Updated Oceanic Niño Index Graphics


With renewed interest in La NIña I have updated the graphics depicting the Oceanic Niño Index (ONI). Most notably and to make the data more readable, separate time-series graphs for the periods of 1950-1989 and 1990-present have been added. This is in addition to the previous 1950-present time-series. See https://ggweather.com/enso/oni.htm.






Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Phone: (650) 712-1876
Email: jnull@ggweather.com
Web: http://ggweather.com
Twitter: @ggweather

 

 

 

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La Niña 2020 - No Sure Thing

The NOAA Climate Prediction Center (CPC) announced this morning that we have entered into a weak La Niña pattern that is expected to last through the winter. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml



But, what does that mean as far as this winter's rainfall across California and the United States? Given past climatology, it can mean almost anything!  This is dramatically illustrated by the past two La Niñas, both weak, which occurred in the winters of 2016-2017 and 2017-18. Their rainfall patterns looked dramatically different, especially in the West and California. See https://www.ggweather.com/ca_enso/ca_lanina.html and https://ggweather.com/enso2016/us_lanina.html 

For comparison in California, below are the seasonal percentage of normal rainfall for the 11 La Niña events since 1950:


 

And likewise, the precipitation anomalies for the United States.


The bottom line is that with most ENSO (El Niño Southern Oscillation) events, and especially with weak La Niñas, there is no clear signal that can be used as a predictor. What is often forgotten, is that while ENSO events have a strong influence on a given year's weather, they are not the only game in town and the impact of ENSO is influenced by a myriad of other interacting factors like the North Atlantic Oscillation (NAO), Pacific North America (PNA) pattern, Arctic Oscillation (AO), Pacific Decadel Oscillation (PDO), Madden-Julian Oscillation (MJO), etc. STAY TUNED!

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 Rain and Fire Climatology

The recent spate of large fires has pushed 2020 in the unenviable position of already having the 3rd most acreage burned. And there are are still over four months of the year left, and the dry Fall months with offshore winds are often when things get worse. To date, 2020 has seen approximately 1.35 million acres burned, behind only 2018 which had 1.85M acres burned and 2008 with 1.38M acres.



The potential for a bad year has been exacerbated by the exceptionally dry 2019-2020 rainfall season, that statewide averaged just 73% of normal (21.78").



And looking at the combination of the two shows a real bias toward drier years having more acreage burned across Califonia. 

However, there are almost as many caveats as there are datapoints. First, the impacts on the state are more than just the number of acres, but where they are and do they include inhabited areas. And, the rainfall dataset is for a single statewide average and does not take into account if the rain fell more in areas that saw more acres burned.  But the pattern is compelling and should be a cautionary tale for dry years.

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

 

 

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Comparing 2006 and 2020 California Heat Waves


How does the current heatwave compare to the July 2006 event? A quick analysis shows them to be quite similar. Both show the West being dominated by large 500 millibar highs centered over the Great Basin. For the dates chosen, in the approximate middle of the hottest 7-day periods, the current high is only slightly stronger. 



There are many metrics that can be used to compare, but here I have looked at the warmest 7-day period, based on daily maximum and forecast temperatures for July 2006 and August 2020. In most of the nine stations examined, the values are comparable. The largest outliers were in Redding and Livermore which were 5 degrees hotter in 2006, and Fresno which was 4 degrees hotter. 
 

There are many caveats to be applied to such a simple analysis, including the influence of nighttime minimums, the humidity given the recent influx of subtropical air, changes in population, etc.


Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Phone: (650) 712-1876
Email: jnull@ggweather.com
Web: http://ggweather.com
Twitter: @ggweather

 

 

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Hottest Week of the Year?

For many locales across the United States, the last week of July is the hottest time of the year. And given the prolonged heat over much of the country this summer, just the idea that there is cooler weather ahead may be reason to rejoice.

But a few locations, especially along the West Coast, wait until August and even September to peak. And San Francisco is certainly the latest of any major United States city by not reaching its normal highest maximum temperature of 70.4 degrees until September 24th. Yet, just 40 miles away, San Jose has one of the earlier hottest days on average, peaking out at 83.8 degrees on July 10th. 

US Daily Normals can be found at:  https://ggweather.com/normals/daily.htm

  



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

 

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"Stealth" El Niño Comes to an End


"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


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Geographical Analysis of San Francisco Rainfall

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

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California Rainfall Deficit Update

With only a few exceptions, the recent rain across the state has just kept up with the previous March 1st normal-to-date figures. The picture is pretty discouraging if the amounts needed (i.e., deficits) to reach normal by the end of the rainfall season (i.e., June 30) are examined. For example, from the table below, the Northern Sierra 8-Station Index (8SI) needs another 29.25 inches to reach their end of season normal of 54.52 inches. The 8SI March 24 to June 30 normal is 9.42", so 310% of that amount would be needed.

The good news is that the state's major reservoirs are near, or in some cases above, historical averages to date. See https://cdec.water.ca.gov/reportapp/javareports?name=rescond.pdf

The other good news is that the prospect of a very early fire season has been somewhat mitigated in the short-term.

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

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Does a Dry February Mean a Wet March or April


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

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