Snow Probability on Christmas Day (climatology 1991-2020) -White Christmas in North America - Christmas Snow Chance in US, Canada, Mexico

Chances of a White Christmas in North America: Declining Snow Cover Trends on Christmas Day

Cities and landscapes blanketed in a white layer: snowy scenes give Christmas extra charm. But how often does this picture-perfect scenario happen in North America? This article delves into the chances of experiencing a white Christmas in Canada, the US, and Mexico. We’ll look at current trends: Is the likelihood of snow on Christmas Day decreasing across the continent? Whether you’re planning to spend your holidays in a snowy destination or are just curious about snowfall trends near you, we’ve got the answers you’re looking for.

Check the white Christmas probability in Europe here.

To figure out the chances of a white Christmas in North America, we used ERA5 reanalysis data. This time, we’re focusing on snow cover on the 25th of December, the day when Christmas is mainly celebrated in Canada, Mexico, and the US. We’ve picked 21 UTC for our checks, which is 4 PM Eastern Standard Time and 1 PM Pacific Standard Time. Also, the way we define a ‘white Christmas’ here is a bit different from our Europe study. We’ll explain more about this in the ‘methods’ section at the end.

Odds of a white Christmas in North America

The chances for a white Christmas in North America vary strongly from north to south, with odds for a snow cover on Christmas Day generally being higher in the North, as shown in the figure below. However, elevation also plays a crucial role, with higher mountains also showing increasing odds for a snow blanket on Christmas.

Snow Probability on Christmas Day (climatology 1991-2020) -White Christmas in North America - Christmas Snow Chance in US, Canada, Mexico
Snow Cover Probability on Christmas Day climatology 1991 2020

White Christmas probability in Canada

Most of Canada, in terms of land area, almost always sees a white Christmas. But, since most Canadians live in the southern part of the country, their experience varies. In southern Alberta, some areas have a snow probability just under 50%, meaning a snowy Christmas happens about half the time. Around Toronto, the chances also hover around 50%. Vancouver, with its milder climate, has a much lower White Christmas probability (around 10%), though it’s hard to pinpoint this on the map due to the data’s coarse resolution. As you move further north, into the less populated territories and provinces, a white Christmas becomes almost a certainty.

White Christmas probability in the US

In the US, snow on Christmas is a sure thing in the highest parts of the Rocky Mountains. We also find high Christmas snow cover probabilities in the far north, reaching values over 80 or 90% regionally along the border with Canada. We see a lower white Christmas probability in a strip from Montana to Kansas.

In southern and coastal states, the chances for a white Christmas plummet to near zero. That means most places in e.g. Florida, Louisiana, Texas, and California rarely, if ever, see snow on December 25th. Interestingly, there’s a tiny light blue spot in southern Texas, near the Gulf of Mexico, indicating a slim 1-5% chance of snow – more on that later.

White Christmas probability in Mexico

In most of Mexico, a white Christmas is virtually unheard of. The chances are zero in the vast majority of the country. However, some mountainous regions in the north occasionally see snow, with the highest probabilities locally reaching up to 30%. But for most of Mexico, a snowy Christmas is either extremely rare or simply doesn’t happen.

Change in snow probability on Christmas Day in North America

Looking back to the climatological period from 1951 to 1980, the picture of white Christmases across North America changes quite a bit. During that period, snow was far more common, as can be seen in the figure below. In Canada, almost the entire country, except for some regions near the U.S. border, would typically see a snow-covered Christmas. In the northern U.S., the odds were also significantly higher, with some states bordering Canada almost always waking up to a white Christmas. In the mountainous regions of Mexico, the area with a non-zero probability of snow was more extensive than it is today. So, while snow was still a rarity in Mexico even then, a larger part of the country had some chance of experiencing it.

Snow Probability on Christmas Day (climatology 1951-1980) -White Christmas in North America - Christmas Snow Chance in US, Canada, Mexico
Snow Cover Probability on Christmas Day climatology 1951 1980

Decreases in White Christmas Probability in Northern US and Southern Canada reaching up to 30 %pt

The contrast between our two climatological periods (1951-1980 and 1991-2020) clearly visualizes the changes in white Christmas probability between those two time periods. Most notable are the declines in the U.S. Northeast, like the New England region, and in Canada’s Nova Scotia, where the chances of a snowy Christmas have plummeted by over 30 percentage points. Similar decreases in white Christmas probability can be seen in states like Montana, along the west coasts of both Canada and the U.S., and in Alberta’s southern regions. The decreasing probability is given by red colors in the figure below. The drop in snowy odds in the Mexican mountains is also evident.

A percentage point is a unit of measurement used to express differences in percentages. E.g. moving up from 40 percent to 44 percent is an increase of 4 percentage points (although it is a 10-percent increase in the quantity being measured).

Change in snow probability in Christmas in North America - Change in White Christmas Chance US, Canada, Mexico

Higher snow probability in southern states?

Contrary to the substantial snow losses in the aforementioned regions, some southern regions, including parts of Nevada and Northern Texas, show an opposite change – an increase in white Christmases. This trend is not just a projection from ERA5 data. Historical records, like this National Weather Service archive for North Texas, also support the observation of more frequent white Christmases in the recent period, tallying 8 events from 1991-2020 compared to 6 events between 1951 and 1980.

