California Drought: Happy New Year?
Using the IRI Data Library to Predict and Track the 2016 Water Year
By Elisabeth Gawthrop
International Research Institute for Climate and Society
NOTE: We’re updating this post monthly with the latest forecasts from our Maprooms.
October 1st marked the first day of the new “Water Year” for California, which is like the state’s fiscal year for water management. Most of the rain during the water year falls in the first half. Water stored in those six months is used during the second half, when much less rain falls. California is in the midst of what scientists say is the driest spell in 500 years. The 2015 water year, which just ended, was the fourth consecutive dry year, with 2014 being the 3rd-driest year since records began 120 years ago. Moreover, the 2015 water year was the warmest on record in California, beating the previous record in 1995–96 by almost a degree and drying out soil and increasing evaporation of the water that did make it to reservoirs.
Much of California’s water supply comes from its northern half, where snowfall accumulates in the mountains and acts as a natural reservoir of water. As the snow melts throughout the dry summer, it replenishes the reservoirs that in turn supply cities and farms throughout the state. One of the most striking statistics from the past year is that, according to California’s Dept. of Water Resources, the statewide snowpack on April 1, “held only 5 percent of the average water content on that date in records going back to 1950. The previous low record of 25 percent of average was set in 1977, during one of California’s most significant droughts and was tied in 2014.” The department also stated that “the winter average minimum temperature for the Sierra Nevada region was 32.1 degrees Fahrenheit, the first time this value was above water’s freezing point in 120 years of record-keeping.”
The current El Niño event, forecasted to be one of the strongest since 1950, has raised hope of more rain for the state during the upcoming winter. How much water would California need to replenish its deficit? The answer isn’t straightforward, but this NOAA blog post suggests precipitation totaling ~150% of an average year might end the drought. But total amount isn’t the only relevant metric — other key factors for drought mitigation include how quickly rain falls and where. Southern California can only retain so much water at a time, and, as mentioned above, the precipitation in the mountains and reservoir areas is key.
Unfortunately for drought relief, El Niño has historically increased rainfall in the southern part of the state, but it doesn’t significantly impact the northern part. According to the Department of Water Resources, however, a significant amount of precipitation fell statewide during the strongest El Niño events in water years 1983 and 1988. Moreover, the NOAA Drought Task Force reports that sea-surface temperature patterns in the Pacific and Indian Oceans contributed to the drought. The El Niño changed these patterns, which will have a cascading influence on atmospheric conditions for much of the globe, including California. The Task Force also states that the precipitation effects are likely to be greater during the later winter months compared to the early winter. For more on the chance for El Niño to improve drought conditions, see its recent report (IRI’s Bradfield Lyon and Lamont-Doherty Earth Observatory’s Mingfang Ting, Richard Seager and Bor-Ting Jong are co-author’s, along with NOAA and NASA scientists).
So, what does the current forecast reveal about California’s wet season? While there are many ways to statistically skin the climate cat, we’re going to use the IRI Data Library to show average and forecasted seasonal precipitation over California as the water year progresses. We’ll also show the seasonal observations from the last two years, for comparison. And, we’ll update with this year’s observed precipitation, as those numbers come in.
Forecasts Issued October 2015
Here are the November -January seasonal average, forecast and past observations.
There is a slightly better chance for southern California to see more rainfall than average during this season compared to the October-December seasonal forecast shown last month (see below).
As mentioned above, the El Niño precipitation signal is expected to be strongest later in the winter, so these increased odds are likely due to the expected influence of El Niño.
So, let’s take a look at the updated forecast for January-March.
It’s similar to the one issued last month, but with slightly better odds for more precipitation in parts of northern California. That’s good news for helping alleviate drought conditions.
Parts of southern California are still forecasted to have about an 85% chance of above-average rainfall.
Forecasts Issued September 2015
Here are the October-December seasonal average, forecast and past observations.
For the first part of the wet season, there are not increased odds for either below- or above-average precipitation. At least not getting below-average rainfall would be a plus for the state. But, as discussed above, the El Niño precipitation signal isn’t expected to be strong until later in the winter.
So, let’s take a look at the data for January-March. Here are that season’s average precipitation, forecast and past observations.
During this season, it gets a little more interesting. Much of the state has at least a 60% chance of getting more precipitation than average, with the southern quarter of the state having at least an 80% chance.
The flexible forecast maproom also allows users to customize maps using specific thresholds, such as this map for January-March season, which shows the chances of being in the top 30% of wettest seasons.
According to the September-issued forecast at left, much of the state has at least a 50–50 chance of being in the top 30%.
