AgWeatherNet Weekly Weather Outlook for June 26 to July 2, 2020

Robust upper trough to be major driver for unsettled conditions and much cooler temperatures

Mark Ingalls, AgWeatherNet Intern
Craig Oswald, Field Meteorologist, 509-786-9256
Michelle Daughtrey, AgWeatherNet Intern


The region of anomalously high surface pressure near the west coast that helped to bring above average temperatures throughout the state this week will lose its grip on the region this weekend. This area of high pressure looks to move west and north, encroaching on Alaska, and help build a strong trough downstream and directly over the Pacific Northwest bringing a round of MUCH cooler temperatures and some rainy weather.

Friday night, an upper low descends south from Yukon Territory, reaching Washington during the day Saturday. This low and its associated trough will linger over the American West for much of next week and will be the primary driver for conditions in the Pacific Northwest during this period.

Before the end of the day Friday, many will have noted that high temperatures peaked around 100 in the Tri-Cities area with other lowland eastern Washington locations being near or above 90, including Yakima, Wenatchee, and Spokane. To the west of the Cascades, things will have been much cooler with temperatures along I-5 ranging from 70 around Bellingham to 77 in Seattle and 86 in Portland. High temperatures gradually cool west from I-5 toward the Coast. Winds are expected to ramp up this evening in advance of the approaching upper low and breezy to windy conditions will last for most of the state through Saturday. The windiest locations will be the leeward side of gaps in the Cascades such as the Kittitas Valley and the east end of the Columbia Gorge, but generally breezy conditions can be expected throughout the Columbia Basin and southwest Washington, especially near the coast.

Saturday comes with cooler conditions statewide. For lowland Eastern Washington and the I-5 Corridor, afternoon highs can be expected to drop about 10-15 degrees. This trend continues into Sunday east of the Cascades with another 10-15 degree drop, bringing the Tri-Cities, Yakima, and Wenatchee down to the low 70s with mid to upper 60s for Spokane and the Palouse. Overnight lows will remain in the mid 50s for the I-5 Corridor, but eastern lowland locations will see a 10-15 degree drop between Friday and Saturday night.

The drop in overnight temperatures in Eastern Washington can be largely explained by the low bringing in drier and cooler air from the north. While shower activity is expected to increase with this low, it doesn't have substantial moisture to work with once settled into the region. In the Columbia Basin, AgWeatherNet sites were observing dew points around 60 Friday morning, but these will drop to around 40 on Saturday. While conditions look dynamically favorable for a few isolated thunderstorms to development, the lack of moisture will limit any widespread thunderstorm activity and make this primarily into a rain shower event.

For Western Washington, rain showers may start as early as Friday night with chances increasing first in Neah Bay and progressing toward the south west with rain likely in most westside locations by Saturday evening. An embedded thunderstorm or two may also be possible with this activity, particularly Sunday. Shower activity moves into and east of the Cascades primarily Sunday night and into Monday. However, a few stray scattered rain showers have the potential to develop in areas of eastern Washington as early as Saturday night. It appears that much of the rain on the eastside will land in mountainous areas like the Okanogan as well as generally east of a line from the Tri-Cities to Moses Lake to Omak. This will leave much of drought stricken Central Washington out of most of this event's precipitation activity, which has been the story for that area for the majority of 2020.

The largest contributor to precipitation values in Eastern Washington on Monday is anticipated to be a deformation zone that sets up on the northwest side of the low as it is positioned over Central Idaho. This will look similar to May 20, when a similar deformation zone set up across eastern Washington roughly along and east of US 395. Observed precipitation on that day was highly variable since the boundary remained in the same place for the span of several hours. For example, Walla Walla received over 1.50" of rain while the Tri-Cities Airport in Pasco saw 0.50", with the Richland Airport seeing 0.11" and Prosser 0". The amount of rain on Monday should be sharply contrasted like it was on May 20, but there is some uncertainty on where the western edge of the rainfall will be. If the low centers itself somewhere near Salmon, Idaho (which looks to be the most likely scenario as of Friday morning), only far eastern portions of Washington can expect significant rainfall. If it is further west, such as near McCall, Idaho, locations like Walla Walla, Ritzville, and the Tri-Cities stand to get in on the action.

