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Faculty of Graduate Studies

Osamu Kabayama

  • BSc (University of Victoria, 2022)

Notice of the Final Oral Examination for the Degree of Master of Science

Topic

Synoptic Controls on Summer Extreme Precipitation and Hydrological Response in the Okanagan Valley, British Columbia

Department of Geography

Date & location

  • Thursday, September 10, 2026

  • 3:00 P.M.

  • Virtual Defence

Reviewers

Supervisory Committee

  • Dr. David Atkinson, Department of Geography, University of Victoria (Supervisor)

  • Dr. Shannon Fargey, Department of Geography, UVic (Member) 

External Examiner

  • Dr. Tobi Gardner, Watershed Protection Division, Capital Regional District 

Chair of Oral Examination

  • Dr. Douglas Briant, Department of Biology, UVic 

Abstract

This study examines summer extreme precipitation in the Okanagan Valley, British Columbia, and how these events are reflected in hydrological response in the Mission Creek watershed. Although the role of large-scale circulation in winter precipitation is relatively well understood, the atmospheric drivers of summer extremes in this semi-arid, snow-influenced region remain less clear, especially when localized convective storms are involved. 

To address this problem, a diagnostic circulation-to-environment framework was employed to link large-scale atmospheric patterns with observed high-elevation precipitation and reservoir natural supply response. Daily Sea Level Pressure (SLP) anomalies from the NCEP/NCAR Reanalysis were classified using a 6 × 4 Self-Organizing Map (SOM) for the June–October period from 2007 to 2022. Extreme precipitation events were identified from high-elevation records at the Mission Creek Snow Pillow (1,780 m) using the 97th percentile threshold, yielding 69 events. 

The results show that summer extreme precipitation in the Okanagan is associated with two main groups of synoptic patterns. Low-Pressure Patterns (LPP) are linked to broad low-pressure systems, Atmospheric River-related moisture transport, and cut-off lows, which produce widespread precipitation through broad lifting and moisture transport. High-Pressure Patterns (HPP), by contrast, are linked to persistent high-pressure conditions under which strong surface heating, local instability, and local moisture supply can support localized convective extremes. Thunderstorm observations from Kelowna Airport support this convective interpretation.

Reservoir response was evaluated using records from the Black Mountain Irrigation District, a local irrigation and water-supply district in the Okanagan Valley. Extreme precipitation did not consistently produce an immediate increase in BMID Natural Supply, a reservoir-accounting estimate of inflow over each reporting interval. This weak short term correspondence indicates that watershed response depends on more than just rainfall. Forest disturbance, represented by Equivalent Clearcut Area (ECA), showed a weak positive relationship with peak flow, while SWE had a more consistent relationship with peak flow. However, peak flow per unit SWE showed no clear linear relationship with ECA, suggesting that the effect of forest disturbance is not simple or linear. 

Long-term Snow Water Equivalent (SWE) records from 1970 to 2025 showed no statistically significant trend in total snow-season length. SWE phase transition dates showed an overall tendency toward later timing, with the strongest statistical support for later snowmelt initiation. Later snowmelt initiation may increase the chance that snowmelt overlaps with summer extreme precipitation, including rain-on-snow risk under LPP conditions and heat-driven melt or convective rainfall overlap under HPP conditions. Together, these findings show that summer hydrological extremes in the Okanagan Valley cannot be understood from precipitation alone. The main contribution of this study is a workflow for tracing atmospheric extremes from large-scale circulation to mountain precipitation and reservoir Natural Supply response, and for identifying where that connection is preserved, weakened, or delayed by watershed conditions.

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