Death Valley made national news on Sunday as it recorded a preliminary high temperature of 130°F. This is the hottest temperature recorded across the globe since 1931 and the hottest in the U.S. since 1913. This is also the highest temperature ever recorded on Earth in the month of August. Just about the entire country west of the Great Continental Divide is experiencing extreme heat as well. Many areas have recorded 100°-110°+ temperatures since Sunday including triple digit temperatures recorded as far north as Washington state and parts of southern Montana. This excessive heat is the result of a large high pressure system in the upper levels of our atmosphere. While this system brings extreme heat out to the west, it is also responsible for several days of dry, sunny conditions here in the Stateline.
This high pressure system aloft has created a large ridge, or an arching shape, in the jet stream over the northwest U.S. Air underneath this ridge compresses and warms causing rapid heating down at the surface. A large trough, or dip, in the jet stream is parked over the Mid-Atlantic leaving the upper Midwest centered right in between the two. In this part of the jet stream on the east side of a ridge and west side of a trough, wind tends to rapidly decelerate and wind particles converge. As they do, air is driven down toward the surface and creates a high pressure system at the surface. High pressure systems tend to bring stable, dry air to an area which limits the development of rainfall and cloud cover. In this case, a very strong, large system of high pressure has developed in the central Plains and is making its way into the Midwest. It will center itself over the Stateline while it continues to push east bringing lots of sunshine and virtually no rain chances through Friday.
This isn’t the best news considering the Stateline is falling way behind in rainfall for the month. So far this August, through the 17th, Rockford has fallen 2.88” short of the normal month-to-date rainfall total of 2.60” having only received 0.52” so far. Since meteorological summer began on June 1st, Rockford is 3.88” shy of the normal amount of rainfall from June 1st through August 17th. A good-sized portion of the Stateline is under “abnormally dry conditions” per the National Climate Mitigation Center, though no droughts have been declared in Illinois as of Tuesday morning.