25th Anniversary of Dec 1995 West Coast Mega Storm

See: https://ggweather.com/dec1995/

Today and tomorrow (Dec 11-12) mark the 25th anniversary of what is arguably the strongest storm to hit the San Francisco Bay Area and the West Coast in the past 70 years. It is the only time in that period that saw SF Bay Area winds in excess of 100 mph at sea level and over 130 mph on Mount Diablo. And at the same time, San Francisco recorded over 5 inches of rain in a 30-hour period while Kentfield had in excess of 11 inches. It is the strongest storm, and benchmark of the Bay Area Storm Index (BASI) with a rating of 10.0.

Dr. John Monteverdi (Emeritus Professor of Meteorology at San Francisco State University) and I have put together a webpage with some of the highlights from this remarkable event.  See https://ggweather.com/dec1995/





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

 


Posted

A Climatological Look at Dry Autumns

A Climatological Look at Dry Autumns

  

With a very dry start to the 2020-2021 California rainfall season, and no rain in the forecast through the end of the month, this is a look at what has happened in similar seasons. San Francisco, with the longest continuous rainfall record in the state back to 1849, is used for comparison.

To date, San Francisco has only had a paltry 0.54", which is just 12% of the normal July through November average of 4.55".

Below is a breakdown of the 16 seasons that had less than one-inch to begin the season. Of the previous 15 seasons, 5 of them finished the season above normal rainfall, and 10 finished below normal. Likewise, for seasons with a moderate La Niña, there was one season (1995) with above normal SF seasonal rainfall and two that ended below normal.

But caveat emptor, this data is climatology and NOT a forecast. 




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






This email was sent to jnull@ggweather.com
why did I get this?    unsubscribe from this list    update subscription preferences
Golden Gate Weather Services · PO Box 3373 · Half Moon Bay, CA 94019 · USA

Email Marketing Powered by Mailchimp

Posted

21st Century La Niñas and California Precipitation



Below is a graphical catalog of the eight La Niña Sea Surface Temperature Anomalies (SSTA) that have occurred since the turn of the century; along with the corresponding California precipitation anomaly, and the previous year's ENSO category.

In broad strokes, over the seven previous La Niñas, four were DRY (2007-8, 2008-0, 2011-12, 2017-18) and three were WET (2005-06, 2010-11, 2016-17) across most of California. And it is interesting to note that all three of the wet years followed El Niño years, while the four dry years followed La Niñas.

The latest forecast from CPC and IRI are pushing this winter into the Moderate La Niña category, and briefly into the Strong range. Last year there was a weak El Niño.



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

 

Posted

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

 

 

 

Posted

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

 

Posted

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

 

 

Posted

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

 

 

Posted

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

 

Posted

"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


Posted