It took until February 23rd for southeast Michigan to receive its first winter storm warning of the winter season. The forecast was generally 4-8 inches, with 2-6 inches near the Ohio border and also around Saginaw. Before then, there has been only a few 1-3 inch snow falls but nothing worthy of a warning, not even an advisory. So it’s needless to say that the approaching winter storm was highly anticipated by weather enthusiasts. Unfortunately, the winter storm turned out to be a bust for most of southeast Michigan.
The first image below represents the snowfall totals for southeast Michigan. The southern half received 1-3 inches of snow and the northern half received 3 or more inches. It is easy to see that most of the area was far under the 4-8 inches that was being forecasted.
February 24, 2012 snowstorm totals. Courtesy of NWS Detroit/Pontiac
The next image below represents the snowfall totals for Michigan and parts of Wisconsin/Illinois. The bulk of the snow fell in an area from the Wisconsin/Illinois border northeast through center lower Michigan. Some notable totals in central Michigan are 14.5 inches in Clare County and 11 inches in Kalamazoo County. A good portion of western central Michigan received more than 6 inches of snow.
February 24, 2012 snowstorm totals. Courtesy of NWS
So what went wrong for southeast Michigan? First look at the image below. The left image represents the forecasted location of the low pressure system for Friday morning at 7am. The low’s center was forecasted to be in northern Ohio near the southwest portion of Lake Erie. The image on the right is the actual surface map for Friday morning at 7am. There are two important features worth noting. First, the low’s center was further west over northern Ohio. Second, the low pressure system already occluded (represented by the purple front that goes from the low’s center to the cold/warm front intersection). In short, a system becomes occluded when the upper level wave starts to become negatively tilted which moves the upper level divergence further west. The low occludes and moves west as well in an attempt to stay under the area of divergence.
Comparison of forecasted surface map and actual surface map for Friday 7am. Courtesy of HPC
Now take a look at the image below which shows the actual surface map for Friday morning at 10am. The low’s center continues to move to the northeast as anticipated but since its position was further west than forecasted, the low’s center now moves into southeast Michigan instead of northeast Ohio.
Surface Map for Friday 10am. Courtesy of HPC
The low’s center now in southeast Michigan is the reason why the area received much less snow. First, warmer air was able to move into the area allowing any precipitation to be between a very wet snow, sleet, or even rain. Second, the dry slot was able to move into southeast Michigan. The dry slot is sinking dry air that gets caught into the low’s counterclockwise circulation which causes the dry air to move towards the low’s center. The dry air will dry up the lower atmosphere thus not allowing any precipitation to form in the area. The water vapor image below from Friday at 945am shows the dry slot in southeast Michigan. The radar image below from Friday at 915am confirms the water vapor image by showing a precipitation free area along the eastern portion of southeast Michigan.
Water Vapor image for Friday at 945am showing dry slot in southeast Michigan. Click on the image to make it larger. Courtesy of RAP.
Radar Image for Friday at 915am that shows the dry slot prevents precipitation from forming. Courtesy of RAP
With the low’s center moving through southeast Michigan, which was further northwest that forecasted, the precipitation pattern typical with a low pressure system will shift northwest as well. The image below represents the typical precipitation pattern with a low pressure system. The image shows the heaviest snow to the northwest of the low’s center, with lighter snow the further north. Also, it shows mixed precipitation along the occluded front and a combination of freezing rain, sleet, and rain along and north of the warm front. The original forecast was for the area of heavier snow to be over southeast Michigan which would have given the area the 4-8 inches that was being forecasted. Instead, this area of snow was over the Wisconsin/Illinois border northeast into central Michigan. This matches up with the image above that showed much this area received 6-10 inches with locally higher amounts in some spots.
Image represents the typical precipitation that occurs around a low pressure system. Courtesy of PSU
So even though there was a winter storm warning for southeast Michigan, there were concerns from the NWS Detroit/Pontiac office that this scenario might unfold. Their Wednesday afternoon forecast discussion mentioned the following:
“ALTHOUGH THERE IS SOME QUESTION AS TO HOW FAR NORTH THE DRY SLOT WILL WORK INTO THE STATE...AND IF THIS HAPPENS HOW FAR THE WARM SECTOR WILL MAKE IT NORTH...”
Even though this was a concern of theirs, it wasn’t enough of a concern to prevent them from issuing a winter storm warning. This is a good example that shows how difficult it is to determine the exact track of a low pressure system even a day before the event. As this past storm shows, a change in its track by only 50 or so miles can make a big difference in what happens. For me, it’s unfortunate that this occurred because it could have been our one and only chance for a significant snowstorm this winter. It is almost March so spring is right around the corner, and looking at the long range forecast models, it looks like temperatures will continue to be warmer than normal preventing any snowstorms from moving into the area. Hopefully we will have one more opportunity for a good snowstorm.
