1
Heavy Cold-Season Precipitation in British Columbia, Washington and Oregon
MSC/COMET Presentation, 23 February 2001
Gary M. LackmannDepartment 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 16-18 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 ft3 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 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
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 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
14
Precipitation Totals, 17-18 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
n
iiqfQGPVq
1**
iq*1'
pp
ff r
110
2
0
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:
