Author: cw3e-web

CW3E AR Update:Post-Event Summary: 7-10 April 2016

CW3E AR Update: 7-10 April 2016 Post Event Summary

April 13, 2016

CW3E and WRCC give a post-event storm summary about two cutoff lows that made landfall over Southern California during 7-10 April 2016. Precipitation was widespread throughout the Southeastern United States with 72-hr accumulations generally ~0.5-3 inches. While the low pressure system entrained moisture from the tropics, spatial characteristics of IWV >20 mm and IVT >250 kg m-1 s-1 did not meet the necesarry requirements to be considered an Atmospheric River.

Click here for a pdf file of this information.


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Odds of Reaching 100% Water Year Precipitation – April Update

Odds of Reaching 100% of Normal Precipitation for Water Year 2016 in California (April update)

April 10, 2016

Contribution from Dr. M.D. Dettinger, USGS

March is over (and the precipitation totals are in), so here is the update to the historical odds of getting to normal (and other fractions of normal) precipitation this year. Notice that the odds from the March 1 situation (in parens; click here for March report) are included alongside the April 1 odds, so that you can see whether March wetness made much difference. In fact, the odds haven’t changed too much in most areas because, although March was wettish (except down south), those additions were balanced in these April odds by the fact that we have almost run out of time this year to get much more precipitation.

Bottom lines (both of which you probably already know): We’re not going to fill the precipitation deficits we accumulated over the past 5 years with this year’s precipitation (except, long-odds, maybe in eastern NV). In southern California, the precipitation situation is especially grim and even the odds of making it to 75% of normal this year are well less than even.


 

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 (for all years, for only El Ninos, or for only La Ninas) 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. Contact mddettin@usgs.gov for more information.

[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: 7-10 April 2016 Outlook

CW3E AR Update: 7-10 April 2016 Outlook

April 7, 2016

A weak low pressure system and high Integrated Water Vapor (IWV) values have brought light-to-moderate precipitation over Southern California this morning. A closed low is expected to develop off the coast over the next two days and will produce moderate precipitation over Southern California on 8-10 April 2016. IWV and Integrated Water Vapor Transport (IVT) values will be above AR thresholds but spatial characteristics of IWV/IVT will not meet the requirements of an AR. Coastal Southern California will experience AR conditions (IWV >20 mm and IVT >250 kg m-1 s-1) and light-to-moderate precipitaton, but this event is not considered an AR. For up to date AR forecasts visit the CW3E AR Portal.

Click here for a pdf file of this information.


 

 

Publication Notice: Predictability of Horizontal Water Vapor Transport Relative to Precipitation

CW3E Publication Notice

Predictability of Horizontal Water Vapor Transport Relative to Precipitation: Enhancing Situational Awareness for Forecasting Western U.S. Extreme Precipitation and Flooding

March, 2016

Lavers, D.A., D.E. Waliser, F.M. Ralph and M.D. Dettinger, 2016: Predictability of horizontal water vapor transport relative to precipitation: Enhancing situational awareness for forecasting western U.S. extreme precipitation and flooding. Geophysical Research Letters, 43, doi:10.1002/2016GL067765 (Please click here for personal use pdf file)

The following paper has just appeared in Geophysical Research Letters. It was motivated by the critical role of horizontal vapor transport in determining the strength and distribution of extreme precipitation in the Western U.S., and by the fact that this transport is the defining characteristic of atmospheric rivers, which are key to many extreme events in the region. The work was carried out primarily at CW3E in response to interest from State and local water agencies to explore new methods to predict extreme precipitation events. While the findings are based on U.S. West Coast domains, the results are applicable to other west coasts of mid latitude continents where cool season orographic precipitation is a key process. The results support the use of water vapor transport as a variable to monitor for earlier awareness of extreme hydrometeorological events.

(e) The average interannual predictability (r2) across the 30°N–50°N, 125°W–120°W region. (f) The predictability throughout the forecast horizon calculated using all winter forecasts (n = 2796) at 38°N, 122°W. From Lavers et al. (2016).

Contacts: David Lavers (david.lavers@ecmwf.int) and F. Martin Ralph (mralph@ucsd.edu)

Abstract

The western United States is vulnerable to socioeconomic disruption due to extreme winter precipitation and floods. Traditionally, forecasts of precipitation and river discharge provide the basis for preparations. Herein we show that earlier event awareness may be possible through use of horizontal water vapor transport (integrated vapor transport (IVT)) forecasts. Applying the potential predictability concept to the National Centers for Environmental Prediction global ensemble reforecasts, across 31 winters, IVT is found to be more predictable than precipitation. IVT ensemble forecasts with the smallest spreads (least forecast uncertainty) are associated with initiation states with anomalously high geopotential heights south of Alaska, a setup conducive for anticyclonic conditions and weak IVT into the western United States. IVT ensemble forecasts with the greatest spreads (most forecast uncertainty) have initiation states with anomalously low geopotential heights south of Alaska and correspond to atmospheric rivers. The greater IVT predictability could provide warnings of impending storminess with additional lead times for hydrometeorological applications.

