First Alert Weather In-Depth – “Overrunning” determines the type of precipitation

ROCHESTER, N.Y. – If you are as old as me, then you probably remember the ice storm dating back to 1991. Hard to believe that it was 32 years ago. But just the mention of ice accumulation can make folks a little nervous. But we want to emphasize that we do not see a storm of the same magnitude.  The simple reason is that this storm will not last as long. The ’91 storm had a duration of 24 hours of moderate freezing rain, but this storm should be a for a much shorter duration.

The principles and the mechanisms of these two storms are basically the same. In meteorology this is what is called “overrunning”. This is a process that starts with lots of warm, moist air that rides-up over top of a layer of cold air. This wedge of below freezing air is very important to the type of precipitation.

If we look at a cross sectional view of the atmosphere, we can see examples of how the temperature plays such an important role.  If we have warm air from the surface all the way up through the atmosphere, the precipitation will be just rain. However, if there is a shallow wedge of cold air at the surface, then the raindrops become “supercooled,” which means the drops do not freeze until they make contact with a surface. That is the scenario for freezing rain. If the air at the surface has a thicker wedge of cold air, then rain drops do have time to freeze as sleet pellets. In the last example, the entire column is below freezing and that produces mainly snow at the surface.

Obviously, whatever the type of precipitation we receive, travel conditions will be a problem. Slick roads will be increasingly more likely as we get later into Wednesday afternoon into Wednesday night. The News 10NBC First Alert meteorologists have posted a yellow alert as road conditions will deteriorate. As usually the case, it is important to take it slower on area roads.