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Western US weather

Busy monsoon week ahead in the Southwest; Hottest weather of the month in the interior Northwest

By Brian Thompson , AccuWeather meteorologist
7/19/2019, 9:42:08 AM

There will be two notable shifts happening in the next week or so across the West:

-- Monsoon thunderstorms will increase in coverage across the Southwest, along with building humidity.
-- Heat will spread northward across the interior Northwest.

Before we get to those, it's worth noting that the disturbance moving through the Pacific Northwest turned out to be a nice rainmaker across Washington state from Wednesday into Thursday. Many areas picked up more than a quarter inch of rain, with some higher elevations receiving more than an inch. 

That's a good amount for the middle of July, and the coverage of 0.25-inch plus rainfall amounts was more than I expected when I mentioned it in passing in the last couple of blogs. 

An unusually strong jet stream may have had a hand in the rain event. Take a look at this graph from Quillayute, Washington, which sits along the Pacific coast. 

The winds measured at jet stream level (about 30,000 feet) came in around 140 knots, or just over 160 mph. The red line that goes up and down shows the highest values ever recorded at that time over the course of the year. Look how high the dot is (showing Thursday morning's reading from the weather balloon) compared to the previous summer's records. This was a fast-moving jet stream for the winter, and virtually unheard of in the summer, coming in at about 25 knots higher than the previous record in mid-July.

This fast-moving jet provided more dynamics than you would typically have for a disturbance moving through in July, which may have helped lead to some higher rainfall totals.

This rain added to what has been a pretty fruitful month for rain in the Seattle area and much of western Washington. Rainfall is running above average for the month, which is good news considering that much of the area is experiencing drought conditions. The latest Drought Monitor this week continues to show the sizable area of drought in western and northern Washington, far northern Idaho and northwestern Oregon.

Drought Monitor 7/18

Unfortunately, there isn't much rain on the horizon as a large ridge builds back across much of the West, which will send heat northward into the interior West early next week.

This is not a setup that brings heat into Seattle and Portland, as the ridge axis will be too far to the east, which will keep an onshore flow going most of the time along the western side of the ridge. Even so, temperatures will come up some.

What the building ridge will do, however, is crank the temperatures up east of the Cascades toward the northern Rockies. Boise, Idaho, has yet to hit 100 degrees this year, but temperatures are forecast to be close to 100 from Sunday to Tuesday.

This whole area has generally been running pretty close to average this month without any major bouts of heat, so it seems like we're overdue for a warmup.

The heat and low humidity all across this area will create a higher fire danger later this weekend into next week. The wind overall should not be that strong, but it can be locally gusty. The fire danger has already been running high -- the Powerline Fire northeast of Yakima, Washington, has burned over 7,700 acres.

Yakima has seen just 0.04 of an inch of rain this month. They only average 0.22 of an inch for the month, so this is far from a significant anomaly, but it just goes to show how dry the brush is across much of the area east of the Cascades. 

Thunderstorms should be very sparse over the weekend, but any storm that forms will have the potential to produce lightning that can start new fires.
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Thunderstorm coverage will likely increase some into next week as some monsoonal moisture gets pulled northward. Little impulses of energy moving around the ridge will have the potential to lead to more numerous thunderstorms as far north as Idaho and Montana, but the exact timing of this is tricky from this far out.

Speaking of monsoonal moisture, the monsoon is going to start to kick into a higher gear starting this weekend across the Southwest, leading to more widespread thunderstorms each day.

The most widespread impact felt will be the increase in humidity at the surface. This will make it feel more muggy outside and also not allow the nights to cool off as much. Take Palm Springs, California -- overnight lows are currently running around 70, but as the humidity increases, so do the nighttime temperatures.

As I like to do sometimes, I'll take you under the hood and show one of our forecasting tools to illustrate this point. This is a statistical computer model forecast for Palm Springs for the next week or so. I've circled the key points I'm trying to make:

Palm Springs Forecast Explanation

-- The model is forecasting a low of 70 Saturday morning, when dew points will be in the 40s. Dew points in that range are pretty low, indicating that it's pretty dry at the surface.

-- Look ahead to Wednesday, when the dew points are significantly higher, closer to 60 F. The overnight low is now forecast to be 82, a full 12 degrees higher than Saturday.

I cropped out the bottom part of the forecast, but this product also shows the potential for precipitation rising from virtually zero at the beginning (when the air is dry) to closer to 20-30 percent by next week. Monsoon thunderstorms are much more common in the higher terrain surrounding the Coachella Valley, but Palm Springs is a place that can occasionally get a thunderstorm during the monsoon season. This is the sort of pattern where that could happen.

Here's a look at the precipitable water (amount of moisture in the atmosphere) around midweek. Look at how far north the greens get, up into Utah and Idaho. 

Precipitable Water Forecast Wednesday

Where the brighter yellows and oranges show up across southern Nevada, Southern California and western Arizona, this is where the most moisture will be available.

With this in mind, thunderstorms will be breaking out on a daily basis from the high terrain of the Four Corners back through Nevada and into the central and southern Sierra and the high ground of Southern California. As I mentioned earlier, with the moisture streaming northward, portions of Idaho and Montana could even see some showers and storms developing.

Flash flooding is the big concern across the higher terrain, and anyone hiking or spending time outdoors in these areas should keep an eye to the sky and be ready for rapidly changing weather conditions. 

Some of these storms will wander into the deserts toward the end of the day and into the evening, which is when you're most likely to get downpours and thunderstorms into the Phoenix area, along with the potential for strong winds and dust storms. Las Vegas will also be in a spot where there can be some showers and thunderstorms.

With the increase in humidity and afternoon cloud cover, the afternoon temperatures will come down a touch in places like Las Vegas and Phoenix.

The ridge staying in place over the West next week will keep the monsoon active through much of the rest of the month.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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Western US weather