Another winter storm is on the way for most of southeast Michigan, but it is ice instead of snow that will be the main concern. The map below shows the current (as of 11:30am) watches/warnings across southeast Michigan. The counties that border Ohio (Monroe and Lenawee) do not have a watch or a warning as they are expected to receive mostly rain. The row of counties to their north (Wayne and Washtenaw counties) are currently under a winter storm watch because there is still uncertainty with what to expect in this area. All counties remaining to the north (Macomb, Oakland, and Livingston and to the north) are currently under a winter storm warning since there is more confidence that they will experience freezing rain, sleet, and snow.
Image courtesy of NWS Detroit
Right now, a low pressure system is organizing to our west in the central plain states. As this system strengthens, it will move towards the east and pass to our south through the Ohio Valley tonight and tomorrow. This low pressure system will have really good upper-level dynamics along with plenty of moisture that will be transported in our direction from the Gulf of Mexico.
The image below shows the basic set up that will take place for us overnight. Tonight at about 2am, the center of the low pressure system is expected to be in northern Iowa. It will have a cold front that trails back into the southern plains and a warm front that will be positioned in Illinois/Indiana/Ohio. You can hardly see it, but at the top of the image is the arctic high pressure system. This system with its anticyclonic (clockwise) rotation will cause southeast Michigan to experience cold surface winds from the northeast. At the same time, warm, moist air from the south will move northward in our direction towards the warm front (thanks to the counterclockwise circulation around the low pressure system). Once the warm air reaches the warm front, it will be forced to rise due to the cold air north of the warm front (cold air is more dense than warm air, thus forcing the warm air to have to go in another direction…up). This process is known as overrunning. This warm, moist air will continue to rise vertically the further north it moves horizontally from the warm front, which will cause most of southeast Michigan to have warmer temperatures above the surface than we will at the surface.
Image courtesy of HPC
This situation can always be shown well on a skew-t diagram (image below). This image represents the temperature (red vertical line) and dew point temperature (green vertical line) from the surface to the top of the troposphere (the atmosphere we live in). The black slanted line represents the freezing temperature (32F/0C). If the red temperature line is to the left of the black slanted line, then the temperature in that part of the atmosphere is below freezing, and if the red temperature line is to the right, then the temperature in that part of the atmosphere is above freezing. Looking at the two forecast soundings below (left is NAM and right is GFS for Wednesday at 2am), we are going to focus on the lower part of the atmosphere (between 1000mb-700mb). In the lowest part of the atmosphere, the temperature is below freezing which represents the cold air from the arctic high pressure system, but above this layer of air, the temperature is above freezing which represents the warmer, moist air that is from the Gulf of Mexico. Now in this type of setup, the precipitation will fall from the clouds as snow and then melt into liquid when it reaches the layer of air that is above freezing. As the liquid falls into the layer at the surface that is below freezing, it will do one of two things. If the layer that is below freezing is big enough, the liquid will have time to partially melt, thus forming sleet. However, if this layer is very small, then the liquid will remain liquid but freeze instantly once it reaches the surface, thus falling as freezing rain.
Image courtesy of PSU ewall
Now what we can expect and where is always difficult in these types of situations because it is very hard to determine where the rain/freezing rain line will form and also where the freezing rain/sleet/snow line will form. The NWS Detroit meteorologists are basically forecasting rain south of I94, sleet and freezing rain between M59 and I69, and snow north of I69. This is because south of I94, the warmer air will be closer to the surface (and the arctic high will have less of an impact further south), allowing temperatures to remain above freezing. As I mentioned earlier, Wayne and Washtenaw counties have winter storm watches because it is difficult to determine if they will see more rain than mixed precipitation. I suspect that they will see a combination of both and will be given a winter weather advisory sometime this afternoon. The counties that are around M59 to I69 have the biggest threat for a big ice storm. They could see 0.25-0.50 of an inch of ice depending on if more freezing rain and less sleet falls. This much ice accumulation can cause tremendous weight on power lines and tree limbs making power outages a threat. To the north of I69 where the cold air from the arctic high will play more of a major role (and overrunning will not play much of a role), they will see pretty much all snow, with 5-10 inches possible depending on the location. The map below shows the amount of liquid precipitation expected by tomorrow at 8am. Most of southeast Michigan is in the 0.50-0.75 inch range. Remember this is liquid precipitation, not the amount of snow or ice expected. Also, the warning is to 8pm and the map below only shows to 8am, so we could receive high totals.
Image courtesy of HPC
Hopefully this is the last ice storm we see this year. I can use one more big snow storm though! Spring would be nice also.
UPDATE:
1. The counties with the winter storm watch now have an winter weather advisory. This area can expect rain changes to freezing rain over night with minor ice accumulation.
2. I forgot to mention that this morning's model runs have trended towards the heaviest precipitation being a bit further north than previous runs. If this occurs, then we could expect the rain, sleet, and freezing rain to be on the lighter side of the original forecast. Also, this northward movement could bring the dry slot into southeast Michigan which would cause the precipitation to end earlier for the areas effected.
The area with the best chance for accumulating ice is still expected between M59 and I69.
