Snowfall Autopsy and the Silent Killer (Plus a Look Ahead)

Quick update on our last snow event this past weekend. As expected (mostly),  very little snowfall accumulation was observed at sea level with the exception of Squamish.

Quite the learning experience for forecaster's and highlights how dangerous it is to rely on a single piece of guidance such as the one above

Quite the learning experience for forecaster's and highlights how dangerous it is to rely on a single piece of guidance such as the one above

Areas inland saw a slushy mix including portions of Surrey, and especially in the Fraser Valley.

Some guidance performed well (HRRR, Euro deterministic), while other models such as European ensembles (models run many times over with small changes), the University of Washington mesoscale models, SREF (short Range-Ensemble forecasting), and the WPC Probabilistic simulation did not perform well and overdid snowfall amounts closer to sea level.

The culprit for the WPC Probabilistic forecasting being so erroneous is likely the weight towards the European ensembles -- have a look:

A multi-model ensemble is utilized to create a distribution of values around the WPC accumulation at each grid point. The typical constituency of this ensemble is as follows:

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

Source: http://www.wpc.ncep.noaa.gov/pwpf/about_pwpf_products.shtml

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57 Total members

Out of the 57 total members that make up the probabilistic ensemble, over half indicated a decent snowfall for #YVR, which was tough to ignore completely, especially with the respected European ensembles on board for a significant event. 

the 4KM-WRF also showed plenty of snow down to sea-level which was also a challenge to completely ignore, as this computer model has typically handled snowfall events fairly well in the past. 

In the end, I made the decision that very little snow would accumulate at sea level, but left open the opportunity as models were not unanimous.

Looking Ahead

#FOGCOUVER

Fog forecast for early this morning, highlighting the serious threat of fog for the Pacific Northwest. 

Fog forecast for early this morning, highlighting the serious threat of fog for the Pacific Northwest. 

Remember, fog can be especially dangerous, and in the Pacific Northwest fog is the number one killer (think automobile accidents), outnumbering all other severe weather threats. I'll admit, it's shocking and goes against intuition.

A fog advisory is in effect, and should always be taken seriously. Although fog isn't as exciting as a severe wind storm and its threat is more covert, precautions should always be taken. 

Current fog advisories (6:00 AM PST)

  • East Vancouver Island
  • Metro Vancouver
  • Sunshine Coast

"When people think of severe weather, they often think of electrical storms, but really, the limited visibility of fog and snow squalls are the real severe weather and pose some of the greatest hazards for travel"

- Meteorologist Kerry-Ann Lecky Hepburn

What's the root cause of all this fog, Vancouver? Well the recent rains have provided a true soaking, with lots of trapped low level moisture.

Don't believe me? Take a look at a recent atmospheric profile derived from satellite data.

Of note, take a look at the high freezing level. Typically temperatures decrease with height, but the red line moving towards the right shows temperatures increasing with height, a sign of stability. 

Of note, take a look at the high freezing level. Typically temperatures decrease with height, but the red line moving towards the right shows temperatures increasing with height, a sign of stability. 

Some of the highest resolution models indicate that YVR will be socked in all afternoon (see below), and by this afternoon low level clouds will likely persist. Models in the past have struggled with showing when fog will dissipate or mix out due to the limitations of vertical resolution of weather models, but slowly they are improving.

Do the UW weather models agree?

High resolution NAM, showing completely sunny conditions for #YVR today... A likely outlier and a potential epic failure. 

High resolution NAM, showing completely sunny conditions for #YVR today... A likely outlier and a potential epic failure. 

High resolution GFS doing a little better of a job but is still likely missing fog for Northern Georgia Strait and Puget Sound

High resolution GFS doing a little better of a job but is still likely missing fog for Northern Georgia Strait and Puget Sound

Long Ranger

Long Range (Day 7-14) includes a very strong signal for above normal temperatures for coastal B.C

Long Range (Day 7-14) includes a very strong signal for above normal temperatures for coastal B.C