CW3E AR Update: 17 January 2017 Outlook

CW3E AR Update: 17 January 2017 Outlook

January 17, 2017

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Multiple ARs Forecast to Make Landfall Over Next Week

  • Three ARs are currently forecast to make landfall over the U.S. West Coast at different times over the next 5 days
  • The first AR is forecast to make landfall over the Pacific Northwest today and is associated with strong AR conditions (IVT 750–1000 kg/m/s)
  • The second AR is forecast to make landfall over Southern CA from 20 – 22 Jan and is associated with strong AR conditions (IVT 750–1000 kg/m/s)
  • The third AR is forecast to make landfall over Central CA and could potentially be associated with strong AR conditions (IVT 750–1000 kg/m/s)
  • Forecast uncertainty is currently high with both AR 2 and 3
  • Weather Prediction Center precipitation forecasts currently range from 3–9 inches with AR 1, 2–4 inches with AR 2, and 2–5 inches with AR 3


 

 

 

 

 

 

Summary provided by C. Hecht, and F.M. Ralph; 3 PM PT Mon 17 Jan. 2017

CW3E AR Update: 11 January 2017 Outlook

CW3E AR Update: 11 January 2017 Outlook

January 11, 2017

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A look back at how the forecasts changed ahead of and during the active AR landfall period from 8-11 January (dProg/dT) and for next possible AR landfall next week

Forecasts initialized 0600 UTC 4 Jan – 0600 UTC 11 Jan


 

 

 

Summary provided by B. Kawzenuk, C. Hecht, and F.M. Ralph; 3 PM PT Wed 11 Jan. 2017

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 AR Update: 6 January 2017 Outlook

CW3E AR Update: 6 January 2017 Outlook

January 6, 2017

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Update on Strong AR over the weekend

  • Forecast confidence of AR conditions have improved since yesterday’s update
  • Moderate AR conditions could last over 24-hrs over some locations in Central CA
  • 1–3 Day Precipitation forecasts by the Weather Predication Center are as high as 15.5 inches over high elevations of Sierras
  • 19 river gauges are currently forecast to rise above flood stage by the California Nevada River Forecast Center
  • Extended forecasts show another AR making landfall next week, though the forecasts are more uncertain

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

Valid 1200 UTC 6 Jan – 0000 UTC 12 Jan 2017


 

 

 

 

 

 

Summary provided by C. Hecht, B. Kawzenuk, and F.M. Ralph; 5 PM PT Fri 6 Jan. 2017

Products presented in this outlook are meant for informational purposes and are not intended to support decision making operations

CW3E AR Update: 5-10 January 2017 Outlook

CW3E AR Update: 5-10 January 2017 Outlook

January 5, 2017

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Strong AR forecast to impact California this weekend

  • Forecasts continue to suggest the landfall of a strong AR over the weekend
  • Updated forecasts indicate that AR duration may decrease, which suggests lower precipitation amounts in some locations
  • Several more river gauges are expected to rise above flood stage
  • Extended forecasts predict another AR early next week potentially bringing more precipitation

Click IVT or IWV image to see loop of 0-144 hour GFS forecast

Valid 1200 UTC 5 Jan – 1200 UTC 11 Jan 2017


 

 

 

 

Summary provided by C. Hecht, B. Kawzenuk, and F.M. Ralph; 3 PM PT Wed 5 Jan. 2017

CW3E AR Update: 4-10 January 2017 Outlook

CW3E AR Update: 4-10 January 2017 Outlook

January 4, 2017

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Strong AR forecast to impact California this weekend

  • A strong AR with IVT as high as 1000 kg m-1 s-1 is forecast make landfall over the West Coast this Weekend
  • Precipitation from an AR currently impacting CA combined with the precipitation from AR this weekend are producing 7-day precipitation forecasts as high as 20 inches in the higher elevations of the Sierra Nevada Mts.
  • The hydrologic impacts associated with an event of this magnitude could be numerous and several rivers are already forecast to rise above flood stage in portions of northern/central California and western Nevada

Click IVT or IWV image to see loop of 0-138 hour GFS forecast

Valid 1200 UTC 4 Jan – 0600 UTC 10 Jan 2017


 

 

 

 

 

 

 

Summary provided by C. Hecht, B. Kawzenuk, and F.M. Ralph; 3 PM PT Wed 4 Jan. 2017

CW3E AR Update: 15-17 December 2016 Outlook

CW3E AR Update: 15-17 December 2016 Outlook

December 15, 2016

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  • AR conditions could last up to 24 more hours over northern CA resulting in as much as 1–3 more inches of precipitation
  • Flood warnings have been issued by the NWS at locations in Oregon and northern California
  • Visit weather.gov for point specific watches, advisories and warnings

Update on AR Impacting California

  • The AR has begun to move southward as precipitation continues to fall over northern California
  • 48-h precipitation accumulations range from 1–10 inches over portions of northern CA


 

 

 

 

Summary provided by C. Hecht, and F.M. Ralph; 1 PM PT Wed 15 Dec. 2016

CW3E AR Update: 14-17 December 2016 Outlook

CW3E AR Update: 14-17 December 2016 Outlook

December 14, 2016

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  • NWS precipitation forecasts for the rest of the event continue to predict higher precipitation amounts of ~10 inches in 3 days over the higher elevations of the Sierra Nevada Mts. with other locations in northern CA receiving an additional 2-10 inches
  • CNRFC river forecasts are now predicting several river gages to rise above flood stage

Update on AR Impacting California

  • Precipitation has begun falling in portions of northern CA and southern Oregon where 24-h precipitation accumulations range from 1 to 4.9 in.
  • AR conditions are expected to continue over northern California for the next 48-hrs before moving southward


 

 

 

 

Summary provided by C. Hecht, and F.M. Ralph; 1 PM PT Wed 14 Dec. 2016

CW3E AR Update: 13-17 December 2016 Outlook

CW3E AR Update: 13-17 December 2016 Outlook

December 13, 2016

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  • Current NWS precipitation forecasts predict higher precipitation amounts of over 10 inches in 3 days over the higher elevations of the Sierra Nevada Mts. with other locations in northern CA receiving 2-10 inches
  • Recent heavy precipitation has primed soil conditions and river flows to raise concern for flooding in some locations in northern California, as seen in NWS river forecasts

Landfalling AR to impact California

  • A moderate-strength AR is expected to make landfall in north/central California tonight, and could reach “strong” AR level
  • The AR propagates southward later in the week bringing AR conditions to portions of southern California

Click IVT or IWV image to see loop of 0-90 hour GFS forecast (Valid 1200 UTC 13 Dec – 0600 17 Dec)


 

 

 

 

Summary provided by C. Hecht, and F.M. Ralph; 12 PM PT Tue 13 Dec. 2016