With persistent dry weather in March, rain-dominated rivers in coastal areas of British Columbia have been experiencing extremely low, and in some cases record low, seasonal levels, according to the River Forecast Centre.
The low levels are primarily due to the extremely dry weather in regions that typically experience wetter weather through February and March. Cold also played a role, as freezing temperatures at low elevations inhibited runoff.
Warmer temperatures towards the end of March led to melt of low elevation snowpacks. In coastal areas, this led to modest increases in streamflows, which still remain well below normal for this time of year.
In the BC Interior, warm temperatures have led to the early onset of freshet, and rivers have been receiving snowmelt runoff.
Early-April streamflow is generally above normal or well above normal across the Interior, reflecting the onset of freshet two to four or more weeks ahead of normal.
The Climate Prediction Centre (CPC) at the National Oceanic and Atmospheric Administration’s (NOAA) National Weather Service (NWS) has declared that El Niño conditions are present and sea surface temperature anomalies in the equatorial Pacific Ocean have strengthened since the beginning of February.
NWS continues to forecast a high likelihood of El Niño continuing through spring 2019, and potentially extending into the summer and fall periods. Typically, El Niño is linked to warmer winters across British Columbia, with a trend towards a lower than normal snowpack. While this year’s low snowpack follows this trend for El Niño winters, low snowpacks are not the result of warmer winter temperatures typically expected during El Niño winters; instead, this year has featured persistent colder than normal temperatures and extremely dry weather through February and March, leading to the observed low snowpack across the province.
Seasonal forecasts from Environment and Climate Change Canada favour a high likelihood above normal spring temperatures (April-May-June) across British Columbia, particularly along coastal areas.
Annual snow accumulation in British Columbia usually reaches maximum levels in mid-April, therefore the April 1 survey usually provides a good snapshot of the overall annual snowpack that will provide river runoff for the freshet season.
Currently, the snow accumulation ranges from well below normal to normal across the province. At this stage in the season there is no elevated flood risk present in the current snowpack across the province. Normal seasonal flood risk is expected in the Peace, Upper Fraser and North Thompson. With below normal snowpack in most regions, reduced flood risk is expected.
On the Fraser River, the overall basin index is 80 per cent of normal; a peak flow in the range of 6500-8000 m3/s at Hope is likely with higher flows possible if adverse weather patterns, in particular heavy rainfall, emerge in the spring. While snow is one significant aspect to seasonal flooding in B.C., weather during the freshet season also plays a key role, and flooding is possible in years with near normal or low snowpack.
In areas with low snowpack, key flood risks shift towards heavy precipitation events, either short-duration events or prolonged periods of wet weather.
It is important to note that May and June are wet months through the B.C. Interior with the potential for extreme precipitation pattern. In the Rockies and northeast B.C., upper-low weather patterns can extend the flood season into July. Therefore, it is important to note that precipitation poses a real flood risk through the spring even with limited snowpack.
Seasonal volume runoff forecasts are near-normal (85-105 per cent) for the Upper Fraser, Middle Fraser, North Thompson, and Skeena. Seasonal volume runoff forecasts are below-normal (<85 per cent) for the South Thompson, Thompson, Similkameen, Bulkley, Cowichan Lake, Okanagan Lake, Nicola Lake and Kalamalka-Wood Lake.
The snowmelt component of seasonal runoff for Vancouver Island, South Coast, and Lower Fraser is below normal and may lead to low flow issues in the summer.
Well below normal snowpack in the Northwest and Stikine is an early indication of the potential for below normal seasonal runoff. Hot weather in the second half of March has led to an early onset of the freshet season. Snowpacks at low elevation have melted, and snowmelt has been observed at mid-elevation (<1500m).
Snow density has been increasing over the past couple of weeks at higher elevations, and melt is expected to occur soon at higher elevation, unless a significant cold weather pattern emerges in the next couple of weeks. Current conditions indicate the potential for an extremely early spring melt and freshet season; these current conditions typically occur 2-6 weeks from now.
With an increased likelihood of above normal temperatures persisting through the spring, the trend of early melt is expected to continue. This suggests that the next two to four weeks may be the critical window for freshet for medium-sized and mid-elevation rivers across the province, and early-to-mid-May for larger rivers.
This is much earlier than is typically experienced, and with previously experienced warms spells in March and associated low snowpacks throughout the province, there is the potential that snowmelt will be unprecedentedly early this season.