Graduation date: 2007Many meteorologists rely heavily on their experience of past weather events to supplement their forecasting tools, such as using past experience to help decide between conflicting numerical weather prediction models. Experiential knowledge is an important piece of forecasting, however it is highly subjective and variable from one forecaster to another. In efforts to replace this experiential knowledge with a more objective record, this thesis explains how a synoptic map-pattern climatology was created and how it could be used as a contribution to weather forecasting methods. Creating the map-pattern climatology was a two step process: first atmospheric flow patterns at the 500-millibar level for the selected region were categorized by type; and second surface level weather data were correlated with the map-types found. The atmospheric flow patterns over the Pacific Northwest region were found using a correlation-based method. Each day during the winter months of December, January, and February for the past 58 years was categorized by map-type. This resulted in 5,235 days that were classified into 20 distinct map-type groups. Once the map-types were found, temperature, precipitation, and snowfall data from stations throughout Oregon and Washington were then correlated to the map-type data. The weather data were calculated for three spatial scales: an average of Oregon and Washington combined; average by Oregon climate zone; and for a single city. It was found that each map-type is associated with a unique weather environment at the surface. As an additional study, the distribution of map-types during El Niño and La Niña years was studied in combination with the surface weather data. It was found that El Niño and La Niña years have quite different map-type distributions compared to normal years, and the weather resulting at the surface was also far from average. Once completed, this Pacific Northwest climatology provided a record of the most common flow patterns and resulting weather at the surface. If distributed in a format in which current atmospheric flow data could be compared to historical map-type data, this synoptic map-pattern climatology could be used as a forecasting tool. In order for this information to replace the previously relied upon experiential knowledge, it would be necessary to know the forecast day’s projected map-type along with the historical surface weather data associated with that map-type. If distributed in this way, weather forecasters would have easy access to what has happened historically with any given map-type, thus replacing their variable and subjective experiential knowledge with an objective and accurate record.