CW3E AR Update: Post-Event Summary: 5-6 March 2016

CW3E AR Update: 5-6 March 2016 Post-Event Summary

March 10, 2016

CW3E gives a post-event storm summary about a moderate strength Atmospheric River that made landfall over Northern California on 5 March 2016. The AR was an R-Cat 1 event and produced over 10 inches of 72-hour precipitation.

Click here for a pdf file of this information.


 

 

 

 

Northern Sierra Precipitaiton: 8-Station Index

~7.5 inches of precipitation measured by the 8-Station Index

Increased water-year-to-date accummulation from average to ~115% of average

Lake Mendocino Storage

Increase in storage of ~8000 af

Storage remains above the Target Water Supply Storage Curve

SSMI Integrated Water Vapor: 0000 UTC 3 March – 0000 UTC 7 March

  • AR developed over the Central North Pacific and made landfall over Northern California
  • AR entrained moisture from previous decaying AR off California coast prior to landfall

GFS IVT Analysis: 0000 UTC 3 March – 1200 UTC 7 March 2016

Cazadero, Ca Snow Level Radar

Heavy bright-band precipitation began ~0600 UTC 5 March

Large drop in snow level post-frontal

Bodega Bay 0200 UTC Sounding

  • Saturated atmosphere up to ~550 hPa
  • IWV = 31.9 mm
  • IVT = 878.3 kg m-1s-1
  • Winds >50 knots throughout the troposphere
  • Majority of water vapor flux between 950-700 hPa

Event Summary

  • A moderate strength Atmospheric River made landfall over Northern California during 5-6 March 2016
  • Maximum 48-h precipitation of >10 inches occurred over the Sierra Nevada
  • Widespread precipitation >6 inches occurred over the Coastaland Sierra Nevada Ranges
  • Several rivers reached above flood stage in Northern California

CW3E R-Cat Alerts

CW3E R-Cat Alerts: Automated Notification of Heavy Precipitation Events

March 10, 2016

The Center for Western Weather and Water Extremes (CW3E) has set up an automated e-mail notification service that informs users of heavy precipitation events in near real-time. Along the West Coast, such events are often the result of land-falling atmospheric rivers, which transport substantial moisture into the area. Tracking, analyzing, improving the forecasting, and anticipating the impacts of such extreme events are a core element of CW3E’s mission.

The “Rainfall Category” or “R-Cat” 3-day precipitation classification of Ralph and Dettinger (2012) is a simple, effective measure of strong precipitation events, which can have a large impact on the Western U.S.:

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

An R-cat email alert includes a short summary of the 3-day total precipitation, location of the event (based on sources such as the Global Historical Climatology Network [GHCN] and the National Weather Service’s co-op precipitation stations), and a map showing the location of the event(s). Here is a recent example:

————————————————————————————————————
Station: BRUSH CREEK RS USC00041130
Location: (39.695, -121.345)
Date: 2016/03/07 (third day of event)
Event strength: R-Cat 1
3-day total precip (mm): 206.3
Individual days precip (mm): 76.5 102.1 27.7
 
————————————————————————————————————
Station: QUINCY USC00047195
Location: (39.937, -120.948)
Date: 2016/03/07 (third day of event)
Event strength: R-Cat 1
3-day total precip (mm): 202.7
Individual days precip (mm): 59.7 119.9 23.1

The email R-Cat alert service can be subscribed to by sending a message with the subject line “subscribe” to rcatalert@cirrus.ucsd.edu.

For more information, please contact David Pierce, dpierce@ucsd.edu.

CW3E AR Update: 10-11 March 2016 Outlook

CW3E AR Update – 10-11 March 2016 Outlook

March 10, 2016

A storm predicted to make landfall over northern California on 10 March 2016 has characteristics of an Atmospheric River. The AR is showing weak-to-moderate strength with an average landfall duration of 12-24 hours. This AR is about 75% as strong as the last significant AR landfall over northern California (March 5-6, 2016). The storm has the slight potential for R-Cat 1 rainfall in favored mountain areas. For up to date AR forecasts visit the CW3E AR Portal.



Odds of Reaching 100% Water Year Precipitation – March Update

Odds of Reaching 100% of Normal Precipitation for Water Year 2016 in California (March update)

March 10, 2016

Contribution from Dr. M.D. Dettinger, USGS

The Febraury 2016 precipitation observations are now in, and have allowed for an update to the calculation of the odds of reaching 100% of normal for the water year across three key climate divisions of California and Nevada. These odds have decreased slightly across all of California and Nevada as a result of a very dry February. The odds of reaching 100% of normal Water Year precipitation in the key northern California climate division that encompasses the Sacramento River, and the State’s largest water supply reservoirs, decreased from 52% as of the end of January 2016, to 38% as of the end of February 2016. A series of Atmospheric Rivers have made landfall over Northern California and produced heavy precipitation throughout early March and will most likely cause changes in these odds when updated at the end of March.


