Wednesday, February 17, 2021

Significant Winter Storm to Impact Area

Odds are increasing that a high impact winter storm will impact the region beginning early tomorrow morning and continuing to tomorrow evening. Uncertainty still exists with regards to the potential for mixed precipitation, as well as where the heaviest banding will setup. Nonetheless, most if not everyone in the region will see eventful winter weather...and some will see substantial impacts to travel and activities. 

Below, you'll see the 06z GFS model. As we get closer and closer to the event, I'd suggest weighing more heavily on the mesoscale models like the NAM, RGEM etc. 

The 06z GFS run from 2/17. This indicates a period of snow and then mixing for the DC area.

Next is the 06z NAM run followed by the 06z RGEM run (that is a Canadian regional model). 

The 06z NAM run from 2/17. Another possible progression to the storm system.

06z RGEM model (a Canadian produced model). 

At this point in time, I think it's safe to say that the favored areas to the north and west of the I-95 corridor will do the best in terms of snow totals. Sleet and freezing rain mix potential will cut down the snow totals closer to the metro area as well as to the south and east of the corridor. 

That said, the less snow any one location sees, the more sleet and icing will be possible. Areas near Fredericksburg and running into Southern Maryland have the potential for a SIGNIFICANT icing event after an initial burst of snow. In these areas, there's even the chance at icing accretion of in excess of a quarter inch. This could lead to power outages in areas hardest hit. 

Conditions may vary greatly over relatively short distances. Thus, your exact location will have large impacts to your exact forecast. I'll do my best to break it down here - apologies if your exact location is not mentioned. 

I-95 Corridor (including areas like Silver Spring, Laurel, and running up towards Baltimore)

2-4 inches of snowfall seems probable. After that, some accumulation of sleet (possibly significant) may occur to give a "crust" to the snow. Additionally, a little glaze (up to a tenth of an inch or so) will be possible to top it all off. A true, "kitchen sink" of precipitation types and NOT fun for driving or even walking. 

North and West Zones (including areas like Frederick, Westminster, Sugarloaf Mountain, Damascus etc - think places like Frederick and Carroll County in MD and Loudoun in VA)

4-8 inches of snowfall with isolated totals to 10 inches. This area may not see much mixing at all - but a little sleet is possible - especially the further south and east you get (closer to the first zone up above). This area should see a storm system more heavily weighted towards snow. Power outage risk should be much lower for these places.

South and East Zones (including areas like Fredericksburg, Charles County, Calvert/St. Mary's)

1-4 inches of snowfall with the most likely being in the 1-3 range. Sleet may become the predominant precipitation type pretty quickly in this area. Even then, a transition to freezing rain may occur much sooner than the other areas listed. This is dangerous as it will create the potential for a significant ice storm component. At least a tenth of an inch of ice, with the potential for four tenths (perhaps even a half inch in spots) will be possible here. This will be enough to cause potential power outages and MAJOR travel concerns. Folks in this area should at least PREPARE for the potential of outages. 


Of course, all of the forecast hinges on how deep and how far north the warm layer above the surface gets. Confidence is high that this storm will have a good supply of fresh cold air to work with at the surface. But there will be some warmer layers of the atmosphere up near the 700mb level that will be the cause of the mixed precip. If these layers are only thin and not too warm, sleet may be predominate in the mixing. However, if the warm layer is on the warmer or thicker side, freezing rain would become more probable. These are factors that are difficult to forecast even at shorter ranges. 

Stay tuned for any refinements or updates to the forecast. This should be a pretty significant/high-impact winter storm for the region. There's the potential for more "waves" behind this system. It's possible these will favor plain rain, though.