CW3E AR Update: 04 April 2017 Outlook

CW3E AR Update: 04 April 2017 Outlook

April 04, 2017

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AR conditions Forecast for Entire U.S. West Coast

  • An AR is currently impacting the Pacific Northwest while another AR is forecast to make landfall over Northern CA on Thursday
  • A mesoscale frontal wave that develops during the second AR could prolong the duration of AR conditions but uncertainty is currently high
  • 1–5 day precipitation forecasts are >6 inches over the high elevations of the Coastal Mts., Northern Sierra Mts., and Trinity Alps
  • Freezing levels are forecast to start at ~7,000 feet before dropping to ~3,000 feet, causing this to be a snow event for higher elevations
  • Wet soil and the potential for rain on snow at lower elevations raises the concern for flooding in eastern California and northern Nevada

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

Valid 1200 UTC 04 April – 0600 UTC 09 April 2017


 

 

 

 

 

Summary provided by C. Hecht and F.M. Ralph; 1 PM PT Tuesday 04 April 2017

CW3E AR Update: 16 March 2017 Outlook

CW3E AR Update: 16 March 2017 Outlook

March 16, 2017

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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).

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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

CW3E AR Update: 28 February 2017 Post Event Summary and Outlook

CW3E AR Update: 28 February 2017 Post Event Summary and Outlook

February 28, 2017

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Summary of the ARs that impacted the U.S. West Coast over the past week

  • Landfalling AR brought weak-to-moderate AR conditions to portions of Southern CA for ~24 hours between 27 and 28 February
  • >6 inches of precipitation fell over the high elevations of San Diego County with lower elevations receiving 1.5–4 in.
  • The San Diego River rose to ~14.15 feet at 2 am 28 Feb, 2.8 feet above flood stage, and the 3rd highest peak all time
  • The heavy precipitation led to several road closures, multiple mudslides, hotel evacuations, and flooded businesses

Click IVT or IWV image to see loop of GFS analysis

Valid 0000 UTC 26 Feb – 0600 UTC 28 Feb 2017


 

 

 

 

 

 

 

Three ARs expected to make landfall over the U.S. West Coast over the next ten days

  • The first AR is expected to make landfall over the Pac NW ~1800 UTC 2 March 2017 with weak strength (IVT=250–500 kg m-1s-1). Weak AR conditions may propagate over N CA. prior to dissipation.
  • A second AR is expected to make landfall over N CA. at ~0000 UTC 5 March 2017. Coastal areas of N CA may see several hours of moderate strength AR conditions.
  • Long range forecasts indicate the potential for a third weak AR during 8-10 March 2017, however there is large uncertainty in the models beyond forecast day 5.
  • Large scale pattern beyond forecast day 9 indicates the potential for a return to active AR landfall conditions over the Pac NW
    Highest precipitation and impacts from these events is predicted to be over the Olympic and Cascade Mtns. in WA and Coastal Mtns. In NW CA.

Click IVT or IWV image to see loop of 0-180 hour GFS forecasts

Valid 0600 UTC 28 Feb – 1800 UTC 7 Mar 2017


 

 

 

 

 

 

Summary provided by C. Hecht, B. Kawzenuk, and F.M. Ralph; 1 PM PT Tues 28 Feb. 2017

CW3E AR Update: 22 February 2017 Post Event Summary

CW3E AR Update: 22 February 2017 Post Event Summary

February 22, 2017

Click here for a pdf of this information.

Summary of the ARs that impacted the U.S. West Coast over the past week

  • Three separate ARs made landfall and impacted the U.S. West from 14–21 February 2017
  • Over 20 inches of precipitation fell over some of the high elevations of the West Coast
  • There were 291 total storms reports made to NOAA NWS during the three ARs
  • A summary of the ARs and their impacts are discussed in this post event summary

SSMI Integrated Water Vapor (IWV)