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 (for all years, for only El Ninos, or for only La Ninas) 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. Contact mddettin@usgs.gov for more information.

[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: 5-7 March 2016 Outlook

CW3E AR Update – 5-7 March 2016 Outlook

March 4, 2016

A storm predicted to make landfall over northern California on 5 March 2016 has characteristics of an Atmospheric River. The AR is showing moderate strength with an average landfall duration of 12-24 hours. The storm has the potential for R-Cat 1 rainfall in favored mountain areas. For up to date AR forecasts visit the CW3E AR Portal.


CW3E AR Update: 3 March 2016 Outlook

CW3E AR Update: 3 March 2016 Outlook

March 3, 2016

CW3E gives an outlook on current and upcoming Atmospheric River (AR) events along the U.S. West Coast. A moderate AR is currently impacting central California producing light precipitation over the northern Sierra Nevada and Central Valley. Two ARs are expected to make landfall over the Pacific Northwest in the next seven days, with both propagating south over California prior to dissipation. Long range forecasts also show the potential for another strong AR to make landfall in the Pacific Northwest next week.

For the latest river and precipitation forecasts and observations visit the California Nevada River Forecast Center (CNRFC) or the U.S. Geological Survey (USGS). For the latest AR forecasts visit the CW3E AR Portal.

Click here for a pdf file of this information.


 

Current Conditions: 3/3/2016
Northeast Pacific SSMI IWV: 1400 UTC 29 Feb 2016 – 1400 UTC 3 March 2016


 
GPS Integrated Water Vapor: 1515 UTC 3 March 2016
 
 
 
   
IWV values greater than 2 cm
throughout central California
-Current AR conditions

Current California Precipitation Conditions
The North Sierra 8-Station and Tulare Basin 6-Station Precipitation Indices are average for this time of year.


 

 
Forecasts
AR Landfall Probability Tool: Initialized 0000 UTC 3 March 2016
Hover mouse over description below to see forecast product or click to open. Images courtesy Jason Cordeira; Plymouth State University

IVT >250 kg m-1 s-1
IVT >500 kg m-1 s-1
Inland IVT >250 kg m-1 s-1
Inland IVT >500 kg m-1 s-1

  • Periods of AR conditions forecasted along most the U.S. West Coast on forecast days 0-1, 1-2, 2-5 and 4-5
  • Strong probability of AR event on forecast days 7-8
  • All forecasted ARs are expected to penetrate inland over Oregon and northern California


 


 
CNRFC River Forecasts
2000 UTC 3 March 2016
 
 

  • 1 river forecasted above flood stage
     -Navarro River
  • 7 rivers forecasted above monitor stage

    Russian River CNRFC Forecast


     
    GFS Ensemble MJO Index Forecast; 3 March 2016 – 17 March 2016

    		  

    • A continuation of MJO activity is expected
    • Propagation through Phases 8 and 1 and possibly 2
    • Significant decrease in amplitude

    GFS IWV: 0600 UTC 3 March 2016 – 1800 UTC 10 March 2016
     
      >

    • AR currently impacting northern and central California
    • AR to make landfall at 1800 UTC 4 March 2016 over NW Pacific
    • AR to make landfall at 0600 UTC 9 March 2016 over NW Pacific
    • All three ARs propagate south over California prior to dissipation

    GFS IVT: 0600 UTC 3 March 2016 – 1800 UTC 10 March 2016
     



     
    CNFRC Precipitation Forecasts: Produced 3 March 2016
    Max 72-h precip >6 inches over Northern Sierra Nevada and California Coastal Range
    Max 6-day precip ~11 inches over Northern Sierra Nevada and California Coastal Range

     
    7-day Precipitation Forecasts: Ending 4:00 am PST 10 March 2016
    Max 7-day precip ~290 mm over Northern Sierra Nevada, Olympic Mountains and Vancouver Island
    Max 7-day precip ~270 mm over Northern Sierra Nevada, Olympic Mountains and Vancouver Island


     

     
     

    Summary

    • Current dissipating AR impacting central California
    • Two moderate strength ARs expected to make landfall along the U.S. West Coast over the next five days
    • Another AR may potentially make landfall late next week
    • 7-day precipitation totals up to 300 mm in multiple locations over the U.S. West Coast


    For more information about AR updates or forecast products visit the CW3E Home Page or contact Brian Kawzenuk, Marty Ralph, or Scott Sellars at CW3E.