What happened today, plus forecast

Nice stretch of weather ahead for metro Detroit


DETROIT – Before getting to the forecast, I need to explain what happened today.

Tuesday's computer models were very accurate in projecting the timing of a cold front sagging southward across the area on Wednesday. No arguments there -- the front moved south into Ohio and Indiana right on schedule late Tuesday night.

What the computer models didn't have a handle on was the band of rain that developed BEHIND the front.

To give you an idea of how blind the models were to this, I checked one of our highest resolution computer models mid-morning Wednesday, and it still had the rain diminishing early in the afternoon. Those of you who know me know that I don't get many forecasts wrong and, when one does go awry, I'm very upset and want to get to the bottom of things. I spent a lot of time today trying to figure out what happened, and I think I know the answer.

There's no way I can give you the full technical explanation -- it involves a lot of physics and thermodynamics -- but I believe that today's rain band that developed behind the front and just sat there was due to something called frontogenesis. In general terms, frontogenesis is the change in the magnitude and orientation of the temperature gradient in a layer aloft due to directional and speed changes in the wind field. I won't go any farther, except to tell you that frontogenesis causes vertical motion and, since rising air gives us clouds and precipitation, an area of frontogenesis sometimes generates a band of precipitation.

As I mentioned earlier, the cold front itself was well south of us. The rain band behind the front was caused by an area of frontogenesis that didn't diminish until mid-to-late afternoon -- and this is when the rain literally dissipated right overhead (it didn't move out, it just fell apart once the frontogenesis diminished).

I apologize for any inconvenience today due to the rain that wasn't "supposed" to be here, but the computer models just didn't have any grasp on the frontogenesis. Had I had even the slightest hint about this yesterday, I certainly would have mentioned the possibility of rain in my forecast.

Yesterday's and today's rain has prompted a Flood Warning to be issued for the Clinton River at Sterling Heights and Clinton Township. The river level is rising, and is expected to crest this evening about 6 inches above flood stage, causing minor flooding. The river should fall back below flood stage tomorrow.

Alright, onto the forecast, which will make you smile!

Tonight, expect partly cloudy skies, with the possibility for some fog to develop. Lows in the low 50s, with a very light wind.

Mostly sunny to start your Thursday, then becoming partly cloudy. Very comfortable with highs just making it into the mid 70s, although a northeast wind will keep those of you downwind from Lake Huron only in the 60s. Becoming mostly clear Thursday night, with lows in the mid 50s.

Then mostly sunny on Friday, Saturday, and Sunday – absolutely perfect weather for the Detroit Grand Prix! Highs will start in the upper 70s on Friday, and rise to the low 80s by Sunday. Overnight lows will be very comfortable, in the 50s.

Things then turn more humid, and more unsettled Monday through Wednesday, with a chance of showers and thunderstorms each day.

Remember to follow me on Twitter: @PaulGrossLocal4

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