DC SNOW FORECAST TIMELINE: Wintry mess to impact region Sunday

WASHINGTON - One thing to note right off the bat here. This will be a VERY difficult forecast. We are still two days away from the start of this storm. There is a lot that can change when it comes to a storm in just 48 hours. Minor shifts in the storm track could lead to either significantly more, or significantly less snow across all parts of our region. 

So please stay tuned for updates and expect some things to change prior to the storm arriving on Sunday afternoon. 

That being said, general expectations today are that for most of the region, the storm will start as a snow event, before changing over to sleet, freezing rain, and eventually rain in the DC region.

The question always with a storm like this is how long will the snow last prior to the changeover? And how far to the west will the rain/snow line get. 

That is going to be key for how much snow could postnatally fall in DC. Even once the change occurs, we have a number of other concerns, mostly that a combination of ice and gusty winds could lead to power outages on Sunday night.

As we have mentioned several times before, the key to this system at onset is how cold temperatures will be. Sunday mourning temperatures are expected to range from the middle teens to the lower 20s, making it one of the coldest mornings of the winter so far. 

This following a Saturday where the region is expected to stay below freezing all day long. The ground will be plenty cold and ready for snow, so it is not a question this time about whether the snow will stick this time around. The chief question is how long will it snow before the precipitation changes over to sleet and freezing rain. 

The entire reason why the change will occur in the first place has to do not with the cold air at the surface, but what is occurring above our heads at about 5000 feet. As the storm intensifies, it will develop a feature known as a low level jet. 

This jet will be screaming warm air and moisture in from the southeast, and even if temperatures at the surface remain above freezing…once the warmth sneaks in about the surface it will change our precipitation over to sleet and then, if surface temperatures at the surface are below freezing, freezing rain. 

Icing is a major concern with this system, around the I-95 it would perhaps be an even bigger concern than the snow could end up being. The threat is there that if we chance from snow to rain too quickly, it could freeze on contact, and then we are talking about a sheet of ice over the region. It could accumulate in some spots a tenth to a quarter of an inch. 

This accumulation on trees and power lines, and some gusty winds as the storm intensifies, means the threat for some power outages will be possible during the peak of this storm. Remember if you are seeing ice pellets falling from the sky, it's sleet, not freezing rain. Freezing rain looks and feels like regular rain, it only freezes once it makes contact with a surface that is below freezing. 

Let's talk timing. This is something that can most certainly change between today and Sunday, so be sure to stay tuned for more updates throughout the day tomorrow. 

But currently most models agree the snow will push into the region sometime between 2-5pm on Sunday. The faster that the precipitation arrives, the more prolonged the snow will be, and therefor the more snow we are likely to see. If the precipitation comes in too late, we will see a shorter snowfall duration, and therefor less of it.

By the evening hours, as the storm approaches and intensifies, the warm air will start to move into our region aloft. This will quickly march the rain/snow line northward as the low level jet intensifies. 

This could also be the time frame of the greatest concern for icing across the area. Winds will likely gust in excess of 40mph in some places, and the wet snow, followed by ice and winds could lead to some down trees and powers outages in the region. 

By the late night and overnight hours, the center of the storm is likely to pass near DC. Warmer air wrapped around this storm will likely push mixed precipitation all the way back to the I-81 corridor and cities like Front Royal, Hagerstown, Martinsburg and Harrisonburg could all see some mixed precipitation cut back on their snow totals out in that region. 

There will likely be a "dry slot" that rolls in overnight as well, ending precipitation for a period of time, before cold air wrapping back around the storm could bring some light snow back, even to DC, by Monday morning. 

Snow totals for a storm like this are notoriously difficult to pinpoint. To me, the hardest part of this forecast is between the I-81 corridor and the I-95 corridor. Someone in that zone is going to boom with higher than expected snowfall totals, and someone else is going to bust and get less than expected. That is the very nature of these types of storms. 

Areas east of I-95 who were spoiled by last Monday's snow event will likely get the short end of the stick this time around. This storm has all the hallmarks of being a Appalachian runner, where the storm stays mostly snow, and the highest totals end up in the mountains. Ski resorts in the region should do very well with this storm. 

This morning, winter storm watches were issued in the areas where the National Weather Service was most confident on the most impactful winter weather. A winter storm watch means the risk of 5" of more of snow exists, and also the threat for enough icing to damage tress or power lines. 

The latest discussion notes that areas closer to DC were left out of this watch at this time because confidence was lower. In a boom scenario, 5" or more of snow in the DC metro is certainly possible, but at this time does appear unlikely. The one thing that would get us into winter storm criteria locally would be the icing aspect. They noted that they could see these watches expanding eastward later on Friday because of this icing risk.

As you can see, there are a lot of moving pieces with this storm. Track, timing, and intensity of precipitation and surface temperatures are all going to play a MAJOR role in who gets what on Sunday into Monday. Between the mountains and the Chesapeake Bay, there is going to be a SIGNIFICANT gradient in snowfall, between those that get absolute nothing and those that are going to be shoveling snow. 

The details should continue to get more and more refined the closer to Sunday we get, so please check back for forecast adjustments. Stay safe! We will keep you ahead of the storm.