We review the climatology of White Christmas conditions in Boulder, including a discussion on why Christmas Day has the second highest occurrence of snow on ground out of the entire year. We also discuss the upcoming pattern shift which will favor widespread snowfall in the Mountains and maybe a little white stuff in Boulder, too.
At a Glance:
- We review the two possible definitions of a “white Christmas” and examine Boulder’s Christmas weather in years’ past
- Why does Christmas Day have the second highest occurrence of snow on the ground for the entire year?
- The forecast for 2021 brings heavy snow the Mountains thanks to an atmospheric river event, with resulting dicey holiday travel
- Not much precipitation is expected east of the Mountains, but there is a small chance of some rain and snow in the Denver Metro area Friday into Saturday
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White Christmas Climatology
There are two acceptable meteorological definitions for the occurrence of a “White Christmas.”
- 1″ of snow present on the ground at any point during Christmas Day — this is the “official” definition as denoted by the National Climatic Data Center
- At least 0.1″ of snow must fall and accumulate on Christmas Day
Surely we all prefer the second option, with actual snowflakes flying outside as you open your gifts and stuff your face with holiday food! Old, disgusting brown snow on the ground doesn’t really tend to spread the holiday cheer, now does it?
The climatology map below from the NCDC puts the Front Range at about 40% chances of having 1″ of snow on the ground on December 25th, with near 100% odds in the western Mountains of Colorado.
Percentage chance of having snowfall on the ground on Christmas (Credit: NOAA). This is a climatology, not the forecast or specific odds for 2021.
When just considering the last 30 years, Boulder has had a 21% occurence of accumulating snowfall on Christmas Day, but a much higher 53% occurrence of some snow on the ground. The graphic below compares all occurrences of snow on Christmas Day and surrounding days since 1991. The longer-term averages dating back to the 1940’s for Boulder, 17% snowfall and 37% snow on the ground, suggest we have been relatively more fortunate in the last few decades. Though, snowfall data in Boulder prior to 1990 or so is not all that reliable or consistent.
Christmas Day snow occurrences by year are shown in the graphic below. We haven’t officially had falling snow on Christmas since 2014 or snow on the ground since 2017.
Similarly, here’s a summary graphic of how the last 70 Christmas Days have panned out in Boulder. For reference, a black coloring means that year met none of the specified conditions (i.e. it was dry, but not terribly cold or warm).
Here are some additional Santa Day tidbits for Boulder…
Top 5 Snow Depths on Christmas Day Since 1948:
- ~15″(1982, unofficial)
- 13″ (2006)
- 11″ (2011)
- 9″ (2007)
- 7″ (1983)
Top 5 Snowfalls on Christmas Day Since 1948:
- 6.7″ (2012)
- 5.9″ (2007)
- 2.9″ (1997)
- 2.5″ (1964)
- 2.2″ (2014)
How the Last 6 Christmas Days Played Out in Boulder:
- 2020: Christmas Day 2020 was absolutely gorgeous in Boulder with a high of 59 degrees under full sunshine. There were a few small snow events earlier in the month, but that was long melted before the 25th spelling out yet another “brown” Christmas.
- 2019: December 2019 was one of the least snowy December’s in Boulder history with only 3.4″ for the entire month. Christmas Day was no exception with no snow falling or present on the ground. There were quiet conditions in Boulder and partly sunny skies. The high was 50 degrees.
- 2018: No snow fell in Boulder for nearly three weeks leading up to Christmas 2018 and the day itself was rather mundane. The high was 44°F with mostly sunny skies and light winds.
- 2017: Two quick-moving light snow events dropped 7″ of new snow in Boulder in the few days leading up to Christmas 2017. With temperatures largely staying below 40 degrees, this snow persisted to Christmas Day with a depth of 4″ reported on the ground. No snow fell in Boulder on Christmas, though a favorable northwest flow pattern produced widespread accumulating snow in the Mountains this day. The high in Boulder was a cold 26°F after a morning low of 11°F.
- 2016: A strong storm system was pushing across Colorado/Wyoming this year, but the track was too far north and the Front Range got downsloped. A few convective rain/snow showers were reported in eastern Colorado, including brief thundersnow at DIA (no accumulation) which resided in the warm sector in the morning. However, it was dry and extremely windy in Boulder with gusts up to 80MPH around the area Christmas evening! The 4″ of snow that fell about a week before Christmas 2016 was long gone so there was no snow on the ground this year either.
- 2015: Light snow fell on the evening of Christmas 2015 and spilled over into the 26th totaling 2.6″. However, since Boulder’s record-keeping for the date ends at 6PM local time, there was no snow “officially” reported for the day. There was snow on the ground though as nearly a foot of snow pummeled Boulder on December 15th and due to seasonally chilly conditions leading up the holiday, 2″ of that nasty snow remained on the ground in Boulder for Christmas.
