Coolest weather since early June to slide into the West next week

A significant cooldown is coming to the West heading into next week. For many areas, it will be the coolest air since early June.

The change will end the stretch of excessively hot weather across the Southwest over the past month or so as the monsoon continues to be sluggish.

Across much of the interior West, it’s been a toasty start to September.

Through Thursday, Downtown Los Angeles is running 7.6 degrees above average for the month. Las Vegas is already running 7 degrees above average for September, with Phoenix 6 degrees above average. Much of the Sacramento and San Joaquin valleys have been 5 to 8 degrees above average so far this month.

In the Northwest, Boise is running nearly 11 degrees above average through Thursday, with Spokane at better than 7 degrees above average for the month. Much of western Montana has been unseasonably warm as well.

The monsoon has shown some signs of life this week, with a thunderstorm even reaching San Diego on Wednesday. This recent stretch has brought some pretty humid weather too, as the dew point in San Diego has jumped above 70 degrees a couple of times in recent days.

Unfortunately, the monsoon will struggle to get going over the next week as the flow overall becomes more onshore across the Southwest.

This change in flow will be thanks to a series of fairly deep troughs moving through the West, which will bring a more westerly flow overall.

While the Southwest monsoon has been quieting down again, there have also been some rounds of thunderstorms farther north across the Northwest, which have contained quite a bit of lightning:

The lightning has helped to start dozens of fires, the vast majority of them being small. The showers and thunderstorms have since moved northeastward into Washington state Friday morning.

More unsettled weather is coming to the Northwest through the middle of next week thanks to these troughs moving through.

The most noticeable impact across the West from these troughs will be the drop in temperatures, which will be felt everywhere.

The drop across the Southwest will be very noticeable too:

Phoenix has not been below 95 degrees since May 29, at the end of the great Memorial Day weekend cool spell. For many areas, it’s been at least early June since it’s been as cool as what is on the way next week.

In addition to the cooldown, it will be a pretty unsettled stretch across the Northwest. Round one will come Sunday and Sunday night with showers and thunderstorms across Washington, Oregon, Idaho and much of Montana and Wyoming.

There can be some locally heavy downpours with these showers and storms. Farther south, into Nevada and Utah, there will be a greater risk for dry thunderstorms with very little rain. This could lead to some wildfires due to lightning strikes. In the Northwest, this threat will not be quite as high thanks to the cool air, clouds and more significant moisture. Even so, any thunderstorm that does not have much rain with it will have the potential to start a fire given how dry many areas have been.

Another trough swinging through will bring another opportunity for showers and even a few thunderstorms from Tuesday into Wednesday. Again, the main area impacted by the wet weather will be from Washington and Oregon eastward through Idaho, Montana and northern Wyoming.

Over time, this pattern could yield a decent amount of rain, which would be very welcome news. Here is what the GFS (American) model is showing for total precipitation through the end of the day Wednesday:

The European model is fairly similar but also brings more significant rain farther south and east into areas east of the Cascades and into southern Idaho.

While it’s important to not focus on specific precipitation totals that the models are forecasting, this certainly leads to some confidence that this will be a pretty wet stretch for early September across the Northwest. There will be the potential for over an inch of rain through next Wednesday over much of the area.

As the last of the series of troughs moves out around the middle of next week, the upper-level flow will flatten out later next week, which will put an end to the wet pattern in the Northwest.

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