TO the left is the CPC outlook for the 28th through July 2nd showing about half of the US with excessive heat. The central US will see temperatures breaking 100° Our area will see temps in the 90s with dew points in the low to mid 70s. This will be a very uncomfortable airmass unless you like tropical weather and like to sweat. The airmass will be in Michigan from Friday through next Monday.
The National Weather Service issues excessive heat warnings within 12 hours of the onset of extremely dangerous heat conditions. This warning is generally issued when the maximum heat index temperature is expected to be 105 degrees or higher for at least two days and nighttime air temperatures will not drop below 75.
Heat waves form when high pressure aloft (from 10,000–25,000 feet) strengthens and remains over a region for several days up to several weeks. This is common in summer (in both Northern and Southern Hemispheres) as the jet stream ‘follows the sun’. On the equator side of the jet stream, in the upper layers of the atmosphere, is the high pressure area.
Summertime weather patterns are generally slower to change than in winter. As a result, this upper level high pressure also moves slowly. Under high pressure, the air subsides (sinks) toward the surface. This warmer sinking air creates a high level inversion that acts as a dome capping the atmosphere, inhibiting convection, thereby trapping high humidity warm air below it. Typically, convection is present along the periphery of the cap where the pressure becomes less. This peripheral convection, however, can add to the high pressure dome by ventilating the upper level outflow of the thunderstorms into it. The end result is a continual build-up of heat at the surface that people experience as a heat wave.
A low pressure system will track across Lower Michigan tonight into Wednesday morning. This will bring the scattered showers to the area this afternoon with showers and a few thunderstorms tonight. Seems the heaviest rainfall is likely to be near and north of I-96 tonight. The showers may linger into the mid morning hours of Wednesday.
As the system moves of out the area on Wednesday some additional showers and thunderstorms are possible Wednesday afternoon, mostly east of US-131. Thursday should be a pleasant sort of day with partly cloudy skies and highs in the 80s.
We still expect our heat wave over the weekend with high in the lower to mid 90s and heat index values in near 100. It now looks like a cold front may come through the area Sunday evening or early Monday. That will bring the threat of afternoon thunderstorms Sunday and bring slightly cooler temperatures Monday and Tuesday.
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