DISCUSSION: With the first winter storm of the season well on its way to impact the northern Great Plains with mixed wintry precipitation, it’s time to take a general look at the differences between the various types of precipitation that can occur during winter storms. The National Weather Service has issued a Winter Storm Warning for most of western and central North Dakota, stating that “a wintry mix of rain, freezing rain, sleet, and snow will begin the storm this evening before changing into all snow later tonight.”
Why will the storm start with a wintry mix? What determines the precipitation type? For the most part, it depends on temperature. We all know that water freezes at 0°C (32°F), but that doesn’t mean that if it is 0° outside that it will automatically snow. The atmosphere is a little more complicated than that; temperatures at various heights in the atmosphere contribute to how precipitation falls. For instance, in Bismarck, North Dakota where the storm is expected to hit, the most recent temperature profile (Figure 1 above, retrieved using BUFKIT) shows that from the surface up to 750 millibars (roughly about 2 kilometers high), temperatures (red line) are above freezing. With this current temperature profile showing the lowest 200 mb above freezing, rain is expected since the air is too warm for snow.
As the Bismarck WFO forecasted, colder air is expected to move in, allowing for temperatures at the surface to drop and forming a cold layer, which is seen above in Figure 2 (retrieved from BUFKIT). Precipitation that is falling around this time (3Z or 10 PM MST) will fall as freezing rain, since the snow will melt in the warm layer (the pocket of air that is above freezing and in this case, is above the cold layer at the surface). As long as this warm layer doesn’t get above 3°C (about 37°F), precipitation that is falling will initially fall as snow but melt partially in the warm layer, reaching the ground as freezing rain. However, if the warm layer is above 3°C, the snow will melt completely, becoming rain, and the cold layer at the surface would not be cold enough to refreeze the rain. Coincidentally, if the warm layer’s maximum temperature were between 0.25° and 1°C (about 32.5°-34°F), and the cold layer were still there, there would be the potential for sleet, which is possible at 5Z (or midnight MST) (see Figure 3, above).
This wintry mix is forecast to continue into the night until around 7Z (or 2 AM MST), which is when the vertical temperature structure changes and becomes completely below 0°C at all levels, allowing for snow to fall. Snow will continue to fall, getting heavier at times, throughout Monday and Tuesday before tapering off on Wednesday. To learn more about other high-impact weather events from across North America, be sure to click here!
-Meteorologist Katie McCracken