FastCast—Monday, Mar. 27 to Friday, Mar. 31:
Despite being a week into spring, Monday morning will feel more winter-like, with lows dipping to the upper 20s to low 30s across the lowlands. For those with sensitive plants, be aware that frost is likely. However, high temperatures through Thursday will be on the warmer side. Expect partly to mostly cloudy days with lowland highs in the upper 50s to low 60s, sometimes cooler by the water, and warmer near the foothills. Overnight lows will be in the upper 30s to low 40s. A strong low pressure system will develop off the OR/CA border by Tuesday morning. This will cause a large difference in pressure across the Cascades, bringing strong gap winds. Winds will increase late Monday, with peak winds from mid-morning to early evening on Tuesday. As usual, these winds will be localized, with 30-45 mph gusts expected in Whatcom & San Juan Counties, 25-35 mph gusts from Mount Vernon to Everett, calmer conditions from Everett to Seattle, and gusts of 35-45 mph from South Seattle to Puyallup, with a very sharp cutoff. The strongest winds will reach 40-55 mph, in the Cascade Foothills from Gold Bar to Orting. Conditions will calm by early Wednesday morning. Rain will return to the area on Friday afternoon, with temperatures dropping as well. Friday’s highs will only reach the upper 40s to low 50s.
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A cold Monday morning is expected for Washington state, as seen below in the European model forecast.
Expect morning lows in the lowlands to drop to the upper 20s to low 30s, with warmer lows along the Sound and on the coast. Eastern Washington will drop into the mid 20s to low 30s.
Conditions will get interesting on Tuesday. A strong low pressure system will set up off the Oregon/California border, as seen below in the UW WRF model forecast for 4 AM Tuesday.
The strength of this storm (record breaking for this time of year), will cause a strong cross-Cascade pressure gradient, which will accelerate winds through the mountain gaps. I added a black arrow to the map to show how our winds will pull toward the low center.
How strong will the winds be? Let’s take a look at the high-resolution NAM forecast, showing winds at 10 AM Tuesday.
The NAM is calling for winds gusting 30-40 mph for most areas from the Fraser River Gap (Lower Mainland of BC & Whatcom County) all the way south to Puyallup, with some areas of lower winds, mainly Western Skagit County, Whidbey Island, the NE Olympic Peninsula, and the Northern Kitsap Peninsula.
Meanwhile, the HRRR high-resolution forecast has stronger winds for the region, seen at 11 AM Tuesday.
This forecast shows winds gusting up to 45 mph for the Fraser River Gap, with 25-35 mph gusts for areas from Everett to Seattle. The strongest winds in this forecast are in the foothills, from Gold Bar to Orting, with gusts of 45-55 mph, and from South Seattle to Puyallup, with gusts of 35-45 mph.
Some tree damage and isolated power outages are possible with this event, most likely in Whatcom County and along the Cascade Foothills.
One positive of these winds is that they are downslope winds, which compress the air as they descend the slopes, bringing warmth! This is seen in the European model forecast for Tuesday’s highs.
Expect a warm day across the lowlands on Tuesday, with highs in the upper 50s to low 60s, warmest near the foothills, where the effects of downslope winds are greatest.
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