ATMOSPHERIC RIVER: Wind Event With Mountain Snow

Well. Things are about to get mighty active. A heavy rain event is imminent, especially for higher terrain and westward facing slopes; mountain snow + large coastal waves accompanying the powerful frontal system as well.

Current IR Sat Image of monster BC storm approaching the coast from a WNW trajectory; typically atmospheric rivers mean rising freezing levels, but not so in the case; massive snows on route to the Coastal Mountains!

Current IR Sat Image of monster BC storm approaching the coast from a WNW trajectory; typically atmospheric rivers mean rising freezing levels, but not so in the case; massive snows on route to the Coastal Mountains!

This system is fueled by a beefy jet stream moving across the Pacific that is looking particularly long for an atmospheric river; a pineapple express tends to be from a more SW origin and tap into the subtropics near Hawaii. 

A lot of comments about this storm and in typical west coast fashion, another fall storm evokes shrugs, so let's make this a little more interesting -- I'll break down some of the finer details and hopefully touch on what this means for you in some way or another:

Skiers/Snow Boarders: What do you care about?

  • Snow Amounts
  • Freezing levels
  •  Long range weather outlook for planning purposes (hint: more on the way next week!). 

 

It's simple. High snow amounts + low freezing levels = many happy skiers. 

(I promise that's the only equation in this post)

 

Probabilistic Outlook for Whistler Mountain and Coastal Mountain Range:

What are the chances of receiving more than 30 inches or more than 75 cm over the next few days?

Answer: Very High or Likely

Courtesy Weather Prediction Center

Courtesy Weather Prediction Center

The light blues and oranges indicate indicate 70-90% chance that those regions will see snow of epic proportions blanket the terrain, generally above 1600 metres over the coming days

What goes into such a model?

---------------------------------------------------Nerd Alert Line

21 NCEP Short-Range Ensemble Forecast (SREF) members
25 ECMWF ensemble members, randomly selected

1 NCEP North American Mesoscale (NAM) 12Z (day) or 00Z (night) operational run
1 NCEP Global Forecast System (GFS) 12Z (day) or 00Z (night) operational run
1 European Center for Medium-Range Weather Forecasts (ECMWF) latest operational run
1 Canadian Model (CMC) latest operational run
1 ECMWF latest ensemble mean
1 NCEP Global Ensemble Forecast System (GEFS) latest ensemble mean (6-h SLRs)
5 NCEP GEFS members, randomly selected

---------------------------------------------------Nerd Alert Line Ends

Almost like diversifying a stock portfolio there's a variety of deterministic and ensemble (probability) model forecasts that go into create such an intricate a balanced approach to forecasting snowfall. The impending snowfall will more than likely actually exceed 100 cm, as the ensemble approach can often smooth over extreme heavy rain and snow events.

Take a look at what could be more realistic:

50th percentile showing roughly 100 cm of snowfall forecast over a mere 72 hours; consequently, serious snow event on the way to parts of Whistler Mountain. 

50th percentile showing roughly 100 cm of snowfall forecast over a mere 72 hours; consequently, serious snow event on the way to parts of Whistler Mountain. 

Freezing Level 

Snow/Rain line likely to peak around 1600 meters on Friday while fall on Saturday -- mid-mountain temperatures hover around the 0-1°C in yellow time period. Highlighted region depicts the time period where mixed precipitation is most likely. 

Snow/Rain line likely to peak around 1600 meters on Friday while fall on Saturday -- mid-mountain temperatures hover around the 0-1°C in yellow time period. Highlighted region depicts the time period where mixed precipitation is most likely. 

Surfers/Kite Surfers/Para-gliders

  • Marine Conditions,
  • Wave Heights/Wind Directions
  • Impacts to BC Ferries & travel; especially to Tofino on Vancouver Island 

This will primarily be a section about wind. While you can avoid the snow simply by living at sea level, often the wind is unavoidable tearing umbrella inside out and wrecking havok along the BC Coast during the fall and winter months.

This is what we call a classic BC fall storm with strongest winds generally coming out of the SSE: 

Wolf Reid, Climate.Washington.edu/stormking

Wolf Reid, Climate.Washington.edu/stormking

A trailing cold front or vigorous frontal system poised to slide down the coast will funnel strong SSE winds up the Strait of Georgia and also into the Lower Mainland early Thursday afternoon. The high atmospheric water content makes this storm unique, but the micro-features of the topography will make certain wind-prone regions very windy tomorrow; yes some of you will undoubtedly lose power, so as a precaution charge your devices.

Forecast Swell:

WAM model.

WAM model.

Yes, the powerful 960 hpa low pressure system generates swells and significant wave heights of 10-13 metres in the dark red or possible waves in excess of 40 feet. Tofino, by the time the swell march towards Vancouver Island anticipate waves 3-4m in the immediate coastline with large 6-7m waves dangerously coast to the western shores of Vancouver Island. 

While northern portions of Georgia Strait will see their peak winds early in the morning, by mid-afternoon local gales surround the Lower Mainland. Delta, Surrey, the Gulf Islands, and Boundary Bay in particular need to pay attention including even areas of the Fraser Valley. High tide is after 4 pm local time, so some minor beach erosion may be enhanced, but we're luckly not in a sensitive King Tide at the time. Winds generally 40-60 km/hr over wide swath but southern section watch out for those gusts of 80-90 km/h

Model agreement is generally good, as we look into a second opinion; latest local NAM weather model run for tomorrow afternoon (see below) as strong frontal system and relative baroclinic zone moves across the coastal mountain range early tomorrow afternoon. Winds can also be enhanced to due the orientation of Puget Sound, the fetch (distance wind travels across the water), and also the downsloping of winds from the Olympic Mountains in Washington State.

Wind flow near the Olympic Mountains often plays a key role in locating some of the stronger winds in the Pacific Northwest; it's a common misconception that the terrain in BC makes weather forecasting so difficult but you can use it to your advantage.

Mountains don't move (that fast..) and rain shadows and local convergence zones can be quite predictable. 

Shaded area in red I reckon has a 50/50 shot of exceeding a 90 km/h gust at this point in time (Tsawwassen could be a likely candidate)

Shaded area in red I reckon has a 50/50 shot of exceeding a 90 km/h gust at this point in time (Tsawwassen could be a likely candidate)

Some of the cooler air aloft would allow some of the air to descend following basic parcel theory at the dry adiabatic lapse rate, would would potentially mix down some stronger winds aloft; something to watch tomorrow evening near southern Vancouver Island; an event back in January comes to mind, but those strong winds tomorrow might have a tougher time reaching the surface

Currently, the low level jet over southern Vancouver Island is modeled to be near 110 km/h and as mentioned it's possible some of these stronger winds will be able to 'mix down' and become surface gusts to 80-90 km/h 

Rainfall Forecast

*Urban flooding & local stream flooding possible*

Front likely to shift into Washington State with remaining showers still lingering, but heaviest rains tapering into Friday;Thursday evening torrential downpours for areas NE of the Lower Mainland; watch for local flash flooding during this time.

Local rainfall rates approaching 10 mm/h in some cases with terrain enhancement is entirely possible.

The problem is rain intensity will remain quite heavy before easing later Friday morning with heavier rains sliding east into the Fraser Valley for Friday, as system is relatively snow moving.

Areas near Georiga Strait should also expect much less total rainfall accumulation (40-60 mm)

24 rainfall projected rates in excess of 100 mm of on windward mountain slopes. Less for lower elevations. 

24 rainfall projected rates in excess of 100 mm of on windward mountain slopes. Less for lower elevations. 

Cheers! Tweet using the hashtag #BCstorm