WHAT IS AN ATMOSPHERIC RIVER (AR)? HOW DO ALASKA FORECASTERS MONITOR THESE IMPACTFUL EVENTS? Dr. Marty Ralph Aaron Jacobs Director, Center for Western Senior Service Hydrologist, Weather and Water Extremes National Weather Service, (CW3E) Juneau Alaska Outline CW3E Mission What is an atmospheric river (AR)? What are the conditions that lead to ARs? AR Research using Observations and Globally Situational Awareness Impacts from ARs on Alaska Communities How are ARs, monitored, assessed and forecasted in Alaska? Ongoing AR research in Alaska ATMOSPHERIC RIVERS OVER ALASKA SSMI/SSMIS/AMSR2-derived Integrated Water Vapor (IWV) Valid: 00 UTC 01 August 2019 – 16 UTC 05 August 2019 Provided by: B. Kawzenuk & C. Hecht First Multi-year Catalog of AR Events Created Used RNW 2004 Method & Satellite IWV Data •Long, narrow plume of enhanced 16-Feb-04 atmospheric water vapor in warm North coast sector of winter storms •Identified via integrated water vapor (IWV) in numerical models or observed in SSMI/S Satellite IWV >2cm: estimates >2000 km long •Example from 2014 illustrates AR South coast landfall in California IWV >2cm: <1000 km wide Method from F.M. Ralph, P. Neiman, G. Wick (2004), Mon. Wea. Rev. (“RNW method”) Neiman, P.J., F.M. Ralph, G.A. Wick, J. Lundquist, and M.D. Dettinger (2008), J. Hydrometeor. 4 SSM/I Integrated Water Vapor (cm) Flooding on California’s Russian River: Role of atmospheric rivers Ralph, F.M., P. J. Neiman, G. A. Wick, S. I. Gutman, M. D. Dettinger, D. R. Cayan, A. White (Geophys. Res. Lett., 2006) ARs can CAUSE • SSM/I satellite data shows atmospheric FLOODS and river Atmospheric • Stream gauge data show regional extent of Rivers,PROVIDE Floods and high stream flow covers 500 km of coast WATERthe Water Russian River floods are Resources of associated with atmospheric SUPPLYCalifornia rivers Mike Dettinger, M. Ralph, , T. Das, - all 7 floods over 8 years. P. Neiman, D. Cayan (Water, 2011) 5 Region for which atmospheric river events are a dominant cause of extreme precipitation, flooding and contribute to water supply in the Western U.S. (Ralph et al. 2014) Region of major atmospheric river influence Lake Mendocino - Pilot study ARs as “Drought busters” ARs contribute to breaking >40% of CA droughts Dettinger 2013 6 The frontal wave increased the Phasing of tropical and extratropical duration of AR conditions where conditions leading to entrainment of the extreme precipitation tropical water vapor into the AR occurred Conceptual Model of Synoptic Conditions Associated with an AR “Tapping” the Tropical Water Vapor Reservoir, which leads to a more extreme rainfall event on the US West Coast Marty Ralph, Paul Neiman, George Kiladis Conceptual Model of Synoptic Conditions Associated with an AR “Tapping” the Tropical Water Vapor Reservoir, which leads to a more extreme rainfall event on the US West Coast Derived from Ralph et al. 2011, Mon. Wea. Rev. Where do Atmospheric Rivers Make Relationship Between Coastal Extreme Surface Landfall Globally? Winds and AR Landfall? Locations (dots), and frequencies (dot sizes) of landfalling atmospheric rivers Percentage of coastal extreme surface winds Guan and Waliser, 2015 (JGR) events that are associated with landfalling atmospheric rivers (color fill), and frequency of occurrence (dot size). Waliser and Guan, 2017 (Nat. Geoscience) Emerging Topic: Impacts of ARs on Polar Regions The Role of Atmospheric Rivers in GRL 2014 Extratropical and Polar Hydroclimate, JGR- Atmos 2018 Deanna Nash, D Waliser, B. Guan, H. Ye and F.M. Ralph Dropsonde Observations of Total Integrated Water Vapor Transport within North Pacific Atmospheric Rivers F.M. Ralph, S. Iacobellus, P.J. Neiman, J. Cordeira, J.R. Spackman, D. Waliser, G. Wick, A.B. White, C. Fairal, J. Hydrometeorology (2017) Method/Data: Uses 21 AR cases observed in KEY FINDING 2005 - 2016 with full dropsonde transects. An average AR* transports 4.7 ± 2.0 x 108 kg s-1 of AR edges best defined by using water vapor; equivalent to 2.6 times the average IVT = 250 kg m-1 s-1 discharge of liquid water by the Amazon River NASA GLOBAL HAWK Conclusions*: • Average width: 850 km Synthesis from 21 observed ARs; Used in the Glossary of • 75% of water vapor transport occurs below 3 Meteorology’s Definition of “Atmospheric River.” AIR FORCE C-130 km MSL; < 1% occurs above 8 km MSL • Average max IVT: ~800 kg m-1 s-1 21 aircraft *Represent averages For transects of ARs the Northeast PaciFic used here Ocean in the January- March season Background image denotes weekly AR Uses a total of 304 frequency during cool dropsondes seasons (Nov -Feb). Forecasting Challenges in Alaska & British Columbia Model-averaged Peirce skill score (PSS) Error in IVT for Re-Forecast Hits 1–7 Day Forecast Magnitude Error Forecast models tend to mis-forecast IVT magnitudes by 100 to 250 kg m–1 s–1 AR landfall location by 