Field Crops Team
HARVEST ALERT
FACT SHEET #1 Fall
1997
Climatological First Freeze Dates
Jeff Andresen, Agric. Meteorologist,
Geography, Michigan State University
With so many crops lagging behind normal in development, the date of first freeze will likely be a critical factor in determining final yield performance this season. Figures 1a, b, and c give the average (50th percentile), 10th percentile, and 90th percentile date of first fall freezing temperatures (32°F or lower at the 5-foot instrument shelter height), respectively. The 10th percentile dates refer to the earliest 10 percent of observed first freeze events while the 90th percentile refers to the date at which 90% of all first freezes have occurred. With the first and last of these dates for a given location, one could determine a representative range of first freeze dates, from the 10th percentile to the 90th percentile, which statistically comprises 80 percent of all possible events in the period of record. Surprisingly, at most locations, this range of dates is only about two to three weeks. It is important to note that these statistics refer to temperatures taken at a 5-foot level inside an instrument enclosure. Actual temperatures near the surface may be several degrees colder, especially under clear, calm, stratified conditions, even though the 5-foot shelter/enclosure temperature is still above freezing.
The average date of first fall freeze in Michigan depends greatly on proximity to the lakes, with several weeks separating the earliest occurring events in interior sections of the Upper and Lower Peninsulas to the latest events at stations right along the lakeshore in the south. Unfortunately, there is no accurate way to predict when first freeze events will occur very far in advance (i.e. much more than several days in advance). However, certain weather patterns are necessary for the first below-freezing temperatures of the season. They include: a large Canadian-origin area of high pressure, the center of which moves near or across Michigan; and the clear, calm conditions that are normally associated with such high pressure areas. For a strong Canadian-origin air mass to move into the Midwest, follow the National Weather Service 6-10 day outlook and extended outlooks, watching for a northwesterly or northerly configuration of the jet stream across the central and eastern U.S. This is the only meteorological pattern capable of bringing unseasonably cold air into the region at that time of the year.
Click for Michigan maps showing 10% chance, 50% chance and 90% chance of first fall freeze.
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