Accumulating snow back in the forecast

Now that we are finally back into spring we get to talk about accumulating snow again… sounds kind of weird doesn't it? Well, it's perfectly normal. In fact, the metro typically gets a few inches of spring snow every year. But before you scoff, walk away, and start pouting, I have some good news for those that are looking forward to warmer weather: the snow doesn't last long this time of year. It's often here one day and gone the very next, especially with smaller storms like the one on the way here.

Spring snow is a pretty amazing marvel because there are SO many components that are working against accumulation, and that includes this storm. So now it's just past the Vernal Equinox and that yellow ball in the sky continues to get higher and higher every afternoon. This leads to more heat, or what we can shortwave radiation penetrating the atmosphere and hitting the surface. This leads to heat and insulation of the surface. So look at the diagram below. This shows the amount of insulation (heat) from the sun at different latitudes throughout the year…


Listed are the major milestones including the equator (0°) and then 30, 60, and 90 degrees. We are around 45° so I added that line in pink. You can see that our heat from the sun is dismal from November to February. But we really start to ramp up the fire in March and that catapults us to the Summer solstice in June when the sun is at its peak. Because of this extra heat, it is much harder for snow to not only accumulate when that yellow ball is above the horizon, but also to hang around for more than a few hours at a time.

Now because of the higher sun angle, the ground begins to thaw as well. Over the last couple of weeks, the area has been blessed with 50's, 60's, and 70's which has helped the topsoil rise above that freeze mark. In fact, a chart shows by just how much. This is from a sampling station in western Minnesota. Check out the frost depth over the winter as compared to now…


As shown, the frost depth hit 37" in this spot on March 9th (some locations across the state were closer to 50") but because of the warmth the last dozen days or so, the ground has begun to thaw. Notice though that it still has to thaw from the top down, so March 16th the top 11" were unfrozen, but 11" to 37" were still a block of ice. This is important for 2 reasons. First, because the topsoil is now above 32°, when snow falls it will melt from the top down and the bottom up because the sun will warm the top and the above freezing ground will warm the bottom. Second, liquid water cannot penetrate a frost layer, so any moisture that fall won't seep into the ground past the 11" mark in this location. This is critical knowledge for spring runoff (how much will flow into area rivers) and also for soil moisture storage for farming. You think 37" is deep? Now compare this year in the chart above, to last year in the chart below in the same location…


So the age old question is how much are we getting? Well, because we not only have to account for location, intensity, and track of the band of precipitation, we also have to account for atmospheric and ground melting and a host of other factors that will prevent accumulating snow, so let's just say it's a bit complex. Here's an example to help. The image below is what we call a meteogram for MSP Airport through Monday afternoon. Each colored line represents a different forecasting computer model. The X axis is time and the Y axis is overall accumulation.


This shows that forecasting computer models are predicting anywhere from a half an inch to nearly 5 inches of snow, depending on which one you pick. Think that's a wide range?? Well, honestly I have seen worse. But that's not the whole story. Computer models don't take into account ANY melting factors. So we have to base a forecast on what we see using this and a host of other variables as guides. NOT so easy. But tell you what… it looks like many in central and southern Minnesota have a pretty good shot at a couple inches of snow so get ready for the area to look a little more wintry heading into Sunday afternoon.

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