AgWeatherNet Weekly Outlook for September 25 to October 01, 2020

Active weather coming to an end by Sunday with a dry week to follow

Mark Ingalls, AgWeatherNet Intern
Craig Oswald, AgWeatherNet Field Meteorologist

Synopsis:

An atmospheric river aimed at the Pacific Northwest will be pushed northward over the weekend as a ridge of high pressure builds offshore. The ridge will slowly move east as it builds, putting it directly over Washington on Tuesday and Wednesday and slightly east on Thursday. The result will be climbing temperatures and mostly sunny skies across the state, though haze will be possible in more populated regions with minimal wind to keep the air clean.

Discussion:

The widespread rain showers that spread east of the Cascades from western Washington this morning (Friday), in conjunction with a passing frontal boundary, will continue to diminish through the afternoon and evening. As the front continues east, widespread rain in western Washington will continue to give way to scattered showers.

Conditions are also favorable for the Puget Sound Convergence Zone to become active Friday afternoon and evening, adding to observed rainfall values in this narrow band of more intense precipitation. Given the trajectory of incoming wind and moisture, it appears that it will develop in Snohomish County, but this doesnt mean all of Snohomish County will be affected by it. The Puget Sound Convergence Zone is typically aligned from west to east stretching from near the sound itself into the Cascades but is only about 10 to 15 miles wide of its north-south axis.

On top of rain, breezy conditions are expected with this frontal passage. Most of Washington can expect breezy to windy conditions through Friday night with winds generally southerly or southwesterly (locally variable based on topography). Gusts may reach to 30 mph in more exposed locations west of the Cascades, with gusts to 45 mph in the windiest parts of Eastern Washington. Windy conditions for Eastern Washington will continue into Saturday with gusts possibly reaching 40 mph in Adams and Grant Counties.

Shower activity will taper off in Eastern Washington lowlands Friday evening and into the overnight hours, while showers are expected to continue in mountain areas and Western Washington lowlands through Saturday. A shortwave embedded within the mean flow will brush Washington on Saturday evening bringing a final shot of rain showers to the northern third of the state, especially in the Olympics, Cascades, and upland areas of the Okanogan.

High temperatures will climb from five to ten degrees below average statewide on Friday to be about five degrees above average west of the Cascades and near average to the east on Monday. The high amplitude ridge continuing to build through the week will place highs in the region around ten degrees above average to start October, with the warmest parts of the state expected to be near or above 80° beginning Tuesday.

Low temperatures on the west side are expected to remain fairly static with little variation from this weekend. Clearing skies and declining dew point values will allow for cooler conditions in Eastern Washington Sunday, and Monday, mornings, with lows potentially falling to around five degrees below average in spots. A widespread hard freeze event is not expected, but frost may develop in some more well protected valleys  particularly north of US 2 and toward the northeast corner of the state where mid 30s are the potential with clear skies and light winds as expected. The Okanogan and upper Yakima Valley will also be under similarly cool upper air Sunday morning, but with clouds expected to linger, temperatures in the cooler locations will likely remain around 40. Warmer air looks to move in from the south during the day Sunday, and will lead to an increase in overnight lows for the Okanogan and Upper Yakima by Monday morning and the northeast corner of the state by Tuesday morning.

Relative humidity values are expected to be low in arid parts of Washington and Oregon to start next week, but with the ridge in place strong winds are not expected. This leads to the expectation that while people east of the Cascades should continue to be fire aware, extreme fire behavior like what was seen on Labor Day is not expected during the period.


