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Postscript to the storms: "The DC Split?"

Josh Larson @ 8:27 PM

Recent grumbling abounds regarding the fact that people living in the immediate DC metro area have largely missed out on storms and significant measurable precipitation over the past 12-hours, continuing a pattern that has lasted for the past 5-7 days, and that has seemingly been present at other times this summer as well as during past summers.

Here is 24-HOUR DOPPLER-ESTIMATED PRECIPITATION over the DC metro area; the areas highlighted in red have seen the most rain over the past 12 hours; note that the majority of Washington, DC, Montgomery County, MD, eastern Fairfax County, VA, and Prince Georges County, MD received less than 0.30", with this number actually being on the high side in many locations:

Even more surprising, check out 7-DAY DOPPLER-ESTIMATED PRECIPITATION. Much of the most-immediate DC metro area has seen less than 0.25", while significant portions of southern Maryland, western Maryland and West Virginia, northeastern Maryland, Delaware, and most of Pennsylvania have seen a widespread 2-4" of rain over this period, with some locations an excess of 8". Recall that OVER A FOOT OF RAIN HAS FALLEN over the past couple of days in some locations in the Mid-Atlantic.

So what gives, you may ask. My Dad, no meteorologist himself, begrudgingly has termed this uneven rainfall distribution the "DC Split." Fellow weather aficionados and I have frequently discussed this seemingly-reoccurring phenomena. Unfortunately, there's no neat or satisfying answer. A lot of it is simply random, as weather tends to be. Part of it has to do with local topography, and elevation differences - which affect cloud cover, moisture availability (dew points), and instability - part of it has to do with local thermal and convective boundaries, sometimes altered by the mountains or the bay, and some of it has to do with the specific dynamics of the "shortwaves" of energy that have been pushing across the region.

If anyone has other theories as to the uneven distribution of storms and precipitation over the DC Metro area recently as well as more chronically, feel free to comment.

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