Current Winter Setting a New California-Wide Record Precipitation Accumulation

Current Winter Setting a New California-Wide Record Precipitation Accumulation

March 7, 2017

Fueled by a string of strong atmospheric rivers (ARs), California’s current winter-to-date accumulated precipitation has hit a new record high level, eclipsing the previous record set during the strong El Niño winter of 1982-83.

The winter began with an unusual early season AR, which contributed 6% of normal annual California-wide precipitation over the period Oct 14-17. Strong AR activity continued in Jan and Feb 2017, with exceptionally strong precipitation Jan 8-10, which produced 14% of normal statewide annual precipitation in just three days and reached R-cat 4 intensity. (R-cat levels measure intense precipitation events; a fuller description of R-cat levels and this event can be found here). The AR during Feb 7-9 produced 9.5% of total annual California precipitation. Together, the latter two AR events produced nearly a quarter of an entire normal year’s precipitation in just 6 days, with each event including extreme intensity AR landfalls in the state.

The figure below shows the water year (Oct 1st – the following Sep 30th) that holds the record for maximum precipitation in California accumulated since the beginning of October for each day of winter. The current water year, 2017, broke the old record in early February and has continued to be the record-holder up to the current date (first week of March). Currently, 1982-82 holds the record for the maximum state-wide accumulated precipitation at the end of May in observations that go back to 1948. The accumulation so far this year is above the pace of 1982-83, but 1982-83 received a significant amount of precipitation in March and early May.

This figure shows California statewide accumulated precipitation estimated from 96 stations distributed across the state, but similar results are seen in the “Eight Station Index”, which uses eight stations in the Sierra Nevada selected for their importance to the state’s water supply. The eight station index is likewise currently at new record levels of accumulated winter precipitation, superseding the previous record-holding winter of 1982-83.

The southern portion of the state, including the greater Los Angeles region and San Diego county, are unusually wet so far this winter but not at record breaking levels. For instance, the Los Angeles region received substantially more precipitation in 2005, which led to widespread flooding, infrastructure damage, and several deaths.

The record-breaking precipitation has led to high values of snow cover, as shown by the yellow line (winter of 2016-2017) below. In the central and southern Sierra Nevada, current values are almost twice what is seen at the typical peak of snow accumulation on April 1st, and significantly above the high values seen during the El Niño winter of 1997-98 (dashed blue line). Snow is an important component of California’s water supply, since it holds the precipitation from intense winter storms, releasing the water more slowly via snow melt.

Contact: David Pierce and Marty Ralph

CW3E AR Update: 13 February 2017 Outlook

CW3E AR Update: 13 February 2017 Outlook

February 13, 2017

Click here for a pdf of this information.

Two ARs Forecast to Impact the West Coast

  • Potentially moderate AR conditions to impact the Pacific Northwest and Northern CA on 15–16 Feb.
  • Up to 5 inches of precipitation could fall over the next 72-hrs over the Coastal Ranges
  • Although forecasts are more uncertain, a second AR is likely to impact most of CA from late 16 Feb through early 18 Feb
  • 7-day precipitation forecasts are as high as 7.6 inches over Northern CA and 6.5 inches over Southern CA

Click IVT or IWV image to see loop of 0-132 hour GFS forecast

Valid 1200 UTC 13 Feb – 0000 UTC 19 Feb 2017


 

 

 

 

 

 

 

 

Summary provided by C. Hecht and F.M. Ralph; 3 PM PT Mon 13 Feb. 2017

R-Cat 2 Strength AR

Recent AR Reaches R-Cat 2 Strength

February 9, 2017

Sign up to receive an automated “R-Cat Extreme Precipitation Alert” email from CW3E showing he most extreme precipitation events over the previous 3 days (only on the rare days when there is extreme precipitation). The attached map shows the locations of 30 such reports from the storm that hit the West from 5-8 February 2017.

The maximum three-day precipitation during the most recent Atmospheric River event was 373.9 mm (14.72 inches) at a location called Four Trees on the western slopes of the Sierra Nevada Mountains, about 950 meters (3,100 feet) above sea level, in Plumas National Forest.

That made this an “R-Cat 2” precipitation event on CW3E’s scale. “R-Cat” stands for “Rainfall Category,” a simple scaling system invented by CW3E’s Marty Ralph and Mike Dettinger (see brief journal article here or here.

The landfall of a strong atmospheric river (AR) followed by a second AR in California over the last few days produced extreme precipitation over much of Central and Northern California. The second AR is currently over central and northern CA and is expected to produce more heavy precipitation over the next 24 hours (see CW3E AR Update: 9 Feb Outlook). This event was identified and reported in real-time by a new tool developed by David Pierce and Marty Ralph at CW3E that automatically monitors rain gauges across the Western U.S. and sends an email alert when extreme precipitation events occur to anyone signed up for the service. The service is free and is intended to provide information to interested individuals in a timely manner.

