The well-advertised warm up is on its way here, but questions still loom about just how warm we really get. I'd say low 40's are pretty much a guarantee statewide, but there is the real potential for widespread 50's across the area. So why don't we know for sure? Well, like most things in Mother Nature, it's complicated.
There is no question about the amount of warm air heading for Minnesota… it will likely be the warmest since early December, however this may not translate to higher temperatures at the surface. So the way the atmosphere works is that warmer air is less dense than colder air, which means it rises very easily. In order to transport warmer air to another region, you have to have winds that will carry it in that direction. But, the warm air is less dense than the air it is moving into, therefore it rises. This creates a warm layer of air aloft (above the surface) which is often called an inversion, or temperatures rising with height. In order to get warmer temperatures at the surface, we have to transport this warmer air from 3, 4 or even 5 thousand feet high, down to the ground. This act is called mixing. Mixing occurs when winds at higher levels of the atmosphere are the same direction as the surface winds AND are also strong enough that thermals (the rising of air as it heats from the afternoon sunshine) are able to take air from the ground and surge it upward allowing air that's above the ground the freedom to move to the surface. This is why you get wind gusts. That seems complicated enough right?? Well, we have to add in one more factor; the snow cover.
Snow is the X factor to all of this. It often creates a mess for forecasters because of its cooling properties. Just a couple inches of snow on the ground can make a day where the atmosphere is primed to allow highs in the 70's turn into a 35 degree day. It's because the snow not only reflects abundant amounts of solar radiation back into space (heat that we get from the sun) but also creates this cold layer of air right at ground level because it is 32 degrees. It would be like sticking a 32 degree blanket on your bed… everywhere else in the room is warm, but the air on and just above that blanket is far cooler.
So how does all of this work in the atmosphere?? Well, let me show you. The following 2 images are what we call atmosphere soundings. A sounding is an instrument that gets launched into the atmosphere by the National Weather Service that takes measurements as it rises to the top of the atmosphere. The red and green lines you see going from top to bottom are the temperature in Celsius (red) and dewpoint in Celsius (green) of the atmosphere with height. So imagine the sensor going from the surface to the top of the atmosphere… every measurement it takes is on this graph. It also takes wind speed measurements which can be seen on your right… not only wind direction but wind speed. Now, this is what we call a forecast sounding… or what the computer models think a profile of the atmosphere will look like in MSP at 3pm Sunday afternoon and then again on Monday afternoon.
Do you see the LARGE variation in temperatures (the red line) at the bottom of the graph (close to the surface)? That is what we call our temperature inversion, our warm layer aloft. Notice the height lines going parallel from the surface starting at 1 representing 1000 feet and going up from there. The height lines show that temperatures at about 2000 feet are some 10°C higher than at the surface. So the big question is, can these temperatures get mixed down to the surface?? That remains to be seen, but here is going to be the big problem for Sunday. Look at the winds and more specifically the wind direction. On Sunday, they are different directions… granted it's not much different with surface winds out of the southwest and 2000 foot winds out of the west, but a little difference is all it takes. But now look at Monday. Even though temperatures just above the surface are a little cooler, with the winds lining up pretty nice, these warm temperatures may make it down to the surface and COULD give us the warmest day since November!