Guide to........forecasting snow

[Paul]

We keep hearing a lot about these different meteorological terms, but which ones are the most reliable? I will attempt to shed a little light. (without going into a great technical explanation of which each is).

Thickness (TT) (typically 1000-500mb) Is widely used in forecasting snow, the 528DAM is banded about so frequently, but i have always found it never to be as clear cut as that, TT below 522DAM is really the figure i would look for to use as a benchmark, however sometimes even below this it will rain or sleet, and in some cases thickness well in the 530's will give snow. But why ?, it all relates to the lowest third of the atmosphere, that below 850mbs, (this area is crucial in snow/rain forecasting. It is often a better guide to use 1000-850mb thickness as an indicator for snow, simply because more often that not it is this area which is more important than above.

850mb Temps the temp of the 850mb layer is widely used -5, -10 etc, this is more simple, as it is what is says it is :- the temperature at the 850mb level. For snow usually at least -5 must be present, but again this doesn't always work.

The crucial thing we are missing up to now is the temps of the lowest layers of the atmosphere, near the surface, the boundary layer. It is these lowest layers that are important. This is where WBulb & 850mb WBPT comes in, (Wet Buld potential temperature). Dave & Sam have already spoken of the Wet Bulb Temp, the 850mb WBPT is simply the potential temperature of the wet bulb calculated at the 850mb level (this is more for fronts) Forecasters will use a mixture of all three to gauge the risk of snow, but typically it will be the Wet Bulb which will have greatest weight.

Today for example we have thickness below 528mb / 850temps are at or below -5C in most places, yet the snow is above 300-400m. Why? Well wet bulb temps are above freezing (between 1-4C), so snow is only likely where the wet bulb falls to below freezing. (the wet bulb freezing level), however in the heaviest showers this is dragged down with the constant precipitation (the atmosphere aloft is cold as i have said above, so with heavy enough rain/hail this is dragged down, the snow level is also dragged down so eventually it snows at the surface temporarily) Over the next 24-30 hrs the wet bulb temps drop steadily, and more and more showers turn to snow. The frontal system, despite moving in with very cold air aloft will have an atlantic maritime warm sector, it will reintroduce wet bulbs above freezing for most of western britain overnight into thurs, thus rain is more likely, despite the low 850mb temps and TT. Only inland away from the sea will the wet bulb remain sub zero, so here on the eastern edge will it fall as snow, and poss more generally as it moves away south and lower wet bulbs feed south.

Another situation is when you have high thicknesses and higher 850mb temps, but a cold undercut (continental typically), here wet bulb will be several degrees below zero, so snow is more likely than rain as the cold air at the boundary layer undercuts the warm air aloft. (this was responsible for the blizzrds in the westcountry in feb 1978, thicknesses here were well above 528 and 850mb temps in the SW nearer to 0 to +1C.

Taken from the UKww forum - with their permission, originally posted by 'PJB'

http://www.ukweatherworld.co.uk

[Methuselah]

thanks paul. i've always "known" somehow that the wet-bulb temps are important - now i can explain it properly. :lol:

[J10]

I thought I would bring this to the top and many people may find this interesting in the current weather.