The Sager Weathercaster is a weather forecasting slide rule and table set, originally developed by Raymond M Sager in the 1940s. Early versions are also known as the “Kenyon Weathercaster” and “Guest Weathercaster”. By inputting the current wind direction, barometric pressure, pressure trend, and cloud cover; a forecast covering a 30-mile area for the next 12-24 hours is generated. The forecasts are said to be in the 80-90% accuracy range. The device remains popular amongst sailors and outdoorsmen, who may have limited access to accurate weather information in remote areas or the open ocean via internet, satellite, or radio.
The Sager Weathercaster is no longer commercially produced (to my knowledge); however they do show up on the used market and on sites such as ebay now and then.
How the forecasts were created
The Sager Weathercaster was developed in a time before numerical weather prediction- while Raymond Sager is long deceased, some information from his son found on youtube implies heavy use of historical trends and statistical modeling, similar to the earlier Zambretti Forecaster also written about on this site.