Beyond this weekend's burst of activity, the center of the low moves further south taking much of the precipitation with it. There will still be relatively cool air aloft thanks to the deep trough associated with this low, so periodic shower activity is expected to linger in mountainous portions of Washington through much of the week. This will be primarily in the form of isolated showers in mountains generally north of I-90, but a few stray showers might form in mountains to the south as well.

For lowland locations Tuesday through Thursday, sunny to partly cloudy skies outside of rain showers should allow daytime highs to be near seasonable. For the I-5 Corridor, this represents the low to mid 70s, upper 70s for Spokane and the Palouse, and near to low 80s for the Columbia Basin and Yakima Valley. Lows should also be near average for this time of year.

By Wednesday, another upper disturbance looks to impact the state. This one will be significantly weaker than the previous weekends. Since it's expected to remain in southern British Columbia, most of the showers it produces will be in the northern half of Washington, primarily through upland areas. More details on this can be expected in the Monday update.

8 to 14 Day Outlook  July 3, 2020 to July 9, 2020
  • Slightly cooler than average temperatures and near average precipitation are expected during the 8 to 14 day time frame.   

Important Points about Next Week's Weather

South Central Washington
  • Wind:  Mostly W/NW and windy/generally breezy through the weekend. A few gusts as high as 40-45 mph at times, particularly Saturday and Saturday night. 
  • Precipitation:  Low chance for a stray rain shower or two Saturday night. A few scattered rain showers expected to begin building in through the day Sunday. More consistent scattered showers likely by Sunday night.  
  • High Temps:  Upper 90s Fri, mid 80s Sat, low 70s Sun. 
  • Low Temps:  Low 60s Fri night, low 50s Sat night, low 50s Sun night. 
  • Sky Cover:  Sunny Fri, mostly clear Fri night, mostly sunny Sat, mostly clear Sat night, partly sunny Sun, mostly cloudy Sun night. 
Next Week
  • Wind:  W/NW winds easing slightly Monday yet remaining a bit breezy. Switching to SW Tuesday night and continue to be generally breezy at times. Turning calm to light and variable Friday.  
  • Precipitation:  Scattered showers remaining through Monday with greater activity to the east. Trending mostly dry through Monday night. Maintaining a low chance for a stray rain shower or two through Thursday with greater threat remaining through eastern parts of the region. Dry Friday.  
  • High Temps:  Upper 70s Mon, low 80s Tue, low 80s Wed, mid 80s Thu. 
  • Low Temps:  Mid 50s Mon night, low 50s Tue night, low 50s Wed night, mid 50s Thu night. 
  • Sky Cover:  Partly sunny Monday, turning mostly sunny Tuesday and remaining so for the week.  

North Central Washington
  • Wind:  Mostly W/NW and windy/generally breezy through the weekend. A few gusts as high as 40-45 mph at times, particularly Saturday and Saturday night throughout the Basin. 
  • Precipitation:  Low chance for a stray rain shower or two Saturday evening through upland areas. Chance for rain increasing through the overnight.  Showers expected to continue to build in through the day Sunday with more consistent showers by Sunday evening.  
  • High Temps:  Low 90s Fri, near 80 Sat, low 70s Sun. Cooler in high terrain. 
  • Low Temps:  Upper 50s Fri night, low 50s Sat night, mid 50s Sun night. Cooler in high terrain. 
  • Sky Cover:  Clearing skies through Friday night. Sunny Saturday. Clouds building in again Saturday night with cloudy skies for the remainder of the weekend.  
Next Week
  • Wind:  W/NW winds generally easing Monday yet remaining a bit breezy through the Basin until Friday. Turning calm to light and variable Friday.  
  • Precipitation:  Rain showers to trend lighter more scattered in nature through Monday with greater activity remaining through eastern portions of the region, however. Another surge of shower activity from the north expected by Tuesday night, especially through upland areas with the Basin only to see a few passing rain showers at best. Conditions to remain unsettled with on and off showers through Thursday with greater activity through upland areas and less through the Basin. Dry Friday.  
  • High Temps:  Low 80s Mon, low 80s Tue, upper 70s Wed, upper 70s Thu. Cooler in high terrain. 
  • Low Temps:  Mid 50s Mon night, mid 50s Tue night, mid 50s Wed night, mid 50s Thu night. Cooler in high terrain.  
  • Sky Cover:  Mostly sunny to partly cloudy.  

