From the ground a winter storm just looks like a cloudy gray day! Luckily, winter storms are usually easily detected by satellite, and areas of precipitation are visible on radar. Different from thunderstorms, we usually can tell well in advance that a winter storm will be coming.
Satellite evidence
Meteorologists look for jet streaks and dry slots, areas of greatest spin, shear zones, areas of sinking air, and cold cloud tops, among others. All of these features are recognizable with enough practice! Comma clouds are a little easier seen on satellite by the untrained eye, and can be associated with winter storms.
Radar evidence
Radar can give us clues as to what type of precipitations is falling. Sleet shows up well on radar because it is a solid ball of ice. Sometimes it can be mistaken for heavy snow on radar, so forecasters must use both surface observations and radar to make accurate forecasts. One problem is that extremely light snow can go undetected because snowflakes have lower moisture and higher air content than other types of precipitation (remember snow flakes have holes). Forecasters must combine both surface observations and information from Doppler radar to determine where and how fast the snow is falling and accumulating. Bright bands in radar data are caused by melting snow or ice as it falls to the ground. The areas of melting snow and ice cause stronger echoes and thus overestimation of precipitation. A bright band also indicates the lowest elevation of any wet snow. The bright band is created because snow falls slowly, and reflects less than water. When the snowflakes start melting, snowflakes get covered by water - and look like large raindrops - reflecting a lot more of the radar beam. When the melting finishes, the soon-to-be raindrop shrinks and speeds up, and the reflectivity decreases. When snow melts into rain, the region where this melting occurs reflects the radar beam more than the snow above or the rain below. With more of the beam reflected, it creates an area on radar that is brighter than the surrounding precipitation.
Doppler radar can show the wind direction too, which is helpful when forecasting near mountains and large bodies of water. If the radar shows wind blowing up the mountain (upslope), forecasters know that automatically, one of the ingredients is in place of the development of precipitation: lift. Doppler radar can also tell you the depth of the cold air. If you know the slope of the terrain and the moisture content of the air, you could have a better guess towards snow accumulation rates. If the radar shows wind blowing over a large section of a body of water (fetch), then they know that another ingredient is present for the formation of precipitation - moisture.
Radar velocities can help identify the location of cold fronts because there is usually a sharp change in wind direction and will show up as a discontinuity on Doppler radar.
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