CW3E AR Update: 22 February 2017 Post Event Summary

CW3E AR Update: 22 February 2017 Post Event Summary

February 22, 2017

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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

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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

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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

CW3E AR Update: 9 February 2017 Outlook

CW3E AR Update: 9 February 2017 Outlook

February 9, 2017

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Update on AR Currently Impacting CA, OR, and WA

  • Precipitation continues to fall over portions of Northern California, Washington, and Oregon
  • Another 3–5 inches of precipitation has fallen over some locations over the last 24-hrs
  • AR conditions will last for several more hours over Northern CA before the AR propagates southward over Southern CA
  • Up to 5 more inches could fall over the High Sierra’s and 1–2 inches forecast at lower elevations and portions of Southern CA


Click IVT or IWV image to see loop of 0-84 hour GFS forecast

Valid 1200 UTC 8 Feb – 0000 UTC 12 Feb 2017

The AR will continue to impact Central and Northern CA, OR, and WA before weakening and propagating southward over Southern California


 

 

 

 

 

 

Summary provided by C. Hecht and F.M. Ralph; 1 PM PT Thurs 09 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.

CW3E AR Update: 8 February 2017 Outlook

CW3E AR Update: 8 February 2017 Outlook

February 8, 2017

Click here for a pdf of this information.

Update on AR Currently Impacting CA, OR, and WA

  • Precipitation continues to fall over portions of Northern California, Washington, and Oregon
  • 6–10 inches of precipitation has fallen over some locations over the last 72-hrs
  • AR conditions continue to persist over Northern CA, OR, and WA as the second AR made landfall this morning
  • Another 5–8 inches are forecast for the higher elevations of Northern CA, OR, and WA as flooding remains a concern


 

 

Click IVT or IWV image to see loop of 0-128 hour GFS forecast

Valid 1200 UTC 8 Feb – 0000 UTC 12 Feb 2017

The southwesterly oriented AR is forecast to bring IVT magnitudes as high as 1000 kg m–1 s–1 and IWV values as high as 28 mm to the West


 

 

 

 

 

Summary provided by C. Hecht and F.M. Ralph; 5 PM PT Wed 08 Feb. 2017

CW3E AR Update: 7 February 2017 Outlook

CW3E AR Update: 7 February 2017 Outlook

February 7, 2017

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Two ARs to Impact the West CoastUpdate on ARs Currently Impacting and Forecast to Impact West Coast

  • As much as 6.5 inches of precipitation has fallen over the high elevations of CA over the previous 48 hours
  • Several rivers and streams are currently experiencing flooding in the San Francisco Bay Area
  • The second AR is forecast to make landfall at ~4 AM on Wednesday 8 February
  • An additional 5–10 inches of precipitation could fall over already wet soils, raising concern for additional flooding


 

 

 

Click IVT or IWV image to see loop of 0-127 hour GFS forecast

Valid 1200 UTC 7 Feb – 1800 UTC 11 Feb 2017

The next AR is currently forecast to make landfall at ~4 AM on 8 February 2017 with maximum IVT ~1000 and IWV ~34 mm


 

 

 

 

Summary provided by C. Hecht and F.M. Ralph; 2 PM PT Mon 07 Feb. 2017

CW3E AR Update: 6 February 2017 Outlook

CW3E AR Update: 6 February 2017 Outlook

February 6, 2017

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Two ARs to Impact the West Coast

  • Two moderate to strong ARs are forecasted to make landfall in Northern CA and the Pacific Northwest
  • Due to the timing of both ARs, AR conditions could last over 90 hours in some locations
  • Weather Prediction Center forecast shows 1-5 Day precipitation amounts up to 17.5 inches over higher elevations in Sierras
  • 15 river gauges are currently forecast to rise above flood stage by the California Nevada River Forecast Center

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

Valid 1200 UTC 6 Feb – 1800 UTC 11 Feb 2017


 

 

 

 

 

Summary provided by C. Hecht and F.M. Ralph; 3 PM PT Mon 06 Feb. 2017