"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

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

Posted

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

Posted

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

Posted

SF Record Dry Days and Updated California Period of Record Rainfall

There is lots of obvious interest in the lack of rain, with many California location going ZERO for February; even with an extra Leap Day.

1. It is likely that there will be no rain in SF through the end of the month; which will make a total of 32 dry days since the City's last measurable precipitation (i.e., 0.09" on 1/28). This will be tie it with 2013-2014 as the 4th longest winter dry spell. Previously:
 
For context, here are some other record San Francisco dry and wet spells from Climate of San Francisco (https://ggweather.com/sf/)

2. Also with heightened interest in the historical context of this year's rainfall I have put together Period of Record monthly rainfall for over 50 California locales; including the Sierra Nevada Indices and the seven state Climate Divisions. See https://ggweather.com/monthly/


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

 

Posted

Updated California Rainfall by the Numbers

Lagging rainfall statewide totals projected through the end of February paints a dim picture climatologically for the outcome of the 2019-2020 rainfall season. Forecast models remain dry through at least February 29th, except for some possible light convective showers over southern California and the Southern Sierra Nevada, but any of the expected rainfall there will not shift totals to any appreciable degree.









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

 

Posted

Defining DROUGHT ...  It's NOT Just Rainfall




Defining Drought … It’s Not Just the Rainfall


Whenever California has the potential for a below normal rainfall season, like now, the D” word starts making its way into the media and everyday conversation. But what exactly constitutes a “drought”. There is no simple answer and it certainly depends on who you ask and where they live.

The American Meteorological Society defines drought as “A period of abnormally dry weather sufficiently long enough to cause a serious hydrological imbalance”. The important takeaway from this broad-brush definition is the use of term “hydrological imbalance” and not rainfall deficit. This is especially true in California where the State’s diverse infrastructure means water falling in the northern half of the state strongly impacts hydrologic imbalances many hundreds of miles away in the south.

And there is no single metric, be it the Drought Monitor, reservoir levels or seasonal rainfall deficits, that is the right tool for everyone. The needs of a water district manager are very different from those of a farmer, a local industry, a ski resort operator or a homeowner.

Broadly, drought is subdivided into four categories: meteorological drought, hydrological drought, agricultural drought, and socioeconomic drought.

Meteorological drought is a measure of the “degree of dryness” resulting from rainfall and snowfall deficits. There is additionally a very important temporal aspect which impacts the other types of drought by its dependence on not only the degree of deficit but also its longevity. These deficits can be measured as the number of days without rain or the percent of an average amounts of precipitation over days, months, years of even decades.

Hydrological drought is a function of the water supply available from rivers, reservoirs and groundwater; and very importantly the infrastructure to distribute that water. The temporal aspect is even more important with hydrological drought as there can be significant time lags between when precipitation occurs, and it impacts surface or subsurface supplies. [This is one of the reasons that meteorologists in California and the West use July 1 to June 30 rainfall season as opposed to the hydrologist’s October 1 to September 30 water year (http://ggweather.posthaven.com/rainfall-season-vs-water-year)]  

Agricultural droughts operate on a short time scale as a precipitation deficit during even a short growing season can have significant impacts. These impacts are exacerbated (or mitigated) by crop type, the availability of stored water (i.e., hydrologic drought) plus soil type and moisture.

Socioeconomic drought is the impact on human activities and the related economies and is a function of all of the previous three types of drought as well as metrics like population change and water usage patterns.

A variety of indices and other metrics have been developed to attempt to quantify drought, though one that is good for agricultural drought may not be as adept at capturing socioeconomic impacts.

The bottom line is that drought has many intersecting layers and the effects of any or all of these drought types is dependent upon the user and his location. Drought is complicated!

Additional Resources:


Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Phone: (650) 712-1876
Email: jnull@ggweather.com 
Web: http://ggweather.com
Twitter: https://twitter.com/ggweather
Facebook: https://www.facebook.com/Golden-Gate-Weather-Services-151100414926621/

Posted

California Rainfall Season to Date by the Numbers

Except for December, the current California rainfall season has been rather disappointing across most of the state. And, climatologically the prospects of most regions in the state getting back to normal by the end of the season (i.e., June 30th) are not particularly promising. Only Eureka and Los Angeles have reached or exceeded normal more than half the time for similar years; while San Francisco and the 3 Sierra Indices have reached or exceeded normal only between 5% and 20% of the time. 

