Heat returns to the West; Rain to end the month in the Northwest?

After a mid-month cooldown, heat is going to be the story again across California and the Southwest this week.

A ridge nosing into the Southwest this week will bring a push of hot air into the area.

Notice that this is not a huge ridge that impacts the entire West, as this one is more focused more toward the Southwest. This ridge is part of a larger Omega block in the jet stream, which is shown well graphically by the National Weather Service office in Phoenix.

The core of the heat will be focused in Arizona, southern Nevada and inland areas of California. That being said, it will be running on the warm side in the Bay Area through midweek and will get hot along portions of the Central Coast.

The most interesting feature on the weather map this week, though, is the potential for a potent upper-level low to slide up the West Coast late in the week.

The models have been pretty consistent over the past few days of the general area where this low will move, but the details of what will result of this system are still unclear.

One thing that is clear is that the low will help erase the heat across much of the West and get temperatures back closer to average heading into the end of the month and the beginning of June.

The path of the upper low is a bit unusual, especially for late May. This is where the GFS is forecasting it to be on Friday afternoon...

And then into the Pacific Northwest by Saturday afternoon.

The European shows a similar idea, just a little slower. This could be a sizable rain event for portions of the Pacific Northwest with widespread showers and even a few thunderstorms somewhere in the Friday night to Sunday morning timeframe.

The models do seem to agree that western Oregon may have the best chance of seeing a good soaking.

Let's look at the GFS ensembles for Eugene, Oregon, over the period that this upper low is moving through.

For those new to the blog, ensemble forecasts use many runs of the same model with slightly initial conditions, as the starting data that goes into the models is not perfect.

The black line shows the average of all the ensemble members, which shows an average of about 0.8 of an inch through Sunday. The increase after that is from another potential system into the first couple of days of June.

There will be the potential to pick up more than inch of rain in some spots in areas that see multiple downpours. Any rain is good news in this area, where there are severe drought conditions in some places.

There could even be some hail with some of the storms if everything comes together perfectly. We'll fine tune the details on this and take another look at this system later in the week.

Looking into the longer range as June gets underway, a general trough in the jet stream should prevent any significant heat from getting into the West.

The GFS today has been showing the potential for a deep trough to barrel into the West Coast next Monday and Tuesday, potentially bringing a shower threat down to the Bay Area. This would be unusual - SFO has not seen rain in the month of June since 2017.

The European model is showing a more benign trough with very little rain. This seems to make more sense climatologically, so that's the way we'll lean for now. Again, we'll take another look at this later in the week.

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