Thursday, January 16, 2014

"It Never Rains in Southern California"

Residents and CoCoRaHS observers throughout California (and not just southern California) may be humming this Albert Hammond song from 1972 with complete justification.  The western U.S. has been suffering through drought conditions for much of the last year, but conditions have worsened in the past three months. Currently 90 percent of California is in at least severe drought, and 63 percent is in extreme drought.

Winter is usually the wet season in California as storms roll in off of the Pacific and produce rain along the coast and snow in the Sierra Nevada. The snow is particularly important for California and the west. Snowmelt in the spring and through the early part of summer is counted on to fill reservoirs that supply water to communities and irrigation for agriculture. The winter snowpack in the northern and central Sierra provides about a third of California's water supply.

Satellite images of the Sierra Nevada range
in January 2013 (left) and January 2014 (right).
Credit: NOAA Environmental Visualization Laboratory
The water equivalent in the snowpack in the Sierra is only 19 percent of normal and that is raising concern for the spring and summer. When snow survey teams headed out on the first survey at the beginning of January found more bare ground than snow in some areas. The current level is the lowest on record along with January 2012.

This series of charts shows the water content of the snowpack as a percent of of the April 1 average. Note that this season to date (pink line) is at or below the driest season on record (1976-1977)
Source: California Department of Water Resources
What has happened to the storms that normally bring rain and snow to California? They have been kept at bay by a persistent ridge of high pressure over the eastern Pacific and western U.S.  The corresponding downstream trough over the central and eastern U.S. has been responsible for the cold and snow so far this winter.

Here is the climatological normal 500 millibar level (about 18,000 feet) pattern for the period from December 1 to January 15. The upper level winds flow parallel to the contours on the map Note how in the contours are fairly flat in the western U.S., which means winds tend to steer Pacific storms into the region.

Here is the mean 500 millibar pattern for the last 45 days. Notice the ridging of the contours in the eastern Pacific. Winds steer Pacific storms northeastward over the ridge into Alaska and western Canada, deflecting them away from the west coast.

It is still early in the snow season, and a few large, wet storms in through March could help the situation. However, the U.S. Drought Outlook indicates that drought conditions will persist through the end of April. In addition to water supply problems the extreme drought conditions will mean a volatile wildfire season.

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