8 to 14 Day Outlook, October 02 to October 08, 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: SW and windy with gusts to 45 mph in some locations. Trending to light and variable through Sat night and Sun.
  • Precipitation: Low chance for a light rain shower or two through Friday night. Dry conditions expected Saturday with a low chance for a brief sprinkle Saturday night. Dry Sunday.
  • High Temps: Upper 60s Sat, near 70 Sun.
  • Low Temps: Mid 40s Fri night, low 40s Sat night, low 40s Sun night.
  • Sky Cover: Partly cloudy, clearing to mostly sunny Sun and mostly clear Sun night.
Next Week
  • Wind: Light and variable.
  • Precipitation: Dry conditions expected.
  • High Temps: Mid 70s Mon, upper 70s Tue, near 80 Wed, low 80s Thu.
  • Low Temps: Mid 40s Mon night, mid 40s Tue night, upper 40s Wed night, upper 40s Thu night.
  • Sky Cover: Sunny and mostly clear.

North Central Washington
Weekend
  • Wind: W/SW and breezy to windy, especially through the Basin where gusts to 45 mph are possible. Trending to light and variable through Sat night and Sun.
  • Precipitation: A few scattered rain showers to linger, primarily in upland areas through Friday night. Trending mostly dry Saturday with a return of scattered rain showers Saturday night. Dry Sunday.
  • High Temps: Mid 60s Sat, upper 60s Sun. Cooler in high terrain.
  • Low Temps: Near 40 Fri night, low 40s Sat night, low 40s Sun night. Cooler in high terrain and deep valleys.
  • Sky Cover: Partly to mostly cloudy. Becoming mostly sunny and clear Sunday.
Next Week
  • Wind: Light and variable.
  • Precipitation: Dry conditions expected.
  • High Temps: Low 70s Mon, upper 70s Tue, low 80s Wed, low 80s Thu. Cooler in high terrain.
  • Low Temps: Upper 40s Mon night, mid 40s Tue night, mid 40s Wed night, mid 40s Thu night. Cooler in high terrain and deep valleys.
  • Sky Cover: Sunny and mostly clear.

Eastern Washington
Weekend
  • Wind: SW and windy with gusts to 45 mph in some locations. Trending to light and variable through Sat night and Sun.
  • Precipitation: A few scattered rain showers to linger through Friday night. Trending mostly dry Saturday with a return of scattered rain showers Saturday night. Dry Sunday.
  • High Temps: Low 60s Sat, low 60s Sun.
  • Low Temps: Mid 40s Fri night, upper 30s Sat night, mid 30s Sun night.
  • Sky Cover: Partly to mostly cloudy. Becoming mostly sunny and clear Sunday.
Next Week
  • Wind: Light and variable.
  • Precipitation: Dry conditions expected.
  • High Temps: Upper 60s Mon, mid 70s Tue, mid 70s Wed, upper 70s Thu.
  • Low Temps: Mid 30s Mon night, upper 30s Tue night, low 40s Wed night, upper 40s Thu night.
  • Sky Cover: Sunny and mostly clear.

Western Washington
Weekend
  • Wind: S/SW and breezy with windier conditions near the coast and sound. Gusts to 30 mph in some locations. Trending lighter and more variable through Sat night and Sun.
  • Precipitation: Rain showers trending more scattered and less widespread in nature through Friday and Friday night. Resurgence of shower activity Saturday afternoon. Trending dry Sunday. More consistent shower activity through the Olympics and Cascades.
  • High Temps: Mid 60s Sat, upper 60s Sun. Much cooler through the Cascades and Olympics.
  • Low Temps: Upper 40s Fri night, mid 40s Sat night, mid 40s Sun night. Much cooler through the Cascades and Olympics.
  • Sky Cover: Mostly cloudy to cloudy. Trending to partly cloudy through Sat night. Partly sunny Sun. Mostly clear Sun night.
Next Week
  • Wind: Light and variable.
  • Precipitation: Dry conditions expected.
  • High Temps: Low 70s Mon, mid 70s Tue, upper 70s Wed, upper 70s Thu. Much cooler through the Cascades and Olympics.
  • Low Temps: Upper 40s Mon night, low 50s Tue night, low 50s Wed night, low 50s Thu night. Much cooler through the Cascades and Olympics.
  • Sky Cover: Sunny and mostly clear.

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