California’s rainy season is now a month past its mid-point (which would be around February 1st) and the drought prognosis is mixed: not as good as most hoped but it could be worse. A wet and snowy January has been followed by a very dry and very warm February (it was San Diego's warmest on record). March will hold the key as to how the season ends up. Herein is an end-of-February update on the status of reservoir, snowpack, and precipitation levels/amounts and how it compares to the last time I blogged on this subject six weeks ago January 7th.
The latest California Drought Monitor map (as of February 23rd) shows a modest improvement to the state’s overall drought situation since the rainy season began in earnest three months ago. Map NOAA, USDA et al.
Precipitation
As of the end of February seasonal precipitation (since July 1, 2015) is running about 80% of normal statewide with a general bias toward wetter conditions in the north versus drier conditions in the south (with the notable exceptions of San Diego—which received tremendous tropical storm-related rainfall in August and September-and Fresno which enjoyed its wettest January in 21 years). After a promising start to the rainy season in December and January, February fell flat and has finished as one of the driest such on record. In fact, February was so dry that it erased most of the precipitation and snowpack surplus that had developed earlier in the season. Below are two tables comparing seasonal precipitation for select cities as it stood on January 7th (the previous time I blogged on this subject) and February 28th. As one can see there has been a modest decline in precipitation percentages of normal across the board (and a very significant decline in Los Angeles and San Diego).
Season-to-date precipitation totals and normals for select California cities as of January 7, 2016 (top table) and as of February 28, 2016 (bottom table). The sites are arranged geographically from north (Eureka) to south (San Diego).
What has been most disappointing about this winter (so far) is how El Nino has generally under performed precipitation-wise in California. There have been plenty of Pacific storms that have raked the West Coast but the focus has been primarily on the Pacific Northwest. In fact, Seattle has experienced its wettest climatological winter (Dec-Feb) on record. If we look at the history of the past strong/very strong El Niño’s (since 1950) we can see that the precipitation’s geographical distribution has been unusual this El Nino season so far (for the November-February time period).
Oceanic El Nino Index since 1950. The seasons experiencing strong or very strong El Niño’s are highlighted in red and were those of 1957-1958, 1965-1966, 1972-1973, 1982-1983, 1997-1998, and 2015-2016. Graph courtesy of Jan Null, Golden Gate Weather Services.
Lets compare the November through February precipitation totals for each of those El Nino seasons in Seattle, San Francisco, and Los Angeles.
November-February precipitation totals for Seattle, San Francisco, and Los Angeles for each and all of the strong/very strong El Nino seasons since 1950. The anomaly of this season (2015-2016) is immediately apparent. This El Nino has seen Seattle with 157% of normal precipitation and Los Angles with 44%. In the previous five El Nino seasons Seattle observed an average of 97% of normal precipitation whereas Los Angeles saw 166% for the November-February time frame.
Here is another graphic that illustrates just how unusual this El Nino season has been for the Los Angles basin area so far this winter.
Percentage of normal to date 'water year' precipitation (which begins October 1st--'seasonal' precipitation year begins July 1st in California) for the Los Angles basin area during the past five strongest El Nino’s compared to this season. Graph courtesy of Zack Labe, Univ. of California-Irvine.
Reservoirs and Snowpack
As bad as the actual precipitation figures outlined above may look, the reality in the reservoirs and mountains is not so grim. For one thing most of the heaviest precipitation that has fallen so far this season has occurred in the far northern reaches of the state which is where the largest and most important reservoirs are located. If we compare the reservoir capacities as they stood on January 7th to where they are now (February 28th) we can see a major improvement in the levels at Shasta Reservoir and Lake Oroville, the two largest and most important water-holding facilities in the state.
California reservoir conditions as of January 7th (top map) and February 28th (bottom map). As one can see there has been a tremendous increase in the water levels at both sites over the past six weeks (from 51% of normal to date on Jan. 7 at Shasta to 83% now and from 47% normal Jan. 7 at Lake Oroville to 75% now). Maps provided by the California Department of Water Resources.
Snow water content for the northern, central, and southern sections of the Sierra Nevada and other mountain ranges as they stood on January 8th (top map) and February 29th (bottom map). Although it has fallen below average again (as of the end of February) it is, at least, close to normal and, compared to last year, a huge improvement. Maps from the California Department of Water Resources.
As can be seen above, the Sierra and Trinity Mountain snow packs have fallen below average (for the date) again statewide as of the end of February. On January 7th the statewide average snow/water equivalent stood at 107% of average at that time and now it is down to 85%. Although this is not great it also isn't so bad. Much of the lost snow pack melted during the mild February but much of the runoff flowed into the state's reservoirs (hence the reservoir's relatively healthy condition at this time). Furthermore, it is altogether possible to have additional tremendous snow accumulations in March and April.
In fact, on that upbeat note, current forecast models indicate a return to a wet, possibly very wet, pattern beginning around March 5-6 and continuing for at least two weeks (the GFS model is calling for 8"-10" of rain in San Francisco between March 5th-16th, we shall see!). The storms should affect the entire state, including at last, the bone dry south. How cold the storms might be, and thus how much snow for the Sierra might accumulate, has yet to be determined.
Christopher C. Burt
Weather Historian
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