AgWeatherNet Weekly Outlook for August 21 to August 27, 2020

Unsettled conditions through the beginning of the week

Mark Ingalls, AgWeatherNet Intern
Craig Oswald, AgWeatherNet Field Meteorologist
Joe Zagrodnik, AgWeatherNet Postdoctoral Research Associate

Synopsis:

The cold front crossing Washington today (Friday) will continue to produce shower activity west of and in the Cascades and breezy conditions to the east. Cooler temperatures on Saturday will follow the front before a transition to more SW flow warms things by a few degrees. A weak shortwave trough looks to impact the region Monday and Tuesday, again bringing a slight chance of shower activity to west side and mountainous areas with breezy conditions for the mountain gaps. Zonal flow takes hold Wednesday and Thursday, keeping things mainly dry.

Discussion:

A cold front that was centered around 128° West (the Washington Coast is about 124° West) mid-morning on Friday will move across Washington on Friday and Saturday. Rain showers in western lowlands will taper off throughout the day. The Puget Sound Convergence Zone is expected to be active Friday afternoon. Model guidance suggests that this narrow band of convective shower activity will form near Everett and migrate south before dissipating along the King/Snohomish County Line during the evening hours. For the Cascades and Olympics, some light showers may linger into Saturday. Storm total precipitation values may reach several inches in the Olympic rain forests, with lower values elsewhere.

As the front crosses the mountains, winds will increase. These will be strongest downwind of the mountain gaps, such as in the Kittitas Valley and the east end of the Columbia River George, but a notable breeze is expected to be observed in the Columbia Basin. Isolated gusts to 40 mph will produce pockets of blowing dust, particularly downwind of freshly harvested fields and construction areas. Relative humidity values should experience a modest increase behind the front which, the Storm Prediction Center still notes an elevated fire risk for the Columbia Basin and adjacent valleys thanks to the anticipated stiff breeze and dry fuels.

Temperatures Saturday will be about the same as Friday for Western Washington but noticeably cooler conditions can be expected east of the Cascades. Highs along the I-5 Corridor look to range from about 70° along the British Columbia border to near 80° in the Portland/Vancouver area. Along the Columbia, Snake, and Okanogan Rivers, highs in the mid-80s can be expected, which are a few degrees below average. Outside of these locations, places like Yakima and Moses Lake can expect lower-80s and Spokane and the Palouse should see upper-70s to near 80°. There isnt much impact to overnight lows expected during this period.

Temperatures climb by a few degrees in Eastern Washington and the greater Portland area Sunday with highs holding within a few degrees of seasonal averages throughout much of the state through the week. Another shot of precipitation for the Olympics and Cascades (with isolated lowland showers West of the Cascades) comes in the form of a shortwave trough embedded within the mean flow Monday.

This will quickly ride the eastern periphery of a broader trough offshore and progressing eastward, taking it into British Columbia. This shortwave coupled with the generally southwest flow pattern may be enough to initiate modest thunderstorm activity near Bend, Oregon that may progress northeast. There is a low chance that one of these may stray out of the Blue Mountains into the southern Columbia Basin or the Palouse. Rainfall values from this event should be low. Breezy conditions can again be expected in Eastern Washington as this disturbance passes. Some mountain showers may linger into Tuesday in the mountains.

Beyond Tuesday, the upper flow becomes zonal. This should keep things dry throughout the state, though points in Western Washington may rotate between partly cloudy and overcast skies with most of the cloudiness being limited to the morning hours. There seems to be reasonable model consensus in a robust upper-level low impacting the region next weekend. If this occurs, it should bring cooler temperatures across the state. Disagreement remains over the track of this low, however, so how much precipitation it may bring remains to be seen. If it were to follow the European models track, its path would pass north of Prince George and Edmonton keeping us fairly dry. The GFS brings it quite a bit closer to Washington, moving roughly near Kelowna and Calgary, which would bring in more rainfall. More on this in next weeks update.


