The Central Coast snowpack is sitting at 46 per cent above normal as of April 1, up an additional 25 percent since the last reading March 1. The 2021 snowpack throughout British Columbia is above normal, with many regions way over their normal values.
The average April 1 percentile for all sites within the Central Coast region is the 91st percentile. This means that historical April 1st values for these sites in aggregate were below current values 91 per cent of the time in the historic record, and snowpack was higher than current values nine percent of the time. The region with the highest average percentile is the Northwest (99th percentile). The region with the lowest average percentile is the East Kootenay (45th percentile).
The report indicates that La Niña conditions began in fall 2020 and are beginning to ease. According to the Climate Prediction Center (CPC), there is an 80 per cent chance of transitioning from La Niña to ENSO-neutral conditions during May-July 2021. La Niña occurs when oceanic temperature anomalies along the equatorial Pacific Ocean region are below normal for an extended period. Historically, La Niña conditions create cooler temperatures for British Columbia and wetter weather in the South Coast and Vancouver Island during the winter and early spring months. Recent La Niña years (2017, 2012, and 2011) have seen significant flooding.
By early April, approximately 95 per cent of the annual B.C. snowpack has typically accumulated. The risk of spring flooding is elevated due to the above normal snowpack across the entire province. While snowpack is one risk factor for freshet flooding, snowpack alone cannot predict whether flooding will occur or not. Spring weather is also a critical flood risk factor, where the timing and severity of temperature and rainfall patterns are important drivers of flooding irrespective of snowpack levels.
The first week of April was relatively cool with a couple of minor storm systems moving through the province. The second weekend of the month is forecast to remain cool with another low-pressure system affecting many parts of the province. A major change in weather pattern is forecasted starting April 12, in which a high-pressure ridge will likely result in very warm temperatures for the province.
By early April, nearly 95 per cent of the annual BC snowpack has typically accumulated. Peak provincial snowpack usually occurs in mid-April. Warmer temperatures beginning the week of April 12th are expected to initiate the freshet season with low elevation snow melt and corresponding rises in streamflow, particularly in smaller creeks and low elevation areas.
While snowpack is one risk factor for freshet flooding, snowpack alone cannot predict whether flooding will occur or not. Spring weather is also critical, where the timing and severity of temperature and rainfall patterns are important drivers of flooding irrespective of snowpack. Spring freshet poses a seasonal risk across the BC Interior.
Scenarios that could exacerbate flood risk this year include prolonged cool weather followed by a rapid shift to persistent hot weather (particularly in May), or persistent wet weather or extreme short-term rainfall. Favourable scenarios would include continued dry weather and seasonal temperatures.