THREE HOT DAYS, THEN RAIN TO THE SOUTH

THE HEAT UP

Some pretty typical late summer heat is on store for California this week. The warming will kick in Monday and continue through Wednesday, with inland highs in the upper 80s to upper 90s, about 10 degrees above average for this time of year.

This will be a statewide heat event, with temps running well above average from Oregon to the Mexican border.

This will not be an extreme heat event, and the NWS may not issue heat advisory products, but anyone working outside needs to take frequent breaks and hydrate well.

THE RAIN

Tropical storm (former hurricane) Mario is moving northwest off the coast of Mexico. It will bring a packet of moisture to the southern California coast by Wednesday.

The European, WPC (below) and most other models keep the bulk of the rain south of us, but clouds and some showers may arrive across the Bay Area by midday Thursday.

Here is the WPC for total rain by Friday afternoon.

The GFS model and European AI are different. They bring showers and possibly thunderstorms all the way north to the North Bay Area region by late Thursday. The other models don’t agree at present.

In any case, wherever the rain and T-storm end up, a big issue across California at this time of year is a very dry lower atmosphere. This can set the stage for “dry thunderstorms”, which are high-based storms in which lightning can occur, while most rain evaporates before it reaches the ground. Obviously this raises the risk of fire starts.

You can see all of the yellows and oranges in this low level water vapor image. That shows just how dry our atmosphere is. When tropical moisture moves in, it almost always flows in above this dry layer, making it hard for rain to occur.

With all of that said, the NWS has issued a slight chance of showers for our North Bay region for late on Thursday, continuing into Friday. Again, most of the action should be focused to our south, but fire officials will be on alert statewide.

We’ll keep you updated as conditions change.

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