Say it ain’t snow? With the roller coaster weather, we have been experiencing lately, and the busted forecast for the weekend snow potential, many of you are probably skeptical about our snow chances this week. I’m going to break it all down for you with a full forecast discussion.
First, let me start with a quick rant (this is relevant to the credibility of this forecast). We live in an era where it is really easy to “hype up” a storm on social media. Meteorologist and weather enthusiasts will often get excited when they see a chance of snow. Our first thought is to share it with someone. It takes about 30 seconds to take a screenshot of model snow map and share it on twitter. It only takes about another 30 seconds for that map to be all over the internet with false information. Our atmosphere is actually very complex. The top of our troposphere, is actually about 10km up. Pressure decreases with height as we go higher up in our atmosphere so we generally look at different levels of atmosphere as different pressure levels. This forecast discussion will focus on 4 levels; 1,000 mb (the surface) , 850 mb (low level jet and moisture), 500 mb (trough’s, ridges, and upper level forcings) , and 350mb (upper level jet(. In order for our region to experience the “perfect storm” everything needs to fit together from the top of the atmosphere, down to the surface.
Second, let me talk a little bit about how models work. A forecast model is just like any other model. Let’s take a model of our solar system for example. When a student does a science fair project on our solar system, they will typically build a model using various balls or objects of different sizes and color them to look similar to what our solar system looks like. When you look at the model, it is obvious that it is not to scale. We can’t fit our solar system in a box, however, the model still gives us a really good understanding of what our actual solar system looks like. There will be some errors in the size of the planets, the chemical composition, and other factors, but it gives us an understanding. Weather models work the same way. Our atmosphere is actually really complex. The equations that govern how our atmosphere works are very complex and difficult to solve. Some equations are so complex that we have to make assumptions in order to solve, or represent them in our model. In order to solve these equations, we have to tell the atmosphere what the weather is like at the time the model is run. If we tell the model that the temperature is 50 degrees at a certain point (let’s say Washington D.C), but the temperature was actually 52 degrees, then our model is off before it even starts. This happens with some variables more than you would think. This is why we use ensemble forecasts. That small error in temperature, will propagate and grow over time. This is essentially a long way of me explaining that even with our technology today, it is hard to model our complex atmosphere so forecasts are generally only accurate within 48 hours of an event. The discussion I am about to get into is from the European model forecast for Wednesday morning. The model seemed to initialize well, but the actual event is still a few days away so this forecast could change significantly
Now, let’s get into an actual forecast. So as I mentioned, we like to look at the atmosphere from the top down. The image below is the 350mb wind speed. This is the top of the atmosphere or what we refer to as the upper level jet. The darker red and orange colors are where the fastest winds are. This is known as a jet streak. The dynamics and meteorology behind a jet streak is actually quite complex, but the simple way of explaining it is that your best chance for snow, is along the jet streak. If you look at the surface map that depicts precipitation map, you will notice that the snow (or blue area) is mainly in northern Illinois and Indiana which happens to be on the edge of the jet streak. The jet stream essentially drives atmospheric motion. The cold air and moisture made its way down from Canada and took a ride along the jet stream. This is why California has been hammered this week.
The next map is the 500mb geopotential height and relative vorticity map. This is not the ideal set-up for a big synoptic scale snow storm for our region. We typically look for what we call a trough, which is an elongated area of low pressure. The upper level energy is actually to our west. It’s carrying a lot of moisture with it, so we are within the track to get some snow. The vorticity is essentially a measure of spin in our atmosphere. Stronger vorticity, would actually allow our surface low to strengthen. Our region is not within the vorticity max, so we aren’t expecting a huge storm here but there is still a lot of moisture in this system so we can still expect a decent amount of snow/rain.
Our next 2 maps are our 850mb maps. The first map shows our relative humidity, or the amount of low level moisture feeding into the storm The next map shows the low level jet stream. In order to get snow, you need 2 things; temperatures at or below freezing, and source of moisture. You can see that the relative humidity for our area is quite high. You can also see the low level jet is feeding in a lot of gulf moisture and coastal moisture from the Atlantic. This shows that there is definitely enough moisture for precipitation. The question is, how much of it will be snow and how much of it will be rain?
This is where our last map comes into play. Shown below are two surface maps, 7am, and 12pm, with the 1,000- 500mb geopotential thickness overlaid. Thickness is essentially a measure of the distance between two pressure levels. This is usually proportional to temperature. We specifically look at the 5,400 geopotential thickness line, or the 540 line. This is generally the equivalent to the freezing line, or the rain/snow line. If you look at the first map, for the 7am, we are below the red 540 line but by 12pm, we are well above it and we start to transition to a mix and eventually over to rain. This is the most difficult part of the forecast. If the cold air sticks around a little longer, then the snow totals could be higher. If we transition to rain quicker, our snow totals will be lower.
The final map is a snow totals map. This map assumes a 10:1 ratio. This means that for every 10 inches of snow, we assume 1 inch of liquid precipitation. When we get mixed precipitation, or heavier snow, this ratio can be off. This model shows anywhere between 2 inches in the southern portions of our area to near 7 inches in our more northern areas. My prediction is that the rain will cut down snow totals. My early predication would be 2-4 inches for the immediate D.C metro area. With that said, this discussion is based on one model, one run, and it’s more than 48 hours out. A lot could change. I hope you enjoyed this discussion and found it educational.
Early school cast: As of right now, I would say no school Wednesday for Fairfax, Arlington and Loudon county
