Mountain thunderstorms wind down across the West this week; Cooldown coming to the Northwest
While the West Coast has been getting back to more typical late-spring weather of late with temperatures bouncing back and the marine layer returning, the weather has been active across parts of the interior.
Thunderstorms have been erupting daily across the high terrain of the West, and have brought flooding downpours and frequent lightning. Here is a plot of lightning strikes from just Sunday, showing over 10,000 lightning strikes in a 24-hour period.
You can even find the lower end of the San Joaquin Valley pretty easily by looking at the gap in the lightning strikes from the storms that popped in the surrounding mountains.
The storms have also turned severe in some cases. In fact, the NWS in San Francisco issued its first severe thunderstorm warning (for a very rural part of Santa Clara County) in over three years. That was the longest streak without one in the county.
With the upper-level trough still sitting over the Southwest for the next few days, these daily thunderstorms will continue to pop. This means the threat of more drenching downpours across the higher terrain. Flash flooding will be the main concern with the storms that fire.
Some of the strongest storms could also contain hail and gusty winds.
As the trough drifts eastward during the first half of this week, the thunderstorm coverage will decrease across California a little each day, especially by Tuesday and Wednesday.
A ridge will try to build into California around Wednesday and Thursday, but a trough moving into the Pacific Northwest will squash that. Even so, since this trough will be more progressive and more focused in the Northwest, we don't expect quite the coverage of daily thunderstorms that we're seeing now.
The question is how far south will the trough dig. The European has been digging the trough a little farther south than the GFS, which would bring the thunderstorm threat into Northern California and Nevada. The GFS solution would keep things more bottled up in the Pacific Northwest.
This trough will also help bring some cooler weather into the Northwest as well. We've talked about how cool it's been lately in the Southwest, with places like Phoenix and Las Vegas ending the month of May nearly 6 degrees Fahrenheit below average.
In California, the departures were not quite as significant, but it was still cool. Downtown Los Angeles was 2.5 degrees below average for the month, while San Francisco was a degree off the average.
In the Northwest, though, it's been a different story. Seattle finished the month 4.5 degrees above average, fueled by a very warm and dry stretch of weather early in the month. Highs will only be in the lower and middle 60s by late this week in Seattle and Portland while the trough is overhead.
Despite this being a pretty sharp trough, it doesn't look like it will be a big rainmaker.
Moderate drought (the tan color) is being indicated across much of western Washington in this past week's Drought Monitor. Abnormally dry conditions are shown in yellow, while no drought is occurring from the Yakima Valley to the Walla Walla Valley.
In recent weeks, the "moderate drought" area on the weekly drought monitor has expanded from the northern Cascades back through the Puget Sound and across the Olympic Peninsula thanks to the dry spring. Seattle received just 32 percent of their normal rainfall in the month of May.
With this in mind, any rain is good news for the area.
Some weak ridging is expected to build in after that, which could limit rain chances in the Pacific Northwest following the trough this week.
Even with the ridge trying to build in, there may be some ripples of energy that move through. These may re-ignite the thunderstorm chances across the higher terrain of the West at times.
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