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Heavy Cold-Season Precipitation in British Columbia, Washington and Oregon

Heavy Cold-Season Precipitation in British Columbia, Washington and Oregon. MSC/COMET Presentation, 23 February 2001. Gary M. Lackmann Department of Marine, Earth, and Atmospheric Sciences North Carolina State University. 1.

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Heavy Cold-Season Precipitation in British Columbia, Washington and Oregon

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  1. Heavy Cold-Season Precipitation in British Columbia, Washington and Oregon MSC/COMET Presentation, 23 February 2001 Gary M. Lackmann Department of Marine, Earth, and Atmospheric Sciences North Carolina State University 1

  2. The “Pineapple Express”: A Worst-Case Scenario for West Coast Flooding • What is the “Pineapple Express” (PE)? • Characterized by • anomalous subtropical moisture transport • warm temperatures, heavy precipitation • rapid snowmelt, lowland flooding • Directly affects • British Columbia • Washington, Oregon, Northern California • Indirectly affects much of North America? 2

  3. Outline I. A Brief Climatology: The Pineapple Express  Methodology: stream and rain gauge data  Limitations of compositing  Composite patterns and implications II. Case Study: Flood of 16-18 January 1986  Methodology: Piecewise moisture transport  A moisture transport feedback  Anticipation of model biases 3

  4. I. A Brief Pineapple Express Climatology • Objectives: • Identify planetary- and synoptic-scale common denominators for cold-season heavy precipitation • Seek identifiable precursors • Determine “character” of moisture transport • Provide context for more detailed case studies • Methodology: • Use daily precipitation data and stream gauge data to identify events • Examine individual events, stratify case sample • Generate composites for 6-day period bracketing event 4

  5. Methodology • A. Atmospheric Composite: • 27-year data sets from • Olympia (OLM), • Seattle-Tacoma Apt (SEA), • Stampede Pass (SMP), WA • Astoria (AST), OR • Case selection criteria: • Daily precipitation > 12.5 mm (0.5”) 24 h -1 and • Maximum Temp. > 10 C (lowland) or > 5 C (mountain) • B. Runoff Composite: • Tolt River discharge values > 4,000 ft3 s-1. 5

  6. Methodology and Case Selection Results • Six-day composites generated from NCEP CD • Anomalies: deviations from 27-year weighted climo • 46 cold-season events from 1962-1988: • November 18 • December 12 • January 8 • February 5 • March 2 • Tolt: Less sensitivity to temperature criterion • November 3 • December 11 • January 17 • February 5 • March 2 6 5

  7. Composite 500 height and SLP evolution 7

  8. Composite 500 height anomaly evolution 8

  9. Tolt Composite 500 height anomaly evolution 9

  10. Composite SLP anomaly evolution 10

  11. Composite 850 height anomaly evolution: Part I 11

  12. Composite 850 Temp anomaly evolution: Part II Large-scale Chinook effect? Are Pineapple Express events precursors to large-scale warming trends east of the Rocky Mountains? 12

  13. Case Study Methodology • Representative case selected from 46-case sample: The flood of 17-18 January 1986 • Series of cyclones moved from eastern Pacific towards Washington and British Columbia • Severe flooding occurred as result of snowmelt, heavy rain • Questions: • Which flow anomalies are responsible for moisture transport? • QG dynamics versus orographic lifting? • Piecewise moisture transport via PV inversion 13

  14. Precipitation Totals, 17-18 January 1986 14

  15. Case Study Methodology: PV • Piecewise moisture transport: • Quasigeostrophic form of potential vorticity (PV) is given by • q partitioned, piecewise geopotential obtained via inversion • where 15

  16. 00 UTC 17 January 1986

  17. January 1986 Case Study Results: • Moisture transport due to transient, cyclonic systems • Lower-tropospheric, diabatically produced PV anomalies dominate transport • Feedback hypothesized involving LLJ, diabatic PV redistribution, and warm-sector moisture transport • Models must accurately represent cold-frontal precipitation in order to account for this feedback

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