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.

Voting:
Good = 16
Bad = 0

Tags:   narr   firstfreeze