FastCast--Wednesday, Dec. 27 to Saturday, Dec. 30:
Due to storms offshore, the Northwest will be impacted by strong winds in the foothills and near gaps in the terrain, while the same offshore storm brings high surf to the coast. Overnight Tuesday into Wednesday morning, expect wind gusts near the gaps to reach 40-45 mph, with some isolated gusts up to 55 mph. These winds will cause areas of tree damage and power outages. On the coast, waves will reach 20-25 feet, bringing dangerous surf, large breaking waves, and potential coastal flooding and beach erosion. Easterly offshore flow will bring warm temperatures to the lowlands. Highs on Wednesday will reach the low 50s, but will warm significantly to the mid to upper 50s on Thursday and Friday. Temperatures will begin to decrease on Saturday, with highs back in the low 50s. Low temperatures will remain in the low to mid 40s. Additionally, expect 0.2-0.5" of rain across the lowlands through Thursday morning.
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Continue reading the full blog below!
Two different weather hazards are impacting the region...gap winds and high surf. Let's take a look at the forecast!
We'll start with the European model forecast for sea level pressure early Wednesday morning.
Notice the difference in pressure across the Cascades, with low pressure offshore. The cross-Cascade pressure difference is what fuels gap winds, and the deep low pressure offshore is what causes the large swells and high surf.
Now, let's take a look at the wind forecast, starting with the European model.
This forecast shows gap winds (E-SE winds) gusting 40-50 mph, strongest along the foothills from Eatonville to US-2. The lowlands, mainly from Auburn to Seattle, could gust 20-35 mph. The coast will also be breezy, with SE winds gusting 30-40 mph.
Let's compare this to the HRRR high-resolution forecast, also showing peak gusts through Wednesday.
This forecast shows winds in the foothills gusting 30-40 mph, weaker than the European model. The lowlands gust 25-35 mph, except 35-45 mph for Whidbey Island, Whatcom County, and the San Juans. The coast gusts up to 45-50 mph in this forecast.
Now, let's switch gears to looking at the forecast for wave heights. Below is the European model forecast for wave heights on Thursday morning.
The European model shows 20-25 foot waves reaching the Washington and Oregon coasts from late Wednesday night through Thursday.
Let's compare this to the GFS forecast, seen below, also showing waves on Thursday.
The GFS forecast shows similar waves of 22-28 feet reaching the Northwest coast from late Wednesday to Thursday.
There is good agreement on this forecast, so expect high surf, waves running far up beaches, potential coastal erosion and flooding, and be very aware on jetties and around logs.
Next, here is a look at total rain through Thursday morning, from the European model.
Expect 0.1-0.4" in the lowlands (isolated higher amounts), with 0.5-1.5" on the coast and Kitsap Peninsula. Most rain will fall by Thursday morning.
Finally, a look at high temperatures on Friday (Thursday will be similar), from the European model.
Due to warming offshore flow, expect lowland highs to reach the mid to upper 50s, with some areas reaching the low 60s. The coast will reach the low to mid 50s. Areas from Skagit County northward will be sheltered from the offshore flow, keeping high temperatures in the upper 40s to low 50s.
Stay safe in areas impacted by gap winds and high surf, and enjoy our interesting weather!
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