Wednesday, December 18, 2019

What are the chances of a White Christmas in Victoria?

It's just one week until Christmas, so the long range forecast is accurate enough to know that the chances for a White Christmas this year in Victoria are virtually nil.  The photo below was taken on the last White Christmas in Victoria, not long ago in 2017.  It won't look like that this year.



So what is the historical likelihood of having a White Christmas in Victoria?  Environment Canada defines a White Christmas as having 2 cm or more of snow on the ground on Christmas morning.  Using that definition, there have been just 6 White Christmases in Victoria since 1914 when the Victoria Gonzales weather station opened: 1937, 1971, 1990, 1996, 2008, and 2017.  So 6 out of the last 106 years have seen snow on the ground on Christmas morning, meaning that over that period there has been a 5.7% chance of having a White Christmas.

Interestingly, the frequency of White Christmases in Victoria has actually increased in recent years.  Prior to 1990, there were just 2 White Christmases in the 76 years from 1914 to 1989 - that's just a 2.6% chance.  But in the last 30 years (1990-2019), there have been 4 White Christmases, resulting in a 13.3% chance.  Amazingly, White Christmases have been have been more than 5 times as frequent in Victoria over the past 30 years than they were in the previous 76 years!  What's even more surprising, is that this has happened as the overall amount of snowfall in Victoria has decreased: from 1914 to 1989 average annual snowfall at Victoria Gonzales was 29.9 cm, while from 1990-2019 the average annual snowfall in Victoria was just 16.4 cm.  So why has the frequency of White Christmases increased?  It's really just up to random chance - the very infrequent snowfalls that we get in Victoria have just happened to occur at or just before December 25, giving us White Christmases.  2017 was a good example of that fortuitous timing of Christmas snow: it snowed overnight on Christmas Eve, providing a White Christmas, but it was melting by evening and all gone by Boxing Day.

Victoria averages 4.7 days per year with 2 cm or more of snow on the ground.  On any given day between November 1 and March 31, there is a 3.1% chance of having snow on the ground.  This likelihood peaks during the last 10 days of December and the first 10 days of January, as shown in the chart below.  During that 20-day period, which includes Christmas, the chance of having snow on the ground ranges between 6% and 9%.

Average Frequency of Snow on the Ground in Victoria (1955-2019)














How does Victoria compare with other locations Canada?  The chart below compares the of having a White Christmas for several cities across Canada.  To make the comparison, I have used the 1955-2019 period, for which data is readily available for a number of locations.  In Victoria, there were 5 White Christmases during this 65-year period, resulting in a 7.7% chance.  Vancouver had 6 White Christmases during this period, or a 9.2% frequency.  As shown in the chart, other Canadian cities have significantly higher chances of a White Christmas: in Toronto it's 47%, while Calgary and Halifax have a 56-58% chance.  White Christmases are almost assured in most other major Canadian cities, ranging from 75% in Montreal to 98% in Winnipeg.

Chance of a White Christmas for Selected Canadian Cities
Given that Vancouver averages 8.6 days per year with at least 2 cm of snow on the ground versus 4.7 days for Victoria, it's surprising that the chance of a White Christmas isn't much greater in Vancouver.  Again, this seems to come down to random chance.  While Vancouver only had one more White Christmas during the 1955-2019 period compared with Victoria, those White Christmases have been much snowier in Vancouver.  For Victoria's 5 White Christmases, the average snow depth was 7 cm, while for Vancouver's 6 White Christmases, the average snow depth was 20 cm.

So if White Christmases are pretty rare in Victoria, what does the weather typically look like on Christmas Day?  Based on the 30-year "normals", the average high temperature is 6.8 C while the average low is 3.1 C.  The daytime high on Christmas Day in Victoria is between 5 C and 10 C 67% of the time; it's between 0 C and 5 C 23% of the time and between 10 C and 15 C 10% of the time.

