AgWeatherNet Weekly Outlook for September 11 to September 17, 2020

Smoky weekend followed by a chance of precipitation next week

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

Synopsis:

The trough that was keeping winds easterly over the Pacific Northwest has moved eastward, placing a broad ridge over the Pacific Northwest. A transition to onshore flow is expected with a southwesterly wind pattern, bringing smoke from Oregon and California with it beginning Saturday. That smoke, coupled with cooler air coming off the Pacific, will begin moderating temperatures across the state – starting with areas west of the Cascades. A frontal passage associated with an upper wave looks to bring chances for precipitation next week. The best chance for most locations comes Monday and Tuesday, but rainfall values in areas with fires do not look substantial enough to drastically improve firefighting efforts. Lower chances of rain continue through the rest of the week

Discussion:

As the lower and mid level wind pattern shifts from offshore to onshore, smoke will become denser across Washington for the weekend. Typically, onshore flow would help to remove smoke from locations in Western Washington, but the southwesterly tilt of it (rather than due west) will bring smoke north from Oregon and California. Smoke will also be sourced from over the Pacific, where several days of offshore winds have pushed it out to more than 1,000 miles into the ocean in places.

Reduced air quality throughout much of Washington will be the result of this change, and denser smoke will likely inhibit daytime heating at least through the weekend, but possibly lasting longer. More details on the smoke itself can be found in Joe's recent blog post on the subject.

Cooler temperatures resulting from onshore flow will first reach the west side of the state, with highs cooling to near average for this time of year. In Eastern Washington, highs in the basin will remain near or above 90° through Saturday with cooler temperatures with increasing elevation. No precipitation is expected through the weekend, but low clouds and fog are possible west of the Cascades. These may last longer into the day than usual because of the smoke.

On Monday, an upper level trough approaches the region, bringing with it a chance of rain showers. There may be enough moisture embedded in this to even bring light rain showers into the Columbia Basin, with rain likely in Western Washington and mountains across the state. Rainfall values will not be enough to put a dent in the drought conditions in Central Washington. While rain is also expected in the Willamette Valley and western foothills of the Oregon Cascades, it also doesn't look significant enough to bring an end to the fires gripping that region leaving smoky conditions as a possibility for most of Washington.

Breezy conditions are expected Monday with the initial passage of this wave, particularly in the mountain gap regions in the lee of the Cascades and through the Strait of Juan de Fuca. Increased winds could intensify hazardous fire conditions, but the moisture embedded in this system should keep relative humidity values high enough that critical fire conditions like what was seen on Labor Day aren't likely. Still, extreme fire behavior will be possible for existing fires as well as fires that may start.

Rain chances continue in parts of the state Wednesday and Thursday. This should be limited to Western Washington and mountainous regions of the state, but with southwesterly flow in place there is a small chance that a thunderstorm will form in Central Oregon on Thursday and wander into far Eastern Washington. Precipitation values will also be low during this period, but the consistent moisture will be a welcome relief to those fighting fires.

The Climate Prediction Center issued a La Niña Advisory this week. This means that La Niña conditions are occurring and are expected to continue to do so for at least three months. La Niña occurs when sea surface temperatures in the equatorial East Pacific run at least 0.5°C cooler than average. This may seem like a small difference, but it can have a big influence on global weather patterns – particularly for continents lying near the Pacific. For the Northwest, this makes conditions more favorable for cooler and wetter weather through the winter. This isn't guaranteed, though, as there is a lot more going on that drives regional weather patterns than just equatorial sea surface temperatures.


8 to 14 Day Outlook, September 18 to September 24, 2020
  • Above average temperatures and below average precipitation are favored for the period.

