Metro Detroit forecast: The calm before the warm, and the storm

Tracking both dangerous heat and severe weather


DETROIT – Everything remains on track with my forecast, so let’s get right into the details. 

A Heat Advisory has been issued for all Local 4 counties except Sanilac County until at least 10 p.m. Friday.  As I’ve been discussing all week, very hot daytime temperatures combined with warm overnight lows that don’t allow our bodies to recover from the heat will develop Thursday and continue into the weekend.

Skies should remain mostly clear overnight, and humidity will start ramping up during the night as higher moisture-content air streams into southeast Michigan.  Lows in the upper 60s (that’s 20° Celsius for our Canadian neighbors).  Very light south wind.

Mostly sunny to partly cloudy on Thursday, and becoming hot and humid.  Highs in the low 90s (33° Celsius), with the humidity making it feel like 97° (36° Celsius).  Southwest wind at 10 to 15 mph.  Thursday’s sunrise is at 6:16 AM, and Thursday’s sunset is at 9:03 PM.

Thursday night we need to remain alert and have our weather radios set, because a special nocturnal cluster of strong to severe thunderstorms (called a Mesoscale Convective System, or MCS -- I’ve talked about them a lot this summer) will develop up north and then move almost directly due south downstate. At this point, it appears as if the greatest risk for severe weather is across the western half of the Lower Peninsula, but we definitely have a risk, too, with the primary threat being a potentially widespread damaging wind event.  Here’s how our 4km RPM model depicts the scenario playing out:

Overnight lows Thursday night will only fall into the low to mid 70s (23° Celsius), and remember that it’ll take all night to get to that temp -- it’s going to be oppressively warm and muggy.

Storms should be gone by sunrise Friday, so I think we’ll get through the day dry.  However, something that needs to be monitored is that these MCSs sometimes leave boundaries behind them -- kind of like mini-cold fronts -- and those can trigger scattered thunderstorms.  So keep an eye on the radar on the free Local4Casters app if you have outdoor plans Friday afternoon. 

One thing that is guaranteed is that it’ll be dangerously hot and humid.  Highs should reach the mid 90s (35° Celsius), with the humidity making it feel possibly as hot as 105° (40° Celsius).  By the way, if the Heat Index is expected to be 105° or greater for three hours, then the National Weather Service likely will upgrade the Heat Advisory to an Excessive Heat Warning.

Continued warm and muggy Friday night, with lows in the low 70s (22° Celsius).

Mostly sunny, hot and humid on Saturday (but not as humid as on Friday), with highs in the low 90s (33° Celsius), and the humidity making it feel like 95° (35° Celsius).

Continued warm and muggy Saturday night, with lows in the low 70s (22° Celsius).

Becoming very hot and humid once again on Sunday, with a good chance of thunderstorms.  Highs near 90° (32° Celsius), with the humidity making it feel like 98° (36° Celsius).

Thunderstorm chances continue Sunday night, with lows in the low 70s (22° Celsius).

A cold front finally crosses the area early Monday, with thunderstorms ending by mid-to-late morning.  Becoming mostly sunny and less humid, but still hot, with highs near 90° (32° Celsius).

Then mostly sunny and less humid on Tuesday and Wednesday, with highs in the mid 80s (30° Celsius).

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