The Next Winter Storm Looms
February 19, 2013 8:08 PM
It was another wicked cold day Tuesday as temperatures dropped dangerously cold when you combine them with the 15 to 30 mph breeze that continued through much of Monday night and early Tuesday. Unfortunately, it will be another bitterly cold night Tuesday, but this time winds wont be quite as strong however actual air temperatures will dip into the teens or even 20’s below zero in some areas. But most of our attention is on the upcoming winter storm that will likely consume much of the central U.S. spreading wintry weather across more than a dozen states with several more expecting rain and thunderstorms through Friday. Our area will of course get the wintry side of the storm and snow totals will highly depend on snow ratios. An explanation along with the snowfall potential can be found below.
The question now isn’t IF we are going to get some snow but HOW MUCH? Snowfall looks all but certain at this point, but the amount of snow is still a big question depending mostly on the strength of this storm AND on the overall snow ratio. The snow ratio refers to the amount of snowfall produced by the same amount of liquid. Basically we are talking about the different between the heavy wet snow (the snow that’s good for snowballs) and the dry powdery stuff (the snow that’s good for skiing). Snow is heavy and wet when it has a higher moisture content and is powdery when it has a lower moisture content. So, very heavy wet slushy snow would have a snow ratio of 8 to 1… meaning that every 8 inches of snow equals 1 inch of liquid water. If you have dry powdery snow… its typically around 20 to 1… or 20 inches of snow for every inch of liquid. In South Dakota, we find one of the highest varieties of snow ratios, meaning that any type of snow can fall here. Temperatures play a huge role in creating these snow ratios. Colder air is dryer and holds less moisture, which would lead to higher snow ratios, and warmer air is just the opposite. Well what makes this storm so difficult is that there will be some pretty cold air that gets sucked into the northern side of this. That leads to many questions… how will this affect snow ratios?? How high will they be?? This is why we always forecast a range of expected snowfall amounts because there is no way to know for sure, however we can take an educated guess and often times come out pretty close.
Let me show you just how much snow ratios can affect overall snowfall amounts. Below is the snowfall forecast from one computer model. The first image shows snow totals if the snow ratio is a standard 10 to 1. The second image shows snow totals using an algebraic theorem that tries to conclude an overall snow ratio for specific points. This means that each point has its own snow ratio based on temperature and snowfall intensity, but typically this will show ratios much higher than 10 to 1, ranging anywhere from 12 to 1 to upwards of 25 to 1.
It’s a pretty substantial difference. Over our area alone, it could mean the difference from getting 6 inches or getting more than 10. In parts of Nebraska and Kansas, it could mean the difference from getting 12 versus getting close to 20. So these are just some of the big dilemmas we have to face when forecasting snowfall.
Here is one place to look when expecting some accumulating snow This is a forecast produced by the Hydrometeorological Prediction Center (HPC) showing the percent chance of receiving specific amounts of snow. The following four images show the chance of receiving at least 2, 4, 8, and 12 inches between Wednesday and Friday evenings.
According the HPC, nearly the entire area is guaranteed at least 2 inches of snow with the exception of the far north. Much of central and southern South Dakota have at least a 60 percent chance of receiving 4 inches, including the city of Sioux Falls. But then it quickly tapers of with areas just south of I-90 possibly getting as much as 8, with northern Nebraska the best candidates here. When it comes to a foot or more, the chances are zero for most of us, with roughly a 10 percent shot near the Nebraska border.
This mirrors what we saw a little earlier with the heaviest snow falling in southern Nebraska and northern Kansas.
Want another product to look at snowfall potential? Here ya go… this is from our in house computer model we call Futurecast.
Continues to paint a very similar picture. Here is a close up of our area.
Remember that these totals on Futurecast are only through 7pm Thursday. We will likely see accumulating snow past that timeframe so many of these totals will be low.
After all of this, here is what I am forecasting.
My forecast, along with everyone else’s for that matter, will highly depend on snow ratios. Right now, I’m expecting around 15 to 1 in many areas which is how I derived most of these numbers. The storm has been trending further south over the last 24 hours or so. If this trend continues, then snow totals will be lighter than what you see here. But it works in the opposite way as well so these can change if the overall storm track shifts from its anticipated course. Stay tuned to KDLT and KDLT.com for the very latest.