Happy Christmas Eve everybody! I hope this post finds you all in good health and cheer, or at least as good as things can be considering the times. I was fortunate enough to witness the “Grand Conjunction” last night with Jupiter and Saturn . If you missed it this year, have no fear – it will occur again in 2080. I’ll be 87 then!
We currently have mostly sunny skies over our area with just a few high clouds streaming overhead in advance of a system slated to affect us on Christmas. These high clouds will increase throughout the day, and unfortunately, we will not be able to see the Grand Conjunction tonight. Still, we can’t be picky in December – we’ll take whatever sunshine we can get. Any day with a touch of blue sky is a winner in my book.
Credit: CIRA/RAMMB
The Washington Snowstorm
Before I delve too deeply into the Christmas forecast and outlook for the upcoming week, I want to touch upon the crazy weather that Western Washington experienced on Monday 12/21, the day of the winter solstice. Seattle, and many other places in Northern and Central Western Washington, had a record daily high, record daily precipitation, and lowland snow all in the same day. Sea-Tac, for example, had a high of 59 (a record) while experiencing 0.5″ of snow. This was all due to an extraordinarily strong cold front arriving from the NW, ushering out the warm, moist, subtropical “atmospheric river” and replacing it with much cooler air originating from the Gulf of Alaska.
Credit: University of Washington
The temperature contrast with this front was impressive, but still, the cool, onshore flow alone was not nearly cold enough to give lowland snow to the area. Instead, Western Washington witnessed extraordinarily heavy precipitation rates along this cold front that helped lower the snow level all the way to sea level. Heavy precipitation lowers the snow level because snowflakes will melt as they fall into warmer air, and this change of phase from solid to liquid requires energy. The technical name for this is the “enthalpy of fusion” (also called the “latent heat of fusion”). For water, the enthalpy of fusion is 333.55 Joules/gram (or 79.72 calories/gram), so the melting of snow literally takes energy out of the atmosphere and lowers the temperature as a result. As a side note, hurricanes derive their energy from the “enthalpy of vaporization,” where water vapor releases heat (energy) as it rises, cools, and condenses into liquid cloud droplets in the eyewall. The more energy, the stronger the hurricane, which is why hurricanes prefer warm ocean water and a moist environment.
Retrieved from socratic.org
Take a look at the radar imagery of this front and the temperature graphs drops as it passed through. According to WSU post-doctoral researcher Joe Zagrodnik, the 10-degree hourly drop in temperature at Sea-Tac from 3-4pm was tied for the largest hourly temperature drop of the past decade.
Pretty clear picture of the temp drop and windshift that occurred about 15 min ago. Rainfall was quite intense as well… pic.twitter.com/yjeZuM9zQW
— Lynn McMurdie (@McMurdieLynn) December 21, 2020
Credit: University of Washington
All of this rain caused urban flooding. Rain rates exceeded 2″ per hour in spots, bringing back memories of the slow-moving, intense cold front from the “Hanukkah Eve Storm” that dropped around an inch of rain over Central Seattle between 4 and 5 pm on 12/14/2006. I remember that squall very vividly… unfortunately, my mom had the camera at the time so I wasn’t able to get any footage.
Epic flooding on Avalon Way in West Seattle @NWSSeattle @westseattleblog pic.twitter.com/EX0gPC6y9H
— Joe Zagrodnik (@joejoezz) December 21, 2020
And finally – the snow. Snow totals varied substantially throughout Western Washington and I wasn’t able to find any sort of extensive, organized “snow totals” bulletin, but from what I gather, amounts ranged from 4-5″ over the Northern Interior and above 500-600 feet to just non-sticking snow at sea level. My folks live at sea level on southern Whidbey Island and just saw some non-sticking snow, but my friend and fellow WeatherTogether blogger Wachi Suyaruenkaew measured around 4 inches at 560′ in Clearview, WA. The snowflakes were very large and heavy – this occurs when the surfaces of snowflakes melt and the snowflakes become “sticky” and form large “clumps” as they collide on their way down.
Credit: Wachi Suyaruenkaew
Your Christmas Forecast
White Christmas’ are rare in the lowlands. I don’t know the exact statistics for Portland, but as a native Seattlite, I remember hearing that there was about a 10% chance of there being a “White Christmas” on any given year, which is defined as having 1 inch or more of snow on the ground. The last White Christmas for the Pacific NW was back in 2017, and I remember this one very vividly as I had Christmas Eve dinner at my folks’ house in Seattle and then took the train down to Portland in the snow. It felt like I was riding the Polar Express, except when I reached Portland, I was greeted to a nasty freezing rain instead of gentle snowflakes.
This year is looking like a “Gray Christmas,” which, as you might expect, is the most common type of Christmas in these woods. Rain should arrive after sunrise, and periods of rain will persist throughout the day, with highs only reaching the low/mid 40s. However, the Cascades could see 6-12″ of snow above 3000-4000 feet, so folks living up by Government Camp will definitely see a White Christmas!
Credit: University of Washington
Showers persist for Boxing Day and we’ll calm down a bit Sunday through Tuesday morning as another ridge of high pressure moves over the area. But on Tuesday afternoon, another system should swing through the area, and we should stay in an unsettled pattern through the rest of the week. This pattern looks like a very typical La Nina pattern, with a ridge of high pressure in the Northern Pacific and a trough over the West Coast, giving us cooler/wetter-than-average weather and gobs of snow in the mountains.
Credit: StormVista Weather Models
It’s too early to tell how January will turn out, but if past La Ninas are any clue, the odds are tilted towards a cool and wet month with plenty of mountain snow. La Nina’s influence tends to be much stronger after the New Year, and as an avid skier, I pray it will in 2021!
Have a warm, joyous, and safe Christmas!
Charlie