While a warming climate explains the diminished snow probabilities in the traditionally snowy north, the reasons behind the southern increases are less straightforward.

This notable study found a significant correlation between snow frequency in December in the south-central Plains and southwestern U.S. – precisely the area where we see an uptick of white Christmases – and El Niño events. The number of El Niño events during both climatological periods is similar, however, the later climatological period did have stronger El Nino events. This could potentially explain the increased frequency of Christmas snow in these regions.

Another aspect to consider is the potential role of random variability. Given that white Christmases are historically rare in the southern regions, even a few snowy events due to yearly weather fluctuations can significantly impact the overall trend. Essentially, in these areas, the ‘noise’ of interannual variability could be overshadowing the ‘signal’ of long-term climate trends, making it challenging to attribute these occurrences solely to climatic changes.

It is not only the ERA5 data that suggests more white Christmases occurred in the most recent climatological period in southern and southwestern parts of the US. This archive of the National Weather Service confirms that at least in North Texas, more white Christmas events occurred in the later climatological period. This list counts 8 white Christmas events in the latter climatological period (1990-2020), while counting 6 white Christmas events in the 1950-1980 period.

The white Christmas of Southern Texas: 2004

As promised, I would come back to the faint increase in snow cover probability in southern Texas. Upon first seeing this data, I suspected a data error, as snowfall along the Gulf of Mexico seemed unlikely to me. I decided to investigate and came across an extremely interesting weather event: the white Christmas of Southern Texas in 2004! This Christmas miracle unfolded as a cold air outbreak, accompanied by intense precipitation, blanketed the extreme southern region of the US, and even parts of Mexico, in several inches snow on Christmas Eve. For those interested in this extraordinary weather event, here is some worthwhile reading material:

Satellite Snow Cover - White Christmas Texas 2004
A Snow cover visible from in Southern Texas and just over the border with Mexico on Christmas Day 2004

Image source: Nasa Worldview

Trends in Christmas Day snow probability US and Canada

While our earlier analysis comparing two climatological periods gave us insights into shifting snow probabilities, it’s important to remember that these comparisons don’t necessarily equate directly to long-term trends. Yearly weather variations can significantly influence these periods. To gain a clearer picture, we computed trend lines for snow probabilities across North America for each data point, based on data from 1970-2022. This method, while more balanced, has its own limitations, which we will address in the methodology section.

What do these trends reveal about snow probability on Christmas Day? In the northern US and southern Canada, the trend toward fewer white Christmases is strikingly strong. Some regions here show a decrease exceeding 10%pt per decade. As for the southern states, where we previously noted a slight increase, this trend appears less pronounced when looking at the data through these trends. The signal indicating more frequent white Christmases still exists, as indicated by the blue colors in the figure below, but is considerably weaker than the decline observed in the north.

Snow Cover Probability Trend 1950-2022 on Christmas Day (climatology 1991-2020) -White Christmas in North America - Christmas Snow Chance US, Canada, Mexico - United States

Conclusion: Snowy Christmases in North America – A Changing Landscape

To conclude, our exploration of white Christmases in North America reveals varied and shifting probabilities. For those seeking a guaranteed snowy holiday, Canada’s vast northern expanses and the highest peaks of the Rockies are your surest bet. In the US, higher probabilities linger in the far north, but a notable decline in white Christmas probability is evident in the northern states and southern Canada. Meanwhile, Mexico remains largely snow-free, with only its northern mountains occasionally witnessing a white Christmas. The southern states of the US, though traditionally experiencing fewer snowy Christmases, show a slight increase in recent years, potentially influenced by factors such as El Niño events. However, as the climate continues to change, the overall trend suggests that snowy Christmases may become rarer across much of North America, making the magical white holiday scenes a less common occurrence in the decades to come.

Methods

For this examination, we utilized the ‘Snow Depth’ variable from the ERA5 dataset, a comprehensive reanalysis dataset from the European Centre for Medium-Range Weather Forecasts (ECMWF). Reanalysis involves combining past observations with modern forecast models to recreate historical weather patterns. This method provides a more complete and consistent picture of the atmosphere over time.

We downloaded this variable for the 25th of December at 21 UTC for all available years. The original unit of the data is water equivalent (m). This number was multiplied by 10, assuming a snow water ratio of 10. Consequently, this number was multiplied by 100 (to convert to cm). Our threshold for a white Christmas is set to 1 inch of snow (2.5 cm), as this is the official threshold for a White Christmas by the National Weather Service (https://www.weather.gov/iwx/whitechristmas). This differs from the 1 cm snow depth threshold we used for Europe. We selected daytime rather than the early morning (as in the official definitions), because most people celebrate Christmas in the afternoon. Note that the official definition of a white Christmas in Canada differs slightly, with a threshold of 2 cm counting as a white Christmas.

The trends in each grid cell are determined using a linear fit on the binary snow depth data, representing the presence or absence of snow. While linear fits are generally better suited for continuous data and logistic regression might be more appropriate for binary data, the linear fit method was chosen for its simplicity and computational efficiency. This approach offers a broad view of the snow cover trends, indicating general increases, decreases, or stability over time, though it is less precise in estimating exact probabilities or changes.

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