Important Points about Next Week's Weather

South Central Washington
Weekend
  • Wind: Generally S/SW and light to breezy at times, especially though the Basin.
  • Precipitation: No precipitation expected.
  • High Temps: Low 90s Fri, near 90 Sat, upper 80s Sun.
  • Low Temps: Low 50s Fri night, near 50 Sat night, mid 50s Sun night.
  • Sky Cover: Smoke and haze. Otherwise clear to partly cloudy.
Next Week
  • Wind: Mostly SW and light to breezy Mon and Tue. Transitioning to mostly light and variable by Wed and remaining so for the week.
  • Precipitation: Low chance for a light rain shower or two by Monday night. Low chance for showers lingering through Thu with a stray weak thunderstorm possible for far eastern parts of the region Thu.
  • High Temps: Mid 80s Mon, low 80s Tue, mid 80s Wed, mid 80s Thu.
  • 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.

North Central Washington
Weekend
  • Wind: Generally light and mostly S/SW yet highly variable through areas of complex terrain. Breezy conditions expected at times through the Basin.
  • Precipitation: No precipitation expected.
  • High Temps: Low 90s Fri, low 90s Sat, near 90 Sun. Cooler in higher terrain.
  • Low Temps: Upper 50s Fri night, near 60 Sat night, low 60s Sun night. Cooler in higher terrain.
  • Sky Cover: Smoke and haze. Otherwise clear to partly cloudy.
Next Week
  • Wind: Becoming more consistently S/SW through Mon and breezy. Remaining breezy Tue. Turning light and more northerly yet variable by Wed and remaining so through the week. Continued periods of breeziness through the Basin however.
  • Precipitation: Chance of light rain Mon night and Tue, primarily through upland areas. Continuing on and off in the mountains and upland areas through Thu with greater chances near the Cascade crest.
  • High Temps: Mid 80s Mon, low 80s Tue, mid 80s Wed, mid 80s Thu. Cooler in higher terrain.
  • Low Temps: Mid 50s Mon night, mid 50s Tue night, mid 50s Wed night, mid 50s Thu night. Cooler in higher terrain.
  • Sky Cover: Areas of smoke and haze. Otherwise partly to mostly cloudy.

Eastern Washington
Weekend
  • Wind: Generally S/SW and light to breezy at times, especially though the Basin and Palouse.
  • Precipitation: No precipitation expected.
  • High Temps: Mid 80s Fri, mid 80s Sat, upper 80s Sun.
  • Low Temps: Upper 40s Fri night, upper 40s Sat night, low 50s Sun night.
  • Sky Cover: Smoke and haze. Otherwise clear to partly cloudy.
Next Week
  • Wind: Mostly SW and light to breezy Mon and Tue. Transitioning to mostly light and variable by Wed and remaining so for the week.
  • Precipitation: Low chance for a light rain shower or two by Monday night. Low chance for showers lingering through Thu with a stray weak thunderstorm possible Thu.
  • High Temps: Upper 70s Mon, upper 70s Tue, upper 70s Wed, near 80 Thu.
  • Low Temps: Low 50s Mon night, near 50 Tue night, upper 40s Wed night, low 50s Thu night.
  • Sky Cover: Smoke and haze. Otherwise partly to mostly cloudy.

Western Washington
Weekend
  • Wind: Generally light and W/SW while becoming variable at times. Becoming breezy Sunday night.
  • Precipitation: No precipitation expected.
  • High Temps: Mid 70s Fri, mid 70s Sat, low 70s Sun. Cooler in higher terrain and on the coast.
  • Low Temps: Near 50 Fri night, upper 40s Sat night, low 50s Sun night. Cooler in higher terrain.
  • Sky Cover: Smoke and haze. Otherwise clear to partly cloudy.
Next Week
  • Wind: Transitioning to S Mon and remaining light to breezy at times through Tue. Becoming mostly light and variable through the end of the week.
  • Precipitation: Low chances of rain starting Mon, becoming more widespread and consistent Mon night. Light showers lingering through the week, especially in the mountains.
  • High Temps: Mid 70s Mon, mid 70s Tue, upper 70s Wed, mid 70s Thu. Cooler in higher terrain and on the coast.
  • Low Temps: Mid 50s Mon night, mid 50s Tue night, mid 50s Wed night, mid 50s Thu night. Cooler in higher terrain.
  • Sky Cover: Areas of smoke and haze. Otherwise partly cloudy to overcast.

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