A Preliminary Evaluation of Heavy Snow Conceptual Models for East Vancouver Island (EVI) Rodger WU and Brad Snyder Pacific Storm Prediction Centre, EC, Vancouver, BC, Canada Ruping Mo and Paul Joe National Lab for Coastal and Mountain Meteorology, EC, Vancouver, BC, Canada Vancouver British Columbia

Strait of Georgia Pacific Ocean East Vancouver Island (EVI) X X Fraser Valley 1.Large N-S region 2.Most complex terrain 3.Subject to outflow 4. Dense population Vancouver Coast Mountains

Back ground picture was taken Dec 15, 2008 at Nanaimo where 38cm snow was reported (courtesy of randsco.com) snow events (>=5 cm/24 hrs) in the past 10 year 15 extreme snow events (>=30 cm/24 hrs) in the same period EVI is prone to heavy snowfalls ( Jan 2000-Dec 2009 ) 22

Forecast challenges Warning criteria

Four principal patterns indentified (total 81 events from Jan 2000-Dec 2009) 1.Surface low 2.Warm front 3.Trowal 4.Unstable airmass The Surface Low: Extreme snow producer & forecasting trouble maker

10 Key ingredients investigated and four conceptual models developed 1. Sources of moisture 2. Arctic front 3. Surface low track 4. Georgia Strait convergence zone 5. Low-level flows 6. Vertical wind profiles 7. Stability 8. Conveyor belts 9. Banded structures 10. Upper level features Combined with pattern analyses, four conceptual models were developed to help forecasters understand physical processes and make better warning decisions: 1.The surface low 2.The warm front 3.The trowal 4.The unstable airmass

L Arctic air 500hpa Surface 700hpa Outflows 500hpa trough Arctic front 250hpa jet Surface Low GST C. Z. Enhanced vertical motion Moisture transfer Unstable Open cells Moisture pick-up Destabi- lization Heavy snow A B

A cross section along line AB Pacific Vancouver Island Ranges Strait of Georgia Fraser Valley Enhanced vertical motion Moist, unstable flows Dry, cold outflows

Preliminary evaluation of conceptual models using 10 snow events from winter For each event Identify pattern and determine relevant conceptual model Identify key ingredients depicted in the model Determine if these ingredients play roles on heavy snow over EVI

Four key ingredients Key ingredientConceptual Model Says 1. Arctic frontModerate 2. Low centre West of VI - intensitybelow 1000 hPa 3. GSTCZ*Srn Sxns - intensity35-45 knot 4. Instability - Inversion top 4KM or higher Case 1 20-Nov-10 (29 cm) Moderate West of VI 998 hPa Srn Sxns 35 knots 7 km Case 2 9-Jan-11 (18 cm) Weak West of VI 1018 hPa Srn Sxns 20 knots 5 km Case 3 22-Nov-10 (10 cm) Weak West of VI 1007 hPa Srn Sxns 25 knots 2 km * Georgia strait convergence zone

20 Nov 2010 case: - arctic front & surface low

20 Nov 2010 case: - t he instability CB top 7 km Echo top 6 km KUIL X

20 Nov 2010 case: - GST convergence Shawnigan Lake 29 cm 2 cm 5 cm

Summary Complex terrain enhanced heavy snowfalls over EVI pose great forecasting challenges to meteorologists. Based on 81 snow events over EVI during the years of 2000 – 2009, four principal weather patterns were identified and 10 key ingredients were investigated. Four conceptual models were developed aiming at assisting meteorologists to do better warning decisions. A preliminary evaluation of these conceptual models was performed by using 10 snow events that occurred during the winter of 2010 – The results indicate that these conceptual models have the capacity to provide a reliable forecasting guidance to the meteorologists.

Thank you! Questions?