Tuesday, March 28, 2006

Who'll Start the Rain?

The drought continues in the Washington DC metro area this afternoon. Scattered showers, a few with moderate rain, were mostly confined to the mountains of West Virginia at mid afternoon. A few very widely scattered showers had made their way along and east of I-81, but they were all south of I-66. They were heading to the northeast, but the easternmost ones seemed to be drying out as they got nearer to I-95. Charlottesville reported light rain at 3pm which lasted less than 20 minutes, and the amount was a trace. Sunshine through a broken high overcast was enough to push temperatures into the low 60s at most metro area locations.

The showers, which are associated with a weakening low pressure area pushing eastward from the Ohio Valley, may still manage to reach the metro area this evening, but accumulation amounts are likely to be meager at best.

Regional radar image shortly after 4pm today from weather.com

Tonight and Tomorrow

For tonight, mostly cloudy skies early will give way to gradual clearing by morning with lows around 42. There is a 30% chance of measurable rain. Tomorrow will see more sunshine than today and highs around 62.

Drought Marches On

Unless a downpour parks itself directly over the official rain bucket tonight, we are well on our way to breaking the record for driest March by a margin of over ½". This raises the interesting question of the connection between temperature and precipitation in March. Is a dry March warm because it has more sunshine, or is it dry because there is more cold, dry northwesterly flow?

The chart to the right shows a plot of the average monthly temperature vs. total precipitation for the Washington DC period of record starting in 1871. There is a lot of scatter in the data, so the relationship is fairly weak, but the regression line shows that, on average, March tends to be drier with increasing temperature. In fact, the second-driest March so far (0.64" in 1945) was also tied for the warmest at 55.5°. The driest, 0.57" in 1910, was also fairly warm with an average of 51.2°. What's unusual about this month is that the average temperature has been so, well, average. The monthly mean so far of 46.5 is less than a degree below the long-term average.

CapitalClimate chart from NWS data, photo © Kevin Ambrose

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