On average, Victoria gets 2.6 mm of rain on Christmas Day, although there has only been measurable rain on 51% of Christmases.  The wettest Christmas Day was 1972, with 51 mm of rainfall.  Meanwhile, Victoria averages 2.4 hours of sunshine on Christmas Day, or about 30% of the 8 hours that is possible on December 25.  There has been at least some measurable sunshine on Christmas 63% of the time, so Victoria is more likely to get at least some sunshine on Christmas Day than to get some measurable rain.



Tuesday, December 3, 2019

Fall 2019 in Victoria: Cooler and Drier than Normal

We're now at the end of meteorological autumn, which runs from September 1 to November 30.  Overall, fall 2019 was slightly cooler and drier than normal.  September was close to normal temperatures while October was the coolest in 35 years.  November was slightly warmer than normal.  Normally in the fall, rainfall amounts  in Victoria increase sharply as the season progresses - November averages more than 5 times as much rain as September.  However, this fall, rainfall was much more evenly distributed throughout the season than is normally the case.

Victoria received 190 mm of rain this fall, 84% of the 30-year average of 226 mm.  Victoria received more than double its normal rainfall in September, close to normal rainfall in October, then only 52% of normal rainfall in November.   The 68 mm of rain recorded this November was tied for the 7th driest in the past 30 years, so while it was  drier than normal, it was nowhere near record-setting (just 15 mm in November 1979).


There were 36 days with measurable rainfall during fall 2019, close to the normal 37 days.  The biggest anomaly was  in November when there were just 10 days with measurable rainfall compared with the normal 18 days.  There was a 14-day dry streak from October 26 to November 8 - the longest such streak on record during the month of November at Victoria Gonzales.  Once you get into November, dry streaks are pretty rare in Victoria.  By contrast, the longest October dry streak was much longer at 33 days, ending October 28, 1987.

The chart below shows daily precipitation during fall 2019, and compares the cumulative precipitation with the 30-year average.   Cumulative precipitation was above average for most of the fall, thanks to the wet start in September and early October.  This was followed by the 14-day dry stretch noted above during late October and early November.  Some rain did return in mid-November, but not enough to bring seasonal rainfall to normal levels.  The wettest day of the fall was November 17, with 30 mm, accounting for nearly half of rainfall during the entire month of November.


Daily & Cumulative Rainfall in Victoria, Fall 2019

Victoria is usually much drier than surrounding locations in the region, but that was less case this fall.  The Victoria Airport in North Saanich normally receives about 20% more rain during the fall than Victoria proper, but this year the Airport received only 4% more (197 mm at the Airport vs. 190 mm at Victoria Gonzales).  Seattle normally receives about 30% more fall rainfall than Victoria, but this year it only got about 16% more rain.  On the other hand, Vancouver received 419 mm, or more than twice as much rain as Victoria, which is fairly typical.

Comparison of Rainfall in the Victoria Region, Fall 2019

The mean temperature this past fall at Victoria Gonzales was 10.7 degrees, cooler than the 30-year average of 11.0 degrees.  This was the first cooler than average fall since 2013 - the last five falls have averaged 11.7 degrees.  For the fall as a whole, the average daily maximum was 13.5 degrees (0.5 degrees cooler than the long-term average) while the average daily minimum was 7.9 degrees (just 0.1 degrees cooler than the long-term average).        

The chart below shows the daily maximum and minimum temperatures at Victoria Gonzales during fall 2019, compared with the 30-year averages.  Generally, daily max temperatures were slightly below the average for most of the season, except for a brief heatwave in early September and a warmer than average period in mid-November.  Daily minimum temperatures fluctuated on either side of the average.  The warmest temperature of the fall was 25.1 degrees on September 5.  The coldest temperature of the fall was a chilly -0.8 degrees on November 30, the last day of the season.  There were 2 days this fall (November 29 and 30) when the minimum temperature went below freezing.  The average first frost at Victoria Gonzales is December 26, so frost in November is earlier than normal.


Daily Max & Min Temperatures in Victoria, Fall 2019