CW3E AR Update: 05 June 2017 Outlook

CW3E AR Update: 05 June 2017 Outlook

June 05, 2017

Click here for a pdf of this information.

Late Season AR Forecast to Impact West Coast

  • An unseasonably strong AR is forecast to impact the Pacific Northwest and Northern CA over the next couple of days
  • As much as 4.1 inches of precipitation is forecast to fall over the higher elevations of the Coastal Mountains in CA and OR over the next week
  • With higher freezing levels forecast during landfall, there is a potential for rain on snow and increased runoff
  • Due to the combination of snowmelt and the landfalling AR, several rivers in the Pacific Northwest are forecast to rise above flood stage

Click IVT or IWV image to see loop of 0-114 hour GFS forecast

Valid 1200 UTC 05 June – 0600 UTC 10 June 2017


 

 

 

 

 

 

Summary provided by C. Hecht and F.M. Ralph; 1 PM PT Monday 05 June 2017

CW3E Update: Flood Risk From Snow Melt

CW3E Update: Flood Risk From Snow Melt

May 23, 2017

Click here for a pdf of this information.

Anomalously Warm Temperatures Expected to Contribute to Melting Snowpack and Elevated Runoff

  • High temperatures of 60–80˚F (16–26˚F) are forecast for the higher elevations of the Sierra Nevada Mountains over the next several days
  • The higher than normal water year to date precipitation over much of the West Coast has created snow packs that are much greater than normal over most of the Sierra Nevada Mountains
  • The combination of anomalously high temperatures and snow pack is forecast to lead to increased runoff and potential flooding
  • The National Weather Service has issued flood watches and warnings for several locations in California
  • Loop of GFS Forecast Surface Temperatures

    Valid 1200 UTC 23 May – 1200 UTC 30 May 2017


     

     

     

    Summary provided by C. Hecht B. Kawzenuk and F.M. Ralph; 12 PM PT Tuesday 23 May 2017

Atmospheric Rivers: Recent Developments and Applications in California

Atmospheric Rivers: Recent Developments and Applications in California

May 19, 2017

In Sacramento on Tuesday, May 23rd, CW3E director, F. Martin Ralph will be presenting a seminar about atmospheric rivers and their impacts to California legislative and agency staff. The seminar, Atmospheric Rivers: Recent Developments and Applications in California, will provide updates on the impacts of ARs on the current water year and the ongoing research to better understand and better forecast ARs. Dr. Ralph is looking forward to sharing all of the exciting research being done at CW3E with the group.

Key California precipitation index hits record yearly level after only 7 months

Key California Precipitation Index Hits Record Yearly Level After Only 7 Months

April 13, 2017

Click here for a pdf of this information.

A key index of California precipitation observations at eight stations in the Northern Sierra Nevada has set a new record high level after less than 7 months, beating the previous record that took 12 months of accumulation to set. A series of atmospheric rivers (ARs) that brought heavy precipitation to the state, especially in Jan and Feb, largely accounts for the record total accumulation.


 

 

 

 

 

 

Summary provided by F.M. Ralph, D. Pierce, C. Hecht, M. Dettinger, and D. Cayan; 2 PM PT Thursday 13 April 2017

CW3E AR Update: 11 April 2017 Outlook

CW3E AR Update: 11 April 2017 Outlook

April 11, 2017

Click here for a pdf of this information.

AR Currently Impacting West Coast

  • An AR is currently impacting Northern CA producing widespread precipitation over the region
  • A second AR is forecast to merge with the current AR, prolonging AR conditions over Northern CA
  • .25 to .88 inches of precipitation has already fallen across portions of Northern CA with >2.5 inches forecasted for higher elevations
  • This event could produce enough precipitation to make water year 2017 the wettest year recorded by the Northern Sierra 8-station index

SSMI Integrated Water Vapor (IWV)

Valid 10-11 April 2017

Click IVT or IWV image to see loop of 0-51 hour GFS forecast

Valid 1200 UTC 11 April – 1500 UTC 11 April 2017

A second AR with a separate parent low-pressure system is forecast to merge with the current AR, which could prolong AR conditions over portions of Northern CA


 

 

 

 

Summary provided by C. Hecht and F.M. Ralph; 1 PM PT Tuesday 11 April 2017

How Many Atmospheric Rivers Have Hit the U.S. West Coast During the Remarkably Wet Water Year 2017?

How Many Atmospheric Rivers Have Hit the U.S. West Coast During the Remarkably Wet Water Year 2017?

April 6, 2017

It has been well established that much of the west coast receives roughly 30-50% of its annual precipitation from landfalling atmospheric rivers. One of the goals of CW3E is to provide timely information on atmospheric rivers and their impacts on water in the West. The analysis presented here is based upon examination of AR conditions on each day from 1 October 2016 through 31 March 2017. Research-based criteria for AR identification have been used, especially the strength of integrated vapor transport (IVT). ARs are also ranked according to a simple scale introduced in 2016 (see inset in the graphic for the scaling).

As would be expected, one reason this winter has been so wet in the west is the large number of landfalling ARs. In addition, a large fraction of these events has been strong, or even extreme, in magnitude, and have caused serious flooding, and incidents like the Oroville Dam spillway issue.

Contacts: F. Martin Ralph, Chad Hecht, Brian Kawzenuk

There have been 45 total atmospheric rivers that have made landfall over the U.S. West coast from 1 October to 31 March 2017. Of the 45 total ARs, 10 have been Weak, 20 have been Moderate, 12 have been Strong, and 3 have been Extreme (Based on IVT magnitude). 1/3 of the landfalling ARs have been “strong” or “extreme”.

The large number of ARs that have made landfall over the U.S. West Coast have produced large amounts of precipitation. The Northern Sierra 8-station index is currently at 83.4 inches, which is just 5.1 inches below the wettest year on record with seven months remaining in the water year. The graphic below, from the California Department of Water Resources, highlights this information.

Odds of Reaching 100% Water Year Precipitation – Apr Update

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

April 6, 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 March 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 March 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 AR Update: 16 March 2017 Outlook

CW3E AR Update: 16 March 2017 Outlook

March 16, 2017

Click here for a pdf of this information.

Two ARs Forecast to Impact West Coast Over the Next Week

  • First AR to make landfall over Oregon Friday morning and primarily impact the Pacific Northwest and Northern CA
  • Second AR is forecast to impact Oregon and Northern CA beginning Monday Morning
  • Coastal Oregon could potentially experience strong AR conditions around 8 PM PDT Friday associated with first AR
  • Second AR could bring moderate AR conditions to Northern California but forecast confidence is currently low
  • Precipitation forecasts range from 2 to 4.8 inches over the high elevations of Northern California and the Pacific Northwest

Click IVT or IWV image to see loop of 0-150 hour GFS forecast

Valid 1200 UTC 16 March – 1800 UTC 22 March 2017


 

 

 

 

 

Summary provided by C. Hecht and F.M. Ralph; 1 PM PT Thurs 16 March 2017

Odds of Reaching 100% Water Year Precipitation – Mar Update

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

March 8, 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 February 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 February 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)

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