ATMOSPHERIC CIRCULATION AND DAILY PRECIPITATION IN WALES

Daily precipitation affinity areas are established for Wales by means of S-mode Principal Components Analysis (PCA) and Cluster Analysis, using data for the autumn-winter (September-January) period, 1982-87, subset according to surface wind direction. Relatively strong PCA simple structures were found for five wind directions clockwise from south to north, using the OBLIMIN rotation for the first four (south to northwest) and the VARIMAX rotation for the last (north). Whilst interpretation of many of the most significant Prinicipal Components (PCs) was difficult in strictly meteorological or climatological terms, the precipitation areas produced by clustering based on the most important PCs, yielded a continuum of change in areal organisation with progressively changing wind direction. Only the far southwest and northwest of the country appear in every area map, although superficially, northeastern, English border and southeastern areas were present on most. The derived areas bear only small overall resemblance to currently used forecasting areas.