Authors: Richard Grotjahn
Publish Date: 2011/02/08
Volume: 37, Issue: 3-4, Pages: 587-604
Abstract
A pilot scheme uses upper air data from a few extreme hottest days to identify those and other extreme hottest days measured by 3 stations sampling the California Central Valley CV Prior work showed that CV extreme heat wave onsets have characteristic large scale patterns in many upperair variables those patterns also occur for the hottest days A pilot scheme uses areas of two upperair variables with high significance and consistency to forecast extreme surface temperatures The scheme projects key parts of composite patterns for one or more variables onto daily weather maps of the corresponding variables resulting in a ‘circulation index’ for each day The circulation index measures how similar the pattern on that day is to the composite patterns in areas dynamically relevant to a CV extremely hot day with a larger value for a stronger match and larger amplitude The scheme is tested on the development period 1979–1988 and on the subsequent 18 year ‘independent’ period 1989–2006 The pilot scheme captures about half of the rare events in the development period with similar skill for the independent period Based only on 16 days of extreme heat in the first 10 years the scheme is not intended to represent the general distribution however the circulation index has similar kurtosis variance and skewness as the observed maximum temperatures Properties of the high end tail of the distribution are notably improved by adding the second predictor The scheme outperforms simply using 850 hPa temperature above the CVMaximum temperatures above 40°C are a regular feature of summer in the Central Valley of California USA hereafter CV The Sacramento Valley SV constitutes approximately the northern third of the CV with most of the rest being the San Joaquin Valley SJV while a small portion is known locally as ‘the Delta’ where the Sacramento and other CV rivers empty through the Carquinez Strait into San Francisco Bay The CV is bounded by high mountains to the east the Sierra Nevada the Cascades to the north the Transverse Ranges to the south and Coastal Ranges to the east Home to 5 million people and regarded as the most agriculturally productive region in the world the social and economic importance provides ample motivation to study CV heat wavesA period of at least 48 h during which neither the overnight low nor the daytime heat index Hi falls below the NWS heat stress thresholds 80 and 105°F At stations where more than 1 of both the high and low Hi observations exceed these thresholds the 1 values are used as the heat wave thresholds
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