While snowmobilers have enjoyed the extended season of powder, large snowpacks may cause trouble if coupled with cool spring temperatures and a hot early summer.
The measured snowpacks in the Upper Fraser drainage basin this spring are the highest on record. The government’s records go back about 60 years.
“(The snow pack) is exceptionally high,” Campbell says.
Dave Campbell, head of the BC River Forecast Centre, says the recurring weather pattern La Nina has caused heavier precipitation throughout the past year. That seems to be backing off, he says, but if spring comes on too quickly, the melt could saturate floodplains and current river banks.
Campbell says the snowpack gives an indication of what the flooding risk will be; the rest is determined by temperatures during the spring. A warm late-spring may increase flooding in vulnerable areas. He says in 2011, the spring was cool and wet.
The Ministry warns of an elevated flood risk along the entire length of the Fraser River from the Robson Valley to the Fraser Valley. The melting season begins now and extends roughly until the end of June.
According to the April 1st snow survey and water supply bulletin, the snowpack which drains into the Canoe River is close to 200 per cent its normal size, with 192mm in the estimated water equivalent of the snowpack. The normal is 92mm.
The McBride snowpack measurement is 150 per cent the normal, with 681mm the water equivalent of the snowpack, with 456mm the normal.
Campbell says snowmelt driven rivers in British Columbia generally reach their peak levels in May and June.
The latest bulletin warns that two forecasted weather conditions 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 to trigger flooding.
Very high snow packs (>135 per cent 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 per cent 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 per cent of normal. This is the 5th highest April 1st snowpack observed since 1953, and is similar to levels observed in 2007. By April 1st, about 95 per cent 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.
The River Forecast Centre monitors snow and weather conditions and 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.