Wednesday, September 27, 2006

Nice Now ... Gaining Rain Later?


Sunny, breezy.
A gentle southerly breeze and sunny skies have brought temperatures to the mid 70s in the Washington metro area this afternoon.

A cold front approaching from the Midwest could bring the possibility of showers by late tomorrow. For tonight's forecast and the outlook through the weekend, scroll down to Dan's post below.

Pictured: Yesterday was a great day for an outdoor lunch at Reston Town Center, by photographer Kevin Ambrose.

Precipitation Distribution: Drought or Naught?

Today is the 11th consecutive day without measurable rain in Washington, and it's been just under 2 weeks since more than 0.1" fell. Although August came in at barely 30% of the long term average, September is already nearly 40% above the average, so we're not talking about the dreaded "d" word here (especially since some folks tend to get quite exercised in the Comments section at any hint of such a mention; you know who you are!)

Looking back at the first 3 quarters of 2006, however, it does seem that there have been some interesting anomalies in the distribution of precipitation. The year so far, at just under 35", is comfortably above the 9-month average of 30". On the other hand, the 62 days with measurable precipitation are well below the average total of 85 for the year through September.

One way to look at this is by defining a "P/E" ratio (precipitation per event). This can be approximated by dividing the total precipitation for a month by the number of days in the month with 0.01" or more. (Strictly speaking, this is stretching the definition of "event", since one event can extend across multiple days.)

The chart shows that 3 of the 4 months beginning in June have had P/E ratios more than 50% above the long-term average. The August P/E of 0.34" per event was 89% of average, but the month's total rainfall of 1.03" was spread over only 3 days, or just 1/3 of the average number of August rainy days. The bottom line is that the year so far is running above average for total precipitation, but that total has mainly been achieved through a significantly smaller than usual number of events. chart from NWS data, photo © Kevin Ambrose

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Seasonal Outlook

Latest seasonal forecast: Click here.

Latest 3-month temperature outlook from Climate Prediction Center/NWS/NOAA.