Valid 14-21 Feb 2017

  • The first AR made landfall between 18 UTC (10 AM PST) 14 February and 00 UTC 15 February (4 PM PST 14 Feb) over the Pacific Northwest before propagating southward over California
  • Maximum IVT at the Coast was between 800 and 1000 kg/m/s, which is considered a strong AR
  • Some locations experienced AR conditions for up to 42 hours during this event
  • Note: The strength of AR conditions noted on this summary was determined based on 6-hourly NCEP GFS analysis periods and observed IVT magnitudes may have been higher at specific locations along the coast
  • The second AR, which developed in conjunction with a mesoscale frontal wave, made landfall ~6 UTC on 17 February (4 PM PST 16 Feb) over Southern CA
  • Some locations experienced AR conditions for up to 24 hours during this event
  • Note: The strength of AR conditions noted on this summary was determined based on 6-hourly NCEP GFS analysis periods and observed IVT magnitudes may have been higher at specific locations along the coast
  • The third AR made landfall at ~6 UTC on 20 February (4 PM PST 19 Feb) over the Northern Ca
  • Maximum IVT at the Coast was between 700 and 800 kg/m/s, which is considered a moderate strength AR
  • Some locations experienced AR conditions for up to 42 hours during this event
  • Note: The strength of AR conditions noted on this summary was determined based on 6-hourly NCEP GFS analysis periods and observed IVT magnitudes may have been higher at specific locations along the coast


 

 

 

 

 

 

Summary provided by C. Hecht, B. Kawzenuk, and F.M. Ralph; 3 PM PT Wed. 22 Feb. 2017

CW3E AR Update: 17 February 2017 Outlook

CW3E AR Update: 17 February 2017 Outlook

February 17, 2017

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Update on ARs Currently Impacting and Forecast to Impact West Coast

  • Precipitation continues to fall over a majority of California
  • The Transverse Mountains (north of Santa Barbara) have received over 3.5 inches in the last 24-hours
  • SoCal is forecast to receive another 2 – 6 inches in the next 24-hours, raising concerns for flooding and landslides
  • Forecast confidence for this weekends landfalling AR has increased over Northern California

NOAA NWS NEXRAD Pacific Southwest Radar Loop

Valid 10:08 AM – 11:18 AM PST 17 Feb 2017




 

 

 

 

 

 

 

Summary provided by C. Hecht and F.M. Ralph; 1 PM PT Fri 17 Feb. 2017

CW3E AR Update: 16 February 2017 Outlook

CW3E AR Update: 16 February 2017 Outlook

February 16, 2017

Click here for a pdf of this information.

Update on ARs Currently Impacting and Forecast to Impact West Coast

  • As much as 6 inches of precipitation has fallen over the past 24-hrs over the high elevations of Northern CA, OR, and WA
  • The second AR, which develops in conjunction with a mesoscale frontal wave, is forecast to impact Southern CA
  • Over 12 inches of precipitation is forecast to fall over the Transverse Ranges, raising concern for flooding and landslides
  • The lower elevations of Los Angeles and San Diego are forecast to receive 1–4.5 inches over the next 72 hours


 

 

Click IVT or IWV image to see loop of 0-126 hour GFS forecast

Valid 1200 UTC 16 Feb – 1800 UTC 21 Feb 2017

  • A mesoscale frontal wave is forecast to develop off of the current AR which will bring AR conditions to Southern CA.
  • The proximity and propagation of the low-pressure system will bring non-AR related precipitation the North/Central CA
  • Another AR is forecast to make landfall over Northern CA from 20 – 22 Feb.


 

 

 

 

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

CW3E AR Update: 15 February 2017 Outlook

CW3E AR Update: 15 February 2017 Outlook

February 15, 2017

Click here for a pdf of this information.

Update on Multiple ARs Forecast to Impact West Coast

  • Precipitation has begun to fall over portions of WA, OR, and Northern CA in association with first AR
  • A second AR, which develops with a mesoscale frontal wave, is forecast to propagate southward and impact Southern CA
  • A third AR could impact portions of Northern CA on 20–22 February, but forecast certainty is currently low
  • Maximum 7-day precipitation forecasts are over 12 inches over the high elevations of Northern CA, OR, and WA
  • Southern CA could see as much as 8 inches over the Transverse ranges, which could be a concern for flooding and landslides

Click IVT or IWV image to see loop of 0-180 hour GFS forecast

Valid 1200 UTC 15 Feb – 0000 UTC 20 Feb 2017


 

 

 

 

 

 

 

 

Summary provided by C. Hecht and F.M. Ralph; 12 PM PT Wed 15 Feb. 2017

CW3E AR Update: 13 February 2017 Outlook

CW3E AR Update: 13 February 2017 Outlook

February 13, 2017

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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