A weak disturbance will track through the D.C. area tonight bringing with it the potential for a few flurries. The only areas that should get measurable snowfall are places such as Western Maryland or parts of West Virginia in the mountains where there could be 1 to 3 inches. Skies should clear out by tomorrow morning then begin to cloud up once again by tomorrow afternoon. That is right about the time when the forecast gets tricky.
Why Freezing Rain? Let's take a look at the set up we have and why we think freezing rain will be an issue across the area. There are numerous factors that come in to play in this case. The first will be the advancing low pressure system. It is expected to move directly over the D.C. area through the time period. That is important because it will put the region in the favorable spot to have the freezing line directly over us at some point. We also have the mountains that will come in to play, as cold air will pool on the eastern side of the mountains (Cold Air Damming) as well as the valleys. This is due to a weak area of high pressure that will be over the northeast. Let's take a look at the surface map early Saturday morning.
- 6Z GFS Forecast Surface Map
This shows the high to the northeast, the advancing weak low and the 6-hour precipitation potential. That high to the northeast is important as it pushes cool and dry air into our region at the surface and along the Appalachians. If we start out dry at the surface, then initial precipitation will probably evaporate before it reaches the ground, further cooling the column of air at the surface until it becomes saturated. Now let's look a little higher up in the atmosphere around 850mb, which is nearly 5000 feet.
- 850 mb Temperatures and Winds at 6Z Saturday
This shows winds out of the west-southwest around 30 kts which will help push in the warmer air above the surface. Let's take another look at this in the forecast sounding. I love looking at soundings and use Bufkit on a daily basis.
- Skew T plot for Saturday morning
This sounding shows a couple of things which we look for in a freezing rain scenario. The red line is the temperature and the green line is the dewpoint. Since they are so close together you know the atmosphere is saturdated. I circled number 1 since you can see the layer below freezing at the surface as the 0 degree line is the dotted line to the right. The numbers to the left show the heights in the atmposphere, so as the temperature crosses the zero line around 925 mb, you can tell it is a shallow layer of cold air at the surface. That is important as rain shouldn't be able to refreeze into sleet before reaching the ground. The second circle for number 2 shows warmer air aloft and for a few thousand feet. This will act to melt anything that falls from the cloud to rain before it enters the freezing layer once again. Number 3 shows the wind direction and speeds, which are all out of the southwest aloft but out of the east at the surface, which tells us that cold air is there because of the easterly flow from the high over the northeast. I'll tell you, this kind of situation is challenging but fun to forecast!
That sounding was as of 5am at Dulles Airport. As we go ahead in time, warmer air begins to get in at the surface as well through the early morning hours which will help change everything over to rain. It is to the north and west where freezing rain may hold on a little longer and their could be some ice accretion on roadways.
- Chance for > or = 0.01" ice Saturday
Here is the graphic that shows the potential for great than or equal to 0.01 inches of ice. This shows about a 50% to 80% chance throughout the the D.C. area. Remember, this is a tiny amount of icing but only a small coating can make a huge impact.
Final Thoughts: I really think any serious icing will be in the Shenandoah Valley from Winchester, VA north to Hagerstown, MD. Closer in to D.C., some light freezing rain will be possible Friday night into Saturday morning before sunrise but will change over to all rain after sunrise. Just try not to head out too early on Saturday and possibly change your plans so you can head out later in the day. This should not be a huge impact storm because of the timing coinciding with the weekend but still take precautions as changes can often happen in these situations.