On the western edge of the monsoon high parked around the Four Corners, a well-developed disturbance has been moving northwest through southwestern Nevada. So far this has brought an extensive cloud shield with it from the Bay Area to the central San Joaquin Valley and over much of western Nevada. Here are two satellite views, the first the water vapor view and the second the visible look.
This disturbance will continue to track northwest tonight so that by dawn Tuesday it is near the Napa Valley. It then moves north to just west of Redding by mid-afternoon. There are likely to be a few showers, and in places a thunderstorm, over western Nevada and up through all the Sierra. Areas west of the Sierra probably will not be totally rain-free. In the lowlands from the northern San Joaquin Valley on north, radar will show quite a bit of returns but some of that will only be falling aloft. That being said I still expect at least spotty showers with perhaps a rumble of thunder with the best chance from the Sacramento Valley to Napa Valley on north. It does not usually rain this time of year when it does this is usually how it happens. This will, unfortunately, have absolutely no effect on the ongoing drought conditions.
Clouds and moisture will increase in southern to south-central Oregon late tonight and tomorrow, and I expect scattered showers and a thunderstorm in these areas too, especially in the afternoon.
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As of the end of June there had been no named storms in the Eastern Pacific basin.
This is some serious and dangerous heat. Outdoor activity is just not at all recommended during the daytime.
Any shower and thunderstorm can contain heavy downpours, heavy enough to cause temporary, low-lying ponding.