Wednesday, October 28, 2009

October Rainfall Extremes Set New Record

See here for more Washington, DC weather records.

Nov. 1 Update: An additional 0.13" on Halloween brings the final monthly total to 5.71", or 2.49" above average. Here are the monthly, seasonal, and year-to-date totals for the region:
Washington National 5.71"/9.02"/35.68"
Washington Dulles 5.70"/7.53"/38.93"
Baltimore BWI 6.24"/9.72"/42.57"
PM Update: Added chart of October 1920-29.

Original post:
With 1.23" of rainfall yesterday, and an additional 0.79" through this morning, the preliminary October rainfall total for Washington stands at 5.59", over 70% above the long-term average for the month. This precipitation excess caps off a decade of October extremes. The 0.02" in 2000 was the second lowest since records began in 1871, and the 9.41" in 2005 was the record highest.

The last 5 consecutive Octobers, and 8 out of the decade 2000-2009, have all been at least 50% above or below the long-term average. The only other decade in which there have been more than 6 such extremes was the 1920s, when there were also 8. This decade's extremes have all been at least +/-55% of average, however, whereas 1925 barely made the 50% level at 50.9%.

Of the 8 extreme Octobers this decade, 5 have exceeded on the up side and 3 on the down side. The 1920s, on the other hand, excelled on the dry side, with 6 of the 8 extreme months below average.

Image (click to enlarge): Washington, DC October 2000-2009 precipitation percentage above or below average; CapitalClimate chart from NWS data

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