The ultimate impact on the D.C. area of the expected Sunday night-Monday storm, and specifically how much snow we see versus rain, sleet and freezing rain, remains a fairly open question. But what we can say is there is enough potential for a wintry mess by sometime Monday morning that you may want to take home what you need today, in case you find yourself working from home on Monday and/or Tuesday.

Monday Snow Probabilities

Chance of at least 1″: 50% (60% north & west burbs)

Chance of at least 4″: 25% (35% north & west burbs)

As we’ve said over the past couple days, the metro area will likely be in or near the wintry mix battleground between primarily snow to the north and primarily rain to the south. The models continue to advertise that the storm will likely start as rain as early as late Sunday afternoon or as late as Sunday evening or overnight, developing first in the far north and west suburbs and then spreading south and east. Rain likely transitions to mixed precipitation overnight (rain, freezing rain, sleet and snow are all possible) from northwest to southeast. Then the big question is if and how early Monday precipitation transitions to mostly or all snow, before ending by or during Monday evening.

Some of today’s models have trended colder and snowier for Monday, while others have trended warmer and more toward mixed precipitation. Even the models we trust the most are still wavering from run to run, and that inconsistency means we still can’t say with much confidence how much of each precipitation type will fall and where. The best we can say is that there is at least the *potential* for icy conditions Monday morning and significant accumulating snow during the day, especially in the north and west suburbs, but perhaps even inside the Beltway if the colder air sags far enough south.

Why such a tough storm to forecast?

The main reason is larger than normal north-to-south temperatures differences along a frontal boundary. South of the front, temperatures across much of central and southern Virginia are expected to be in the 60s during the day on Sunday. North of the front, we’re dealing with another arctic air mass, with temperatures late Sunday falling off into the teens as close as northwestern Pennsylvania. That cold air will be poised to come south; the unresolved question is how quickly. Slight changes to the track of two low pressure areas along the front will help determine how quickly the front and cold air sag southward across the area, ultimately determining how quickly the precipitation transitions through the various precipitation types.

Keep reading for a more technical look at the forecast…

TECHNICAL DISCUSSION

The differences in the last two NAM model forecast valid at 7AM Monday (see below) illustrate how small differences in the surface pattern can garner different timing of the precipitation and can impact which type of precipitation you might be getting Monday morning. The 12Z NAM (left panel) is slower and farther north with the initial surface low having it just off the Maine coast, while the 06Z version had it scoot well off the coast. That faster more eastward track allowed the front to sag farther south prior to the approach of the second low. The 12Z NAM has the second low over southwestern Virginia Monday morning while the 06Z NAM had it a state farther south. The more southward location of the front on the 06Z model run allowed the warm advection/overrunning precipitation to develop across the region much earlier than on the 12Z run. It also allowed the cold air to filter into the region much quicker than on the 12Z run. I doubt that the models have resolved the track question enough to definitively say how much of a wintry mess this storm will be across the area.



The 12Z (left) and 06Z (right) NAM forecasts of surface pressure, 6-hourly precipitation and precipitation type, valid at 7 a.m. Monday. (WeatherBELL)

The models continue to vacillate on the storm track and how far south the front will press during Sunday night and Monday. The 06Z GFS had switched from a storm that would produce a significant amount of sleet, to primarily a freezing rain event with a brief period of snow right at the end of the storm. By contrast, the 06Z NAM forecast a major snowstorm, as did last night’s European model, which suggested the northern suburbs might get hit particularly hard by snow. Last night’s Canadian still held onto to a brief period of rain and then an extended period of mixed precipitation before ending as a few inches of snow.

This morning’s GFS model (see below) has its snowiest look yet as it tracks the low well to our south, allowing the cold air to plunge well to our south and change the precipitation to snow by 7 a.m. It would argue for at least several inches of snow across the area. How heavily it would stick to the roads in and around the city would still be a big question mark, but the model really crashes the temperatures, suggesting a moderate-to-high impact storm with heavy snow totals in the north and northwest suburbs if the model were correct. Today’s European model still shows the potential for several inches of snow, but has backed off from the more extreme totals it showed for the north and northwest suburbs in last night’s run. Meanwhile today’s Canadian model has backed down to only an inch or so of snow.



This morning’s GFS model forecast of surface pressure, 6-hourly precipitation and precipitation type, valid Monday at 7 a..m (left) and 1 p.m. (right). (WeatherBELL)

The 06Z GEFS and 09Z SREF runs were much more bearish than this morning’s GFS or NAM model runs. Most of their ensemble members were predicting a rain-to-mix scenario and were quite a bit warmer than the GFS or NAM.

The bottom line is that all the options mentioned yesterday are still on the table. We could have anything from mostly rain and mixed precipitation changing to a brief period of snow, with little impact to the Monday morning commute, to a messy mix-to-accumulating-snow scenario with major disruptions Monday and lingering impacts possible into Tuesday morning. Stay with us through the weekend as we try to pin down more of the details.