Past IEM Features tagged: firstfreeze
With about every accounting scheme now placing Iowa in the fall season, it is time to start worrying about when the first freeze will happen. Such temperatures are not far away from Iowa this Friday morning with sub freezing temperatures expected in Wisconsin. The featured chart presents an IEM analysis of the accumulated areal coverage over Iowa that has dipped below 32 degrees at least once by a given date of the fall season. This value is computed over a grid with the percentage of grid cells representing the coverage. The orange line represents the highest coverage by a given date and the green line the lowest coverage. The red line was last year's value and blue line is the simple climatology. So by the 23rd of September, at least some limited portions of the state have likely seen such cold temperatures. The 50 percent climatology value is about the second week of October, which nicely matches station climatology for a good chunk of the state.
The forecast for this coming weekend continues to push low temperatures well into the 30s and maybe a first freeze for some parts of the state. For weather forecasters, pattern recognition plays an important role. One of the main patterns forecasters look at is the 500 hPa (mb) height surface, whose height informs temperatures and orientation informs general wind circulations. The featured map presents a composite of this height field on mornings of the first fall freeze each year for Des Moines. The composite clearly shows ridging over the western US and troughing over the eastern. The flow orientation is thus out of the northwest, which is a source of cold air this time of year. Such a pattern is currently expected for the weekend, but having a sub freezing temperature is not a given and only in the realm of possibilities.
Tags: narr firstfreeze