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

A Diablo Winds Primer

A Diablo Winds Primer

Diablo Winds are warm dry winds originating when strong high surface pressure builds over the northern Great Basin. The resulting flow from high pressure inland to lower pressure off the California coast is warmed and dried by compressional heating as the air sinks from the Great Basin, which is nominally between 4000 and 5000 feet, down to sea level. The primary impact area is over and downwind of the Coast Range and Diablo Range, from about Lake County in the north to San Benito County in the south. [This pattern also produces strong north winds down the Sacramento Valley and downslope northeast wind in the northern Sierra Nevada.] 

 

These areas of high pressure in a favorable position in the Great Basin are most common in the fall and winter months as the polar jet stream makes its seasonal progression southward.  Troughs and ridges moving along the jet stream begin affecting the northern portions of the Great Basin.   In certain configurations, the jet stream pattern encourages strong sinking motions and the development of surface high pressure areas in the northern Great Basin.  In that position, these highs can generate Diablo Winds and when they move or extend farther south Santa Ana winds can be the result.



The seasonal timing of Diablo, and Santa Ana, Winds is extremely important in regard to wildfire danger. One of the chief characteristics of California’s predominantly Mediterranean climate is its protracted dry period from about May through October. Consequently, these strong, warm and dry winds occur after months of very little, if any, precipitation and when fuels (i.e., grasses, shrubs and forests) are at their driest.

The exact trajectory and strength of Diablo Winds is due to the strength and location of the high in the Great Basin and likewise the strength and location of lower pressure to its southwest. During a Diablo Wind event this trajectory will often begin as a more northerly wind and shift to one from the northeast. This flow is significantly altered as it flows over and down first the Sierra Nevada and then the Coast Range, with the channeling of the wind over ridges and down canyons sometimes drastically increasing its velocity. The vertical temperature patterns associated with the Diablo Wind weather type often have the result of constraining the wind flow against the mountains, almost as if there were a “lid” just over the top of the coastal mountains.  Winds pushing from northeast to southwest are then squeezed through and augmented at the ridge line and on the lee, southwest, slopes in what is known to meteorologists as the “hydraulic jump” phenomenon.

One common “rule of thumb” diagnostic for Diablo Winds in general is to look at the pressure gradient (i.e., difference in pressure) between Winnemucca in northern Nevada and San Francisco; with the higher the difference meaning stronger wind speeds. This comes with the large caveat that the exact location of a Great Basin High and offshore low can dramatically alter the resultant wind speed and direction.

The current and previous 24 hours of Winnemucca (WMC) to San Francisco (SFO) pressure gradients can be found in the lower half of https://www.wrh.noaa.gov/mtr/versprod.php?pil=OSO&sid=001, under SFO-WMC. The more negative the number, generally the stronger the offshore winds. 
Historically, here are several significant SFO-WMC surface pressure gradients days:
    Oct. 20, 1991:   -13.1 mb    Tunnel Fire
    Oct. 8, 2017:     -17.8 mb    North Bay Fires
    Oct. 24, 2019:   -16.3 mb    Kincade Fire

The etymology of the term “Diablo Winds” dates to shortly after the 1991 Tunnel Fire which devastated a large area of the Oakland and Berkeley hills. Myself and another forecaster (John Quadros), working at the National Weather Service office in Redwood City at the time, found that after the 1991 fire, calling offshore wind events in the Bay Area as “the northern California version of Santa Ana winds” was awkward at best, and meteorologically fuzzy at worst. We somehow fell upon the name Diablo Winds as a double entendre because they generally blow from the direction of Mt. Diablo in the far East Bay, and "diablo" translates from Spanish as “devil”; thus, devil winds.

Special thanks to Dr. John Monteverdi for his suggestions and editing.


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

 

 

Not All "Blobs" are Necessarily Equal

 

In recent week there have been numerous articles highlighting the strong Sea Surface Temperature Anomalies (SSTA) that have developed in the Gulf of Alaska and northeast Pacific Ocean; and equating them to the "Blob" that was a dominant feature in 2014 to 2016 period.  However, it is important not to look at these particular SSTA areas in isolation without looking at SSTA patterns in other parts of the eastern Pacific.  (This is a path that has sometimes been mistakenly taken in the past in regard to El Niño, where ENSO temperatures were taken out of context relative to other parts of the Pacific. One example was the Very Strong El Niño event in the winter of 2015-16, which is looked at in the following: https://ggweather.com/enso/compare/.)

While the current (September 2019) SSTA for the northeast Pacific does show significant positive SSTA warming in the northern half of the Gulf of Alaska, it is somewhat weaker and farther north that seen in 2014 and 2016. Also of note this year is the strong positive SSTA area west of California, a feature that was weaker and farther south in 2014 and non-existent in 2016.

The bottom line, is that the strength, location and interaction of these anomalous areas, as well as others in the Pacific, and even globally, all affect the circulation patterns of the West Coast differently; thus great caution should be used in drawing any simple (i.e., 1 + 1 = 2, or "blob" = California drought) conclusions as to their impact on the upcoming winter.

This post is also available at: https://ggweather.com/blob.htm

Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Phone: (408) 379-7500
Email: jnull@ggweather.com
Web: http://ggweather.com
Twitter: @ggweater


Posted

The Santa Clara Valley Rainshadow

 Often when it rains in the San Francisco Bay Area the lowest rain amounts are in San Jose and the surrounding environs of the Santa Clara Valley. To quantify this the normal (i.e., 30-year average from 1981 to 2010) rainfall over the region was analyzed. The data used was from PRISM (Parameter-elevation Relationships on Independent Slopes Model) at Oregon State University. Annual average rainfall was extracted for 600 gridpoints spaced at 0.05 degree (~0.55 miles) intervals over approximately 1300 square miles covering most of Santa Clara and the northern part of Santa Cruz Counties.

Rainshadows are the byproduct of moist air being lifted by the terrain (i.e., orographically) resulting in upward forcing on the windward slopes and thus enhanced precipitation amounts as the air cools and condenses. On the leeward side, the downward forcing causes the air to become drier with less rainfall resulting.

.


In the SF Bay Area, the moist southwesterly flow coming onshore from the Pacific Ocean is often perpendicular to the Santa Cruz Mountains, which are oriented from northwest to southeast. After ascending the mountains and producing sometimes copious amounts of rain, the air dries out as it descends downward into the Santa Clara Valley. This produces about fours time more rain near Ben Lomond (~60 inches) than near San Jose Airport (~15 inches) which is only 22 miles away. (Click here or on the map for full-size image)


Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Phone: (408) 379-7500
Email: jnull@ggweather.com
Web: http://ggweather.com
Twitter: @ggweather