The ridge of high pressure is starting to lose the battle as an upper low and trough approach from the west.
Currently the marine deck at the coast is huge and hardly budging and temps at beaches have been cold, some places struggling to get near 60. Elsewhere, coastal valley temps were mild today and inland conditions while still above average were also on the way down.
There is an excessive heat advisory until 7pm but you have to get out into the Carrizo Plain to find 100+ temps today.
The upper low will continue to creep eastward but not quite be here until later Friday into Saturday. So, tomorrow morning looks like this morning with coastal low clouds pushing into the coastal valleys, some modeling shows more high clouds pushing into the interior as well. But the onshore flow continues to pick up and inland temps will cool Friday, coastal low clouds should also partially clear as the inversion should weaken a lot along with better overall wind pace.
The hardest part of the forecast is to nail rainfall. It won't be much where it happens, outside of NW SLO county coastline where perhaps .25" could fall. In fact the high res modeling shows measurable precip but more from deep marine drizzle rather than along the boundary of the upper low when it moves through. Both the EURO and GFS (mid-range models) like a little rainfall, but under .10" generally.
My gut feeling is that light coastal showers and drizzle could happen but inland air is pretty dry right now, I don't think this does much away from the coast other than cool temps. It was a struggle to decide if a person should use a rain icon for an early and rather light event near the coast. I did, but just for the beaches and near coastal valleys.
One note: the low does look stronger off the coast and some isolated thunderstorms look possible off the coast Friday night into Saturday.
Winds later Friday could be strong in the interior valleys of SLO county, advisories may be needed.
The rest of the weekend looks partly to mostly cloudy and generally mild. Breezy onshore winds will continue especially during the afternoon and early evening.
Another deeper trough moves through later Tuesday into Wednesday, early outlooks liked showers with it but now the majority of models just like it for clouds and keeping things mild to cool into the middle of next week.