Cooler, stormy weather ahead in the Pacific Northwest this week
Originally written Tuesday, June 25th, talking about the final week of June...
The relative quiet of the recent weather will give way to some more turbulent weather in the coming days in the Pacific Northwest.
A large and potent upper-level low will approach the Northwest coast on Wednesday, and slowly move eastward across the Northwest through Saturday before it gets pulled into Canada.
This upper low will have two main impacts. It will lead to a stormier pattern in the Northwest, with showers and thunderstorms developing each day.
The low will also bring a noticeable cooldown to the Northwest and into California as well. This will be most noticeable away from the coast in California. Highs will go from the recent 90s in Sacramento to just the upper 70s by Thursday.
For much of the Central Valley, highs on Thursday will be 5-10 degrees Fahrenheit below average, a noticeable change since many places are running 3-5 degrees above average for the month.
Even in places like Seattle and Portland, highs will only be in the 60s late this week, which is on the cool side for the end of June.
The forecast position of the upper-level low late Wednesday from the GFS, showing what is a large storm system for this time of year.
The cold air aloft due to the upper-level low will create more instability across the Northwest over the second half of the week. Instability is created by the combination of a warm surface and colder air aloft. The temperature difference fuels the instability.
With the strong June sun heating the ground and the upper-level low coming in to bring the cold air aloft, you end up with a natural recipe for showers and thunderstorms. The mountains of the West also provide additional lift for storms to develop.
Wednesday appears to be the day with the most widespread shower and thunderstorm activity across the Northwest.
The combination of the flow around the upper low and a strong jet stream could even lead to severe thunderstorms over Montana Wednesday and Thursday, with the threat of large hail and damaging wind gusts.
A forecast for winds at around 35,000 feet, near the jet stream. This is valid Thursday, which is when severe weather will be a possibility across Montana. The purple area is winds over 125 mph, which is strong for late June. This will be a key dynamic in fueling strong-to-severe thunderstorms.
Isolated severe storms will be a possibility all the way back through Washington and Oregon, mainly east of the Cascades.
Even outside of severe storms, thunderstorms that develop under an upper-level low of this magnitude will have the potential to produce small hail. Hail is more likely in these setups because the cold air created by the low results in a lower freezing level, which makes it more likely for smaller hailstones to reach the ground.
We've also reached the time of year where lightning from these thunderstorms can spark wildfires. Places like Ellensburg, Wenatchee, Yakima and Ephrata in eastern Washington have received virtually no rain this month, so the vegetation is dry and vulnerable to fires.
Outside of the thunderstorm area, gusty winds developing as a result of the storm coming ashore will elevate the fire risk across southern Oregon, Idaho and Nevada - even as far south as northern Arizona.
As of Tuesday morning, Fire Weather Watches are up for much of southern Nevada, including the Las Vegas area, from Wednesday afternoon through Thursday evening.
That increasing wind is not going to be ideal for firefighters battling the Woodbury Fire east of Phoenix, which has now grown to nearly 113,000 acres as of Monday night. It is now one of the biggest wildfires on record in Arizona.
The strongest winds near the fire site will be from Wednesday through Friday.
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