FastCast--Tuesday, May 21 to Sunday, May 26:
A trough swinging through the region will bring quite a change in temperatures and precipitation across the Northwest on Tuesday and Wednesday. Although parts of the lowlands will be rain-shadowed with this system, expect most of the region to receive 0.4-0.8" of rain through Wednesday, with some areas north of Everett and along the coast receiving up to 1.3" of rain through Wednesday. Mountain areas above 5,000 feet could get 3-6" of snow, with the mountain peaks getting up to 1-2 feet. This is important to be aware of if you're in the backcountry or around Paradise, Crystal Mountain, Mount Baker, and the North Cascades Highway. Regarding temperatures, expect highs to drop to the mid 50s on Tuesday, with even cooler conditions possible for some areas. Temperatures will rebound to the low 60s on Wednesday, briefly reach the mid 60s on Thursday, and drop again to the low 60s on Friday as a chance of showers returns. Some showers will linger into Memorial Day weekend on Saturday, with highs remaining cool, in the upper 50s to low 60s, with similar conditions on Sunday. This week, expect overnight lows in the mid 40s.
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Quite a change is ahead as a chilly upper-level low (trough) brings cooler and wetter weather to the Northwest, with rain likely on Tuesday, lingering into Wednesday. Let's take a look at the forecast!
We'll start with the European model forecast for total rain through Wednesday night, seen below.
This forecast shows most of the lowlands receiving 0.4-0.9", with areas from Everett northward and in the foothills getting 1-1.5". The coast will receive 0.9-1.3", areas from Olympia to Portland will receive 0.4-0.6", and the mountains will receive 1-2". Eastern Washington will receive 0.1-0.3", with 0.4-0.8" in the Selkirk and Okanogan Mountains, Blue Mountains, and along the Idaho border.
Notably, the typically rain-shadowed areas will get the full brunt of this storm, which is moving in from the northwest instead of the southwest. This will shift the rain shadow southward, over the Kitsap Peninsula and potentially over areas including Tacoma and Olympia. We see this more in the NAM high-resolution forecast below, also showing rain through Wednesday night.
This forecast shows a more pronounced rain shadow across the lowlands, with areas west of Puget Sound getting 0.05-0.2" of rain (including Shelton, Bremerton, and Olympia), while the metro area gets 0.4-0.7". This forecast shows areas north of Everett getting 0.9-1.2", while the coast and mountains receive 1-3". Eastern Washington gets much less (0.1-0.25") across the entire area, except 0.4-1.5" around the Blue Mountains.
This system will be on the chilly side, so some (very) late season mountain snow is possible, mainly above 5,000 feet. Below is the European model forecast showing total snow through Wednesday night.
This forecast shows areas over 5,000 feet, including the North Cascades Highway, Mount Baker, Crystal Mountain, and Paradise, getting 4-8" of snow, with a potential for more at Paradise. Be aware of potential snow if you are traversing the now-open North Cascades Highway.
Temperatures will also be quite chilly across the region with this system. Below is the NWS NBM forecast for highs on Tuesday.
Notice that lowland highs will only reach the mid 50s, quite chilly for late May. The coast will reach the mid to upper 50s, with the Willamette Valley in the upper 50s to mid 60s, and Eastern Washington in the mid 60s to low 70s.
Highs will increase a bit in the lowlands on Wednesday, as seen below.
Expect highs west of the Cascades to reach the upper 50s to low 60s on Wednesday, with Eastern Washington cooling down as the trough moves east, with highs dropping to the mid 50s to mid 60s.
Finally, what's with this "trough" that is causing this weather? Below is a look at air pressure at 18,000 feet, from the European Ensemble forecast.
Notice the trough moving directly over Washington. In winter, we'd likely be talking about a major cold spell and potentially even lowland snow with this pattern, but in late spring, with the sun near its strongest time, we just get a "chilly" day and rain/high mountain snow moving through.
Stay tuned as we get closer to seeing an accurate forecast for Memorial Day weekend!
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