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  • Will it Snow? by John Holmes


    Hi all

    I promised to give this guide, so here it is.

    Its not complete and it is for those who are new to trying to work out this very difficult question.

    What to look for to get snow at sea level.

    (1) in showers

    (2) frontal weather

    (1) In showers

    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. At 2,000 ft above sea level the chance is reduced considerably.

    (2) Frontal Weather

    Two types of front

    (1) warm front

    (2) cold front

    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.

    This has only touched the surface of trying to forecast will it snow or not but I hope gives, the less experienced, a guide of what to look for.

    On the topic of wet bulb temperature. I have still not worked out how to give Paul sufficient data that they can set up an algorithm to produce charts of wet bulb potential temperature and also wet bulb freezing level charts. If anyone knows how to do this please pm me or Paul as it would be another big step for Net Weather to be able to produce such charts from the GFS run.

    I hope you have lots of opportunity this winter to try this guide out.

    Footnote by Blessed Weather: This article was written by John Holmes, retired Met Office professional, in 2015.

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