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Posted at 11:30 AM ET, 01/29/2010

Slight shift north with snow accumulations

Best chance for accumulation: late Sat. a.m. thru evening

* Very cold weekend: Full Forecast | Farewell to wind record *
* Winter Weather Advisory tomorrow for far southern suburbs (map) *

With models really having locked in on a southern track for a storm now pummeling parts of Arkansas, Tennessee, northern Mississippi and northern Alabama with snow and ice (and Louisiana and southeast Texas with intense thunderstorms), we're becoming increasingly confident that a major snowstorm is not in the cards for the immediate D.C. area.

Confidence remains not as high, however, as to how far north minor to moderate accumulations will reach. A slight shift northward in this morning's model runs was just enough to prompt a similarly slight shift to the north in our accumulation forecast (see map above to the right) compared to our earlier map. The main effect is that areas inside the Beltway now have a slightly better chance of seeing an inch or a little more rather than an inch or less. The best chance for accumulating snow should be between late morning and late evening Saturday.

Why the uncertainty in all this?

Keep reading for the answer, and for our metro area accumulation probabilities. Also, see our full forecast into next week.

Because a hard-to-pinpoint band is likely to set up somewhere near the D.C. area where accumulations will decrease (from south to north) from several inches to possibly no accumulation over a span that could be as small as around 50 miles or less.

The cause of this tight accumulation gradient is a strong high-pressure system that will be centered to our north/northwest -- the same system responsible for our very cold temperatures -- and that we expect will act as a wall stopping the northward progression of accumulating snow. (The sinking motion associated with high pressure tends to dry out the air and limit precipitation).

Considering that a very small shift in the storm's track could have a significant impact on who sees snow and how much, it's still appropriate to provide accumulation probabilities for the metro area, even this close to the storm's arrival:

45% chance: Less than 1"
30% chance: 1-3"
15% chance: 3-6"
10% chance: 6"+

Stay tuned for more updates, and see our full forecast into early next week.

By  |  11:30 AM ET, 01/29/2010

Categories:  Updates, Updates

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