Eastern Washington
  • Wind:  Mostly W/NW and windy/generally breezy through the weekend. A few gusts as high as 40-45 mph at times, particularly Saturday and Saturday night. 
  • Precipitation:  Low chance for a stray rain shower or two Saturday night. Rain showers expected to begin building in through the day Sunday. Widespread rain and rain showers likely by Sunday night.  
  • High Temps:  Mid 80s Fri, mid 70s Sat, mid 60s Sun. 
  • Low Temps:  Near 60 Fri night, mid 40s Sat night, upper 40s Sun night. 
  • Sky Cover:  Clouds increasing through Friday night. Partly sunny Saturday and becoming cloudy by Sunday night.  
Next Week
  • Wind:  W/NW winds easing slightly Monday yet remaining a bit breezy. Switching to SW Tuesday night and continue to be generally breezy at times. Turning calm to light and variable Friday.  
  • Precipitation:  Widespread rain and scattered rain showers to persist through Tuesday with conditions trending drier through Tuesday night. A few passing scattered rain showers likely to remain in the area Wednesday and Thursday. Dry Friday.  
  • High Temps:  Mid 60s Mon, upper 60s Tue, near 70 Wed, low 70s Thu. 
  • Low Temps:  Upper 40s Mon night, upper 40s Tue night, upper 40s Wed night, low 50s Thu night. 
  • Sky Cover:  Mostly cloudy Monday and slowly trending sunny by Thursday.  

Western Washington
  • Wind:  Onshore winds will produce generally light westerly wind throughout the region. Winds to become breezier Saturday with the Strait of Juan de Fuca to experience gusts near 40 mph. Westerly winds remaining breezy Sunday afternoon. 
  • Precipitation:  Scattered rain showers Saturday and Sunday with potential for a thunderstorm or two Sunday late afternoon/evening. Trending dry Sunday night.
  • High Temps:  Upper 70s Fri, mid to upper 60s Sat, low 70s Sun. A North-South temperature gradient produces warmer temperatures moving toward Oregon. 
  • Low Temps:  Mid 50s Fri night, upper 50s Sat night, low 50s Sun night. Much cooler through the Cascades and Olympics. 
  • Sky Cover:   Clouds expected to return Friday evening and stick around for a mostly cloudy Saturday. Clouds will break Sunday afternoon and will continue to clear towards the evening. Mostly clear Sunday night.   
Next Week
  • Wind:  W/SW winds to ease slightly Monday but expected to again become breezy Tuesday. Remaining light to breezy through the week. Calm to light on Friday.
  • Precipitation:  Mostly dry conditions expected early in the week. A stray rain shower or two possible Wednesday and Thursday with a few light rain showers likely through the Cascades and Olympics. Dry Friday.
  • High Temps:  South Puget Sound near 80 Mon, North of Seattle low 70s Mon, mid to low 70s Tue, low 70s Wed, mid 70s Thu. Much cooler through the Cascades and Olympics. Cooler near the coast.  
  • Low Temps:  Mid 50s Mon night, mid to low 50s Tue night, mid to low 50s Wed night, mid 50s Thu night. Much cooler through the Cascades and Olympics. 
  • Sky Cover:  Partly cloudy days and clear nights expected throughout the week.  

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