Warmer Weather Returns

Warmer Weather Returns

Snow in Clearview, WA during Sunday, November 5, 2017
Snow in Clearview, WA during Sunday, November 5, 2017
Credit: Wachi Suyaruenkaew

It was a fringe event once again, but many places around Puget Sound saw a second lowland snow event yesterday morning and afternoon! It wasn’t as widespread as predicted due to the center of low pressure making landfall near Grays Harbor instead of the mouth of the Columbia, resulting in warmer easterly winds during the heavy rain overnight instead of colder northerlies. Nevertheless, as cooler air filled in behind the storm, rain turned to snow in many areas, particularly those with a little bit of elevation. My friends Wachi Suyaruenkaew and Matthew Charchenko received 1.5 inches at Clearview and Novelty Hill (Redmond) respectively, and even Sea-Tac received 0.4 inches. If the low had taken a more southern track, Wachi and Matthew could have seen 3-5 inches of snow out of this storm. And since Sea-Tac saw 1.30″ of liquid equivalent precipitation between Friday and Sunday (thank you Scott Sistek for this tidbit), these two snowstorms would have crippled the city had this setup occurred around Christmas when the air was colder.

Unfortunately, all good things come to an end, and our snowy pattern is no exception. But before we kick out the arctic air completely, Eastern Washington will get one last mini arctic blast tonight.

Sea-level-pressure, 925 hPa temperature, and 10-meter winds at 07:00 am PST, Tue 07 Nov 2017
Sea-level-pressure, 925 hPa temperature, and 10-meter winds at 07:00 am PST, Tue 07 Nov 2017
Credit: University of Washington WRF-GFS model

If you take a look at the image above, you’ll notice that there is an extremely steep cross-Cascade pressure gradient as this cold air dams up against the Eastern Slopes of the Cascades. Cold air from arctic outbreaks is associated with high pressure because cooler air is denser than warmer air and a column of cool air of a given height has a greater mass than a column of warm air of a given height. There are exceptions to the cool air/higher surface pressure relationship, with upper-level extratropical cyclones being a key example. Still, the highest atmospheric pressures (adjusted for sea-level) on Earth are often found in Antarctica and Siberia, and it’s no coincidence that our “thermal troughs” in the summer are associated with our hottest days of the year.

When you get these strong cross-Cascade pressure gradients, air funnels westward through gaps in the Cascades. The easiest gap for this air to travel through is the Columbia River Gorge. The western exit region of the Columbia River Gorge gets tremendous winds in these scenarios, and any Portland weather nut knows exactly the place to go to experience the full fury of these gap flow events. Crown Point is located a few miles east of Corbett, Oregon, and commonly experiences 100 mph winds during major gap flow events. It has had gusts over 30 mph for the last 10 hours, with a peak gust of 44 mph at 5:46 pm. All this occurred with Troutdale-The Dalles gradients in the -2.5 to -3 hPa range… it’s not uncommon for these gradients to reach -10 hPa. When they do, Crown Point looks like this.

This cold pool will erode over the next few days as a front approaches the area and spreads warmer southerly winds into the Pacific Northwest. This front is associated with a large upper-level low pressure system that will drop down south from the Gulf of Alaska into the Eastern Pacific. Although the front looks impressive on satellite, it will stretch and weaken significantly before it makes landfall on Thursday.

7 pm PDT 11/6/2017 infrared satellite and 500 mb heights
7 pm PDT 11/6/2017 infrared satellite and 500 mb heights/temps
Credit: University of Washington Satellite Loops Page

Here’s the position of the trough Wednesday morning. Notice how it’s just sitting there and isn’t providing much “oomph” for any impulses it flings our way.

500 hPa heights and absolute vorticity at 04:00 am PST, Wed 08 Nov 2017
500 hPa heights and absolute vorticity at 04:00 am PST, Wed 08 Nov 2017
Credit: University of Washington

This low will slowly move eastward and deliver periods of rain to the area Wednesday morning through Friday morning. At the same time, another trough will drop down into the Eastern Pacific and spin another weak impulse through during the weekend. Total rainfall amounts from these systems will probably range from 1-2 inches in the lowlands to 2-4 inches in the mountains, with snow levels ranging between 3 and 5000 feet.

Models have shown poor agreement in the extended, but they hint at transition to a more active pattern with WNW flow. This should help us continue to build up a base in the mountains, and I have a feeling some ski resorts will be open for some Turkey Day turns!

 

Have a nice night,
Charlie

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