Metro Detroit weather: Severe threat looms, tornadoes cannot be ruled out

Thunderstorms Tuesday night have capability to become severe

DETROIT – General rain and thunderstorms rolled through the area this morning and early afternoon. These were not expected to be severe, and they weren’t. In fact, they brought beneficial rains to parts of the area that still needed a soil moisture recharge.

However, any thunderstorms that develop later this afternoon and tonight certainly do have the capability to become severe. Here’s the set-up:

We have a very humid air mass overhead, with a strong cold front approaching. That front is the primary trigger for the thunderstorms later today. In many severe weather situations, we know that thunderstorms will develop, and the question is if the necessary ingredients exist to make them severe.

Today’s scenario is a little different: Since the cold front crosses the state during the night, when it’s cooler, the atmosphere is less unstable than during the daytime. The latest high-resolution computer models, in fact, only develop scattered thunderstorms.

So while the thunderstorm coverage could end up somewhat limited, the ingredients are in place for there to be a severe threat. Take a look at this piece of forecast computer model data:

The area circled is wind barbs from the surface up to between 10,000 and 20,000 feet. The arrows show the direction the wind is blowing toward. Notice how the wind shifts with height -- this is called wind shear.

In this particular computer model, the wind shear is quite pronounced. If these conditions aloft verify when any thunderstorm develops this evening, then the storm would easily develop rotation. Now, every rotating storm does not produce a tornado, but the threat is clearly higher when rotating storms (called supercells) develop. However, this is only one model.

Another model that just came in shows much less wind shear, which would significantly reduce the tornado threat. So, with all of this in mind, here is the Storm Prediction Center tornado risk assessment for later today and tonight:

The green area is a two percent chance of a tornado within twenty-five miles of any given point on the map. The pinkish color that encompasses much of Lenawee and Monroe Counties and areas south of there is a five percent chance of a tornado within twenty-five miles of any given point on the map.

Regardless of whether or not any storms generate a tornado, wind 20,000 feet aloft will be blowing at 50 to 60 mph, so severe wind gusts are definitely possible with any storm that fires up, as these storms should easily build higher than 20,000 feet, which means that it will be capable of pulling some of that strong wind down to the surface.

The bottom line is that we have a robust severe storm risk later today and tonight. Now is the time to prepare. Make sure to watch Ben Bailey on Local 4 News First at 4, then at 5 and 6 for the very latest.

If you’re at work, remember that we livestream all of our newscasts, so you can watch live on ClickOnDetroit. Keep your cell phone fully charged in case you lose power. And make sure that your weather radio is turned on, with fresh batteries.

The aforementioned cold front will cross the area late tonight, bringing a sharp change in the weather.  Wind will shift from south to northwest, temperatures will immediately drop, as will dewpoint temperatures -- meaning much drier air is coming in.

So even though it’ll be warm and muggy when you go to bed, you’ll notice the change to a colder, drier air mass toward dawn -- with our West Zone getting it first (lows probably in the mid 50s -- 13 to 14 degrees Celsius), and the far eastside getting it last (lows in the low 60s (16 degrees Celsius).

Any stray showers lingering around first thing Wednesday morning should end rather quickly, with skies becoming mostly sunny during the afternoon. Temperatures may only rise a few degrees from what it is when you started your day. Adding to the feel of the day will be west winds blowing at 10 to 20 mph…you’ll notice those breezes.

Mostly clear and chilly Wednesday night, with lows in the mid 40s (6 to 8 degrees Celsius).

Partly cloudy both Thursday and Friday, with highs in the mid to upper 60s (19 to 20 degrees Celsius).

Weekend forecast

As we’ve explained many times in the past, the long range computer models have great difficulty modeling upper level disturbances several days in advance, and this weekend is no different. It looks as if there will be a couple of them crossing the Great Lakes, but the strength and timing still need to be sorted out.

At this point, it appears that the first disturbance may bring a light shower or two Friday night, but should be long gone by Saturday morning and leave us a nice fall weekend day, with highs in the mid 60s (17 to 18 degrees Celsius).

Some models keep us dry on Sunday, but others try to generate a few light showers with the second of the two upper level disturbances crossing the state. Regardless if we get those showers or not, highs should remain in the mid 60s (17 to 18 degrees Celsius).

About the Author:

Local 4 meteorologist Paul Gross was born in Detroit and has spent his entire life and career right here in southeast Michigan. Paul has researched, written and produced eight half-hour documentaries for WDIV, as well as many science, historical and environmental stories.