Update: Thunderstorms, Snow, and Widespread Rain Ahead

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An interesting weekend of weather is ahead for the Pacific Northwest, with a significant upper-level trough bringing colder conditions to the region. Expect weekend highs to only reach the upper 50s to low 60s, significantly below average for mid-June. 

The biggest threat will be thunderstorms, with storms possible across the region on Saturday, and possible for areas Seattle southward and east of the Cascades on Sunday. 

Let's take a look at the European model forecast lightning flash density, showing the potential areas for thunderstorms Saturday afternoon.

This forecast shows the entirety of Western Washington has a chance of thunderstorms, highest over the lowlands from Thurston to Snohomish Counties. Eastern Washington has an even higher chance of thunderstorms, especially for Okanogan, Ferry, Stevens, Pend Orielle, Spokane, and Whitman Counties.

There is also a chance of isolated thunderstorms as strong showers move in early Saturday morning, from 4-7 AM. Below is the high-resolution HRRR model forecast for lightning flash density at 6 AM.

This forecast shows a chance of thunderstorms as showers move through the region early Saturday. This chance is highest for Island and Skagit Counties.

Instability in the atmosphere will be quite high by Saturday afternoon, and this is a key metric in determining if there is a good likelihood of thunderstorms. Below is the CAPE index forecast (showing amount of instability) on Saturday afternoon from the NAM high-resolution model.

This forecast shows high CAPE for Western Washington, with values of 400-800 possible, and values of 100-500 in Eastern WA. These are significant values for the Pacific Northwest, and indicate that there is a good potential for thunderstorms.

Rain will be widespread in Western Washington this weekend, with most areas receiving rain at times. Below is the NWS NBM forecast for rain through the weekend.

This forecast shows most of Western Washington receiving 0.1-0.3" of rain, with areas under heavier showers (specifically in the Convergence Zone from the King/Snohomish County line to SE Whatcom County) receiving 0.4-0.6". The Cascades will receive 0.75-2.5", highest where Convergence Zone bands move into the mountains.

The colder nature of this system will also drive down snow levels to around 4,500 feet, bringing some accumulating snow to higher passes and recreation areas. Below is the European model forecast for total snow through the weekend.

This forecast shows Paradise and the North Cascades Highway (between Diablo and Winthrop) getting up to 3-6" of snow, with higher amounts possible at Paradise. Stevens and White Passes could see a dusting to 1" of snow.

Finally, it will be another windy day for Eastern Washington on Saturday, with breezy conditions at times for Western WA. Below is the European model forecast for peak gusts through Saturday.

This forecast shows 25-30 mph gusts in the lowlands, mainly from Seattle southward. Eastern Washington will be much windier, with gusts of 40-45 mph for most of the region, except potentially up to 50 mph around the Blue Mountains. This is of concern due to the 1,150-acre Nisqually John Fire, burning west of Clarkston along the Snake River. Winds of 40+ mph will allow this fire to potentially experience significant growth on Saturday.

Remember, with active weather ahead and a chance of thunderstorms, be prepared to seek shelter if you hear thunder/see lightning, and be prepared for gusty winds, heavy rain, and even small hail. If you're in the backcountry, be prepared for cold and potentially snowy conditions. It sure won't feel like June for a few days!