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

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 January 1986  Methodology: Piecewise moisture transport  A moisture transport feedback  Anticipation of model biases

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 ft 3 s -1.

65 Methodology and Case Selection Results Six-day composites generated from NCEP CD Anomalies: deviations from 27-year weighted climo 46 cold-season events from : –November 18 –December 12 –January 8 –February 5 –March 2 Tolt: Less sensitivity to temperature criterion –November 3 –December11 –January17 –February 5 –March 2

7 Composite 500 height and SLP evolution

8 Composite 500 height anomaly evolution

9 Tolt Composite 500 height anomaly evolution

10 Composite SLP anomaly evolution

11 Composite 850 height anomaly evolution: Part I

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?

13 Case Study Methodology Representative case selected from 46-case sample: The flood of 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

14 Precipitation Totals, January 1986

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

00 UTC 17 January 1986

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 January 1986 Case Study Results: