AgWeatherNet Weekly Weather Outlook Monday Update for September 14 to September 20, 2020

Continued smoke and hazardous air conditions with light rain showers

Craig Oswald, AgWeatherNet Field Meteorologist

Synopsis:

A stubborn upper level low and associated surface circulation look to remain offshore of the Pacific Northwest for the remainder of the week. The main effects of this pattern are twofold. First, Washington states’ residence under a southwest flow regime will remain intact, advecting smoke and ash laden air from the widespread wild fires throughout the Oregon Cascades into most of the state. Second, scattered rain showers will continue to move into primarily western Washington through the week.

Discussion:

The upper low and surface circulation currently situated off the coast will remain intact through much of the week. The southwest flow regime associated with this feature will continue to advect smoke and ash form wild fires throughout the Oregon Cascades into Washington. The Basin region of eastern Washington will receive the worst of the smoke conditions through the week according to latest smoke modeling from NOAA Air Resources Lab. The remainder of the state will certainly remain under smoky conditions and hazardous air quality through the week as well. Western Washington may begin to see conditions slowly clear through the week as cleaner air sourced from over the Pacific begins moving into the region (not before all the smoke that had been pushed out to sea was blown back inland over the last several days of course). A few scattered light rain showers are expected to continue to move into western Washington today (Monday) and through the rest of the week and may also help to clear the air. It should be noted however that any shower activity is expected to be light and scattered with most activity relegated to mountain areas and will have a very modest effect in clearing the air. Furthermore, if wind patterns fluctuate from a southwest flow to a southerly flow as weak frontal boundaries associated with the off shore surface circulation encroach upon western Washington, smoke will again move northward from Oregon into western Washington. With this being said, the best course of action would be to plan for smoky, hazy, and extremely poor air quality state wide through the week.

As alluded to, some light and scattered rain showers will continue to move through western Washington through mid week. The majority of this activity will be over the Cascades and Olympics but it is expected low land areas will also see a few showers with very light rain fall totals. This is thanks to higher moisture air being advected by the southwesterly flow associated with the offshore trough and circulation from more equatorial latitudes of the Pacific. This is reflected in the change in relative humidity values which have risen by 40 to 50% statewide compared to where they were last week after the passage of the dry, northeasterly cold front. Areas east of the Cascades look to remain dry through the middle of the week. Temperatures will generally remain near average through the week as dense smoke continues to hamper daytime solar heating, prohibiting daytime highs from climbing exceptionally high.

Weather conditions look to turn much more active state wide starting Thursday night as the upper low situated off the coast begins to move onshore. More widespread rain showers can be expected for western Washington. Scattered showers are anticipated to slowly taper off through Friday and Saturday but a chance will remain until Sunday thanks to added instability aloft. All areas of eastern Washington will also likely see a few scattered rain showers as the passing upper trough and surface cold front, paired with a moistened atmosphere, thanks to the southwesterly flow anticipated through the week, should create enough instability and available moisture to set off a round of showers Friday. More consistent shower activity can be expected for areas of high terrain. A few showers are also expected to linger through Saturday in high terrain with a stray shower possible on the Palouse. Conditions look to return to dry statewide Sunday as zonal flow aloft takes over.

It is likely the passage of Fridays system will help to push and mix some of the smoke out of the air throughout the state. Air quality can’t be expected to completely recover over the weekend, especially with the large fires still burning in northern Oregon and the Okanogan, but it seems reasonable at this time that at least an improvement will be made for the weekend. Furthermore, please take caution as ongoing fires are likely to behave erratically, spread, and/or start new burns with the breezy to windy conditions expected for many parts of the state thanks to the passing system. This topic will be discussed again in Fridays outlook.


Important Points About This Week's Weather

South Central Washington
Remainder of the week
  • Wind: Light and SW yet variable. SW and breezy Friday, transitioning to W, some gusts to near 30 mph.
  • Precipitation: Dry through mid-week. Scattered light rain showers Friday. Trending dry Friday night.
  • High Temps: Low 80s Tue, low 80s Wed, mid 80s Thu, upper 70s Fri.
  • Low Temps: Mid 50s Mon night, mid 50s Tue night, mid 50s Wed night, mid 50s Thu night.
  • Sky Cover: Smoke and haze. Otherwise partly to mostly cloudy.

