A mixed bag of precipitation refers to more than one type of winter precipitation occurring at the same time or different precipitation types occurring during the forecast period. Possible precipitation types include cold rain, freezing rain, sleet and snow. How can different types occur at the same time? The reason is that the temperature profile is on the borderline between different precipitation types. Here are a few examples: Surface temperature very near 32 F: borderline of freezing rain and cold rain Depth of above freezing temperature in surface layer is about the depth needed to melt frozen precipitation: borderline of cold rain, wet snow, wet sleet A shallow layer of warm air aloft is just warm enough to partially melt snow: borderline between freezing rain, cold rain, sleet or snow With just slight changes in the temperature profile, the precipitation type can change from one to another. Also, since precipitation falls at different speeds, multiple precipitation types can occur at the same time at the surface. Snow tends to fall the slowest while cold rain falls the fastest. If the temperature profile is changing from snow to sleet or cold rain, the slower fall speed of the snow can cause snow to still be falling while sleet or cold rain is falling also. Turbulent wind currents can also cause multiple precipitation types of occur since precipitation can make it to the ground that originates in a different location from originating vertically above a location. The temperature structure can changes very quickly, thus just over the course of minutes, the precipitation type can change from one to another. For example, in a cold air advection situation, a cold rain can change over to sleet and/snow. When enough solar radiation is able to penetrate through clouds, daytime heating can cause snow to change over to a cold rain during the day. |