8 to 14 Day Outlook, August 28 to September 3, 2020
  • Above normal temperatures and below normal precipitation are favored for the period.

Important Points about Next Week's Weather

South Central Washington
Weekend
  • Wind: Breezy Fri with W wind, gusts to 35 mph. Otherwise light and variable.
  • Precipitation: No precipitation expected.
  • High Temps: Upper 80s to near 90 Fri, mid 80s Sat, low 90s Sun.
  • Low Temps: Near 60 Fri Night-Sun Night.
  • Sky Cover: Haze possible Fri, otherwise sunny to partly cloudy.
Next Week
  • Wind: SW/Wwind through the week. Breezy conditions possible Mon & Tue.
  • Precipitation: Low chance Mon, otherwise dry.
  • High Temps: Low 90s Mon-Thu.
  • Low Temps: Near 60 Mon Night-Thu Night.
  • Sky Cover: Sunny to partly cloudy.

North Central Washington
Weekend
  • Wind: SW/W wind and breezy Fri with gusts to 35 mph in spots. Otherwise light and variable.
  • Precipitation: Isolated showers Fri, particularly in the Cascades. Otherwise dry.
  • High Temps: Mid 80s Fri & Sat, near 90 Sun. Cooler in higher terrain.
  • Low Temps: Low 60s Fri Night, mid 50s Sat Night, upper 50s Sun night. Cooler in higher terrain.
  • Sky Cover: Partly to mostly cloudy Fri. Sunny to partly cloudy Sat & Sun.
Next Week
  • Wind: SW/Wwind through the week with breezy conditions Mon.
  • Precipitation: Low chance of mountain showers Mon, otherwise dry.
  • High Temps: Mid 80s to near 90 Mon-Thu. Cooler in higher terrain.
  • Low Temps: Low 60s Mon Night-Thu Night. Cooler in higher terrain.
  • Sky Cover: Partly to mostly cloudy Mon, sunny to partly cloudy Tue-Thu.

Eastern Washington
Weekend
  • Wind: W wind with breezy conditions Fri. Gusts to 30 mph. Otherwise light and variable.
  • Precipitation: No precipitation expected.
  • High Temps: Mid 80s Fri, near 80 Sat, upper 80s Sun.
  • Low Temps: Mid 50s Fri Night, near 50 Sat Night, mid 50s Sun Night.
  • Sky Cover: Haze possible Fri, otherwise sunny to partly cloudy.
Next Week
  • Wind: SW/W wind through the week, breezy Mon.
  • Precipitation: Isolated t-storms possible Mon, otherwise dry.
  • High Temps: Mid to upper 80s Mon-Thu.
  • Low Temps: Low to mid 50s Mon Night-Thu Night.
  • Sky Cover: Sunny to partly cloudy Mon-Thu.

Western Washington
Weekend
  • Wind: Generally NW/W wind through the weekend. Intermittently breezy along the Strait with gusts to 30 mph.
  • Precipitation: Widespread rain showers Fri & Fri night. Lingering mountain showers Sat. Dry Sun.
  • High Temps: Mid 70s to near 80 Fri-Sun. Cooler near the coast and in higher terrain.
  • Low Temps: Near 60 Fri Night, mid 50s Sat & Sun Night. Cooler in higher terrain.
  • Sky Cover: Mostly cloudy to overcast Fri, decreasing clouds Sat, sunny to partly cloudy Sun.
Next Week
  • Wind: Generally W wind with variability due to terrain. Intermittently breezy along the Strait.
  • Precipitation: Scattered showers Mon, mainly in the mountains and possibly lingering to Tue. Otherwise dry.
  • High Temps: Mid 70s to low 80s Mon-Thu. Cooler on the coast and in higher terrain.
  • Low Temps: Mid 50s to near 60 Mon Night-Thu Night. Cooler in higher terrain.
  • Sky Cover: Partly to mostly cloudy Mon, sunny to partly cloudy Tue-Thu.

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