To subscribe to this automated CW3E R-Cat Extreme Precipitation Alert via email: just email a message with subject “subscribe” to rcatalert@cirrus.ucsd.edu.

The alerts use a simple new method to identify extreme events, which was published after analysis of decades of daily rainfall showed that 3-day precipitation totals were the most logical choice to characterize events that can have the broadest and largest impacts, especially in the Western U.S. The categorization method is based on 3-day observed precipitation totals (rain and/or the liquid equivalent of snow that fell), where “R-Cat” is short for “Rainfall Category:”

R-Cat 1: 200-299 mm (roughly 8-12 inches) / 3 days

R-Cat 2: 300-399 mm (roughly 12-16 inches) / 3 days

R-Cat 3: 400-499 mm (roughly 16-20 inches) / 3 days

R-Cat 4: more than 500 mm (more than roughly 20 inches) / 3 days

Historically these R-Cat events have occurred nationally with the following average annual frequencies (based on a network of several thousand rain gages that each had to have at least 30 years of daily observations; note that the R-Cat Alert tool does not require sites to have had 30 years of data, so more sites are likely to be found meeting the R-Cat criteria than in the earlier detailed analysis):

R-Cat 1: 48 episodes involving a total of 173 rain gauge sites that exceed the R-CAT1 threshold per year

R-Cat 2: 9 episodes involving 23 rain gauge sites that exceeded the R-CAT2 threshold

R-Cat 3: 2 episodes involving 4 rain gauge sites that exceeded the R-CAT3 threshold

R-Cat 4: 1 episode Involving 2 rain gauge sites that exceeded the R-CAT4 threshold

For comparison, the number of R-Cat 3 or 4 events annually roughly matches the average number of major hurricanes that occur annually in the Atlantic (Cat 3, 4, 5 combined) and the number of the most extreme tornadoes that occur (EF-4 and 5 combined).

Notably, in light of the events of last weekend, it is useful to note that, in the Western US between 1948 and 2010, 44 of the 48 occasions when RCAT3 or RCAT4 conditions were reached coincided with the arrival of an atmospheric-river storm.

Odds of Reaching 100% Water Year Precipitation – Feb Update

Odds of Reaching 100% of Normal Precipitation for Water Year 2017 (February Update)

February 9, 2017

Contribution from Dr. M.D. Dettinger, USGS

The odds shown here are the odds of precipitation in the rest of the water year (after January 2017) totaling a large enough amount to bring the water-year total to equal or exceed the percentage of normal listed. “All Yrs” odds based on monthly divisional precipitation totals from water year 1896-2015. Numbers in parenthesis are the corresponding odds if precipitation through January had been precisely normal (1981-2010 baseline).

Click here for a pdf file of this information.
 

 

 

How these probabilities were estimated:

At the end of a given month, if we know how much precipitation has fallen to date (in the water year), the amount of precipitation that will be required to close out the water year (on Sept 30) with a water-year total equal to the long-term normal is just that normal amount minus the amount received to date. Thus the odds of reaching normal by the end of the water year are just the odds of precipitation during the remaining of the year equaling or exceeding that remaining amount.

To arrive at the probabilities shown, the precipitation totals for the remaining months of the water year were tabulated in the long-term historical record and the number of years in which that precipitation total equaled or exceeded the amount still needed to reach normal were counted. The fraction of years that at least reached that threshold is the probability estimate. This simple calculation was performed for a full range of possible starting months (from November thru September) and for a wide range of initial (year-to-date) precipitation conditions. The calculation was also made for the probabilities of reaching 75% of normal by end of water year, 125%, and 150%, to ensure that the resulting tables of probabilities cover almost the full range of situations that will come up in the future.

[One key simplifying assumption goes into estimating the probabilities this way: The assumption that the amount of precipitation that will fall in the remainder of a water year does not depend on the amount that has already fallen in that water year to date. This assumption was tested for each month of the year by correlating historical year-to-date amounts with the remainder-of-the-year amounts, and the resulting correlations were never statistically significantly different from zero, except possibly when the beginning month is March, for which there is a small positive correlation between Oct-Mar and Apr-Sept precipitation historically.]

Contact: Michael Dettinger (USGS)

Water year 2017 Precipitation

Water year 2017 Precipitation in California

January 24, 2017

The wetness of this winter in northern California has been truly exceptional. Only two past winters on record have had as much precipitation to date as this year has in the Sacramento River watershed, which is key to California water supply. And each of those was a major flood year.

