Entries show station weight (percent), station latitude (degrees N), station longitude (degrees E), and station name for each of the stations in the index.
1 0.95 32.700 -117.200 NORTH ISLAND NAS 2 4.20 33.231 -116.414 BORREGO DESERT PARK 3 5.19 32.734 -117.183 SAN DIEGO WSO AP 4 6.47 33.210 -117.395 OCEANSIDE MARINA 5 11.12 33.378 -116.840 PALOMAR MTN OBSVTRY 6 14.95 33.237 -116.761 HENSHAW DAM 7 57.11 32.990 -116.587 CUYAMACA
The station weight is the product of two terms. 1) the fraction of area the station represents in the total area of interest. So, for example, a station that is located near to another station will have less influence on the final total, while a station that is far from any other station will have more influence. 2) The average annual precipitation at that station, so that a wet station contributes more to the final index that does a dry station. The final weight is the product of these two terms, normalized so that the sum of all the weights adds up to 100%.
Stations are selected based on two criteria. 1) They must have a relatively long (50 year or more) record with few interruptions. 2) They must be updated nearly every day by the California Nevada River Forecast Center.