FRUIT FROST FORECASTS ---THE KEY STATION FORECAST CONCEPT.
Key station forecasts have been part of the fruit frost forecast since the National Weather Service began providing the frost forecasts for central Washington in the early 1930”s. Clearwest continued with the “key station” forecast concept when The National Weather Service terminated the frost forecast service in 1996. Key stations are a number of special minimum temperature reporting sites scattered throughout the fruit growing areas of central Washington. They contain a minimum recording thermometer housed in a standard weather shelter. These stations, until recently, have been mainly read manually, usually by the fruit growers themselves. More recently, automatic stations, such as the Washington State University PAWS network, have been added to the manual network.
When temperatures are forecast to drop to 32 or lower, a specific temperature forecast is made for each of the key stations, as part of the general daily fruit frost forecast. If temperatures are expected to stay above freezing, only a categorical forecast of slightly above freezing (SAF) (33 to 35 degrees) or above freezing (AF) (35+) is made.
The concept behind key station forecasts developed from the knowledge that often a large difference in minimum temperature occurs from place to place on many nights even across a fairly small area. Individual orchards can have locations that differ by 5 degrees or more on the same night. The temperature differences over a larger area can be even more extreme, sometimes up to 15 to 20 degrees on the same night between the warmest and coldest areas in a district. Temperature sensing stations were set up to determine where these various cold and warm areas were located, and then forecasts were made for these sites. A grower could then determine which of these temperature regimes mostly nearly matched his particular orchard location, then “key” off of the forecasts for that station to help determine the likely minimum for his orchard. It was also found that the warm and cold areas migrated around a district depending upon the general weather and wind conditions existing for that night. Experienced growers watched more than one key station, since under certain weather and wind conditions, temperatures in his orchard would be similar to one key station, yet in other weather conditions, it would be nearer to another key station. For example some growers in the lower Yakima valley pay attention to the wind direction reported from Rattlesnake Ridge. They have found that under southwest winds, their orchard is usually warmer and corresponds to one of the warmer key station temperatures in the area, whereas under north or northeast flow, it runs colder and matches more closely one of the colder key stations.
Under key station concept, it is desirable to maintain a key station in the same location for as long as possible, since even a slight movement of the station to another location can result in a different temperature regime. Clearwest has tried to maintain the integrity of the key station network as it existed when the National Weather Service terminated the frost forecasts in 1996. However, for various reasons, some stations have been either terminated or moved, while some have been added. Even if a station remains in the same location, its temperature regime may be altered by physical changes in the local area. In the Yakima Valley, most of the key stations have existed in the same location for many years. In the Wenatchee and Okanogan districts, many of the original key stations have been moved to different locations during recent years. The Central Basin uses only PAWS sites as key stations and these have remained in the same location since first sited.
While the key station concept is useful and Clearwest will continue to make specific temperature forecasts for them, growers should be aware of their shortcomings and always pay attention to the entire forecast. In general, the forecast accuracy for a key station averages about 2 to 3 degrees on cold nights. However, some stations are easier to forecast and our accuracy is fairly good on these, while others are sensitive to local winds and are more difficult. Growers should also pay attention to the general range of forecast minimum temperatures and the level of confidence expressed in the forecast.