    

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

 

Posted

California Mid-Winter Dry Periods


Cue "It's Not Unusual" (by Tom Jones), because an extended period of dry weather in the middle of winter isn't unusual for northern and central California. And given, the current medium range models the next chance of significant rain may be another week and a half off.

Based on San Francisco's daily rainfall, over the past 69 rainfall seasons (i.e., July 1 to June 30) there has been a "dry" period in December or January averaging 19 days. For practical purposes a "dry period" here is defined as consecutive dry days with no rain, or consecutive days broken by no more than two non-consecutive intervening days of very light (* i.e., ≤ 0.08 inches) rain].

The shortest dry spell was 8 days which occurred twice, in 1957-58 and again in 1994-95.  Both of these seasons were during El Niño events!  The longest dry spell was 56 days in 2014-2015, when there was 18.19 inches.  All of these dry periods began in December or January with the exception of the 1964-65 period which was 19 days but did not begin until February 6th.

Even the very wet El Niño seasons of 1982-83 and 1997-98 had intervening dry spells and 22 and 17 days respectively.

Data available at https://ggweather.com/enso/winter_dry_spells.htm

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

Season

Consecutive        *Dry Days

Begin Date

*Intervening Days/Amount

1950-51

13

17-Dec

 

1951-52

12

6-Dec

1/.08

1952-53

13

21-Jan

 

1953-54

13

21-Dec

1/.01

1954-55

16

14-Dec

 

1955-56

20

28-Jan

 

1956-57

33

6-Dec

2/.03

1957-58

8

6-Dec

 

1958-59

12

12-Jan

 

1959-60

13

15-Dec

 

1960-61

36

19-Dec

1/.02

1961-62

21

22-Dec

 

1962-63

42

18-Dec

 

1963-64

19

25-Dec

2/.08

1964-65

19

6-Feb

 

1965-66

21

7-Jan

1/.02

1966-67

30

11-Dec

1/.01

1967-68

23

19-Jan

 

1968-69

13

24-Dec

 

1969-70

12

26-Dec

 

1970-71

29

17-Jan

 

1971-72

21

30-Dec

 

1972-73

15

24-Dec

 

1973-74

11

20-Jan

 

1974-75

22

5-Dec

2/.04

1975-76

25

10-Jan

 

1976-77

26

13-Jan

 

1977-78

16

20-Jan

1/.02

1978-79

15

19-Dec

 

1979-80

26

18-Jan

 

1980-81

16

5-Dec

 

1981-82

12

6-Jan

 

1982-83

22

24-Dec

 

1983-84

15

31-Dec

 

1984-85

11

27-Dec

 

1985-86

21

8-Dec

 

1986-87

15

7-Jan

 

1987-88

28

30-Jan

 

1988-89

11

11-Jan

 

1989-90

36

26-Nov

 

1990-91

17

20-Dec

 

1991-92

17

8-Jan

1/.04

1992-93

14

22-Jan

 

1993-94

20

15-Dec

1/.03

1994-95

8

31-Jan

 

1995-96

15

31-Dec

1/.02

1996-97

20

27-Jan

2/.07

1997-98

17

15-Dec

2/.03

1998-99

21

21-Dec

2/.02

1999-00

27

14-Dec

1/.03

2000-01

22

16-Dec

1/.03

2001-02

18

3-Jan

1/.02

2002-03

19

24-Jan

 

2003-04

11

10-Dec

1/.04

2004-05

17

9-Dec

 

2005-06

14

3-Dec

1/.03

2006-07

12

5-Jan

 

2007-08

10

11-Jan

 

2008-09

18

3-Jan

 

2009-10

12

31-Dec

1/.05

2010-11

14

31-Dec

 

2011-12

49

1-Dec

 

2012-13

16

7-Jan

1/.01

2013-14

36

12-Dec

2/.08

2014-15

56

21-Dec

1/.07

2015-16

9

25-Dec

2/.06

2016-17

10

23-Dec

 

2017-18

13

21-Dec

 

2018-19

11

25-Dec

 

 

Posted