*Blog written at 1:00pm CDT 6/26/13*
Summer is here and that means hot for most in the United States. But along with summer heat, severe weather also poses a threat for many, especially those living in the northern Plains and northern Midwest. Obviously, severe storms can occur any day of the year, but I wanted to focus on where and in what form severe storms occur during the summer months.
You may or may not know that criteria for severe storms, as defined by the National Weather Service, are: tornadoes, and/or at least one (1) inch size hail, and/or wind gusts of at least 58 miles per hour. As stated above, any of these three criteria can occur on any day of the the year. During the summer months, tornado outbreaks (as defined by my definition of >10 different tornadoes in a 200 mile radius in one day) don’t usually occur that often. There are a couple of reasons why this is. Temperatures climb into the 90s or even the lower 100s during peak heating hours of the day in most locations in June, July, and August. While daytime heating is good for thunderstorm initiation, this usually creates a huge dewpoint depression, or the difference between air temperature and dewpoint temperature. Typically, for tornado development, dewpoint depression is often less than 20 degrees, with best opportunity at 10 degrees or less. Of course, this does not mean it is impossible for a tornado to develop with a dewpoint depression of >20 degrees. The main threats when there is such a gap between temperature & dewpoint are wind and hail. I will give a brief explanation why in a bit.
Now, I know what you’re thinking, what about the areas that don’t make it into the 90s and/or 100s, especially during the early summer days? It all has to do with mid-and-upper level patterns. The best areas for tornadoes during the summer months are the central/northern Plains and northern Midwest. Temperatures are relatively cooler in these areas during the early summer days, along with the favorable jet stream location in this area. Typically, sometime during June, a pretty strong mid-level ridge builds into the southern/central CONUS. This not only forces the jet stream north, it acts as a “blocking pattern” in the mid-levels, which meteorologists call the “Omega Pattern”. Find more blocking patterns here. Mid-level troughs/cutoff lows (lower isoheights) are either forced to crest the ridge or retrograde westward. Often, during the summer, an elevated mixed layer (EML) will be present in the lower troposphere, mainly due to the upper level ridge. In simple terms, a warm layer of air is embedded just a few kilometers above ground level, which prevents surface-based storms from forming. Tornadoes are often formed from “surface-based storms”. Storms that form above the EML are called “elevated storms” and pose a major hail and wind threat, with a very slight tornado threat. Elevated storms that form along a boundary layer (cold front, warm front, dryline, outflow boundary, etc.) either clump together and/or form a line of storms. This is how a Mesoscale Convective System (MCS) forms. There are a couple of forms of MCS’s (product of severe storms) that can be recognized on Doppler Radar: bow echoes & squall lines. Bow echoes are usually formed from squall lines.
The reason why wind and hail are the main threats with any MCS is because downbursts are associated with MCS’s. Because cold air is more dense than warm air, rain cooled air a few kilometers above the much warmer ground will crash down, at fast speeds, and spread out once it hits the surface. This creates damaging straight-line winds, sometimes exceeding 100 mph. Updrafts in these storms are very strong, capable of producing very large hail, as well. The stronger the updraft, the stronger the downdraft (wind & hail). Because mesoscale convective systems are usually clustered and elevated thunderstorms, tornadoes are less likely to be produced, but not impossible to form. In fact, tornadoes are more favorable to form at either end of bow echoes, in a area called the bookend vortex. Rotating supercells may also be embedded in the MCS. These systems usually last anywhere from 3-6 hours, on average and can travel hundreds of miles at times.
Derechos are also a form of MCS, but certain criteria are also needed to define a derecho:
1.) Wind damage swath has to extend more than 240 miles (400 km).
2.) Wind gusts of 58 mph (93 km/h) must be along most of its length.
Find more about derechos here.