Saturday Evening Thunderstorm Update

A significant thunderstorm event looks likely across Western Washington from Saturday evening into early Sunday morning. This blog post will be an update on all the threats possible tonight...lightning, rain, and wind.

What You Need to Know:

• Strong thunderstorms will impact the region tonight
• Approximate timing:
• Portland: 4-7 PM
• Olympia: 7-11 PM
• Seattle/Tacoma: 8-11 PM
• Skagit/Island/Whatcom Counties: 10 PM-12 AM
• Vancouver, BC: 11 PM-1 AM
• Storms will contain heavy rain (0.5-1.5" possible), urban flooding is possible
• Severe winds (58+ mph) are a risk per the NWS, and the lowlands may see gusts as high as 40-50 mph, with some isolated parts of WA reaching 55-60 mph (mainly E. Columbia River Gorge)
• Be prepared for tree damage and power outages...charge your devices and secure outdoor furniture

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Let's dive into the details! 

We'll start with the forecast for simulated radar imagery at 8 PM. This tool allows me to compare the NAM and HRRR models next to each other.

The NAM is on the left and HRRR is on the right. At 8 PM, the NAM shows storms still somewhat disorganized and mainly over the Cascades. Meanwhile, the HRRR shows a strong line of thunderstorms moving north into the metro area.

Next, here's 9 PM.

At 9 PM, the NAM shows some storms moving into the Seattle area, with the main band moving in from the SE, located from Chehalis to Pasco. Meanwhile, on the right, the HRRR shows the main storms as far north as Everett by 9 PM, with strong storms still impacting the Sound down to Olympia.

Let's keep going, now looking at 10 PM.

At 10 PM, the NAM (left) shows the main area of storms moving into the lowlands, and also shows another area of strong storms over the Cascades and Eastern WA (from Ellensburg to Ritzville). Meanwhile, the HRRR shows much less over Eastern WA, with storms reaching Island and Skagit Counties by 10 PM.

Finally, here's a look at 11 PM.

At 11 PM, the NAM shows strong thunderstorms slamming the Seattle region, with the northern boundary near Bellingham. The HRRR (right) shows storms reaching the Olympic Peninsula and Bellingham, with more rounds of storms moving NW coming up from Oregon, plus a line of storms still over the Cascades.

Due to the potential for severe thunderstorms with severe wind gusts, the NWS Storm Prediction Center has issued a rare marginal risk for the Seattle and Portland metro areas. Today's outlook is below.

For the first time since May 4, 2017, there is a marginal severe thunderstorm risk for the Seattle region. This threat has been issued mainly due to the potential for severe wind gusts (58+ mph, by definition). 

With this wind being a potential threat, be sure to have your devices charged, take in vulnerable outdoor furniture, and be prepared for tree damage and power outages.

Now, let's switch gears and take a look at wind gust potentials. The HRRR's latest run shows a significant wind event for Western Washington due to the strong thunderstorms moving through.

This forecast is concerning, showing wind gusts of 40-55 mph across the Puget Sound lowlands, from Olympia northward to Skagit County. Whatcom & San Juan Counties could see gusts up to 50-60 mph, and parts of Eastern WA (mainly along the eastern slopes of the Cascades) will gust 40-60 mph, with the strongest gusts, possibly up to 50-70 mph, along the Columbia River Gorge from The Dalles toward Hermiston.

However, this forecast is definitely on the strong side. Below is the NAM forecast, showing an event with lesser winds.

This forecast shows lesser winds across Western Washington, with isolated areas hitting 30-35 mph. The NAM shows Eastern WA gusting 40-50 mph, strongest east of The Dalles.

Another big threat with this thunderstorm event is rainfall. A significant amount of rain will fall in a short amount of time, which is a cause for concern. Below is the HRRR forecast for rain through early Sunday morning.

This forecast shows areas from Chehalis to Whidbey Island receiving 0.5-1.3" of rain, most between Olympia and Seattle. Some areas could receive nearly 1" of rain in a matter of hours.

Let's compare this to the NAM high-resolution forecast, which is even more ominous.

This forecast shows very heavy rainfall over the lowlands, with areas from Tacoma to Whidbey Island receiving 0.75-1.5" of rain, which would cause urban flooding if it verified. The Cascades receive 1-3", which would bring a significant risk of landslides and debris flows in burn scar areas, along with small stream flooding. 

In conclusion, there is a triple threat of lightning, wind, and rain tonight. Be sure to not be outside during the storms...stay either in a vehicle or in a sturdy building. For the latest updates during the storms, follow along in my Western Washington Weather Facebook group. And, of course, enjoy the storms! This doesn't happen too often around here!