Golden Gate Weather Services

"Say what? We had El Niño this winter?".  Yes, between November 2019 and March 2020, the primary metric used to identify El Niño events trickled along the minimum criteria to be classified as a "weak" event.

The Oceanic Niño Index (ONI) is the de-facto standard that NOAA uses for identifying El Niño (warm), and La Niña (cool), events in the tropical Pacific. It is the running 3-month mean Sea Surface Temperature (SST) anomaly for the Niño 3.4 region (i.e., 5°N-5°S, 120°-170°W). If events persist for 5 of these consecutive overlapping 3-month periods at or above +0.5° threshold it is classified as a "historical" warm (El Niño) event.  See https://ggweather.com/enso/oni.htm.


This was the weakest El Niño seen in the data going back to 1950. For this past winter the ONI ranged between 0.5 and 0.6 for the period of October-November-December through February-March-April with an average of +0.52.  The next closest "barely weak" El NIño was in the winter of 1958-59 when the average ONI for the same months was +0.57.

It is very important to note, that SST's also need to couple with the atmosphere for an El Niño to have a significant impacts. And furthermore, that impact can be increased or decreased by the "alphabet soup" of other climatic events like the PDO (Pacific Decadel Oscillation), PNA (Pacific-North American Oscialltion), AO (Arctic Oscillation) and MJO (Madden-Julian Oscillation).

This past winter year is also a good reminder that all El Niños are not the same and that they don't always mean a wet winter for California. That and other El Niño misconceptions are addressed at https://ggweather.com/enso/enso_myths.htm and https://ggweather.com/enso2016/us_elnino.html. 

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

Despite being only about 49 square miles (i.e., approximately 7 miles by 7 miles), San Francisco's rainfall is nearly as diverse as its many cultures. Strongly influenced by its topography and proximity to the Pacific Ocean, its wettest location receives 34% more rainfall than its driest. 

[This is a follow-up to a similar rainfall analysis done looking at the “rainshadow” in the Santa Clara Valley (https://ggweather.posthaven.com/the-santa-clara-valley-rainshadow).]

Normal (i.e., 30-year averages from 1981 to 2010) annual rainfall data was extracted from PRISM (Parameter-elevation Relationships on Independent Slopes Model) for 315 gridpoints spaced at 0.01 degree (~0.55 miles) intervals across approximately 60 square miles covering San Francisco, adjacent waters, and extreme northern San Mateo County. To visualize this, the data was plotted and isohyets (i.e., contours of equal rainfall) were drawn.

San Francisco's rainfall patterns are the byproduct of moist air from the Pacific being forced upward by the terrain (i.e., orographically), resulting in the windward slopes seeing enhanced precipitation amounts as the air cools and condenses. On the leeward side downward forcing causes the air to become drier with less rainfall resulting.

Over San Francisco, the moist southwesterly flow coming onshore with many storms from the Pacific Ocean, is nearly perpendicular to the north-south ridge running pretty much through the center of The City. As it ascends toward this ridgeline the rainfall amounts increase from about 22” along the Great Highway to a maximum of 28.26” annually near Laguna Honda Hospital; just southwest of Twin Peaks. [Note that there is an even higher maximum of 30.17” over the higher terrain of Mount San Bruno in San Mateo County].

The air then dries out as it descends downward into the eastern side of San Francisco with a minima of about 21” along the waterfront between the Bay Bridge and Candlestick Point.

It is important to that this analysis is based upon the 1981-2010 30-year normals, and the “new" 1991-2020 normals which  will be published next year, will be on the order of about 1 inch (~4%) lower.

Click HERE or on the map for full-size image.


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

There has been speculation in the past few days that a dry February portents a wet (i.e., "Miracle") March. However, a look at the years when San Francisco has had less than 0.50" of rain in February does not support that thesis. In fact, only 6 of those 18 years had above normal March rain totals. 

The odds are only slightly better for the combined total of March and April, with only half above normal.


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