The Mechanics of Heat Waves. Updated Forecast.

When forecasting heat waves, there are many variables that are needed to be considered. Some of these include soil conditions, cloud cover, vegetation cover, climate region, low level temperatures, upper level wind flow, and low level wind flow. These are only a few of the many factors that meteorologists take into account when forecasting periods of extremely high temperatures. To start, the definition of a heat wave for the southern Great Lakes region is a period of three days with temperatures of 90 degrees or higher. The standard definition of a heat wave is a period of three days or longer with much above normal temperatures (10 degrees above normal or more). This would make the March warm spell that we had a heat wave, because temperatures were almost 40 degrees above average. The onset of a heat wave comes as an upper level ridge builds into a region. According to the hypsometric equation, upper level ridges (or regions of high pressure) must be associated with a warm column of air. That means, the stronger the ridge, the hotter it has to be at the surface. Not only does the ridge bring a wave of warmer air, but subsidence also brings warmer air into the region. At the far eastern edge of a ridge, subsidence is strongest due to upper level height rises, usually low level cold air advection (because of a passing cold front from the last storm system), and NVA, which is negative vorticity advection increasing with height. All of this leads to subsidence, which warms the low level air mass through time. As the surface high moves east of a region, warm air advection increases into a region, allowing for even warmer low level temperatures. For a temperature forecast, you have to take into account how long the ridge was over a specific region such as a dry region. The current ridge was located over the West for over a week, located right over the Intermountain West and American Southwest desert, which will strengthen the ridge. Since the surface temperatures over the West are actually the low and mid level temperatures when they advect into our area, mixing of these hot temperatures in the presence of solar heating will lead to even hotter temperatures. Soil conditions and type are an enormous factor in summer heat waves. If soils are wet and a clay-type mixture (such as what is usual in our area), then dew points will be quite high during heat waves (75 or higher) while temperatures are usually cooler. If soils are incredibly dry and of a sandy texture (which is what is happening now, our soils have become so dry that some of the soil has turned into a sandy texture), temperatures are quite a bit hotter. I've already talked about the upper level flow, but what may be more important it the low level flow. If low level flow is from the Gulf of Mexico, then a very warm and moist air mass will become present over the area, making outside conditions very uncomfortable. Even in drought conditions, moist flow can bring higher dew points from the Gulf of Mexico, its just that a moist air mass will cause haze in the sky and clouds to form, limiting daytime heating, which is one reason why higher dew points leads to lower temperatures. You also have to take into account cloud cover. This means that you need to thoroughly analyze the jet stream for shortwaves which could induce cloud cover and thunderstorms.

As for the updated forecast, the National Weather Service has just issued an Excessive Heat Warning for temperatures 104-107 tomorrow. Heat indices will approach 110, especially if dew points are higher than expected. Tomorrow night, there will not be much relief with temperatures only falling to near 76 or 77 (which would break records). On Friday, expect yet another dry day as temperatures will soar into the upper 90's, but with slightly cooler low level temperatures. Some meteorologists are predicting rain for tomorrow, but it is highly unlikely due to the fact that the area is moisture deprived and far removed from upper level dynamics. Plus, the storms would have to break through the incredibly strong cap, which is not entirely likely. In fact, expect dry weather for the next week at least. Enjoy the hot days!