By now, word is out that we might see accumulating snow this weekend. To be blunt, confidence is not high with this one. We have two chances over the next few days.
First, we could see a dusting of snow Friday night as a fast moving, weak low pressure system tracks south of us.
The second, and better chance, is associated with a deepening low pressure system that is forecast to develop over the southern plains and head northeast on Sunday. Again, likely passing to our south.
We have a few computer model simulations that give us a prediction of the weather in the longer range, in this case 4 to 5 days out. We use one from the U.S., called the GFS, another from Canada, called the GEMnh and finally the highly regarded ECMWF from the European Weather Agency. Additionally, we also utilize an ensemble solution of the GFS. The ensemble is a collection of 20 different runs of the GFS, all with slightly different intitial conditions as a starting point. Click here: for good background information on the ensemble forecast. The tighter the agreement among the ensemble solutions, the higher the confidence we have in the forecast. If there is a wide variety of solutions, confidence drops.
Okay, so here is the ensemble of the GFS for 6 PM on Sunday. This depicts the storm's low pressure center positions from 12 of the GFS ensemble runs. This is a perfect example of a low confidence solution! We have potential tracks going as far south as the Gulf Coast, or just south of the Ohio River, or just north of the Ohio River. Some have a stronger low than the others. Some are fast, some are slow.
The operational run of the GFS (run at 6 am Wednesday) has the surface low pressure center passing near St. Louis at 6 PM on Sunday. You can see this is the more northern solution of all the ensemble members. The "operational" run is the solution that is shipped out as the official GFS solution.
Here is the weather map from that operational run of the GFS, with mean sea level pressure and precipitation depicted:
If this solution verifies, some areas across southern Wisconsin could see 4 to 6 inches of snow. However, the highly regarded ECMWF favors a more southern and weaker storm system, likely keeping snow amounts much lower. The Canadian GEM model looks like the operational GFS. We also have some temperature concerns, especially near Lake Michigan where warmer air off the lake would likely cause a wintry mix of rain and snow across far southern Wisconsin.
So, as usual, keep up with the forecast. There is huge uncertainty with regard to the details of this winter system. Will it track farther south, will it be cold enough or will it be stronger or weaker than forecast? Naturally, this will become clearer as we get closer to the event.