48ºF

Metro Detroit weather: Relief from heat is on the way

Cold front bears down on area

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DETROIT – Before getting to the forecast, let's first follow up on what we discussed in yesterday's weather article. Some of you were asking us about a sensationalistic headline on the internet which said that a “new weather term could hit Michigan tonight.”

That term is quasi-linear convective system (or QLCS), and we want to assure you that the term is not new, and it is not some once-every-1000-years type of Armageddon storm that destroys humanity.

Is a QLCS dangerous? Of course it is. It’s a line of severe storms that cause damage. Sometimes lots of it. But a derecho, which we’ve written about in the past, is a much more dangerous line of super-severe wind damage that affects a much larger area.

So what exactly is a QLCS? A QLCS is a line of severe storms, but with curves and waves in it. Thus, the term “quasi-linear.” The importance of this type of line of storms is that localized wind shear created by wind flowing out ahead of the storms in areas where there are curls or bows can spin up brief, weak tornadoes.

Take a look at this radar image from last night:

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You can easily see the line of storms coming off of Lake Michigan into western Michigan. Now look closely at the area near the red box:

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See how a big kink has developed in that line? That's a favored area for a possible quick spin-up tornado.  And the red box is, in fact, a tornado warning that had been issued. Now take a look at the same image, but this time looking at the Doppler radar's velocity data, instead of reflectivity (which only shows rain):

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It's important to know that the radar antenna is to the lower right of this image. The radar not only sees the raindrops, but also computes if those raindrops are blowing toward or away from it. Here, the reds are blowing away from the radar, and the greens are blowing toward the radar.

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When we have those reds right next to those greens, we meteorologists call that a couplet, and it indicates rotation. This couplet did indeed drop a tornado that hit south of Alba, which was 70 yards wide and traveled 6.78 miles.

The QLCS spawned one other brief tornado that touched down northwest of West Branch. This twister was only 40 yards wide, and on the ground for less than one-quarter mile (but on the ground long enough to damage two homes). 

Again, when certain conditions aloft exist, lines of severe storms sometimes develop kinks or bows, and those areas are favored for possible brief tornadoes, even if the overall atmospheric set-up doesn't favor tornadoes (like Tuesday night's).

But we here at Local 4 will never scare you with a headline like the one you saw elsewhere yesterday. There are a lot of technical meteorological terms we never or rarely ever use publicly, and QLCS is one of them.

If we had twice the amount of time to do the weather than we currently get, then perhaps we'd use and explain more of these technical terms (like "left exit region/right entrance region," "inverted-V sounding," "convective available potential energy/CAPE," "mesoscale convective system/MCS," "positive vorticity advection/PVA," etc.).

These are all things that we Local4Casters talk about routinely in our weather office.  But we'll NEVER use them in a headline to scare you into clicking on our story.  Clickbait is irresponsible, and not what we do to earn your trust.

Forecast

Alright, with storms out of the picture, let's get to the forecast. A cold front is bearing down on our area, and will usher in a cooler and much drier air mass that will certainly make us happy to open the windows for some outstanding sleeping weather the next two nights.

A few showers are still possible over the next several hours, but they will end this evening once the front moves through. Skies will eventually become partly cloudy overnight, with lows near 60 degrees (15 to 16 degrees Celsius) in our Urban Heat Island closer to Detroit, and in the 50s (13 to 14 degrees Celsius) elsewhere. Wind will shift to the northwest at 5 to 10 mph.

Most of us will have a partly cloudy day on Thursday (aka, Friday Eve), although some high-resolution computer models suggest that a few sprinkles may affect the Thumb later at night into Thursday morning. You'll certainly notice the temperature, as highs only reach the low to mid 70s (22 to 24 degrees Celsius), and the humidity will be SO much more comfortable.

Thursday's sunrise is at 6:57 a.m., and Thursday's sunset is at 8:10 p.m.

Mostly clear Thursday night, with lows in the low to mid 50s (11 to 14 degrees Celsius).

Mostly sunny on Friday (TGIF), with comfortable humidity.  Highs in the upper 70s (26 degrees Celsius). What absolutely spectacular weather for the start of this weekend's big Arts, Eats and Beats festival.

Holiday weekend

Starting Saturday, we enter a period of warmer and more humid weather (but not as bad as the one we're ending right now), with a thunderstorm chance pretty much each day.  Some days, like Saturday, chances are higher.  But many days will feature just a scattered storm chance.  Look for highs in the mid 80s (29 to 30 degrees Celsius) well into next week, with overnight lows creeping back up to near 70 degrees (21 degrees Celsius).


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