Snow Forecasting

Posted: September 28, 2009 in Weather Stuff

A friend of mine has asked me to do a quick introduction in forecasting snowfall (yes, I hear you calling me a weather geek, but I really don’t care :D)

This is a more advanced chart, and if you don’t understand what the 500hPa etc.. charts are, then you’ll find this very hard to understand. This guide is really for people who have an understanding of basic weather charts/terms.

There are two major models we have to look at when it comes to forecasting snow. Firstly you take a look at the GFS, and then you look at the ECM – When these two are both showing the same with (ie colder air) then the chances of it coming off are higher than say if just once of these charts were forecasting colder weather.

When looking at the GFS charts, the ideal ones to be looking at are the 500hPa and the 850hPa charts. Obviously at this stage you’re going to know all about high pressure and low pressure systems, how to read isobars etc.. so the next bit shouldn’t be too difficult for you to understand.

500hPa must be at least -27c for snow to pass through this layer of the atmosphere without melting. As it gets lower, 850hPa temperatures must be between -5c and -7c, although -5c is slightly boarder line and could end up falling as sleet depending on other factors which we’ll discuss a little later. So just a quick review

500hPa –     -27c or lower                                     850hPa –         -5c or lower, but preferably lower than -5c

Example of 500hPa at -27c or lower (Northerly Blast)


Example of 850hPa -5c or lower

As you can see on the 850hPa chart. The temperatures at -5c or lower over much of England and Wales, and -10c across Scotland. This would easily be cold enough to support snowfall. All we need to do now is look at the dew points, as long as they are at 0c or below then snowfall is certainly possible as long as the precipitation is there. There are many other factors involved but these are the basics and all you really need to know at the moment. I might add more about predicting colder patterns, but it does turn slightly more complicated on this bit. If you have any questions then just email me.


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