Where was the most extreme precipitation yesterday in the West?

Where was the most extreme precipitation yesterday in the West?

January 10, 2017

Sign up to receive an automated “R-Cat Extreme Precipitation Alert” email from CW3E showing he most extreme precipitation events over the previous 3 days (only on the rare days when there is extreme precipitation). The attached map shows the locations of 52 such reports from the storm that hit the West from 7-10 January 2017.

The maximum three-day precipitation during the weekend event was 521 mm (20.51 inches) at a location called Strawberry Valley on the western slopes of the Sierra Nevada Mountains, about 1,161 meters (3,807 feet) above sea level, near Interstate 80.

That made this an “R-Cat 4” extreme precipitation event on the CW3E’s scale. This is the top magnitude possible and is very rare. “R-Cat” stands for “Rainfall Category,” a simple scaling system invented by CW3E’s Marty Ralph and Mike Dettinger (see brief journal article here or here.

The landfall of a very strong long-duration atmospheric river (AR) (see second figure) followed by a second AR in California over the last few days produced extreme precipitation over much of Central and Northern California. This event was identified and reported in real-time by a new tool developed by David Pierce and Marty Ralph at CW3E that automatically monitors rain gauges across the Western U.S. and sends an email alert when extreme precipitation events occur to anyone signed up for the service. The service is free and is intended to provide information to interested individuals in a timely manner.

To subscribe to this automated CW3E R-Cat Extreme Precipitation Alert via email: just email a message with subject “subscribe” to rcatalert@cirrus.ucsd.edu.

The alerts use a simple new method to identify extreme events, which was published after analysis of decades of daily rainfall showed that 3-day precipitation totals were the most logical choice to characterize events that can have the broadest and largest impacts, especially in the Western U.S. The categorization method is based on 3-day observed precipitation totals (rain and/or the liquid equivalent of snow that fell), where “R-Cat” is short for “Rainfall Category:”

R-Cat 1: 200-299 mm (roughly 8-12 inches) / 3 days

R-Cat 2: 300-399 mm (roughly 12-16 inches) / 3 days

R-Cat 3: 400-499 mm (roughly 16-20 inches) / 3 days

R-Cat 4: more than 500 mm (more than roughly 20 inches) / 3 days

Historically these R-Cat events have occurred nationally with the following average annual frequencies (based on a network of several thousand rain gages that each had to have at least 30 years of daily observations; note that the R-Cat Alert tool does not require sites to have had 30 years of data, so more sites are likely to be found meeting the R-Cat criteria than in the earlier detailed analysis):

R-Cat 1: 48 episodes involving a total of 173 rain gauge sites that exceed the R-CAT1 threshold per year

R-Cat 2: 9 episodes involving 23 rain gauge sites that exceeded the R-CAT2 threshold

R-Cat 3: 2 episodes involving 4 rain gauge sites that exceeded the R-CAT3 threshold

R-Cat 4: 1 episode Involving 2 rain gauge sites that exceeded the R-CAT4 threshold

For comparison, the number of R-Cat 3 or 4 events annually roughly matches the average number of major hurricanes that occur annually in the Atlantic (Cat 3, 4, 5 combined) and the number of the most extreme tornadoes that occur (EF-4 and 5 combined).

Notably, in light of the events of last weekend, it is useful to note that, in the Western US between 1948 and 2010, 44 of the 48 occasions when RCAT3 or RCAT4 conditions were reached coincided with the arrival of an atmospheric-river storm.

Odds of Reaching 100% Water Year Precipitation – Jan Update

Odds of Reaching 100% of Normal Precipitation for Water Year 2017 (January Update)

January 9, 2017

Contribution from Dr. M.D. Dettinger, USGS

The odds shown here are the odds of precipitation in the rest of the water year (after December 2016) totaling a large enough amount to bring the water-year total to equal or exceed the percentage of normal listed. “All Yrs” odds based on monthly divisional precipitation totals from water year 1896-2015. Numbers in parenthesis are the corresponding odds if precipitation through October had been precisely normal (1981-2010 baseline).

Click here for a pdf file of this information.
 

 

 

How these probabilities were estimated:

At the end of a given month, if we know how much precipitation has fallen to date (in the water year), the amount of precipitation that will be required to close out the water year (on Sept 30) with a water-year total equal to the long-term normal is just that normal amount minus the amount received to date. Thus the odds of reaching normal by the end of the water year are just the odds of precipitation during the remaining of the year equaling or exceeding that remaining amount.

