California, the Pacific Northwest, and the Northern Rockies
Another stormy week was experienced across most of the region, leaving only the southeastern California desert and part of the San Joaquin Valley devoid of significant precipitation. The heaviest precipitation – more than one foot – affected part of the Washington Cascades, interior northwestern Washington, northwestern Oregon, southwestern Oregon, and northwestern California. Amounts exceeded 3 inches in other areas from the Cascades to the Pacific Coast, most of Humboldt and Mendocino Counties in California, the higher elevations in west-central Idaho, and parts of northeastern Idaho and adjacent Montana. The lightest amounts – between 0.5 and 1.5 inches – fell on the interiors of Washington and Oregon, southwestern Montana, and southern Idaho.
Temperatures averaged a few degrees above normal, so the precipitation did not build up snowpack as much as would be expected this time of year. Snowpack water content is generally above normal in central and eastern Oregon, northern Washington, and western Idaho, but remains considerably below normal along the Cascades, in northern Idaho, and across adjacent northwestern Montana.
Dryness and drought improved once again across large parts of the region as a result. Most of Washington, the western half of Oregon, and northwestern California all improved by 1 classification, as did parts of Idaho and a small section of northwestern Montana where precipitation has been heavier than in nearby areas.
Source: Richard Tinker, NOAA/NWS/NCEP/CPC, December 15, 2015