Tuesday, December 23, 2008
Snow versus Rain
Tomorrow represents a classic problem for local meteorologists in winter...snow versus rain. Many winter events are on the margin around here because warm air from off the ocean is always close. Later tonight and tomorrow AM we will initially see snow.. in fact, take a look at the latest satellite and radar imagery....a disturbance is making landfall and associated snow showers are moving into the western Strait and Forks is reporting light snow. In fact, snow appears to be coming in a few hours early..and that is good. But after that the models suggest that by midday the lowland temperatures will be too warm (into the mid 30s) for snow below 300-500 m and we will see rain as a stronger disturbances (an occluded front) moves in. But before it does so, we could get 1-2 inches of snow. However, my colleagues at the National Weather Service have a different interpretation and their latest discussion suggests they believe it will be all snow tomorrow--with 2-4 inch totals. So it will be interesting to see what is going to happen.
Take a look at the model total snow forecast ending at 4 PM to check out your local area based on the latest model run. And clearly the mountains will be getting plenty of new snow.
Surface temperatures don't tell the entire story about snow...and snow can fall when surface temperatures are above freezing by a few degrees (although wet snow). It takes about 1000 ft of above-freezing temperatures to melt snow completely into rain. Thus, the snow level is about 1000 ft below the freezing level. And whether the temperatures will be cold enough can be difficult to determine. For example, if the air is relatively warm, but dry, then evaporation can cool the air down sufficiently to get snow....at least until the air becomes saturated. Another issue is precipitation rate. Precipitation around here almost always starts as snow if you go high enough. When the snow hits warmer air below, it melts and cools the air as it does so. (It takes energy to melt ice!). The heavier the precipitation the more melting and cooling...and if you have heavy precipitation, the snow level can be driven to the surface. So this makes the problem harder...you not only need to be able to predict the incoming air temperatures, but you have to get the humidities and precipitation rates right as well.
You can see why meteorologists need a happy hour once in a while.
Posted by Cliff Mass at 8:21 PM