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FLOOD RISK | B.C. Floods 2018

Significant flooding risk in B.C.: Are you vulnerable?


Tyler Hamilton
Meteorologist

Wednesday, April 25, 2018, 6:58 AM - The cool and wet first half of spring has added significant snow in the alpine, compliments of a lingering weak La Niña that spilled into 2018.

Similar to last year, large-scale flooding is likely, with this abrupt pattern change from one extreme to another over the next week — the record snowpack coupled with the forecast well above-seasonal temperatures is a major cause for concern; it would promote rapid snowmelt at all elevations of the alpine stressing most watersheds. What does this mean for your local community?


MAJOR FLOODING FACTORS

  • Highest snowpack anomaly in the province located at Mission Creek, near Kelowna, with a staggering 162 per cent of normal values
  • Freezing levels have remained below the long-term average for April; this has continued to add significant snow in the alpine
  • False start to summer anticipated over the next several days – this will add the first burst of significant runoff

Keep on top of active weather by visiting the ALERTS page.

The highest snowpack anomalies are located east of Vancouver in the Skagit, Similkameen, Okanagan, and Boundary regions of the province. Although this poses considerable threats, B.C. has just wrapped up a stellar ski season, with several resorts accumulating over 12 metres of snowfall. But, how does the current situation compare and stack up against last year's historic flooding? It's important to look back at the past, to assess the future flooding threat for the province. Using the May 1st snowpack as a benchmark, here's the snowpack relative to normal for this time period one year ago:

The dark blue shades represent the regions of greatest concern, but many watersheds across the province last year were well above the long-term average. But, what's so concerning about this year is obvious as we check the data from April 1st, that showed a significant increase compared to April 2017 – several watersheds are currently near 150 per cent of normal...

Before the B.C. River Forecast Centre releases their May 1st survey, we're already in record-breaking territory for some of the watersheds in southern B.C. including the Okanagan:

The latest snow condition summary was released on April 23rd for the province, and paints a grim picture for the expected flooding this season. In all locations in the province recorded an above normal period of snowfall in April, with several stations (six) reporting record snowpack for this date. Normally, by this time roughly 100% of the snowpack has historically accumulated by this point.


This really puts everything into perspective, as a quick glance shows what an abnormality this year has been. During the past few days, we've likely reached our peak snowpack for this season. It's all mostly melt from here on out, unless a cooler pattern develops in early May. There's also some concern for the Upper Fraser watershed near Prince George as well (~150 per cent of normal), as this will contribute to flooding conditions along the massive Fraser River watershed, even down towards the Lower Mainland. 

In 2017, this region in central B.C was near-to-below normal for snowpack, so this is another factor that must be considered for the spring flooding season, particularly along the Fraser River.

TRICKY FLOOD FORECAST: THE FACTORS

It's tricky at the best of times to predict the exact location of flooding. Spring thunderstorms, fluctuating freezing levels, and precipitation events will all occur before we transition into the dry season; consequently, it will make for dangerous water levels at times. As you're aware, thunderstorms and convective precipitation events can create massive fluctuations in water levels and can pose significant threats to life and property.

Believe it or not, temperatures in the alpine by Friday and Saturday are expected to be in the 90th percentile of what would be considered normal, as temperatures at roughly 1500 m will soar to over 15°C. 

WHAT'S THE WILDCARD?

Higher than normal water levels on the lakes are also a given with these conditions developing over the coming weeks. If we see lower freezing levels in May, paired with a drier, seasonal pattern it would be the best case scenario to limit severe flooding potential and to gradually melt the record snowpack – precipitation is the wildcard. Significant runoff from precipitation, paired with snowmelt, is a worst-case scenario this spring and is already causing problems with flood evacuations.

FALSE START TO SUMMER

Don't get too comfortable with these temperatures, as we'll cool substantially and fall back to reality in May, but it'll be possible to tack on up to 10°C above these normal values for some locations by Friday and Saturday. A false start to summer, indeed.

LONG RANGE OUTLOOK

  • A decaying, upper level trough spins itself towards Washington State this weekend, flinging showers and precipitation into extreme southern portions of the province, lingering into early next week
  • Freezing levels will fall, but it's not all good news – increased shower activity will rise some rivers in the region with the expected rainfall and the added runoff
  • Rainfall not expected to be significant, with most regions recording less than 10 mm – but, there are some discrepancies, so check back in a couple days for some revised numbers; upper lows can be pesky to track

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