200–1000 km out to 7-day lead times over British Colombia and Alaska RMSE of Landfall Location Error Forecast Models tend to perform worse over British Colombia and Alaska compared to the rest of the U.S. West Coast when 1–7 Day Forecast forecasting atmospheric rivers out to 8-day lead times Position Error Beyond 8-day lead times, forecast models perform comparably for all Western North American locations Analysis and graphics from Nardi et al. 2018 Provided by: C. Hecht AR Reconnaissance Cw3e.ucsd.edu A Scale to Characterize the Strength and Impacts of Atmospheric Rivers F. Martin Ralph (SIO/CW3E), J. J. Rutz (NWS), J. M. Cordeira (Plymouth State), M. Dettinger (USGS), M. Anderson (CA DWR), D. Reynolds (CIRES), L. Schick (USACE), C. Smallcomb (NWS); Bull. Amer. Meteor. Soc. (Feb. 2019); DOI/10.1175/BAMS-D-18-0023.1 The AR level of an AR Event* is based on * An “AR Event” refers to the eXistence of AR conditions at a specific location for a specific period of time. ** How long IVT>250 at that location. IF duration is <24 h, reduce AR by 1, iF longer than 48 h, add 1. its Duration** and max Intensity (IVT)*** *** This is the maX IVT at the location of interest during the AR. Determining AR Intensity and AR Category AR 5 – Primarily hazardous IMPACTS The Oroville Event Step 1: Pick a location AR AR 4 – Mostly hazardous, also beneficial Step 2: Determine a time period when IVT > 250 AR 3 – Balance of beneficial and hazardous (using 3 hourly data) at that location, either in the AR 2 – Mostly beneficial, also hazardous past or as a forecast. The period when IVT AR 1 – Primarily beneficial continuously exceeds 250 determines the start and end times of the AR, and thus also the AR Duration for the AR event at that location. Step 3: Determine AR Intensity - Determine max IVT during the AR at that location - This sets the AR Intensity and preliminary AR CAT Step 4: Determine final value of AR level to assign - If the AR Duration is > 48 h, then promote by 1 level - If the AR Duration is < 24 h, then demote by 1 level ) 1 AR Duration (hours) - s On the Web: CW3E.UCSD.EDU 1 - AR “Intensity” On Twitter: @CW3E_Scripps (IVT) 250 IVT(kg m Date and Time Atmospheric Rivers Highlighted in the U.S. Fourth National Climate Assessment, released on 3 November 2017 1. Tropical Cyclones (Hurricanes and Typhoons) 2. Severe Convective Storms (Thunderstorms) 3. Winter storms 4. Atmospheric Rivers (NEW in 4th Assessment) CW3E Atmospheric River Outlooks CW3E Website cw3e.ucsd.edu Points of Contact Director: [email protected] Forecast Products/Outlooks: [email protected] & [email protected] Impacts from ARs on Alaska Communities Heavy rain producing: damaging flooding Heavy rain producing: debris flows Heavy snowfall producing: dangerous driving/traveling conditions, increased avalanche potential Heavy rain producing: beneficial effects Impacts from ARs on Alaska Communities: Heavy Rainfall-Debris flows/Record High Streamflows January 14, 2014, Sitka and Prince of Wales Island (POW) Debris flows resulted from: Flooding and impacts: • Very moist antecedent soil conditions – nearly continuous • Highest ever stage and flow ever recorded on Staney rainfall over previous month (17”-37”), with record daily Creek, 17.55 ft rainfall amounts on Jan 14 of 2.5”-3.5” • Numerous roads were flooded and impassable, along with • Strong wind gusts (greater than 45 mph) which helped to debris flows over roadways isolated several communities generate widespread mudslides along steep and deforested disrupting transportation terrain on POW • Debris flows also caused loss of power and damaged structures • Storm water drainage pumps became clogged with debris Staney Creek river gauge showing record Home in the Hollis area caught in landslide, Landslide on the Klawock-Hollis crest of 17.55 feet, moderate flood along with some flooding, courtesy of state Highway, stage is 17.5 feet (source: USGS/NWS) troopers. courtesy of state troopers, Impacts from ARs on Alaska Communities: Heavy Rainfall/Flooding December 10-14 2013, Ketchikan • 5 days of heavy precipitation (wettest 5 day period since 1902) ranging from 13 to 23 inches, with one day totals from 3 to 5 inches • Spillways on many area dams released water uncontrollably into Ketchikan Creek and produced flooding -- Area dams rose 7 to 14 ft with at least 2 ft over spillway Impacts from ARs on Alaska Communities: Heavy Snowfall November 20-25, 2015, Susitna Valley • Antecedent cold airmass kept precipitation as snow in the northern Susitna Valley • 2 day snow totals exceeded 40 inches and 5 day totals exceed 60 inches • The Parks Highway is closed due to dangerous driving conditions and avalanche mitigation work • Alaska Railroad train is caught in an avalanche. Crew is rescued, but railroad remains closed for 2.5 days Left: Alaska Railroad train covered in snow from avalanche.