10:52pm - update from Chief Meteorologist Dave Hovde:
The newest modeling and hurricane tracking reinforce that current hurricane Kay near the tip of Baja California will likely be a factor in the weekend weather. Rainfall in SoCal looks likely and could be locally heavy. For the Southcoast in SB County rain looks likely with amounts up to .50" (but likely a little less than that). Central Coast rain is not out of the question but amounts should be much lower.
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Our weather has been on repeat for about a week now with inland heat scorching, near records. Coastal valleys are only narrowly missing advisories for the next few days but now there is high confidence that Friday could be another scorcher inland AND at coastal valleys, only beached will remain out of excessive heat concerns Friday.
The cause remains unchanged: a huge and powerful ridge of high pressure is in firm control of weather in The West. The ridge pushed the main branch of the jet stream into northern Canada where the cool air is bottled up for now.
Friday is interesting. The main branch of the jet shows a trough over Montana and North Dakota with an amplified ridge moving back into the PacNW.
At the same time hurricane Kay (or a tropical storm by then) looks to be positioned near the CA/MEX border, offshore. This will also help encourage some offshore flow Friday helping to fuel the expected heat spike.
The heat spike Friday will mean a new excessive heat warning for coastal valleys, with highs potentially in the upper 90s and 100+ for some. The interior is currently covered in an excessive heat warning thru Wednesday PM, but it is likely that will also be extended for the Friday event.
The weekend is also very interesting in that whatever is left of current hurricane Kay will be drifting offshore of the LA Basin. It looks to be close enough to bring high clouds, muggy air, and showers and thunderstorms into California, and even the Central Coast.
I think the Southcoast has the best chance of rain and thunderstorms. I think as far north as Santa Maria the rain and shower chance is something significant. It could rain north of there but is less likely. Max rain potential: I've seen some model output up to and exceeding .50" but I think it'll be less than that (unless thunderstorms are the delivery method in which case higher local potential exists). I think most rain looks to be under .25" but it is really early to predict with more precision due to the nature of hurricane Kay.
The net result will be clouds and lowering pressure will likely result in the needed cooldown. Weekend temps look near average. Stay tuned.
Again, this is still a dangerous heat event. We again remind you to stay informed. Please pay attention to your physical well-being and that of others, specifically those in sensitive age groups or with pre-existing medical conditions. Remember the health of animals and be aware of your power use. Have a plan should an extended power shutdown takes place in your location.