It was an extremely dry month in Napa Valley with most gauges measuring only a trace of rainfall. On average, we receive 21% of our annual rain during the month of December, and a total of 36% by December 31. This December, we received approximately 0.2% of mean annual rainfall, bringing us to 14.5% for the water year (WY).
It was the driest December in Napa since WY 1989-90, although it is not uncommon to receive low rainfall during the month. Below average rainfall has been recorded in 18 of the past 30 Decembers. Less than 1 inch of rain was measured at the Napa State Hospital gauge (December average 5.26 inches) in 6 of 30 Decembers, including recent water years 2011-12 and 2013-14. In our dataset, below-average Decembers are associated with dry years; however, hope of salvation remains. Low December rainfall has occurred in wet years 3 times in the 30-year record.
– Paul Blank, Senior Hydrologist – Paul@NapaRCD.org