Friday, December 7, 2007

Snowpack Raises Weekend Concerns About Ice

It was not by any means an impressive snow system but once again we were clipped last night with a coating of nature's white gold that has brought more slippery roads across the area. It only takes a little snow to cause big problems so make sure to slow down this morning. Notice most areas in and around Lafayette averaged out to about an inch of snow with 1" of snow measured here at WLFI at 3 a.m. The heaviest band of snow shifted about 15 miles south of Tippecanoe County last night. Talk about a tough call. It becomes even tougher to forecast when it is a weak system to begin with and it is being pushed through our area by a jet stream at well over 100 miles per hour. At one point the snow was moving east at 50 mph. Imagine if it was moving at a more typical speed of 15 to 20 mph. We would be digging out from several inches of snow. But this snow-maker moved too quickly to hamper our backs. Some folks can even use the brooms to brush off the snow off the porch or walkways. If you melt down the snow most areas had less than .10" which comes out to a snow ratio of 15:1. It was more like a Colorado snow which meant it was a light, fluffy snow. This brings us to our blog question of the day.

Hey Mike,

Great news cast tonight!! One question though, what is a 20/1 snow?


~Let it snow, let it snow, let it snow!!!!!~

Thanks for the great question Dave. I forecasted a 20/1 snow which is another way of saying we will have a 20 to 1 snow ratio. So exactly what is a snow ratio? Meteorologists use a liquid to snow ratio to forecast snow amounts. Every 1" of rain would equal 10" of snow on average. This is what is called a 10:1 snow ratio or a 10/1 snow. Typically in Indiana every 10 inches of snow yields 1 inch of water when melted down.

But if the atmosphere and ground are very cold like we have seen the last couple days, your snow ratio would go from 10 to 1 to 15 or 20 to 1. This means if your computer models are forecasting .10" of precipitation you would multipy it by 20 to come up with a forecast of 2" of snow. Most areas did not reach 2" of snow last night because our snow ratio was a little closer to 15:1 and our precipitation equivalent was only .07.

So you take the .07" and multiply it by 15 and you come out with 1.05 inches of snow which is what many parts of Tippecanoe County saw last night. The models were all showing about .14" of precipitation over Lafayette but the heavier precipitation shifted just south of us into Montgomery County. So using a snow ratio of 15:1 you can estimate Crawfordsville received about 2.1 inches of snow last night. This is yet another reason why forecasting snow is quite challenging. The storm track and amount of precipitation expected are only a part of the equation.

Speaking of the snowpack you can see how beautiful it looked from space this morning. You can make out Lake Michigan at the top of the picture and the wonderful Wabash River is outlined magnificently in the snow. This snowpack may be beautiful but it could play havoc with our weather this weekend. Earlier in the week it looked like we could melt away much of the snow by late Saturday with warmer temperatures. But with all of the snow we have had this week it has locked in the cold air which means our next system that moves in Saturday night and Sunday will likely begin with a dangerous mix of sleet and freezing rain. I will have more on this tonight. Put away the shovels and brooms, get out the ice scrapers and make sure to have a great day!

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