Why the forecast for this week became so much warmer

If you have been watching the forecast closely since the middle of last week, you probably noticed that forecast temperatures for the early and middle part of this week have steadily climbed. Last Wednesday, we were showing highs in the upper 40s for Tuesday. Well, just 5 days later, we now have a forecast high of 60 (which honestly could be conservative). Now, for some of us, we just accept the fact that the forecast has changed in our favor and we move on enjoying the second week of March and the several days of 60s that now appear in the forecast. But others want to know why there is such a huge change.

 

 

Forecaster confidence is a big piece of the puzzle, but it's the least critical. Many meteorologists are skeptical (myself included) to see any computer model that shows temperatures 20 or 30 degrees above average several days away for 2 reasons; It doesn't happen very often AND it is still a LONG ways away in the forecasting world and EVERYTHING can change before we get there. So that's why you often see the warmer temperatures get warmer with time and the colder temperatures get colder with time if we are expecting some extreme swings. But like I said, that's only part of the puzzle. The rest is pure science…

During the winter months, the sun angle is low, daylight hours are limited, there's snow on the ground… let's just say the atmosphere is never "primed" to warm up… there are simply too many variables working against warmer temperatures. That's why the 50's, 60's, and 70's that we experienced in the early months of 2012 were so astonishing and probably won't happen again in our lifetime. But when we get to March, the sun angle is higher, days are longer, the snow is melting, and the atmosphere has a better chance of getting warmer. But there are still often 2 VERY big variables that prevent temperatures from pushing into the 60's for at least a couple more weeks; snow cover and ground temperature.

With snow on the ground, the makeup of the atmosphere changes… DRASTICALLY. The surface is no longer the warmest layer, which is so typical during the rest of the year. Snow reinforces a very cold layer right at ground level considering it's frozen precipitation… so it's temperature is never over 32 degrees, and is often much colder than that. This prevents the ground from warming because the sun can't heat it efficiently because the shortwave radiation that comes in (which is what warms the ground) is sent back into space because of the reflective nature of snow. That is why it can be so bright on a sunny day with fresh snow on the ground. Along with the cold dense air that's likely already in place, a strong inversion sets up in the atmosphere. An inversion is cold air underneath warm air… the opposite of the typical profile of the atmosphere seen below…

 

In perfect conditions, temperatures would be warmest at the surface and cool about 2 degrees Celsius for every thousand feet you rise. But with snow on the ground, you create a cold pocket of air at the surface which gives you an inversion as seen below…

 

This prevents warming by the sun and mixing by other layers of the atmosphere and keeps the surface cold. Now, once the snow is gone, significant warming can begin. BUT, just how quickly we can warm is also a mystery and varies from year to year and event to event. Just because the snow is gone, doesn't mean that the ground is unfrozen and warm. In fact, because we have had limited snow cover this year, the ground is even more frozen than usual. The frost depth in the Upper Midwest (the depth at which the ground is frozen) is upwards of 4 feet in some spots as seen in the map below…

 

The National Weather Service in Chanhassen measured a frost depth 2 weeks ago of about 32". In most cases, the depth of the frost is more than a foot above average. So because the ground is frozen so deep and surface soil temperatures were near 20° in many cases just a couple days ago, it was EXCEEDINGLY unclear just how quickly and efficiently we would be able to warm even with the atmosphere getting as warm as currently forecast. The only way to know for sure is to see the forecast and then watch what verifies… A K A… see what our high temperatures end up at. Friday was 39°, which was above our forecast. Saturday was 44°, which was also above our forecast. And Sunday was 48° which again was above our forecast. So each day we overachieved giving forecasters more confidence that this not so far away MASSIVE warm up could happen, therefore we continued to raise our forecast high temperatures for this week as confidence grew and computer models painted the same picture, and now it DOES look like 60°+ will happen at least once this week… maybe even a couple of times. Enjoy!


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