Forecasting snow

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Carl M
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Forecasting snow

Post by Carl M » Sun Dec 27, 2009 9:19 pm

This is from another weather forum, so use it for personal use only, that is, don't put it on your website.
Think it might be useful this week!

What to look for to get snow

a] in showers
b] frontal weather

a]
1) Dry bulb temperature below 5C, often 3C is a better mark
2) Dewpoint at or below zero
3) wet bulb temperature, if you have a weather station, no more than about 2C
4) 1000-500mb thickness (DAM) less than 522dm, lower if you are on the coast, but as high as 540dm it is possible in a heavy shower, but unlikely.
5) 850mb temperature of -7C or below, -5C it can occur but not often.
6) on the 850mb chart if the value shown on the contour line is below about 1290dm or 1300dm, then there is a high chance that ppn will be of snow.

(The Met Office use 1293dm for a 50% and 1281dm as a 90% chance of snow)

7) zero degree isotherm or freezing level of 1,000ft or less to give a 50% or higher chance of snow at sea level.
The higher you are the higher the chance of you getting snow. Thus if you live at 2000ft this increases your chances of snow considerably to someone at sea level.

b]
Two types of front
(1) warm front
With this then a dry bulb temperature of 2C or below is needed along with a dewpoint below zero C.
(2) cold front
The dry bulb temperature is not so important as the cold air reduces the temperature, often by 5C or more, in the most active cold front perhaps from 10C to near zero C in a fairly short time. So its what the temperature and dewpoint is behind the cold front which is important.
In (1) warmer air is flowing over the top of cold air, and in the case of (2) cold air is undercutting the warm air ahead of it.

(1) is, I suppose the classic heavy snow situation which with strong winds can give blizzard conditions, even on relatively low ground, assuming all the factors are in its favour. This assumes they are, namely that very cold air lies near the surface but is not being moved away by an approaching frontal system. This stagnates and eventually retreats away again.

In this instance then the above requirements need to be met along with

(a) If prolonged and fairly heavy ppn occurs then what we call the wet bulb temperature will start to lower and this can enhance the probability of snow falling. In this instance if the wet bulb temperature is at 3C or below then the ppn can turn to snow from rain. Also, and this applies to showery conditions also, if the wet bulb freezing level(not easy to find on any chart!) is 2,000 ft or below, then ppn can readily turn to snow.

In light ppn then, often, regardless of any of the above factors being favourable drizzle or light rain will fall not snow.

(2) In this instance then if the air is not all that mild in the so called warm air and the cold air is very cold, with near negative values close behind the front, then even moderate ppn will readily turn to snow as the cold air undercuts the mild air. Use the values above for a guide.
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Paul C
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Re: Forecasting snow

Post by Paul C » Mon Dec 28, 2009 10:39 am

Thanks Carl - something to refer to on borderline events.

I have something on my website that calculates the chance of snow - http://www.bramptonweather.co.uk/betel_ ... table.html
Go down the page a little. I am not sure how it works it out some logarithim (poor spelling) but it has been quite accurate. As I am way below freezing it has minus numbers eg -2000ft at the moment.

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