Autumn Typically Features a Variety of Weather

Late September is when Washingtonians can usually start feeling the change in seasons in earnest.  The month of September begins with an average high of 84 degrees in the Nation’s Capital but that falls to 74 degrees by the end of the month.  However, it isn’t always a seamless transition to autumn and the change in seasons typically yields a multitude of weather.

Such was the case on September 27, 1993.  The day featured high temperatures in the upper 70s but those living in the area at the time will remember what happened in the early afternoon.  That’s when a line of severe thunderstorms brought several tornadoes from Loudoun County, VA to north central Maryland.  Although the spring is when the United States sees the highest frequency of severe thunderstorms, the autumn also sees severe weather as the seasons change.

September 1998 was a warmer than average month in the Nation’s Capital with temperatures reaching at least 90 degrees on ten different occasions.  The final 90 degree day of the month came 16 years ago today when a record high of 95 degrees was set at National Airport (the official weather reporting site for Washington, D.C.).  September 2010 was also a warm month with ten days of 90 degree heat as well.  By comparison September 2011 saw no 90 degree heat.

Fortunately, there is no extreme weather in the seven day forecast.  In fact, tomorrow's weather will feature weather similar to today being mostly sunny and stellar with highs in the low to mid 80s. 

Weather Quiz

Why is the water off the SE Coast of the U.S. significantly warmer in the summer than the water off the U.S. West Coast?

A.    Local ocean currents
B.    Faulty thermometers
C.    Climate change
D.    More hurricanes on the east coast during the summer and fall

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Answer to question from September 18:

Hurricane Isabel was a Category 2 at its time of landfall in North Carolina.  The name Isabel was subsequently retired by the World Meteorological Organization because of the significant damage it caused.  There will never been another storm named “Isabel.”

What is “Radiational Cooling?”

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

Cloud cover, for example, severely reduces the amount of heat that is able to radiate upward into the atmosphere keeping the nighttime temperature warmer.  Being in an urban area also hinders the process as the heat from buildings and automobile traffic keeps the city temperature warmer at night than nearby suburban areas.

Ideal conditions for radiational cooling include clear skies and calm winds coupled with low relative humidity.  Dry air heats up and cools off more quickly than more humid air, especially in rural areas. 

Such was the case yesterday at Dulles Airport in Sterling, Virginia.  Conditions were favorable for radiational cooling Monday night with clear skies, calm winds and low relative humidity.  Yesterday’s low temperature at Dulles Airport was a chilly 42 degrees compared to the average low of 53 degrees on September 23.  Given yesterday’s plentiful sunshine, the high temperature rebounded to a delightful 70 degrees. 

Such cool morning lows, coupled with mild afternoons, are what help the leaves begin to change color in the early autumn.  There will certainly be more such days this fall and you will now have a better understanding of what “radiational cooling” is too the next time you hear the term in a weather forecast.

The Rain in Spain is not Falling

Spain is one of the world’s leading producers of olive oil and wine.  However, a severe drought this year has caused considerable harm to its olive crop. That will lead to worldwide repercussions as the markets are forced to adjust to a reduced supply of olive oil.

The International Olive Council said earlier this year that global demand for olive oil is up, in part, because of increased demand from newer markets such as China and India. The drought was particularly damaging to the olive crop in Spain during May and June when conditions were exceptionally dry across the olive growing regions of Andalusia.  When a similar heat wave and dry period occurred in 2012, the price of olive oil increased by roughly 8%.

Conditions have also been rough in neighboring Portugal due to ongoing drought.  Portugal is prone to periodic drought because of a semi-permanent area of high pressure over the North Atlantic Ocean which can block storms from bringing rainfall to the country.  Like Spain, Portugal has significant olive and grape crops.

Although farmers could see higher profits in the short-term by charging more for the lower volume of olives and grapes they are able to harvest this year, the reduced yield is not good long term.  Like farmers in California where the wine industry is also at risk due to an ongoing drought, increased prices are only a short-term solution.  Not until the weather pattern shifts and the drought ends will agricultural production improve.

This post was filed from Madrid, Spain.

