Snowfall for 2011/2012 season is up, way up!
The BC River Forecast Centre has completed the snow survey for the year and the fact are in - it was a good season, a very good season. Considering that our friends south of the border had a less that normal season, we made out ok. Maybe not Alaskan ok but still better than average. If you are a numbers person, a snow geek or just like to confirm your suspicions then have a read to see the results for yourself....
The April 1st snow survey is now complete. Data from 154 snow courses and 53 snow pillows around the province and climate data from Environment Canada have been used to form the basis for the following reports.
The full version of the bulletin including text, data, graphs and basin index map can be viewed at the River Forecast Centre web site: http://bcrfc.env.gov.bc.ca/bul.....urrent.htm
Weather
La Niña conditions in the Pacific Ocean were observed to weaken through March. Weather patterns through the month were consistent with typical patterns observed during La Niña events. Wetter than normal weather was observed across most of British Columbia, with very wet conditions in the Lower Fraser, Columbia and Kootenay regions. Temperatures across the province were 0.5 to 1.5 ºC cooler than normal.
Snowpack
Combined cooler and wetter weather through March has led to significant growth of snow packs throughout the province. Snow packs are above normal throughout the province (Map 1). Very high snow packs (>135% of normal) are present in the Upper Fraser, Nechako, and Skeena-Nass basins, with record high April 1st snow basin index values in the Upper Fraser and Nechako, and the second highest snow basin index values for the Skeena-Nass (over approximately a 60 year record for each basin). High snow packs (>120% of normal) are present in the Lower Fraser, Columbia, Kootenay, South Coast, Vancouver Island and Peace regions. Snow packs in other areas of the province are slightly above normal. The snow basin index for the entire Fraser River basin is 131% of normal. This is the 5th highest April 1st snowpack observed since 1953, and is similar to levels observed in 2007.
BC Snow Basin Indices – April 1, 2012
Outlook
By April 1st, about 95% of the annual BC snowpack has typically accumulated. For most areas, the transition from snow accumulation to snow melt generally occurs in the middle of April, and therefore the April 1st snow survey is the considered to be the key survey of the year for assessing the impact of snow pack on seasonal water supply and flood risk.
With above normal snow packs through most of the province, above normal spring runoff volume is expected in most basins across the province. Conditions in the Okanagan-Kettle basin have improved since February with the current snow pack at near normal levels. Therefore near normal seasonal runoff is expected for the Okanagan basin.
Spring freshet flooding occurs as a result of rapid snow melt, or a combination of rapid snow melt and additional input from rainfall. Snow pack plays an important role in seasonal flood risk by providing a plentiful source of water available for melt and runoff. In years with elevated snow packs there is an increased likelihood of flooding. However, whether or not flooding occurs depends on the weather conditions during the snow melt season, and flooding cannot be forecast as a result of snow pack levels alone. The greatest risk for flooding results from above normal snow packs combined with well above normal temperatures and/or heavy rainfall during the snow melt season. Given the necessary adverse weather conditions, flooding can occur in years with or without high snowpack.
Snowmelt driven rivers in British Columbia generally reach their peak levels in May and June.
Above normal snow packs often leads to increased seasonal flood risk in many regions of the province. This year, snow packs are at levels of concern for increased flood risk in the Upper Fraser, Nechako, Columbia, Kootenay, Peace and Skeena-Nass basins. The record or near record snow packs in the Upper Fraser, Nechako, Skeena-Nass (including the Bulkley River) creates exceptional seasonal flood risk in these regions, subject to weather conditions during melt.
The current snow pack in the entire Fraser River basin is 131% of normal. The distribution of heavy snow packs within the Fraser Basin is concentrated primarily in the Upper Fraser, Nechako and Lower Fraser basin (eg. Lillooet/Harrison basin), with more moderate snow packs in the Thompson River system. Therefore, there is an elevated flood risk present through the entire length of the mainstem of the Fraser River from the Robson Valley to the Fraser Valley.
Peak flows on the South Coast and Vancouver Island regions are typically associated with major flood events during fall and winter rainfall storm events. Moderate spring flooding can occur in basins with a significant portion of the watershed at high elevation (e.g. Lillooet, Birkenhead), but spring snowmelt driven flows are typically of a smaller magnitude than fall/winter events.
La Niña conditions across the equatorial Pacific Ocean have been weakening and the Climate Prediction Centre with the U.S. National Weather Service (NOAA) is predicting the La Niña event to break down by late-April. NOAA suggests that atmospheric patterns are still indicative of La Niña conditions, and this is consistent with current seasonal weather forecasts from Environment Canada. The current 30-day deterministic temperature forecast from Environment Canada is for cooler than normal temperature across the province, except the far north and north-east. The current 3-month forecasts transition into warmer than normal temperatures through most of the province into the summer, and below normal precipitation over the same period.
Seasonal weather forecasts have some limitations with respect to assessing the potential influence on flood risk or seasonal water supply. This is partly because forecasts are for average weather over extended time periods. Flood conditions arise from extreme weather over shorter time periods, and adverse weather can arise even during periods of seasonally “normal” weather. The current seasonal weather forecast suggests two conditions which may exacerbate seasonal flood risk, especially given the high snow packs in many regions. The first is the forecast for cooler weather through April and into May. This has the potential to prolong the snow accumulation season (and lead to additional snow pack growth) and delay the on-set of the snow melt season (leading to more snow available for melt into the time of year when higher temperatures are expected). The second consideration is the forecast for a transition into warmer than normal temperatures into the late-spring and summer. A rapid transition from cooler to hotter weather during the snow melt season (May – June) is something to watch for because of its potential impact on flood generation.
The River Forecast Centre continues to monitor snow and weather conditions across the Province and when conditions warrant, provides advisories through media releases and on the River Forecast Centre website: http://bcrfc.env.gov.bc.ca/. The May 1st snow bulletin is expected to be released on May 8th, 2012.
Produced by: BC River Forecast Centre
April 11th, 2012