April is a bad month if your name is Frosty the Snowman. The month of April has on average 10 million km2 less snow coverage than March does. That means there are vast quantities of snow melting around the entire Northern Hemisphere. March of 2011 had the highest snow extent since 1985, but April decided to take a different approach. April was the first month of 2011 to have a negative snow anomaly. As a result there was a very large drop in snow coverage from March to April.
The 37 million km2 at the beginning of the April ended with only ~23 million km2 at the end of the month. That is a 14 million km2 decrease in snow coverage in a single month. Of course the average drop is 10 million km2 so large drops are very normal.
The month started off normally, but the negative anomaly dropped as the month proceeded. It stayed within 2 standard deviations throughout the month, but the last two week were pretty close to being outside of that.
As I had been paying attention to the daily maps I was aware that a large early melt was happening in central Russia. The snow vanished early and quickly there with an expanding area of anomaly throughout the mid April time frame.
Just a quick glance at this will tell you that a couple of weeks of this will result in a negative anomaly for the month. That is exactly what happened as well.
From a historical perspective there is a lot of variation in the month of April. The range in the past 39 years is greater than 6 million km2. 2011 is on the lower end of that, but nothing too significant. A fairly middle of the pack negative year with the anomaly being ~2.7% of the average snow coverage for the month.
As expected the anomaly predominately took place in Eurasia as that region has most snow coverage and more variability. North America had a positive anomaly for snow, but not enough to balance out the early thaw in Russia.
Like most anomaly the variation is in timing not in actual amounts. The excessive snow coverage from the winter has now melted in most parts of the world, but in a bit of irony it was snowing where I live this morning as I write this. The snow didn’t stick to the ground, but it was snowing pretty good for a couple of hours. While that is unusual, last year snow managed to stick to the ground as late as May 23rd.
Tags: Snow Extent