How does 2015-2016 rainfall compare to normal?

Given our current state of drought, many are wondering: How does this winter’s rainfall stack up to normal rainfall? 

Napa County RCD’s Senior Hydrologist Paul Blank recently summarized this winter’s rainfall: 

“As of the end of January, Napa Valley has received 57% of our annual rainfall which has almost caught us up and put us on track for an average year. Given this is a strong El Nino year, there’s reason to hope for above-average rainfall in the coming months.”

Here’s more from Paul:

How does this winter’s rainfall stack up to normal?

Precipitation totals for a given location are reported according to the “water year,” which is defined as October 1 through September 30.  So far in Water Year 2015-16, through the end of January, we have received 32.15 inches of rain on Mount Saint Helena* at the top of Napa Valley, and 14.89 inches on the valley floor in the City of Napa*.  Since rainfall varies so much across locations, we cannot evaluate valley-wide precipitation in inches.  It is important to establish long-term averages for each station so rainfall can be assessed in terms of percent of “normal”.  Now that we have 14 years of data for most gauges in the Valley, we can credibly estimate mean annual precipitation, or what is “normal” for each station.  Mean annual precipitation for Mount Saint Helena is 48.44 inches, which means we have received 66% of the annual average as of the end of this January.  In the City of Napa, mean annual precipitation is 25.22 inches and we have received 59% of the annual average so far.  These values can be averaged for all gauging stations to tell us something about how we are doing in general, and as of end of January we have received a valley-wide average of 57% of our total annual rainfall.

To make sense of this number we need to know what is normal.  To do that we look to the rain gauging station at Napa State Hospital which has been in operation for over a century and defines normal rainfall for the Valley.  Although the rainfall amounts recorded at this gauge are specific to its location, studies have shown that it is a good indicator of precipitation throughout the Valley.  The table below lists the percent of annual rainfall that Napa Valley receives during each month.  The cumulative values are shown as the “Long-Term Normal” line with the light blue shading in the graph above.

Since the ALERT System* began collecting these data approximately 14 years ago, we have had six years of above average rainfall (wet years) and eight years of below average rainfall (dry years), including the past four years.  Data for the wettest and driest years during this period, 2005-06 and 2006-07, respectively, are included on the graph above.

As of the end of January, in the current Water Year, we received below average rainfall for the first three months, followed by a wet January in which we received 32% of our average annual rainfall.  For example, in the City of Napa, we received 8.59 inches in January, compared to the normal of 5.05 inches.  For the winter, we have now received 57% of our average annual rainfall total, which puts us on track for an average year.  Strong El Nino years, such as this year, however, are associated with increased late-season precipitation.  Accordingly, there’s reason to hope for above-average rainfall in the coming months. We plan to keep our readers updated as to how we are doing as the winter progresses. Our next update will be published in early March, so stay tuned!

For those interested in more frequent and more technical discussion of California weather, especially regarding this year’s El Nino and the state’s recent drought conditions, RCD staff recommend the blog Weather West.

*How does the RCD obtain rainfall data for the Napa Valley?

Napa County Regional Rainfall and Stream Monitoring System is a  partnership between Napa County, the City of Napa, and other Napa Valley cities, with assistance from the Napa County Resource Conservation District.  Referred to as the ALERT System, it is primarily a flood warning system and includes 17 real-time rain gauging stations spread throughout the Valley at various elevations.  These gauges measure rainfall intensity during storms and alert emergency managers to potential flooding conditions. The ALERT System’s real-time gauging data can be viewed at napa.onerain.com.

– Paul Blank, Senior Hydrologist, paul@naparcd.org