Climate Watch
By Rick Thoman Alaska Climate Specialist Alaska Center for Climate Assessment and Policy International Arctic Research Center/University of Alaska Fairbanks
Modern weather models are generally quite good at forecasting the evolution of the large-scale weather pattern a week or so into the future. Details of individual storms are often, though not always, well forecast three to five days ahead.
However, rain and snow forecasts are usually not as accurate. Some of this is due to the physics of precipitation formation. Large-scale storms provide some of the ingredients that go into creating rain and snow, but important aspects also occur at the scale of individual clouds and the water droplets or ice crystals that make the clouds. The physical processes at this scale can be studied in a laboratory or by specially equipped research aircraft but are far too small for weather models to directly calculate the mathematics that describe these intricate processes. To get around this, for decades weather models have used larger scale approximations, and while these have gotten better with time, the results are not as good as we’d like. One of the outputs of weather models is the rate of precipitation (rain or the amount of water in falling snow), which can be added to total precipitation over specific periods of time. Because of the inherent uncertainty in precipitation forecasts, an important tool for meteorologists is the ability to consider forecasts from many different weather models and different simulations from the same model. An example of this shown in the graphic. This shows the accumulated precipitation (liquid equivalent) for Nome for the week of February 2-9 from 31 different simulations of most tundra environments, snow Global Forecast System, the main nearly always occurs with winds US global weather model. That is directly strong enough to redistribute the useful for rain, but less well snow, sometimes to great effect. for snow. The density of snow varies Everyone in western Alaska has
Barge caught after drifting
greatly, so having an estimate of the experienced storms, when at the end, liquid equivalent in the snow is only the wind has drifted the snow many
loose in Bering Strait
part of the equation and snowfall as feet high on one side of buildings we usually think of it is not directly and the other side is swept near bare forecast by most weather models of snow. Assigning a single number Most weather apps and websites use to the snowfall is possible but is not a simple conversion such as “one really useful without factoring in the inch of precipitation equals ten drifting component, which in turn inches of snow”. There are more sophisticated depends on strength, duration and direction methods available, but of the wind as well as the none account for the fact that, in temperature.