Looking through the atmosphere: How different types of winter precip form

With the potential for a little freezing rain Thursday night and then again Saturday, I thought it would be important to look through the atmosphere to see just how these types of precipitation form.  As we all know, we can get anything from rain to freezing rain to sleet and snow all during the winter months.

The higher up in the atmosphere the colder the temperatures become.  Once you reach cloud level, temperatures are below freezing.  Any precipitation that falls within the clouds will be frozen, but how long and how far down it remains frozen depends on where temperatures warm above 32°.  For snow to fall at the surface, the column of air above will be below freezing (for the most part - temperatures at the surface may be right at or slightly above 32°).  When sleet forms, this usually indicates a presence of a shallow, warm layer a few thousand feet above the surface of the earth.  The frozen precipitation that falls from the clouds encounters that shallow, warm layer and partially melts.  Once that happens, the precipitation then falls within a larger layer closer to the surface where temperatures drop back below 32° causing the precip to refreeze. 

Freezing rain forms when the layer of warm air is a little larger above the surface of the earth, but surface temperatures remain below freezing.  The frozen precipitation falls into the warm layer, melting into rain.  However, at the surface (or very close to the surface) there is a shallow layer of subfreezing temperatures.  This causes the rain to refreeze once it reaches the surface causing freezing rain.  Even a little freezing rain or drizzle can be very problematic.

Rain forms when the column of air is warmer than 32° all the way down to the surface and no refreezing occurs.

As we head closer towards tonight and Thursday, warmer air will move into Northern Illinois and Southern Wisconsin as surface temperatures slowly warm above 32°.  This will occur at the same time a cold front moves south out of Wisconsin.  Temperatures Thursday afternoon should be warm enough to support rain but as the front moves closer to Illinois Thursday evening a transition from rain to freezing rain will be possible.  Ice accumulations are not expected, but this could cause the Friday morning commute to be slick.

The next system of interest will move in Saturday with a main impact of Saturday night through Sunday mid-day.  Again, a wintry mix may occur Saturday before a transition over to snow occurs Sunday.  This snow, however, has the potential to be bigger than the recent snowfalls we've experienced.