Thursday, December 31, 2009

Washington, DC Monthly/Annual/Decade Precipitation Update
Damp December, Ditto Decade (Wettest in Century)

See here for more Washington, DC weather records.

New Year's Update: An additional 0.12" on New Year's Eve brings the final December total to 5.85", more than 90% above average. It's the wettest December in over a quarter century, since the 5.91" in 1983. As noted earlier, it's also the 7th wettest overall.

The final total for 2009 is 45.96", putting the annual average for the decade at 42.58". Six of the years in the decade have been at least 15% above or below the long-term annual average. That equals the maximum number of such extremes also observed in the 1950s and 1980s.

New charts added: Annual precipitation amounts and departures, decadal annual averages.

Tune in next week for temperature analysis.

Original post:
With 0.31" of wintry mix and rain showers so far this New Year's Eve day, the Washington, DC December precipitation total is now almost 90% above average. This is a little ahead of the 87% excess of June, but well short of the 111% in May.

December is the third consecutive month of much above average precipitation and the sixth of the year, with 5 out of the 6 being over 50% above. November, at 46% above, just missed the 50% level. Two of the dry months, February and July, were more than 50% below average, so a total of 7 months this year had more than a 50% deviation. This December is now the 7th wettest in Washington history.

The annual total of 45.84" is over 6" more than the long-term average of 39.35", but still lower than the 46.49" in 2008. Seven years this decade and 6 out of the last 7 years have been above average.

The annual average of 42.57" for the decade 2000-2009 is the wettest in a century and the third wettest overall, behind 43.40" in 1900-1909 and 46.80" in 1880-1889. Actually, the 1870s were slightly wetter with an average of 42.79", but they include only 9 years, since official records began in 1871.

Tune back in over the New Year's weekend for final monthly and annual totals, plus more charts.

Images (click to enlarge):
- Washington DC 2009 monthly precipitation amounts and percentages above or below average
- 2000-2009 annual precipitation amounts and percentages above or below average
- 1871-2009 decadal average annual precipitation amounts (1870s include only 9 years)
All charts by CapitalClimate from National Weather Service data

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