California Rainy Season Ending; January-April rain in San Francisco Lowest on Record

The fourth consecutive severely dry California rainy season is drawing to a close. Rain-bearing low pressure systems typically stop bringing heavy rains to the state by mid-April, as the jet stream shifts to the north in its usual springtime migration. With almost no rain in the forecast for the next seven days, and the 16-day GFS model forecast showing mostly light rains affecting the northern portion of the state 8 - 16 days from now, California has likely seen over 95% of the precipitation that it’s going to get this anemic rainy season. What little precipitation did fall this winter came mainly in the form of rain, thanks to record-warm ocean temperatures off of the coast. This resulted in snow falling only at very high elevations, keeping the critical Sierra snowpack--which provides one-third of the state's water--at record low levels. According to the California Department of Water Resources, snow depths in the Sierras are the lowest on record for this time of year, only 2% of average, and the Southern Sierras have no snow at all--nearly three months earlier than usual. California's eight largest reservoirs are 30% - 83% below their historical average, and the portion of the state covered by the highest level of drought--"Exceptional"--was at 47% this week. The area covered by "Exceptional" drought peaked at a record 58% during the summer of 2014, and this mark may well fall during the summer of 2015.


Figure 1. Aerial view showing recreational boats by the Bidwell Marina at Lake Oroville, California on March 2, 2015. Lake Oroville, California's 2nd largest reservoir, was at 62% of average on April 29, 2015, which was its third lowest level since 1989. Image credit: California Department of Water Resources.

Record dryness in San Francisco
According to wunderground weather historian Christopher C. Burt, with just 1.30” of precipitation in downtown San Francisco this April and no chance of any additional rain to the end of the month, the January-April period will have a total of only 2.89”--easily the lowest on record, and well below the approximately 11.5" of rain the city usually gets. San Francisco got no rain at all in January, the first January on record that has occurred. Here is a list of the top ten driest Jan-April periods on record since 1850 in San Francisco:

1.   2.89” 2015
2.   3.54” 2013
3.   3.68” 1898
4.   4.01” 1976
5.   4.43” 1851
6.   4.75” 1972
7.   4.92” 1862
8.   4.92” 1864
9.   5.08” 1984
10. 5.11” 1977


Figure 2. Predicted precipitation for the 7-day period ending on Thursday, May 7, 2015. With the exception of light rains in the Sierras, California will be dry this week. Image credit: NOAA/HPC.

The long-range forecast: hot, dry, and more intense drought
The latest 3-month outlooks from NOAA's Climate Prediction Center, private forecasting firm WSI, and Columbia University's International Research Institute for Climate and Society all call for above-average chances of hotter than usual weather in California though July, and the U.S. Seasonal Drought Outlook calls for drought to persist or intensify over California. This should be no surprise, given that ocean temperatures along the coast of California are at record or near-record levels for this time of year, and will be slow to change. These record warm ocean temperatures will drive hotter weather and more intense drought this summer than otherwise would occur, and this summer's fire season will likely be severe. Next winter, California has a decent chance of getting a better rainy season, if the current El Niño event manages to intensify into a strong one (an event predicted by several of our better El Niño models.) Still, the Sierras need 1.7 - 2.9 times more precipitation than a usual rainy season brings to bust the drought; some portions of the state need even more than that.


Figure 3. Ocean temperatures off the coast of California were at record or near-record levels for this time of year on April 29, 2015. Ocean temperatures off the coast of Los Angeles and San Diego were more than 4°C (7.2°F) above average, an astonishingly high anomaly. These high temperatures were due to a combination of effects: a very persistent ridge (the Ridiculously Resilient Ridge, or RRR) that has been in place along the West Coast much of the past three years, bringing warm air temperatures and little mixing of cool waters from the depths; a positive phase of the Pacific Decadal Oscillation (PDO), a decades-long natural pattern; and global warming, which has warmed oceans world-wide over the past century-plus. Image credit: NOAA Environmental Modeling Center.

Portlight disaster relief charity raising funds for Nepal earthquake relief
The Portlight.org disaster relief charity, founded and staffed by members of the wunderground community, has connected with disability stakeholders in Nepal, India and Pakistan to seek their input on how best to help our brothers and sisters affected by their devastating earthquake. Portlight has determined that they can be of the most service by raising funds to directly support the relief and recovery efforts of these and other in-country stakeholder organizations. You can donate to this effort at this link:

https://www.commitchange.com/sc/charleston/portlight-strategies-inc/campaigns/portlight-nepal-earthquake-fund

You can follow the progress of the relief effort on the Portlight Blog. Thanks!

FYI, we are having some technical issues with some of the blog comments disappearing, and are looking into the cause.

Jeff Masters