Winter Weather Advisory

The southern half of southeast Michigan has a winter weather advisory that will start at 2am tonight and go until noon on Tuesday.  This advisory is for the possibility of sleet and freezing rain that could make roads icy.

There is currently a surface low pressure system positioned just north of Lake Superior in Canada.  The surface low has a weak warm front to its east along with a trailing cold front.  Ahead of the cold front is a southerly flow that is pumping in warm air into the lower levels of the atmosphere roughly between the surface and 850mb (3,500ft).  This layer of warm air is what will give us a tricky forecast this evening into tomorrow morning.

Image courtesy of HPC

The image below is known as a skew-T diagram and it represents the temperature (red vertical line) and dew point (green vertical line) from the surface up to the top of the troposphere (the layer of the atmosphere that we live in).  Just to get an idea of what this image represents, commercial air planes usually fly at about 30,000 or 35,000 feet which is at 300mb (the millibars is represented on the left).  Anyways, the black line that is on an angle represents the freezing temperature of 0C (32F).  Notice how the temperature line (in red) is to the left of the 32F line (meaning the temperature is below 32F) in the entire atmosphere except for around 850mb.  Here, the temperature is slightly above freezing.  As the warm air continues to pump into our region, the temperature in this layer of the atmosphere will continue to slowly increase.  Because of this, when the precipitation falls from the sky, it will fall as snow until it gets to the layer of the atmosphere that is above freezing.  Here, the snow will start to melt until it falls back into part of the atmosphere that is still below 32F, where it will freeze again.  Now if there is a small layer in the lower atmosphere that is above 32F, then the snow will not have a chance to complete melt before it freezes back up again, giving us sleet.  But if the layer in the lower atmosphere that is above 32F is large, then the snow will have the time to completely melt into rain.  When this happens, there are two possibilities.  The first possibility is the layer of temperatures that is above 32F extends all the way to the surface, which would give us rain.  However, if the layer above 32F doesn’t reach the surface (meaning the surface is at or below 32F), then the rain will freeze on contact with the ground or any other object, thus giving us freezing rain.

 

Image courtesy of SPC

Looking at forecast skew-t diagrams that go into tomorrow morning, it shows that temperatures will be rising over night.  This will give us a chance of sleet and freezing rain for a little while, and then the possibility for rain if the surface air temperature makes it to above 32F.  This forecast will vary depending on location.  Areas to the north will most likely see more snow and there will be a greater chance for rain further to the south.  The areas in between will have the best chance for sleet and freezing rain.  During the day on Tuesday, the temperatures will drop back below 32F giving us a chance of snow.

Below is the 24hr precipitation map that shows how much precipitation is expected tonight and Tuesday.

 Image courtesy of HPC

The next chance for snow will be Thursday evening as a system tracks to our south.  Currently, the system is forecasted to stay far enough south where most of southeast Michigan probably won’t see much snow, but if the system tracks further north then we could get some accumulating snow.