flash flooding often occurs with little or no notice, but flood conditions usually have warning signs. Meteorologists and local authorities will warn of possible flood conditions so you can take proper precautions such as leaving your car and moving to higher ground.