

Sledding at the Capitol during Snowmageddon, Feb. 2010. (Ian Livingston)

The chances that the Washington region will be smacked by a major snowstorm this weekend are increasing. The storm needs to be watched closely this week — it has the largest snow potential we’ve seen since 2010, but as we learned from watching the Nationals last year, potential isn’t always realized.

We remain conservative with snow chances because there is still a lot of uncertainty in any forecast so far.

Next chance of accumulating snow: Friday to Sunday

Chance of at least 1 inch: 70 percent

Chance of at least 4 inches: 40 percent

Chance of at least 8 inches: 20 percent

The forecast models are in unusually close agreement for a storm that is more than four days in the future. All forecast models depict a strong low-pressure center somewhere near or off the coast of North Carolina on Friday night. Such a track all but guarantees that most of the region will have at least some snow.

As always with such storms, how much snow falls will depend on the exact track of the storm and whether the snow mixes with sleet and rain. Accumulating snow is likely across portions of the area, but how much any specific location could get and whether the snow mixes with or changes to rain is still uncertain. A 60-mile shift in the storm track is the difference between snow and rain for the region.

As with many major winter storms that affect our area, the heaviest snow accumulations are most likely in locations north and west of the District.

The National Weather Service is forecasting a moderate winter storm threat from 7 a.m. Friday to 7 a.m. Sunday. “Moderate” is the second-highest level in the new winter-storm product. In the region, it means confidence is medium and potential effects are high. “Potential impacts include significant travel delays and closures,” the NWS says. “Plan ahead to minimize impact on you and your family.”

Three storm scenarios below.



(National Weather Service)

Scenario 1: A crippling storm with double-digit snow in some locations (somewhat likely)

This scenario would play out if a low-pressure system strengthens rapidly and tracks just off the coast of the Carolinas and Virginia. This specific track keeps cold air funneling southward and east of the mountains and means all or most precipitation falls as snow.

Last night’s European UKMET and GFS forecasts were all predicting such a scenario. This morning, the European GFS and the GEFS ensemble mean doubled down on the scenario, simulating snow totals rivaling the epic storms of winter 2009-2010. Even the GEFS ensemble mean is predicting 1.5 inches of liquid water equivalent — which translates to double-digit snow totals, assuming there is no mixing with sleet or rain.



The GFS is forecasting a major winter storm for D.C. this weekend. (weatherbell.com)

Oddly, even with a perfect track, the GFS depicts a brief period during which the precipitation could mix with sleet. But the track of the low- and the position of the high-pressure system to the north argues that, if the model is correct, all or almost all of the precipitation would fall as snow across the city. Mixing would be possible south and east of the city.

If Scenario 1 played out, the city would likely grind to halt because of the harsh travel conditions and extended period of snow that could last from Friday into Saturday. In an extreme scenario, blizzard conditions could develop, and the combination of wind and heavy wet snow could lead to power outages.

Scenario 2: Significant snow west of the city, wintry mix for the close-in metro area (somewhat likely)

In such a scenario, precipitation would begin as snow but change to rain or sleet. The rain or sleet could change back to snow depending on how quickly the storm deepens as it moves away from us. Last night’s Canadian model had jumped on the all-snow bandwagon, but today’s has the primary low tracking to eastern Kentucky and then has the low reforming and tracking over the Chesapeake Bay.

This latest forecast from the Canadian model has snow developing across the area early Friday morning and continuing into the afternoon before changing to rain from east to west late Friday afternoon or evening. It would give Washington several inches of accumulation before the shift. The total snow amount in such situations is always tricky but usually less than expected. Those living well west of the city may see nothing but snow, but both Washington and Baltimore would experience a period of rain.

Such a scenario would present some travel problems Friday, but rain could alleviate them by later Friday night or Saturday.

Scenario 3: The storm tracks too far offshore to bring big snow (least likely)

Of the three scenarios, this is the least likely one.

If the storm tracks far enough south and east off the North Carolina coast, the heaviest precipitation would fall south and east of the Washington region. In such a scenario, only light to moderate snowfall would affect the area, with the heaviest snow occurring south and east of the city. Snowfall could be anything from a dusting to 4 or 5 inches.

Synthesis

Last night’s European ensemble forecast does a good job of showing how much model consensus there is for snow — but still conveys the potential for a miss.

Each of the horizontal lines on the upper panel of the figure above represents a single model forecast of snow. The only differences in each of the model forecasts are small changes in the initial conditions or minor tweaks to the model physics. The scale is shown on the right.

Note that almost all the members are predicting some snow. The magenta or pink bars are indicating that that model that was run was predicting at least 9 inches of snow. The majority of these model forecasts were predicting more than 9 inches. All of those members had a low tracking across the South to near Cape Hatteras.

But there also were a few members that predicted only a few inches or no snow. Those models changed the snow to rain and in one case, delivered all rain. That latter scenario looks unlikely but the one that gives us some snow and then a shift to rain is still a possibility.

Snow lovers take heart — the European model ensemble mean gave Reagan National Airport about 10 inches of snow, assuming a 10-to-1 ratio of liquid to snow, while last night’s and today’s European model was forecasting 20 to 24 inches making the same dubious assumption.