When it comes to tornadoes, 5% is the significant threshold.
With damaging winds, 15% is the significant threshold:
And, for ≥1" hail, 15% is the significant threshold. Hatching is where hail >2" is expected.
Another large area is forecast to be affected tomorrow. Here is the combined probability map:
And, for Monday, here is the combined probability map:
And, it isn't just this severe weather pattern that is unusual.
Much of this region was in drought just a few months ago but copious rains continue to accompany the forecast tornadoes and severe thunderstorms. But, take a look at northern California. It is extremely unusual for heavy rains to fall in late June. Thunderstorms may occur with the heavy rains tomorrow and tomorrow night!