Why is it so hard to forecast lowland snow?

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Meteorologists can get windstorm forecasts right nearly all of the time.  We hit the record high temperatures in July 2009 nearly dead on.  We can give you a skillful forecast of a marine push in the summer.

But snow forecasts, although improving, are often in error.

Why?   Well, there are reasons....a lot of reasons.

(1) You've got to get the amount of precipitation right--really right-- or you have a very bad snow forecast.  On most days, the forecasts hardly talk about the amount of precipitation, just the probability of precipitation.   Most people don't care whether .1 inch or .4 inch of precipitation falls...they would hardly notice it.  But a difference like that would be HUGE for a snow forecast.  Around here there is typically a 10 to 1 ratio of snow versus precipitation in the form of liquid water.  Thus, .1 inch of precipitation would be .1 inch of rain or, if cold enough, 1 inch of snow.  (Precipitation is always reported in terms of liquid water--so snow has to melted before measuring it).  So the difference between .1 and .4 inches of precipitation, which you would be oblivious to of if it were rain, would be the difference between 1 and 4 inches of snow.  That you would notice.

Getting the amount of precipitation exactly right is very hard, much harder than getting the probability of precipitation correct, thus making the snow forecast difficult.  I can tell you why in a future blog.

(2)  But it is worse than that.  The majority of time our temperatures are marginal for snow...we are right on the edge of rain or snow.  The reason...all the warm water (e.g., the Pacific and the Sound) close by.    On most days a few degree error would not even be noticed (like the difference between 58 and 61F), but that is a big error on marginal snow days.  It is very hard to reduce the error under a few degrees.

(3)  But it is even worse than that!   The temperature of the air is affected greatly by the evaporation and melting of precipitation.  And that is dependent on the intensity of precipitation and the relative humidity of the air below!   Remember:  virtually all of our precipitation starts as snow.  Usually it melts before it reaches the ground, but if the intensity is great enough, the evaporation and melting of the snow can cool the air below so that the freezing level (0C) and snow level (about 1000 ft below the freezing level where all the snow does melt) progressively head towards the surface.  So we need to get the intensity of precipitation right, the initial freezing level right, the relative humidity of the air below the precipitating clouds right....you get the message.  And this is hard.

(4)  And there is more.  To get the depth of the snow right, you also need to know the characteristics of the snow--is it dense or light?  That depends on crystal type and temperatures, which can cause the ratio to vary from 5 to 1 for wet snow to 20 to 1 for dry stuff.

(5)  There is another problem.  Snow is so infrequent and takes such unusual conditions to occur that meteorologists have less practice and experience that for more typical weather.  Why unusual?  Well, it is easy to be warm and wet around here (warm being in the 40s and 50s) for air coming off the Pacific,  pretty easy to be cold and dry (when we have high pressure over or to the east of us).   But it takes a lot of doing for us to be cold and wet.  The set up has to be just right.  Meteorologists are like anyone else:  you gain skill with practice.   Only old timers have lots of cases in their heads...and even that is not good enough since the modeling and observation systems have changed so much.

(6)  Still think it is easy to forecast snow?   Well, local meteorologists have more problems to deal with: a multitude of local effects.  The potential for snow is generally much less near local water bodies, particularly near sea level.  Chances increase greatly up on hills...even 200-300 ft can make a world of difference.   We have gaps that allow cold air to move through the mountains (like the Fraser River gap allowing cold air into NW Washington).  Convergence Zones (like the Puget Sound convergence zone) produce areas of enhanced precipitation and sometimes snow.   And their are dozens of local effects I haven't mentioned.

You get the picture.  Snow prediction is hard, real hard, particularly around here.   Forecasting winter snow back east,  say in  North Dakota, is a cake walk---you KNOW is will be cold enough.  Huge simplification.

So unless you are a Subaru advertising executive....have a little sympathy for the poor NW weathermen when a snow forecast goes wrong.  And keep in mind, a lot more technology is coming:  ensemble-based snow prediction, better use of the new dual-polarization radars, and more.  Meteorologists will make progress....and then we can attack even harder problems, freezing rain and shallow fog.
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