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Posted at 10:30 AM ET, 09/24/2009

D.C. Rain Linked to Intensity of Hurricane Season?

By Jason Samenow

* Cooler Friday, Wet Saturday: Full Forecast | NatCast | UnitedCast *

Have you noticed the lack of rain lately? Our rainfall deficit since July 1 is almost 5", and "abnormally dry" conditions are closing in on the region according to the U.S. Drought Monitor. At the same time (as our hurricane expert Greg Postel just explained), the hurricane season has been eerily quiet, with only 6 named storms in the Atlantic basin. So, does the dearth of rain have anything to do with the anemic hurricane season?


In many years, we receive significant rainfall from the remnants of tropical storms and hurricanes after they make landfall. So, intuitively, it would make sense if our wettest (driest) late summer and early fall periods coincided with active (inactive) hurricane seasons.

The logic behind this reasoning: the more hurricane activity, the greater the odds of a storm making landfall, and the greater chance our region would get some of the rain. Over the weekend, I decided to examine the data to see if this hypothesis had any legitimacy. The answer: kind of.

I plotted an index of hurricane activity known as ACE (Accumulated Cyclone Energy) against the rainfall at Reagan National during the months of July-October for the period 1950-2008. There was a correlation, but it was weak (r-squared of 0.17). However, what was most revealing to me was the rank of the wettest (driest) years compared to the rank of the most (least) active hurricane seasons.

Nine (ranks 1, 2, 3, 4, 6, 8, 9, 11, and 12) of the top-15 most active hurricane seasons coincided with the 15 wettest years at Reagan National in the 59 year period (and nine of the top-15 wettest years coincided with the top-15 most active hurricane seasons). Only 3 (ranks 7, 10, and 13) of the top-15 most active hurricane seasons coincided with the driest 15 years. And none of the 15-wettest years coincided with the 15-least active hurricane seasons.

These findings, therefore, make it no surprise that this year has been dry. 2009's ACE is just 40 -- which would make it the 14th least active since 1950 (of course it could still increase if more storms are named).

What this quick and dirty analysis implies to me is that a very active hurricane increases the likelihood of a wet late summer to early fall period in D.C. while an inactive hurricane season increases the chances of a dry period, as has been the case this year.

By Jason Samenow  | September 24, 2009; 10:30 AM ET
Categories:  Local Climate, Tropical Weather  
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I distinctly recall the drought relief brought by Hurricane Floyd to the mid-Atlantic almost exactly 10 years ago (Sept 19, 2009). The drought was one of the worst ever in this region when Floyd's torrential rains came to the rescue. In some areas of eastern Virginia and Maryland the deluge from Floyd dropped as much as a foot of rain (~6" in Metro region).

On the other side, of course, hurricane rains can result in devastating floods

Posted by: SteveT-CapitalWeatherGang | September 24, 2009 1:38 PM | Report abuse

Jason - kudos for the nice research - I was hoping someone would come out not only report the drought but took a crack at explaining it...

Now I have an enginnering degree but a layman in meterology. I did notice metro areas north and south of us all have received better than average precipatation: many parts of Jersey, Philly, Baltimore, Richmond, Charlotte, and etc. It was just DC Metro! No hurricanes for the others either.

Posted by: LoudounGeek | September 24, 2009 7:35 PM | Report abuse

Unfortunately, the "much-needed-rain" crowd seems to be back in business!!!

We have a big swing dance Saturday night at the Spanish Ballroom with the Tom Cunningham Band...and Sue Palka is talking about "as much as TWO INCHES" of heavy rain with possible thunderstorms. [I thought that by late September we are supposed to be OUT of the "dangerous lightning" season for practical purposes around here!] We may just have to make sure that this expected DELUGE does not start flooding out the Orange Line west of Ballston before 7 PM Saturday. Of course I ought to check Metro's website about track maintenance; they have been working on the stretch of track between Ballston and East Falls Church on recent weekends.

Posted by: Bombo47jea | September 24, 2009 11:40 PM | Report abuse

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