Ministry of FLNRO

Snow Survey and Water Supply Bulletin - May 15th, 2016

The May 15th snow survey is now complete. Data from 26 snow courses and 62 automated snow stations around the province, collected by the Ministry of Environment Snow Survey Program and partners, and climate data from Environment Canada have been used to form the basis of the following report.

Click here for the Snow Survey and Water Supply Bulletin as a PDF file.
Click here for the complete May 15, 2016 Snow Survey Data.
Click here for the Basin Snow Water Index Map.

Weather
Weather through the first half of May has been generally warm and dry across British Columbia, with periods of atmospheric instability and showers. Temperatures have continued to be warmer than normal, with extremely warm weather early in the month. Precipitation across the province has been below normal, with some areas in northern BC experiencing wetter than normal conditions.

Snowpack
The provincial snow pack has continued to melt at a rapid rate. Most snow survey locations experienced 100-300mm of snow water equivalent loss over the May 1st to May 15th period, with current melt rates of 10-20mm per day being observed at most Automated Snow Weather Stations.

May 15th snow basin indices have declined since the May 1st indices, with the provincial average dropping from 53% to 39% over the period. The 2016 May 15th provincial average basin index is a new record low (measured since 1980). May 15th indices are extremely low (<60%) across the province, except in the North Thompson, South Thompson, and Upper Columbia, where indices are in the 70-86% of normal range for this time of year.

The low May 15th snow basin indices reflect the accelerated melt of the snow pack this season due to the extremely warm spring, rather than a lack of seasonal accumulation. May 15th snow packs this year are more typical of mid-June conditions, indicating that snow melt this season continues to progress about four weeks ahead of normal.


Table 1:  BC Snow Basin Indices – May 15, 2016


Basin

% of Normal

Basin

% of Normal

Upper Fraser West

NO DATA1

Boundary

55

Upper Fraser East

23

Similkameen

12

Nechako

45

South Coast

57

Middle Fraser

21

Vancouver Island

35

Lower Fraser

49

Central Coast

21

North Thompson

80

Skagit

NO DATA

South Thompson

86

Peace

38

Upper Columbia

70

Skeena-Nass

25

West Kootenay

44

Stikine

1

East Kootenay

12

Liard

NO DATA

Okanagan

35

Northwest

NO DATA

  1.  ‘No Data’ indicates that no snow surveys were conducted within the basin during this survey period

Streamflow
Freshet runoff echoes the pattern in snow melt, with many rivers experiencing flow conditions that are 3-4 weeks or more ahead of normal conditions. Since May 1st many rivers have experienced a transition from well-above normal flows, to near-normal or below-normal flows as of mid-May.

Outlook
Strong El Niño-Southern Oscillation (ENSO) conditions that developed over the equatorial Pacific regions over the past winter are now rapidly declining, and are expected to reach neutral conditions over the next couple of months. The Climate Prediction Centre at the U.S. National Weather Service/NOAA has issued a “La Niña Watch” as modelling is indicating a high likelihood of La Niña conditions developing through the summer and fall of 2016. In the northern Pacific Ocean, temperature anomalies continue to decline offshore, and increase near-shore to British Columbia, indicating a potential warm-phase Pacific Decadal Oscillation (PDO) pattern.

Seasonal forecasts from Environment Canada are indicating an increased likelihood of above-normal temperatures across British Columbia over the May to July period, and into the extended forecast period of the late summer months. The warmer than average seasonal forecast is consistent with historic weather typically observed during positive in-phase ENSO and PDO conditions.

Most rivers have likely experienced their peak flow level for this season, and freshet flows are now receding. In rivers with a large proportion of high-elevation terrain, including the South Thompson River and upper Columbia River, the peak of the freshet is expected over the next few weeks. Seasonal flood risk due to snow melt is now limited across the province, however flooding could still occur from significant rainfall events.

The advanced freshet is expected to put pressure on summer low flows in snow-melt dominated rivers across the province. In many of the smaller and low- to mid-elevation watersheds of the province (e.g. in Central Interior and South Interior), the transition to seasonally lower than normal flows has begun, and the trend is expected to expand to larger watersheds over the next few weeks. The influence of the snowmelt season occurring about a month early this year is expected to continue through the summer. While the volume of runoff is expected to be near normal throughout this year’s freshet in most areas, the majority of this volume will occur through April and May, rather than May and June, as normally occurs. While the impact of this will vary from river to river across the province, the proportion of flows in June, July and August that are derived from snow melt will be greatly reduced. In the northeast and in lower elevation coastal watersheds, snow melt usually plays a minor role in summer flows, and rainfall is particularly important for determining the flows that are experienced through the summer.

For both spring flood risk and summer low flows, snow pack is just one of the important elements that determine whether or not extreme conditions will emerge. Extreme wet or dry weather can significantly impact the likelihood of peak and low flows. If the remainder of the spring and summer has near normal precipitation, below normal flows are likely throughout the province over the summer.

Current forecasts for the week suggest cool and unsettled weather. Significant rainfall is not expected to either pose localized flood risk, or ease the trend towards earlier low flows. The River Forecast Centre is modelling streamflow across the province. Information regarding freshet conditions, including hydrologic model forecasts, is available on the Freshet page on the RFC website. Seasonal information on drought and drought levels is available on the BC Drought Information Portal.

The River Forecast Centre will continue to monitor snow pack conditions and will provide an updated seasonal flood risk and stream flow forecast in the June 1st 2016 bulletin, which is scheduled for release on June 8th.

BC River Forecast Centre
May 24, 2016