Mid-April is the transition point from snow accumulation to the beginning of melting at high elevations. That’s not to say we can’t get accumulation of snow into June in the high alpine, but for the most part, the major accumulation is over.
This makes the April Snow Survey and Water Supply Bulletin from the Ministry of Forests, Lands and Natural Resources’ River Forecast Centre a fairly good guide to how much snow there will be to melt in B.C.’s watersheds this coming freshet.
The report, issued on April 8, indicates that there are areas of concern in the province such as Vancouver Island where the snow pack is only 60 per cent of normal, which could lead to water supply concerns this coming summer, to the Upper Fraser where the snow pack is 136 per cent of normal.
The River Forecast Centre considers anything above 120 per cent to be above normal and the East Kootenays is just skirting that with snowpacks at 118 percent of normal. Last year, at the time of the April report, East Kootenay snowpacks were at 91 per cent of normal.
But weather plays a huge part as well and last year, heavy rains in late June led to severe flooding as the high elevation snow pack came down very quickly.
“It is important to note that snow pack is only one element that influences whether flooding occurs during the spring freshet,” the report says. “Of critical importance are how the snow melts and how much, and when, precipitation is received during the snow melt period. Therefore weather during the melt season is the key driver that determines if flooding will occur or not. Analysis of snow pack should not be considered a forecast of upcoming spring flood conditions; rather it is an assessment of whether or not flooding is more likely to occur. Heavy snow packs lead to an increased likelihood, or risk, of flooding, however flooding can occur during years with normal snow pack and conversely years with heavy snow pack do not always lead to flooding.”
Current medium-range forecasts from the North American Ensemble Forecast System indicate a high likelihood of above normal temperatures through the April 15-22 period, the report says. This extended forecast suggests that the seasonal transition to snow pack melt will likely occur over the next couple of weeks.
The next seasonal water supply and flood risk assessment will be released on May 8.