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Posted at 10:05 AM ET, 03/30/2012

Lack of snow and recent dryness cause for concern for Washington D.C. summer drought?


Abnormally dry conditions are seen across the area in the March 29, 2012 update. March rain is running more than 2” below normal, and the period since November is also running about that far below. Source: U.S. Drought Monitor.
Even with some — generally underachieving — rains last weekend and Wednesday, much of the area remains abnormally dry. The new U.S. Drought Monitor update, released yesterday, even shows the beginnings of a short-term drought around the Maryland and Delaware beaches.

For the month, usually one of the wetter of the year, rain is running over 2” below normal. And that’s on the heels of two somewhat dry months preceding it. Of course, I don’t even need to mention (or probably shouldn’t without ducking) the snow drought we went through this past winter.

Does the recent dry weather portend a drought this summer?

It has been dry, particularly recently, but the winter months in D.C. are historically not that wet a period when compared to the rest of the year . January and February see the least precipitation of any months on average. This January and February (2012) were slightly below normal with deficits of 0.62” and 0.29”.

On the other hand, December had a surplus of 1.85” (although a big chunk of it fell on one day). And while November had a precipitation deficit over 1”, we should remember how wet August-October were thanks to rains from Irene and Lee among other events.


The top-10 least snowy March-November periods. Some winters in D.C. history have had snow before and after this period, but all of the current snowfall normal falls during these months. The summer normal is 10.44"
When it comes to snow, the 2” that fell was third least on record (unless any more falls in April) trailing only the winters of 1972-73 and 1997-98 when 0.1” of snow was recorded officially.

Are there signs that this seemingly precipitation-free period will continue and grow worse?

Not really.

Any correlation between November through March snowfall and resulting precipitation into the spring and summer to follow is basically nonexistent. See the chart on the right. This is, in part, because patterns often change significantly during these periods.

Dry sometimes begets dry, but we are so far just in a short-term spell without major rains following a normally dry part of the year.

Let's look at both rain and snow together during the November through March period, and determine if other years with precipitaion deficits like this one ended up with dry summers.

The precipitation total for the current November-March is 12.38”, so I examined past year within 1” of that amount during the same period. This produced 23 similar November-March stretches, including last year’s 12.77”.
March-November precipitation plotted against summer (June-August) precipitation at Washington, D.C.

Fourteen of those 23 stretches (again, including last year) had above normal summer rainfall, with nine below normal. The lowest summer total of the group was in 1954 when 6.09” of rain fell, and the highest occurred in 1955 when 19.39” deluged the area. Normal is currently 10.44” from June-August.

Lastly, a glance at just March rolled forward to summer again also offers little insight as what to expect. If we get no more rain (and stay at 1.02”), this March will be tied for the 6th driest on record. In case some errant shower park over the area in the next day, let’s consider the 21 Marches with less than 2” of rain on record and see what kind of rains the following summer delivered.

Of those 21 Marches, 10 finished with rainfall above the current summer normal of 10.44”, and 11 finished below (one a paltry .03” below). The most rain seen in those years was 19.88” in 1969 and the least was 4.62” in 1966.

The overall message here is that it’s premature to worry about a drought. However, if the dry spell should continue into the next few weeks and especially months, we may need to re-address the issue!

All data in this post obtained through the National Weather Service Baltimore/Washington.

By  |  10:05 AM ET, 03/30/2012

Categories:  Local Climate, Latest, Droughts

 
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