Western US Daily Summit

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Late summer heatwave taking hold


A strong ridge of high pressure will dominate the weather pattern over the next week, resulting in well above average temperatures across the West, including record highs for many areas. T-storm activity will be very spotty across the Southwest & Southern Rockies in this pattern, while the Cascades & PNW will also see some dry t-storms on Wednesday.

Short Term Forecast

Forecast Highlights:

  • Prolonged heatwave for most of the West from the middle of this week through the middle of next week, with record highs expected to be broken for many areas.
  • Fire danger will gradually increase over the next week, especially across the Far West and Great Basin, in response to an extended stretch of hot and dry conditions.
  • Isolated thunderstorms across the WA Cascades and Colorado/New Mexico on Wednesday as weak disturbances impact these regions. 
  • Weak monsoon with isolated thunderstorms and light/spotty rain across the Southwest for the remainder of this week and into the weekend.

Temperature Outlook:

An impressive late-season heatwave is taking hold across the West and will persist through Labor Day weekend and into early next week. Daily record highs are likely to fall throughout the West over the next 5 days with the hottest temperatures expected this weekend. Some areas across California could potentially see all-time monthly record highs broken for September as well. 

Forecast for Wednesday:

A strong ridge of high pressure is in control of the pattern right now, but a couple of weak disturbances are rotating around the ridge. First, there is a disturbance located over Washington on Wednesday (moving from SW to NE) and another moving into Eastern Colorado and New Mexico (from NW to SE). 

These two features will result in isolated thunderstorms across the Washington Cascades and Interior Northwest, while areas near and east of the Continental Divide will also see isolated to widely scattered thunderstorms.

The storms over the Northwest will be "dry" (< 0.1 inches of rain), while the storms over Colorado will be somewhat wetter but will still only produce light to moderate rain at best, especially compared to recent weeks.

Forecast for Thursday:

On Thursday, conditions will dry out entirely across the Pacific Northwest. Meanwhile, Colorado and New Mexico will also see a significant drying trend with only some isolated high-elevation storms at most (Southeast New Mexico excluded, where there will be more showers/storms). 

Arizona and Southeast California could also see some isolated thunderstorms, but they should be mainly of the dry variety with little rainfall.

Forecast for Friday:

Friday is shaping up to be another quiet day across the West. Arizona will see a slight uptick in thunderstorm activity, but coverage will still be isolated and generally confined to the higher terrain. Colorado will most likely stay dry with only a very slight chance of a storm near the Divide. 

On Friday night, the southern edge of a low pressure trough will dip into Southern BC and Northwest Washington. This could potentially result in some light showers overnight across the Olympic and Cascade Ranges, but confidence is low as some models are projecting shower activity to stay north of the Canadian border.

Fire and Smoke Outlook:

A number of fires are burning throughout the West right now, with most of the activity over the Northwest. Heavy smoke continues to remain confined to areas near and immediately downwind of the fires, while the rest of the Northwest and Northern Rockies can expect occasional periods of light smoke and hazy skies over the coming days.

Fire danger will be on the increase over the next week, however, largely in response to the ongoing heatwave, which will coincide with dry conditions and low relative humidity.

The greatest fire danger will exist over California, the Northwest, and the Northern Rockies. Northern areas could see gusty winds contribute to fire danger on Wednesday, Thursday, and Saturday, while dry thunderstorms (mentioned earlier) will also contribute to the fire danger. 

Extended Forecast

Outlook for Saturday (9/3) to Tuesday (9/6):

Unseasonably hot and dry conditions will be the big story across the West over Labor Day weekend. Head to the higher elevations or hit the water this weekend to beat the heat!

On Saturday, isolated thunderstorm activity will generally be confined to higher elevation areas of Arizona, Southern California, New Mexico, and Southern/Central Colorado where a weak monsoonal flow will be in place.

The Olympics and Cascades in Washington could also see some light showers or even isolated thunderstorms on Saturday depending on how far south the southern extent of a disturbance moving across BC reaches.

On Sunday and Monday, a little bit of monsoonal moisture could creep northward into the Sierra Nevada Range where isolated dry thunderstorms will be possible. Isolated high-elevation thunderstorm activity can be expected across the Southwest on Sunday and Monday as well, while Colorado may see a downtick in thunderstorm chances on Monday.

A similar pattern is expected on Tuesday with isolated thunderstorms possible across the Southwest and possibly the Sierra with light/spotty rainfall.

Outlook for Wednesday (9/7) to Wednesday (9/14):

High pressure will finally start to weaken a bit during this period, resulting in cooler temperatures compared to the week prior. While temperatures are generally expected to remain above average across the West, the excessive heat should come to an end with an overall cooling trend expected.

The pattern also looks a bit more unsettled during this period compared to the week prior. Most models are projecting moisture from a tropical system in the Eastern Pacific to reach the Southwest U.S., which could support a bit of a late-season monsoonal surge into Arizona, SoCal, New Mexico, and Southern Utah/Colorado.

Also, while there are not any particular wet signals showing up further north, we will likely see a couple of disturbances (even if weak) track across the Northwest and Northern Rockies during this period with occasional showers/thunderstorms possible.

Thanks so much for reading! Next update on Friday (9/2).