Lowland Snow for NW Washington on Sunday?

Lowland Snow for NW Washington on Sunday?

Thanks to an atmospheric river last Sunday that brought flooding to many rivers flowing off the Olympics and Washington Cascades and a strong ridge just offshore for the first half of this week, we’ve had mild temperatures to begin February. Temperatures have been particularly warm east of the Cascade crest – in fact, Walla Walla didn’t dip below their average high temperature for this time of the year until February 9th. Fellow WeatherTogether blogger Karl Bonner of The Dalles, Oregon, reported that the average temperature for the first 7 days of February at The Dalles was 54.0 degrees (60.3 for highs, 47.7 for lows). These averages are closer to the averages for the third week of April!

Average highs and lows and measured temperature at Walla Walla over the past two weeks
Average highs and lows and measured temperature at Walla Walla over the past two weeks
Credit: University of Washington Atmospheric Sciences

But our pattern is changing. The aforementioned strong ridge offshore has dissipated, and we are seeing cool, northwest flow on the back edge of a new ridge ahead of a deep low south of the Aleutian Islands.

Infrared satellite with overlaid 500 mb heights and temperatures
Infrared satellite with overlaid 500 mb heights and temperatures
Credit: University of Washington Atmospheric Sciences

As you can see, there is a small “kink” in the 500 mb height lines (the green ones). This kink represents a “shortwave trough,” which simply refers to the nature of the disturbance in the jet stream. Compare this to the much larger and broader “longwave” trough over the Continental US.

This trough will pass through late tonight (Friday)/early Saturday morning and will usher in a cooler airmass. Though we will see bright, sunny skies on Saturday, temperatures will range from the mid 40s in Bellingham to the low 50s in Portland. Though these temperatures are near normal (below to the north, above to the south) for this time of the year, a moderate northerly breeze will make things feel a wee bit chilly, especially considering the warmth we had to begin the month.

A more vigorous shortwave will approach the area from the northwest Sunday morning. With cool air already in place and this system predicted to make landfall near the mouth of the Columbia, there is the potential for some light snowfall in northwest Washington.

The shortwave offshore at 5 am Monday morning
The shortwave offshore at 5 am Monday morning
Sea-level-pressure, 10-meter winds, and 925 mb temperatures at 11:00 am PST, Sun 11 Feb 2018
The surface low approaching the mouth of the Columbia midday Monday. Image shows sea-level-pressure, 10-meter winds, and 925 mb temperatures at 11:00 am PST, Sun 11 Feb 2018
Credit: UW Atmospheric Sciences

The state-of-the-art 1.3 km WRF from the University of Washington shows approximately an inch of snow falling over Whatcom and San Juan Counties on Monday morning, with trace amounts above 500-1000 feet from mid-Snohomish County northward.

3-hour precip/snow ending 7 am Sunday 2/11/2018
3-hour precip/snow ending 7 am Sunday 2/11/2018
Credit: University of Washington
3-hour precip/snow ending 10 am Sunday 2/11/2018
3-hour precip/snow ending 10 am Sunday 2/11/2018
Credit: University of Washington

Because we are now approaching mid-February and the flow with this system is from the northwest and not from interior BC, temperatures will be on the fringe for snow. Moreover, the timing of this system (as shown by the model above) is ideal for snow, as precipitation begins to fall near sunrise when temperatures are near their coldest. As such, a slight shift in timing could preclude any chance of snow for Northwest Washington.

Due to shadowing from the Olympics, the Seattle metro area won’t have any moisture to deal with in the first place, and Portland will see warmer southerly winds ahead of the low with snow levels closer to 2,000-3,000 feet.

 

It becomes very hard to get even a dusting after Valentine’s Day, but it can certainly be done, even for areas south of Bellingham. Portland Int’l Airport saw 0.5 inches of snow from March 21-22nd 2012, and the latest snow downtown occurred on April 14, 1924 with a paltry tenth of an inch. Amazingly, 5.2 inches of snow fell in downtown Portland on April 1, 1936!

Have a great weekend, and enjoy the sunny day tomorrow! If you are in Northwest Washington, hopefully you’ll be treated to some light snow to begin your Sunday.

Thanks for reading,
Charlie

 

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