To arrive at the probabilities shown, the precipitation totals for the remaining months of the water year were tabulated in the long-term historical record and the number of years in which that precipitation total equaled or exceeded the amount still needed to reach normal were counted. The fraction of years that at least reached that threshold is the probability estimate. This simple calculation was performed for a full range of possible starting months (from November thru September) and for a wide range of initial (year-to-date) precipitation conditions. The calculation was also made for the probabilities of reaching 75% of normal by end of water year, 125%, and 150%, to ensure that the resulting tables of probabilities cover almost the full range of situations that will come up in the future.

[One key simplifying assumption goes into estimating the probabilities this way: The assumption that the amount of precipitation that will fall in the remainder of a water year does not depend on the amount that has already fallen in that water year to date. This assumption was tested for each month of the year by correlating historical year-to-date amounts with the remainder-of-the-year amounts, and the resulting correlations were never statistically significantly different from zero, except possibly when the beginning month is March, for which there is a small positive correlation between Oct-Mar and Apr-Sept precipitation historically.]

Contact: Michael Dettinger (USGS)

CW3E Participates in Water in the West: A Science Policy Roundtable

CW3E Participates in Water in the West: A Science Policy Roundtable

December 10, 2016

CW3E was represented in a December 8, 2016 panel discussion hosted by UCSD’s School of Global Policy and Strategy (GPS)’s Science Policy Fellows Program and SIO’s Science Policy Discussion Group, titled “Water in the West: A Science Policy Roundtable”. The purpose of the event was to bring together our different communities and explore the sources of California’s water supply, how water supply can change, how it is used throughout the state and best practices for optimal regulation of its use.

CW3E director Dr. F. Martin Ralph acted as moderator. Other panel members were Dr. Jennifer Burney of GPS, Sandra Kerl of the San Diego County Water Authority, and Dr. Dan Cayan of SIO and a CW3E collaborator. Dr. Scott Sellars, the leader of the Science Policy Discussion Group, was involved in organizing the event. Other CW3E postdocs and graduate students assisted with logistics. The attendance was just over 100 people, with a very engaged audience. Questions were asked on topics ranging from specific projects run by the SD County Water Authority, to potential changes to academic funding sources with the incoming federal administration, to questions on groundwater and atmospheric rivers science. The event was recorded and can be viewed here.

More details are available in the GPS news story.

At podium: Marty Ralph (Scripps/CW3E); Left to right at table: Sandra Kerl (San Diego County Water Authority), Dan Cayan (Scripps/CW3E and USGS), Jennifer Burney (GPS)

Odds of Reaching 100% Water Year Precipitation – Dec Update

Odds of Reaching 100% of Normal Precipitation for Water Year 2017 (December Update)

December 9, 2016

Contribution from Dr. M.D. Dettinger, USGS

The odds shown here are the odds of precipitation in the rest of the water year (after November 2016) totaling a large enough amount to bring the water-year total to equal or exceed the percentage of normal listed. “All Yrs” odds based on monthly divisional precipitation totals from water year 1896-2015. Numbers in parenthesis are the corresponding odds if precipitation through October had been precisely normal (1981-2010 baseline).

Click here for a pdf file of this information.
 

 

 

How these probabilities were estimated:

At the end of a given month, if we know how much precipitation has fallen to date (in the water year), the amount of precipitation that will be required to close out the water year (on Sept 30) with a water-year total equal to the long-term normal is just that normal amount minus the amount received to date. Thus the odds of reaching normal by the end of the water year are just the odds of precipitation during the remaining of the year equaling or exceeding that remaining amount.

To arrive at the probabilities shown, the precipitation totals for the remaining months of the water year were tabulated in the long-term historical record and the number of years in which that precipitation total equaled or exceeded the amount still needed to reach normal were counted. The fraction of years that at least reached that threshold is the probability estimate. This simple calculation was performed for a full range of possible starting months (from November thru September) and for a wide range of initial (year-to-date) precipitation conditions. The calculation was also made for the probabilities of reaching 75% of normal by end of water year, 125%, and 150%, to ensure that the resulting tables of probabilities cover almost the full range of situations that will come up in the future.

[One key simplifying assumption goes into estimating the probabilities this way: The assumption that the amount of precipitation that will fall in the remainder of a water year does not depend on the amount that has already fallen in that water year to date. This assumption was tested for each month of the year by correlating historical year-to-date amounts with the remainder-of-the-year amounts, and the resulting correlations were never statistically significantly different from zero, except possibly when the beginning month is March, for which there is a small positive correlation between Oct-Mar and Apr-Sept precipitation historically.]

Contact: Michael Dettinger (USGS)