A Comparison of the Thermodynamic and Kinematic Features in Recent Blizzard and Near-Blizzard Events in Iowa

The purpose of this research is to determine the validity of the subjective, anecdotal rule that four isobars present across Iowa during a wintertime, snow-bearing, extratropical cyclone is sufficient to create the winds that attend a blizzard (the “Four Isobar Rule”). Using data from the National Centers for Environmental Information's online Storm Event Database, we identified 15 cases of blizzards in Iowa over the course of 10 years (1999-2009 plus one event in 2010). Subjective analyses of mean sea level pressure for those 15 events revealed an 87% success rate of having four isobars present corresponding with the Blizzard events. Conversely, we also analyzed 23 Near-Blizzard events and discovered that approximately 74% did not have four isobars present. Clearly, there is overlap between these types of events. Composite meteorological fields of both case sets were completed using the North America Regional Reanalysis datasets. The composite results are generally consistent with other recent findings on Iowa blizzards. The surface low is often fairly deep (<1000 hPa), occluded or nearly so, featuring a convergent, descending lower troposphere, with 850-hPa winds of 45 kt or greater, and a near-surface layer whose lapse rate is nearly dry adiabatic. The composites of storms in the Blizzard category are slightly weaker than in prior literature, but the composites of storms in the Near-Blizzard category fail to meet any of the criteria for a blizzard in the state of Iowa.