Good afternoon! With winter approaching, I thought it would be valuable to talk about winter weather forecasting, and what makes it unique, FUN, and challenging. There is often confusion between sleet, freezing rain, and snow, and the processes causing these types of precipitation. The goal for this blog is to give you a much better understanding of winter weather!
First things first, what makes winter weather difficult to forecast? There is a lot to consider. Just very small errors in the temperature can have a huge difference in the outcome. Imagine forecasting 0.50″ of rain at 34º. Let’s say the temperature stays 2-3º colder than the forecast. You have the difference between a boring rain vs. a major ice storm. Another important factor to note is that you aren’t just looking at the temperature at the surface to decipher the type of precipitation. It’s all about what is happening aloft!! This is why you can be 20º and have freezing rain, or 38º and snow. Winter forecasts are more likely to bust because just the difference of a degree or two at the surface and aloft can completely change a forecast outcome between rain, snow, sleet, or freezing rain. In the summer, a degree or two difference wouldn’t matter. 🙂
Snow vs. Sleet vs. Freezing Rain:
Snow is the obvious one – where temperatures throughout the atmospheric column are below freezing. There is a catch. Once you’ve determined temps below freezing, you also need to make sure ice crystals are being generated aloft! The whole atmosphere can be below freezing, yet the precipitation falls as drizzle. Wait….what?? It’s true. Snow flakes have specific requirements to develop! They don’t automatically form in the sky once the temperature falls below freezing. The nuclei that ice crystals grow upon to develop snow flakes don’t activate until the temperature reaches at least -10ºC or 14ºF. You need to make sure you cloud (or saturation aloft) is extending into temperatures at least -10ºC to develop snow! Here is what a classic snow forecast sounding profile looks like. Remember where the green and red lines come together, that means you are saturated. Notice your cloud extends well beyond -10ºC, which makes this a good forecast for snow!
The sounding below is an example where precipitation would fall as freezing drizzle instead of snow. You have a case where temperatures throughout the atmosphere are mostly below freezing. However, the moisture is focused in the lower levels of the atmosphere, and not extending to the layers of the atmosphere where snow flakes would generate. You would need larger scale lifting to saturate the upper levels and see a transition to snow.
Sleet – sometimes called ice pellets, are partially melted snow flakes that refreeze before reaching the ground. Often people confuse sleet for frozen raindrops, but they are actually snow flakes that have partially melted, then refreezing into balls of ice. I’ve also seen it confused as hail. To receive sleet, there needs to be a layer of temperatures at least -2.5ºC or colder below a shallow melting layer above that >0ºC (32ºF). Here is a classic sleet forecast sounding below. Note the temperatures briefly warm above freezing, which allows the snowflakes to partially melt. Below this layer, we once again return well below freezing, where these partially melted flakes refreeze into balls of ice as they reach the surface.
Close-up view of sleet.
Freezing rain occurs when the melting layer aloft is thick enough to allow the snowflakes to FULLY MELT ALOFT into raindrops, which then falls onto surfaces that are 32ºF or colder. Think of freezing rain as plain rain that freezes on contact.
Freezing rain is destructive but beautiful. Here is a picture Mike Hollingshead in Springdale, Arkansas from the major ice storm that brought major damage and power outages in January 2009.
Picture from our recent rare ice storm that hit exceptionally early last November in Washington County, Indiana.
If you had to choose between freezing rain and sleet during a mixed wintry event, you want sleet! That is a million times better than freezing rain which will cause way more issues. What causes warmer air aloft than at the surface? Cold air is more dense than warm air, so often warm air can get pulled up and over the cold air below. This is typical of warm fronts lifting into an arctic air mass, or strong winter storm systems pulling in warm air and moisture, such as what occurred this past weekend in the South.
The Magic 540 Thickness Line: I wanted to dive further into how the temps aloft impact precipitation type. The 540 thickness line is often assumed as the rain/snow line. While you can use this as a BROAD first guess, this is not a “precise” way to look at the rain or snow line. What does 540 thickness tell you? The 540 line, or 5400 geopotential meter thickness between 1000 and 500mb tells you the average temperature within that layer is 0ºC . This doesn’t tell you what is happening within the layers of air aloft, which again, is why you must consider the entire vertical profile of the atmosphere. Here is a prime example from an upper low that moved over the southern U.S. last December…..the average temperature between the surface and 500mb in this example for central Mississippi is forecast to be 0ºC (32ºF), but the temperature at the surface is 6ºC (43ºF), and the temperature at 500mb is -10ºC (14ºF). Your average comes out to 0ºC or 32ºF, but that wouldn’t result in snow! Obviously this thick warm layer in the low levels would melt any snow flakes before reaching the surface.
When forecasting snow, you must consider snow to liquid ratios. The “normal” is 10:1, which is the standard used on most forecast data available. This means if you have 10″ of snow and then melt it, it would contain 1″ of liquid. These can vary greatly, however. Colder air masses typically produce dry snows, which are less dense snows that pile up faster. This is typical of clipper systems diving south out of Canada. These have low liquid content with snow to liquid ratios sometimes as high as 30:1. The colder the air, the less moisture it can hold. That said, you also get more snow with less moisture in arctic air than you would with the same moisture content in a warmer environment. Wet snows will have lower snow to liquid ratios. These are more typical of storm systems that are coming out of the Gulf containing warmer air. The high liquid content makes the snow sticky, where the flakes can clump together, hence why you can sometimes see those big, fat snow flakes. These snow to liquid ratios will tend to be lower than the standard 10:1. Here is a simplified model of how storms from different source regions will contain different liquid content, however these can vary greatly with each event and weather doesn’t follow rules.
You can inquire for our forecasting services by going to bamwx.com/contact-us or e-mail [email protected]! We’d love to help your snow-removal business this winter season! Have a great day!
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