Weather Quiz

Hurricane Isabel's track (September 6-19, 2003)

Hurricane Isabel made landfall on September 18, 2003.  Once a Category 5 hurricane, what was Isabel’s strength at the time of landfall in North Carolina?

A.  Category 5

B.  Category 2

C.  Category 4

D.  Category 3

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Answer to question from September 8: 


False.  Although extremely deadly, Katrina’s death toll of more than 1,800 (according to the National Hurricane Center) is less than the all-time deadliest hurricane in U.S. history.  On September 8th, 1900, an intense hurricane made landfall in Galveston, Texas claiming approximately 8,000 lives. That was before hurricanes received names so it’s commonly referred to as the “Galveston, Texas hurricane of 1900.”

Remembering the Twin Dangers of Hurricanes Dennis and Floyd

Hurricane Floyd making landfall in North Carolina (Source: NOAA)

The summer of 1999 was exceptionally hot across the Mid-Atlantic Region.  There were three days of triple digit heat and 45 days of 90 degree heat.  The summer months were also exceptionally dry.  The lack of rainfall in conjunction with the extreme heat created tough conditions for farming and gardening.  However, by the time September 1999 arrived, the tropics began to get active.

Early that September, the remnants of Hurricane Dennis brought the Nation’s Capital appreciable rainfall.  Over a three-day period ending September 6, nearly 2” of rain fell at both National and Dulles Airports.  That was less than two weeks after nearly 5” fell at Dulles Airport over a five-day period ending August 27.  Since the rest of the summer was so hot and dry, so much rain over a relatively short period of time did not produce widespread flooding.  It just helped replenish the moisture content of the soil and fill up local reservoirs.

Little did Washingtonians realize when Floyd became a Tropical Storm on September 8 that major flooding would ensue when the remnants of Hurricane Floyd arrived the following week.  On September 15-16, 1999, National Airport got 3.67” and Dulles Airport recorded 2.56” of rain from the remnants of Floyd.  So in less than a month, there were two significant, multi-day rainfall events.  Since the ground was already saturated after the remnants of Hurricane Dennis passed through earlier in the month, Floyd’s rains caused extensive flooding across the Mid-Atlantic Region and Northeastern United States.

Thanks to Dennis and Floyd, September 1999 catapulted into the Top 5 for wettest Septembers in the Nation’s Capital, with a monthly total of 10.27”.  By comparison, average September rainfall is only 3.31” and the driest September on record in Washington, D.C. was in 2005 when just 0.11” of rain fell.

This post is filed from Lisbon, Portugal, where I've been researching some local weather issues that will be the subject of an upcoming blog post.

The Busiest Time of the Year?

Hurricane Hugo while a Category 5 in September 1989

 

 

 

This is the most active part of the hurricane season based on seasonal averages and climatology.  Early to mid-September is when the ocean temperatures are at their warmest and atmospheric conditions are traditionally their most favorable across the Atlantic.  Hurricane season runs half the year from June 1 through November 30.

Both last season and this season to date have seen below normal activity in the tropics.  In contrast, nearly every season starting in 1995 through 2012 featured a myriad of activity in the tropical Atlantic, including such catastrophic storms as Katrina, Ivan, Charley and Sandy. 

The lack of major hurricanes over the past season and a half is particularly noteworthy.  In fact, Hurricane Sandy was the last major hurricane – Category 3 or above – to form in the Atlantic Ocean back in October 2012.  Hurricane season has the potential to remain active well into October, so the fact that there aren’t any tropical storms or hurricanes out there today doesn’t mean the rest of the season will be quiet.

Some noteworthy storms have occurred on this date in years past.  On September 10, 1961, Hurricane Carla was a Category 5 storm, before making landfall in Texas as a Category 4.  On this date in 1965, Hurricane Betsy made landfall in Louisiana as a Category 4 hurricane.  Also, on this date in 1989, Hurricane Hugo began to develop.  Hugo made landfall in South Carolina as a Category 4 hurricane that caused widespread damage.  Fortunately, there are no Carla’s, Betsy’s or Hugo’s on the horizon today.