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D.C. area sees increasing snow chances Sunday and Monday

The storm may end region’s longest period without at least a half-inch of snow. But there’s a lot of uncertainty about accumulation.

Canadian model simulates snow over the D.C. area on Sunday afternoon.
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Computer models are in reasonably good agreement that the storm slamming California on Wednesday will track across the country and bring snow to the Washington region on Sunday and possibly into early next week.

The storm has a chance at ending Washington’s longest period on record without at least a half-inch of snow: 706 days.

Many models show the potential for about three to five inches of snow, but scenarios that would bring very little or substantially more (into the double digits) also are on the table. To see such a range in possible amounts for a winter storm in the D.C. area is normal for an event four or five days into the future. By Friday or Saturday, forecasts should converge on a narrower range.

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Models also differ substantially on the storm’s timing. Some predict the first flakes as early as late Saturday night while others hold off any snow until Sunday afternoon. Sometime Sunday morning is perhaps most likely. The end time for snow ranges from Sunday afternoon to as late as Tuesday, depending on whether a coastal storm develops and continues precipitation over a long duration.

The other wild card is whether precipitation changes to a wintry mix or even rain and freezing rain after a period of accumulating snow. If the storm takes a track more to the north, as some models simulate, such a changeover would probably occur.

The National Weather Service has placed the Washington region in an “enhanced” threat zone for a winter storm Sunday into Monday.

Chance of at least one inch: 50 percent

Chance of at least three inches: 35 percent

Chance of at least six inches: 20 percent

Note these percentages are intentionally conservative because of the number of days until precipitation begins, taking into account the possibility the forecast could change and that models sometimes overdo snow chances at long lead times.

Discussion

As a zone of low pressure approaches from the west this weekend, models have converged on the idea of a period of snow on Sunday as warmer air from the south and southwest is lifted over a dome of cold air supplied by high-pressure to the north.

For us to get a really big snowstorm would require a secondary low pressure system to develop to our southeast and track up the Mid-Atlantic coast while strengthening rapidly. The European computer model continues to suggest this as a possibility although other models are not as enthused.

Considerable differences still exist in how the various modeling systems are handling the location, evolution and track of the secondary low. Therefore, numerous possible scenarios are still in play but all include some accumulating snow.

The good news for snow lovers is that the American (GFS) forecast still predicts a pretty good thump of snow on the order of three to five inches Sunday before a dry slot moves in and the precipitation changes to light rain and drizzle (or freezing rain/drizzle in our colder areas). The UKMet and Canadian models show a similar scenario. Their simulated snowfall is on the high side of what storms with a similar evolution usually produce but they do suggest that our chance of getting at least one to three inches snowfall is pretty high.

The reason these models do not project more snow is because they project the secondary low to form too late and too far east to lock the cold air in place and keep precipitation going. Instead, after the initial slug of snow, they predict a dry slot to punch into the region as temperatures rise while the secondary low forms and exits to our east.

The European model, on the other hand, projects the secondary low-pressure system to develop rapidly near the coast of North Carolina and slowly move up the Mid-Atlantic shore, resulting in a longer duration snow event. Its latest run brings the secondary low close enough to the coast to draw in some mild air, which could cut back on snow amounts especially along and east of Interstate 95. For our western areas, it shows a huge snowstorm. The storm would also turn into a big wind producer as it cranked up offshore.

The different outcomes presented by the European and American computer modeling systems are well-illustrated in their simulations of the location of the secondary low-pressure zones early Monday in the graphic below.

Of the group of 50 simulations in the European modeling system, the majority position the lows close to eastern North Carolina and southeast Virginia. In the American modeling system’s set of simulations, the majority of lows are centered well offshore the southern Delmarva Peninsula.

Though subtle, the difference in the track forecasts between the modeling systems has major implications for how much snow they forecast. The European model predicts a 60 to 70 percent chance of at least six inches of snow, whereas the American modeling system forecasts only about a 20 percent chance.

The bottom line is that the area will probably experience some accumulating snow Sunday into Sunday night. It could even last into Monday and Tuesday if the secondary storm develops near the North Carolina coast and slows down as it heads north. We’ll be monitoring the forecast over the next several days and fine-tune the forecast for what may be the biggest snow of the year for many parts of the region.

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