Weekend
  • Wind: W and breezy to windy Saturday. Becoming light and more variable by Sunday.
  • Precipitation: Low chance for a lingering light rain shower early Saturday, otherwise dry conditions expected.
  • High Temps: Upper 70s Sat, upper 70s Sun.
  • Low Temps: Low 50s Fri night, mid 40s Sat night, upper 40s Sun night.
  • Sky Cover: Smoke and haze. Otherwise partly to mostly cloudy.

North Central Washington
Remainder of the week
  • Wind: Light and SW yet variable. SW and breezy Friday, transitioning to W, some gusts to near 30 mph through the Basin.
  • Precipitation: A few light rain showers near the Cascade crest through mid week. More consistent and widespread mountain rain showers to begin moving in from the W/SW Thur night. Scattered light rain showers across low elevation areas Friday. Trending dry Friday night with continued showers in upland and mountain areas.
  • High Temps: Upper 70s Tue, mid 80s Wed, mid 80s Thu, near 80 Fri. Cooler in high terrain.
  • Low Temps: Low 50s Mon night, upper 50s Tue night, upper 50s Wed night, upper 50s Thu night. Cooler in high terrain and deep valleys.
  • Sky Cover: Smoke and haze. Otherwise partly to mostly cloudy.

Weekend
  • Wind: W and breezy to windy Saturday, especially through the Basin. Becoming light and more variable by Sunday.
  • Precipitation: Low chance for a lingering light rain shower early Saturday across low elevation areas, otherwise dry conditions expected. Light scattered rain showers to linger through ipland and mountain areas through Saturday. Dry Sunday.
  • High Temps: Upper 70s Sat, upper 70s Sun. Cooler in high terrain.
  • Low Temps: Mid 50s Fri night, near 50 Sat night, near 50 Sun night. Cooler in high terrain and deep valleys.
  • Sky Cover: Smoke and haze. Otherwise partly to mostly cloudy.

Eastern Washington
Remainder of the week
  • Wind: Light and SW yet variable. SW and breezy Friday, transitioning to W, some gusts to near 30 mph.
  • Precipitation: Dry through mid-week. Scattered light rain showers Friday. Gradually trending dry Friday night. More consistent shower activity through the Blue Mtns.
  • High Temps: Upper 70s Tue, low 80s Wed, mid 80s Thu, mid 70s Fri. Cooler through the Blues.
  • Low Temps: Low 50s Mon night, near 50 Tue night, upper 40s Wed night, upper 40s Thu night. Cooler through the Blues.
  • Sky Cover: Smoke and haze. Otherwise partly to mostly cloudy.

Weekend
  • Wind: W and breezy to windy Saturday. Becoming light and more variable by Sunday.
  • Precipitation: Continued low chance for a light rain shower Saturday. Dry Sunday.
  • High Temps: Upper 60s Sat, upper 60s Sun.
  • Low Temps: Mid 40s Fri night, mid 40s Sat night, mid 40s Sun night. Cooler through the Blues.
  • Sky Cover: Smoke and haze. Otherwise partly to mostly cloudy. Cooler through the Blues.

Western Washington
Remainder of the week
  • Wind: Generally light and SW yet variable. Turning W and becoming breezy Thursday night and remaining so for Friday.
  • Precipitation: Light and scattered rain showers through mid-week with more consistent shower activity through the Cascades and Olympics. Widespread light rain to move in from the W/SW Thursday night and turn scattered through the day Friday. Again, more consistent activity through the mountains.
  • High Temps: Upper 60s Tue, mid 70s Wed, mid 70s Thu, upper 60s Fri. Much cooler through the Cascades and Olympics.
  • Low Temps: Low 50s Mon night, low 50s Tue night, mid 50s Wed night, mid 50s Thu night. Much cooler through the Cascades and Olympics.
  • Sky Cover: Smoke and haze gradually lessening. Otherwise partly to mostly cloudy. Overnight areas of fog.

Weekend
  • Wind: W/NW Saturday and breezy. Trending lighter and more variable for Sunday.
  • Precipitation: Scattered rain showers gradually tapering off through Saturday, dry Sunday.
  • High Temps: Near 70 Sat, near 70 Sun.
  • Low Temps: Mid 50s Fri night, near 50 Sat night, near 50 Sun night. Much cooler through the Cascades and Olympics.
  • Sky Cover: Smoke and haze gradually lessening. Otherwise partly to mostly cloudy. Overnight areas of fog.

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