Snow Survey and Water Supply Bulletin - May 15, 2015

Snow Survey and Water Supply Bulletin - May 15, 2015

The May 15th snow survey is now complete. Data from 27 snow courses and 49 snow pillows around the province and climate data from Environment Canada have been used to form the basis for the following report.
Click here for the Snow Survey and Water Supply Bulletin as a PDF file.
  Click here for the Basin Snow Water Index Map.
  Click here for the complete May 15, 2015 Snow Survey Data.

Weather
 
The first week in May saw a cold low pressure system move across British Columbia, bringing cooler temperatures and scattered precipitation. Into the second week of the month, a blocking high pressure ridge developed over northern BC and this pattern has largely persisted. This has brought extremely warm, summer-like temperatures across the northern portions of the province (20-25˚C) and warm conditions through other areas. Thunderstorms over the period have been the primary source of precipitation through the month, leading to sporadic and localized rain events.  Wide-spread rainfall has been limited in May.

Snow Pack
 
Warm weather through early May has led to rapid snow melt across the province. Most snow survey locations across the province experienced 50-200 mm of snow water equivalent loss over the May 1st to May 15th period. Current melt rates at snow pillows range from 5-25mm/day through most of the province, with some regions experiencing up to 30-40mm/day of melt.

May 15th snow basin indices have declined through most of the province since May 1st, and range from a low of 11% of normal in the South Coast and Vancouver Island, to a high of 122% of normal in the Nechako. May 15th snow basin indices have increased since May 1st in the Upper Fraser-East, Nechako, Lower Fraser, Upper Columbia, Boundary, and Central Coast.

Most regions of the province have extremely low May 15th snow basin indices (less than 49% of normal) (Figure 1). This includes the Middle Fraser, Lower Fraser, East Kootenay, Okanagan, Similkameen, South Coast, Vancouver Island and Peace. Below normal snow basin indices (50-79%) are present in the Boundary, South Thompson, West Kootenay and Stikine. Slightly below normal (80-89%) basin indices are present in the North Thompson, Upper Columbia, and Central Coast. Near normal (90-109%) snow basin indices are present in the Upper Fraser and Skeena-Nass basins. Above normal (>120%) snow pack conditions are present in the Nechako basin.

The average of all provincial snow water equivalent measurements for May 15th is 60% of average conditions. This is the lowest province-wide average for the May 15th bulletin in the past 31 years of record.

The extremely low snow pack this season is the result of warm winter and spring temperatures that have led to a higher than normal portion of the winter precipitation falling as rain rather than snow and high rates of early season melt.

Table 1 - BC Snow Basin Indices – May 15, 2015

Basin	% of Normal	Basin	% of Normal
Upper Fraser West	No Data1	Boundary	70
Upper Fraser East	103	Similkameen	29
Nechako	122	South Coast	11
Middle Fraser	41	Vancouver Island	11
Lower Fraser	37	Central Coast	83
North Thompson	86	Skagit	No Data1
South Thompson	73	Peace	47
Upper Columbia	86	Skeena-Nass	91
West Kootenay	55	Stikine	70
East Kootenay	24	Liard	No Data1
Okanagan	41	Northwest	No Data1

1    Data is not available either because there were no scheduled snow surveys or no snow surveys were completed.

Streamflow Runoff
 
Many rivers across the province experienced much higher than normal streamflow over the late-winter and spring period. This was a result of increased runoff due to winter and spring precipitation as rain and rapid early season snow melt.

In some regions of the province, particularly where there is still significant high-elevation snow packs remaining, streamflows remain above normal as unseasonably hot temperatures have been driving rapid snow melt. In unregulated streams in the Nechako River basin, for example, flow rates are currently at 5-year to 50-year return period flow levels.

In the southern part of the province, particularly on Vancouver Island, the depletion of this seasons’ marginal snow pack has transitioned rivers to extremely low levels for this time of year. Flow conditions in mid-May this year are more similar to what is observed in July or August in an average year and is an early indication that there is a risk of extreme low flows this season if the region does not experience wetter than normal summer weather.
Snow packs are still sufficient across some regions of the province to provide on-going seasonal risk for flooding. This is the case for regions that typically experience peak levels later in the melt season, including the Fraser River and Thompson River.

Outlook
 
Current weather patterns are persistent, with a blocking ridge over northern British Columbia leading to limited flow of upper air and continued hot weather with some instability and thundershowers. This pattern is expected to continue through the weekend and into next week, with persistent above-normal temperatures. Current long-range temperature forecasts from Environment Canada indicate a high likelihood of above normal temperatures across British Columbia through the short (1 month) and long (3-month) terms.  Precipitation forecasts, which are less reliable than temperature, suggest normal precipitation over the next three months, with the exception of the eastern third of the province that may experience above normal precipitation.

Flooding is always possible during the snow melt freshet season, even in areas with normal or lower than normal snow packs. Given the snow conditions this year for most of the province, extreme weather, such as extreme precipitation or combined hot and wet weather, would be required to produce flooding or higher than expected flows in most areas of the province.  

In the Nechako and Bulkley Rivers, snow packs are nearing a point where flows on natural systems are expected to peak within the coming week. On the Nechako River near the confluence with the Fraser, the peak is expected to occur later as flows from the Stuart River are forecast to continue to rise. In the larger rivers of the province, warmer weather over the next 5-10 days is expected to continue the on-going rapid snow melt, and steady rises in river levels are expected.

With current weather conditions and outlook, the low flow season is expected to begin early this season. The extent of low flows will largely be affected by late-spring and summer temperature and precipitation. However, with the shift to earlier season low flows, there is the potential for very low summer flows in some regions unless wetter than normal summer weather occurs.

The River Forecast Centre will continue to monitor snow pack conditions and streamflow across the province and provide flood and low streamflow advisories as needed. Snow melt “Freshet” information, including stream flow mapping, is available at: http://bcrfc.env.gov.bc.ca/freshet/index.htm. The next Snow Survey and Water Supply Bulletin is scheduled for release on Monday, June 8th.

BC River Forecast Centre
May 21, 2015 

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