Where did the April snow come from?

If you were in and around the metro during the Thursday evening rush, then you were acutely aware that the much anticipated rain all of a sudden changed to snow. And while a rain/snow mix was in the forecast, large snowflakes and slushy roads were not anticipated…. Well not entirely.

Thursday's random snow event was a combination of many atmospheric components that came together at just the right time, in just the right place, to give us a little showing of white before it pushed east.

This storm was an absolute text book version of a spring storm; strong low pressure pushing northeast, warm front out ahead of it bringing surging warm moist air north out of the Gulf of Mexico, sharp cold front on the south side driving in much cooler air and popping severe storms along and just ahead of it, and then a band of rain changing to snow on the back side of the low across our area… it's circled in yellow.

 

IF this were a winter storm, that band of rain developing over us would have been all snow. But since it's spring and temperatures are warmer, it started as rain… at least at the surface. But higher up in the atmosphere, the clouds were actually producing snow, but because the temperatures near the ground were so warm, melted on the way to the ground and fell as water. BUT… you knew there was gonna be a but… the rain eventually switched to snow. Why did this happen??

It's called dynamic cooling. Whenever it rains, temperatures cool… this is pretty much common sense. But if it rains hard enough and long enough, the atmosphere can cool much further than it otherwise could. This allowed the temperatures Thursday to cool from the mid 40's when it started raining to the low to mid 30's… and therefore, the atmosphere in spots around the metro had cooled enough where the snow no longer melting before it reaches the ground. Think of it a lot like jumping into a cold lake. If you jump in and get right back out… you are cold for sure, but your overall body temperature has changed very little. This would compare to say a quick shower, or even a small thunderstorm. Now jump in that very cold lake and tread water for 10 or 15 minutes. All of a sudden, your extremities go numb and your core body temperature is plummeting which would eventually lead to hypothermia. Well, the atmosphere works in much the same way, but obviously without the extremely dangerous side effects.

So how do we know the rain at the surface will switch over to snow? Well, it's not real easy, but the radar can speak volumes. The follow radar and temperature maps were from Thursday afternoon around 5pm.

 

 

You have a whole bunch of blue on the radar, but then you have a blotch of yellow and red. But look where the yellow and red are located… right near the radar in Chanhassen. This is what we call bright banding. No this is not a thunderstorm; it's actually snow as it melts on the way to the ground. The radar beam sees the melting snow as a GIANT rain drop because it's a flake of snow that is covered in liquid water and takes up more space. It then attributes this larger object as a "really big rain drop" and throws the associated color on the radar that would otherwise signify heavier rain. Hail has the exact same effect on the radar as melting snow does which is why you often see summertime thunderstorms with yellow and red in them… it's not always heavy rain, but can include hail. But with temperatures so close to freezing as noted above, thunderstorms are exceedingly unlikely, so we can guesstimate that it's likely snow that's melting before it hits the ground. And then this is the result…