It's been a hot, mainly dry summer for our area this summer. Some locations have picked up some much needed rain over the past week, but these were hit and miss showers/storms that skipped many cities. While the timing may not be the best (on the weekend), we all stand a decent chance at picking up some widespread, much needed rain showers from Friday through Sunday.
Here's the setup: Rain shouldn't be so hit-and-miss this weekend, as an upper level low pressure system tracks in to the Great Lakes. All of us stand a pretty good chance at measuring much needed rain, perhaps on the order of what now looks like an inch to an inch and a half. Below are projected totals from a few models we use in-house.
This system typically takes a two to three days to move in to and out of the region. We refer to this type of system as a cold core cut-off low. Cold core because it is a mid-latitude cyclone with cool/cold air at its center, and cut-off since there is a disconnect from the upper level steering winds we call the jet stream. Because of this disconnect, the low tracks slowly and literally meanders through the region.
Take a look at the image below. It's from one of our computer forecast models (the GFS, for you weather geeks). This shows our atmosphere about 18,000 feet above the surface, or what we refer to as the 500 millibar level. This upper level chart shows a trough over the Great Lakes with a low pressure system moving in. The image is valid for Friday evening. The yellow and green just off to our north/west indicate a turning motion in the atmosphere called vorticity...which ultimate creates lift. You know the rest...lift leads to cooling and condensing of air, cloud formation, and precipitation.
The next image below shows the same forecast model valid on Saturday evening. Note the "action" directly over Michigan. Lift, spinning motion, and likely cloudy skies with a good chance of scattered showers.
By Sunday morning, the image below still shows Michigan under the influence of the trough and upper level low pressure system producing mainly cloudy skies with a shower or two possible. It doesn't appear as if this system will lift out of the region until at least later Sunday or perhaps Monday.
It's a Catch 22. We need the rain, but it will impact the weekend, a time when most of us look forward to dry, sunny weather for our outdoor activities.
You can get the complete forecast at www.fox17online.com/weather.