Rainfall Season vs. Water Year

 

Unlike most parts of the United States, California’s Mediterranean climate is defined by its “summer drought” given that there is a natural break between one rainy season and the next.  This was recognized by settlers as far back as the Gold Rush and since that time the most common metric to quantify California (and other western states’) rainfall has been a July 1 to June 30 “rainfall season”. Consequently, thousands of reports, studies and analyses related to California rainfall have been based on the rainfall season, including the following from is some of the earliest chronicling of California weather.
 

Conversely, because there is lag of several months to when streamflows in the state’s rivers are at their lowest, hydrologists have historically (and logically for their purposes) used a Water Year (WY) metric from October 1 to September.

However, about two years ago, for reasons that have never been adequately explained, the National Weather Service (NWS) Western Region decided, without the opportunity for comment from the rest of the meteorological community or the public, to start calculating their rainfall products (i.e., Climate Station Precipitation Summary , and others) in regards to the hydrologist’s Water Year. (The remainder of the NWS, by the way uses the calendar year for summarizing rain data, as they do not have a natural summer break in rainfall.)  While this might be nice for consistency with the hydrologic community it puts them out of step with the vast wealth of historic data and others in the rest of the meteorological community.  This was even highlighted by a recent article in the LA Times, “Ideologies clash as weather service realigns rainfall calendar

The numerical differences between rainfall season and water year are slight (i.e., typically only about 3% of the annual amount) given the small amount of rain that typically falls in the months of July, August and September. And that rain is not lost, it is either counted at the end of one methodology’s season or the beginning of the other’s.

As stated about, the problem comes about when trying to compare data published by the NWS in terms of the hydrologist’s WY, with data from past events which have been characterized by meteorologists’ rainfall season.  [To their credit, NWS San Francisco Bay Area automatically generates a table which has both the rainfall season and water year, and it is my understanding the software used was made available, but unfortunately not adopted, by other NWS offices.]

An effort has been made, outside the NWS, to make available to the public and meteorological community data showing rainfall in terms of the July 1 to June 30 rainfall season with products like California Rainfall Season Totals. But this does not address the amount of unnecessary effort expended by everyone, but a very small group, to keep the data consistent and meteorologically logical.
 
Jan Null, CCM
Golden Gate Weather Services
http://ggweather.com
jnull@ggweather.com

 


 

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Not the Wettest January or Season to Date


Yes, January and the Rainfall Season to date (July 1 to January 31) have been wetter than normal, but they are far from "the wettest" or record-setting. Only the 5-station Central Sierra Index had their wettest January on record, but that period of record goes back to only 1913, more than 60 years shorter than most of the other records. Of the individual stations with long periods of record extending back to the mid-19th century, San Francisco's 9.42" was only the 16th rainiest and Redding's 11.45" was their 23rd wettest.

Likewise, the season-to-date number have been impressive, but most have not even been in the top 10 wettest. 


The following are the Top 10 Wettest Januarys.


The following are the Top 10 Wettest July through January.


Jan Null, CCM
Golden Gate Weather Services
jnull@ggweather.com

 

 

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California Precipitation Snapshot; Bay Area Storm Index

The California precipitation picture got a lot prettier over the weekend; especially in Sierra Nevada which saw huge gains. For example the the southern Sierra Nevada (i.e., Tulare Basin) jumped from 103% of normal on January 1 to 165% through yesterday (January 8), while the northern and central Sierra increased to 173% and 172% respectively. See  http://ggweather.com/water/



In the San Francisco Bay Area the Sunday storm ranked as an 8.5 on the Bay Area Storm Index (BASI).  This was based on a 24-hour downtown San Francisco rainfall total of 1.62", a maximum sustained wind at SFO of 44 mph and a peak (below 1500' elevation) wind gust of 75 mph at Spring Valley in San Mateo County above Crystal Springs. This is the highest ranked storm since October 2009 (see archive).  

Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Email: jnull@ggweather.com
Web: http://ggweather.com
Twitter: @ggweather

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December Summary; Seasonal Precipitation Snapshot; La Niña; AMS in Seattle

December Summary; Seasonal Precipitation Snapshot; La Niña; AMS in Seattle


December rainfall in California was pretty much upside down compared to recent months, with near normal rainfall in the north and above normal in the south. San Diego totaled 4.22 inches (276% of their December normal) while Los Angeles had 4.55" (195%). The northern third of the state was close to normal with just rain-shadowed San Jose lagging behind at 58%. [http://ggweather.com/calif/dec2016.htm]

Despite a couple colder than normal periods, only Eureka and Sacramento had monthly average minima of more than 2 degrees below normal. At the same time, LA and San Diego average monthly minima were more than 2 degrees above normal.
[http://ggweather.com/calif/dec2016.htm]

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A snapshot of the first six months of the rainfall season (July 1 to June 30) shows totals well above normal at most locations, with the biggest gains against normal in the south, where Los Angeles and San Diego pushed to 138% and 158% of normal respectively. http://ggweather.com/water/

See also http://ggweather.com/seasonal_rain.htm which is updated daily, in the early evening.

With the potential for extensive rains in the next 10 days, I have updated the Rainfall, River and QPF Page .

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La Niña's cooling in the tropical eastern Pacific may have bottomed and the weekly Sea Surface Temperature Anomalies (SSTAs) for the past month have averaged just -0.4.

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Finally, I am headed for the 97th AMS Annual Meeting in Seattle later this month to present a paper of Heatstroke Deaths of Children in Vehicles. If you are also going to be in attendance, drop me an email and we can try to connect.
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Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Email: jnull@ggweather.com
Web: http://ggweather.com
Twitter: @ggweather

 

 


 

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California Rainfall at the Beginning of Winter

Today, December 1st, begins meteorological winter; the three coldest months in the Northern Hemisphere. And for most locations in California it corresponds to the three wettest months.

After a much wetter than normal October (http://ggweather.com/calif/oct2016.htm) for Northern and Central California the November totals (http://ggweather.com/calif/nov2016.htm) were more of mixed bag, but still concentrated in the north.  Consequently the totals for the first five months of the rainfall season (July 1 to November 30) have the northern half of California above normal (http://ggweather.com/seasonal_rain.htm).  

Given the rainfall bias in the north the Drought Monitor is showing relief there but still 43% of Calif in Extreme or Exceptional Drought.



Jan Null, CCM
Golden Gate Weather Services
http://ggweather.com

 

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A Different Flavor of La Niña?

Unlike those picture puzzles where you are asked to find subtle differences between images, a look at the NASA sea surface height anomaly images for the past three weak La Niñas (1995, 2000, 2011) plus this year (2016), leaves little doubt that what's going on in the eastern Pacific is not subtle at all.  The most striking difference is the anomalous warm water from the equator north to about 25 degrees north. This area warmed in conjunction with last year's very strong El Niño and may have contributed to the atypical winter along the west Coast of North America (see "Differences Between 2015-16 El Niño and Previous Strong and Very Strong Events").




With the current Oceanic Niño Index (ONI) already at weak La Niña levels, tomorrow's updated CPC ENSO Advisory is likely to up the odds for a La Niña this fall and winter. But can we still use the standard "La Niña playbook" of looking at past events as an indicator of broad-scale for West Coast precipitation patterns for this winter?  Last winter the conventional wisdom for the impacts of a very strong El Niño did not work out per previous events. Consequently, it looks like extreme caution should be exercised in looking for patterns based upon the climatological past related to the prospects for current La Niña. 

Jan Null, CCM
Golden Gate Weather Services
http://ggweather.com


Miscellaneous La Niña Resources (use with caution):
La Niña Winter Precipitation & Temperature Climatology  
California Climatology of La Niña Events and Precipitation  
California Precipitation Climatology for Cool Neutral and Weak La Niña Events  

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