Christmas Day has the 2nd highest occurrence of snow on the ground
It may come as a surprise, but Christmas Day, at 57%, has recorded the second highest occurrence of measurable snow depth out of any day in the entire year — only bested slightly by January 4th.
Why is there a spike in snow depth on Christmas Day? Well, it’s certainly not because it snows preferentially more around the holidays. The chances of new snowfall greater than 0.1″ peaks in February and early March, and then again in early April. There is a slight uptick in snow chances in the weak leading up to Christmas, but this is still lower than other times of the year.
We do note that December is technically Boulder’s fifth snowiest month of the year with an average of 12.8″ (based on 1991-2020 data).
How much snow is on the ground doesn’t just depend on newly falling snow. It also depends on old snow sticking around for longer once it has fallen. Thus, both soil and air temperature are also key factors influencing snow depth as both contribute to melting. As it turns out, the few days leading up to Christmas have in fact been the coldest in Boulder’s climatology. Both daily high and daily low temperatures show this same pattern.
The coldest days of the year occurring in December is something observed only in a somewhat small portion of the United States. Just about everywhere east of Boulder and in the Mountains will normally see their most frigid temperatures in January or early February. The map below is from the NCDC.
In addition to the week before Christmas being the coldest of the year in Boulder, it’s also when the Winter Solstice occurs annually. This is the time of the year with the lowest sun angle and shortest days, both of which are good for preventing drooping of any lingering snowmen. Denver’s climatology shows a marginal uptick in total sun hours in December relative to November due to slightly less clouds despite the shorter days, but it’s not much. December is still a very sun-lacking time of year.
To summarize, Christmas Day having one of the highest occurrences of snow on the ground in Boulder is no surprise at all. New snowfall is relatively common during this period. It’s also the coldest part of year and the time when the powerful melting capability of the sun is most reduced. In addition, over the years there may have also been a bias among weather observers to report 1″ of snow depth specifically on Christmas when it didn’t quite measure up. Gauging the depth of old, patchy lingering snowfall can be quite arbitrary as it is. We jokingly wouldn’t blame anyone for fulfilling the hopes of the local community with an official “White Christmas” report!
2021 White Christmas Outlook
The warm and dry pattern is about to change for Colorado as a whole with an impressive atmospheric river event developing across the southwestern United States to round out the week. Deep Pacific moisture is inbound via the subtropical jet stream, or the “Pineapple Express” as some like to call it this time of year!
This moisture will begin to reach our area Wednesday in the form of high clouds, but it really starts to deepen later on Thursday which will spread consistent moderate snowfall into the High Country. A closer look at the surge of moisture on Friday as forecast by the GFS model is shown below.
Near-record moisture levels are being predicted for the entire state of Colorado on Christmas Eve — this will mean widespread snow in the Mountains with the heaviest amounts above 9500 feet in areas favored by southwesterly flow (i.e. the San Juans). Holiday travel will become dicey over any of the passes so plan accordingly Thursday evening into Saturday afternoon. The Interstate 70 mountain corridor tends to be somewhat spared by these type of events, but certainly there will still be several inches of snow to contend with up there.
We talked in-depth about this setup and the resulting snowy impacts in the Mountains in this morning’s Premium forecast discussion, but things will be much quieter around Boulder and Denver despite how much snow will be dumping just up the “hill”.
East of the Mountains, the week will end mild and mostly dry with only minor chances of rain/snow showers Friday through Sunday. The best chance appears to be during the day Friday when some jet-forcing may allow for scattered rain showers across the Denver Metro area. Friday night these showers could turn to wet snow, but any accumulation would be on the lighter side. This potential jet-banding is not easy to pinpoint right now, but do know there is an outside chance of a few snowflakes Friday night into early Saturday. We’d put the odds of accumulating snow in Boulder around 10% right now, and under 5% in Denver for Christmas Eve into Christmas Day. If you’re eager for a White Christmas, root for the Euro model (below left) which currently shows light accumulations from Denver north to Cheyenne. Just about every other model is snowless, though. Insert sad emoji here.
Temperatures will be in the 60’s Wednesday and Thursday, but start to drop with the arrival of a cold front Friday and yet another one on Sunday. Despite this, if you can believe it, each of the next five days will still have above normal temperatures for late-December — we should be in the middle 40’s this time of year!
We’ll continue to monitor the approaching atmospheric river and associated subtropical jet stream with hopes that Mother Nature can pull a snowy rabbit out of the hat for us with a few jet-forced snow bands. As of now, though, it’s not looking good for snow in the Boulder-Denver area. Stay tuned and stay subscribed:
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From all of the BoulderCAST Team,
we wish you Happy Holidays!
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