Rapid Saturday Evening Cooldown to End Warm Stretch

FastCast--Friday, April 19 to Tuesday, April 23:

Friday will be a warm and beautiful day across Western Washington. Highs will likely reach the upper 60s to low 70s, with easterly gap winds bringing warming and gusts of 30-40 mph for the foothills. High clouds will move in on Saturday, with highs reaching the upper 60s to low 70s by mid-afternoon. Then, a strong marine push combined with an incoming cold front will bring a rapid cooldown, with temperatures dropping 15-20° within 3 hours by early evening. Winds will briefly gust 30-40 mph across the region, and showers will move through. A Convergence Zone setting up between Northern King and Skagit Counties will bring a chance of thunderstorms north of Everett on Saturday night. By Sunday, conditions will be partly cloudy, with highs in the mid 50s to low 60s. Monday will be partly cloudy, with highs increasing to the upper 50s to low 60s, followed by mostly cloudy skies and highs increasing further to the mid to upper 60s on Tuesday. Overnight lows through Tuesday will be in the low to mid 40s. By the middle of next week, we'll likely start to cool down again.

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Continue reading the full blog below!

A warm and pleasant day is expected on Friday, followed by another warm day on Saturday, with a marine push/cold front combination on Saturday evening bringing a major cooldown, gusty winds, and showers. Let's take a look at the forecast!

We'll start with the NWS NBM model high-resolution forecast for highs on Friday.

Expect highs in the lowlands to reach the upper 60s to low 70s, with some areas in the North Sound remaining in the mid 60s. The coast will reach the mid to upper 60s, the Willamette Valley will reach the low to mid 70s, and Eastern Washington will reach the mid 50s to low 60s in the north and mid to upper 60s in the south.

Part of the reason that the lowlands will be much warmer than Thursday will be the presence of easterly gap winds (known as "Cascadia" winds). The NAM high-resolution forecast below shows the expected wind gusts through early Saturday.

Notice gap winds gusting 30-35 mph for parts of the lowlands, roughly from Federal Way to Seattle, with areas such as North Bend and Enumclaw reaching 35-40 mph. Also notice that the Portland metro area will have easterly gusts of 35-45 mph, strongest near the Gorge.

Now, let's take a look at Saturday's highs, also from the NWS NBM model.

Saturday will be warmer for most of the region, despite there being near 100% cloud cover due to high clouds. Expect the lowlands to reach the mid 60s to low 70s, with the coast only reaching the low 60s, the Willamette Valley also reaching the mid 60s to low 70s, and Eastern Washington reaching the mid 60s to low 70s.

However, this forecast is somewhat deceptive. Saturday's highs will occur around 2-3 PM, earlier than typical highs. Below is the NAM forecast for temperatures at 2 PM Saturday.

At 2 PM, the lowlands are in the upper 60s to low 70s. However, notice that the coast has dropped to the low 50s. Highs on the coast will occur around midday.

Around 2 PM, a marine push/cold front will be located between the coast and the lowlands, moving quickly eastward. To see this, let's take a look at temperatures by 5 PM Saturday.

By 5 PM, the marine push/cold front will have hit the lowlands, with temperatures dropping rapidly, all the way down to the low 50s in Olympia, the mid 50s to low 60s along the I-5 corridor. Notice how the foothills are still in the mid 60s at 5 PM.

However, by 8 PM (seen below), the marine push/cold front will have advanced through the entirety of Western Washington, bringing much cooler temperatures in its wake.

By 8 PM, all of Western Washington has dropped to the mid 40s to low 50s, with Eastern Washington cooling rapidly into the low 50s to low 60s.

A great visualization of the rapid temperature decrease is found on this map below, showing the NAM model forecast for temperature change from 3 PM to 6 PM Saturday.

This shows that as the marine push/cold front moves across Western Washington, temperatures will drop 15-20° within 3 hours. This temperature drop will occur across the entire region as this feature moves through.

Another impact of this feature will be a brief shot of wind through the region. In addition to strong westerly winds through the Kittitas Valley and some gusty winds across parts of Eastern Washington, gusty southwest winds will move through the lowlands on Saturday evening. Below is the NAM high-resolution forecast showing gusts at 6 PM.

Around 6 PM Saturday, as the marine push/cold front moves through the lowlands, expect winds to increase, with brief gusts of 30-35 mph, with some areas gusting up to 40 mph, especially near the water and in the San Juan Islands.

One final weather impact to be aware of with this feature is that in addition to bringing a brief round of showers from late Saturday afternoon through the evening, a Convergence Zone band between Northern King and Skagit Counties will bring a slight chance of thunderstorms late Saturday night. Below is the European model lightning forecast for Saturday night.

This forecast shows lightning potential over Island, Skagit, and Snohomish Counties. This outlines the potential for thunderstorms in this Convergence Zone band. There is also a very low, but non-zero chance for isolated thunderstorms along the main marine push/cold front as it moves across the lowlands on Saturday evening.

Stay tuned for more information via Twitter and Pacific Northwest Weather Watch YouTube (link on right side of blog) about this dynamic feature on Saturday, and enjoy the sun & warmth while it lasts!