Could We Have A White Christmas?

Could We Have A White Christmas?

I’ve been taking a well-needed vacation from weather blogging and have been spending some wonderful time with family and friends up in Seattle for the holidays. But with rumors of a White Christmas reverberating from Westlake Center to Sea-Tac Airport, I thought I’d weigh in on the potential for some lowland snow on the 24th and the 25th.

A note about lowland snow in Western Washington

Even though the average low temperature for this time of the year is only 35 degrees at Sea-Tac Int’l Airport, getting snow is extremely difficult any time of the year in Western Washington. To get lowland snow, you need to have cold air and moisture in the same place at the same time, but because our cold air comes from dry, arctic airmasses plunging southward through Alberta and British Columbia and our moisture comes from mild storms off the Pacific, Western Washington snowstorms are very rare indeed. Because of the rarity of snow, there are always lots of headlines when snow is in the forecast and news gets out pretty quickly. Unfortunately, since every potential snow shower gets so much hype around here, I’ve noticed that the public can actually come away with a poor overall understanding of the severity of any potential snowstorm in the forecast. And because our snowstorms can vary dramatically in size, scope, and impact, it’s pretty important that people know the type of potential snowstorm they could be dealing with.

Our late Christmas Eve/early Christmas Day snow will be minor and should primarily be confined to areas above 300-500′. The foothills above 500′ and areas north of Everett have a much better chance of seeing lowland snow. I hesitate to say that any snowfall will have minimal effect on traffic – this is Seattle after all – but this is not going to be a debilitating snowstorm that shuts down the city for a day.

4 pm 12/22/2017 infrared Satellite with overlaid 500 mb heights and temperatures
4 pm 12/22/2017 infrared Satellite with overlaid 500 mb heights and temperatures
Credit: University of Washington Atmospheric Sciences

We currently have an amplified ridge offshore, which is subsequently giving us northerly flow aloft and opening up the door for cool, continental Canadian air to spill into the region. Highs only reached the upper 30s to lower 40s around the region today, and temperatures will continue to drop tonight as modified arctic air spills through the Fraser River Valley into the region. Saturday will feature mostly sunny skies with a few high clouds in advance of Sunday’s system, and highs should struggle to reach 40 degrees. With cold air continuing to funnel through the Fraser River Valley, Sunday morning’s lows should dip below freezing nearly everywhere.

SLP, 925 mb temp, and 10-meter wind at 10:00 am PST, Sun 24 Dec 2017
SLP, 925 mb temp, and 10-meter wind at 10:00 am PST, Sun 24 Dec 2017
Credit: UW Atmospheric Sciences WRF-GFS

Things start to get interesting midday Sunday as a surface low pressure system that spun out of a dying cold front approaches the coast. The track of this low pressure system is crucial – too far south and we don’t have enough moisture, too far north and we get mild, southerly winds that will annihilate all of our hopes for a White Christmas. Models have been inconsistent with the track of this low, but the general idea is to bring it somewhere near Astoria, which would be far south enough to keep us on the cool, northerly side of the low but far north enough to provide us with enough moisture.

The University of Washington’s state-of-the-art WRF-GFS shows scattered light snow over parts of the lowlands and no snow over Central Puget Sound, which is a pretty clear indicator of non-sticking snow falling all the way to sea level, with spotty accumulations on the higher hills around the city and even heavier stuff as you get out into the suburbs.

24-hour snowfall ending 04:00 am PST, Mon 25 Dec 2017
24-hour snowfall ending 04:00 am PST, Mon 25 Dec 2017
Credit: UW Atmospheric Sciences

I’ve got a big fat Christmas party to go to, but I’ll have another update on all this snowy goodness tomorrow evening! Keep up those snow dances, and have a great rest of your night.

Charlie

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