AgWeatherNet Weekly Outlook for September 18 to September 24, 2020

Active fall weather to bring an end to widespread smoky conditions plus the possibility of a significant rain event mid-next week

Mark Ingalls, AgWeatherNet Intern
Joe Zagrodnik, AgWeatherNet Postdoctoral Research Associate

Synopsis:

A low pressure system and associated cold front will bring widespread rain showers to Washington state Friday and Saturday. Breezy conditions are possible in the lee of the Cascades with the frontal passage, then a zonal flow pattern will be established. This activity should clear the dense smoke from much of the state. A shower or two is possible Monday into Tuesday, mostly on the west side, but the next widespread rain event looks to be coming next Wednesday or Thursday with another robust frontal passage.

Discussion:

The low pressure system located offshore Friday morning will continue to move eastward, draping a cold front that will cross Washington through Saturday. The system will weaken somewhat as it passes through, but should still be strong enough to produce showers and perhaps a rumble of thunder throughout the state. Rain chances are highest in and near mountainous regions of the state, decreasing to around 20% for the center of the Columbia Basin, roughly along a line stretching between the Tri-Cities and Moses Lake.

Even for spots that stay dry, the vertical mixing and change in wind direction to westerly should be enough to produce significant air quality improvements. Some more sheltered valleys may hold on to stubborn smoke for longer, but daytime solar radiation will still increase in those locations as much of the vertical smoke column is whisked away to the east. Generally hazy conditions remain possible beyond Saturday, with dense smoke directly downwind of larger fires, but with the large-scale flow shifting to become more zonal air quality should remain pleasant through the weekend.

High temperatures for the western half of the state will be a few degrees below average Saturday and near average Sunday. In Eastern Washington, expect highs near average both days. Lows are expected to be well above seasonal averages throughout the state through the weekend. For context, the average high and low for this time of year is 70°/52° in Seattle, 77°/43° in Yakima, and 72°/47° in Spokane.

A weak shortwave trough embedded within the mean flow is expected to pass over Washington Monday into Tuesday. This will be a low-end event, with rain showers limited to the west side. It is possible that locally breezy conditions will be observed with this passage in the Columbia Gorge and Kittitas Valley, but outside of those areas winds should remain fairly light state wide.

The next big headliner comes Wednesday and Thursday when the strongest low pressure system of the fall so far will move into the Gulf of Alaska. A trailing cold front with a strong atmospheric river is expected to reach coastal portions of Washington by Wednesday. The combination of strong forcing from the storm system and a significant influx of water vapor from the atmospheric river brings the possibility of significant rainfall and windy conditions to much of the state, including central and eastern Washington.

Models are still highly uncertain with the timing and intensity of this storm, so more specific information on the forecast for this storm will be detailed in the Monday Outlook. Some models have the greatest impacts aimed directly at Washington State, which would initiate a decent stretch of quality rainfall, but others have it aimed more toward the central or even northern British Columbia coast, which would leave the Pacific Northwest with some rain and wind, but nothing especially significant for this time of year.

With rain chances increasing across the state, it is important to note flash flood risks to fresh burn scars. Fires remove substantial amounts of vegetation that plays a critical role in keeping soil in place. In places where that is gone, there is a risk of flash flooding, debris flows, and mudslides, even in events that have low rainfall values compared to past events that have caused problems in nearby locations. When near a burn scar, monitor local products from the National Weather Service (especially flash flood watches and warnings) to keep situationally aware.


8 to 14 Day Outlook, September 25 to October 01, 2020
  • Above average temperatures and above average precipitation are favored for the period.

Important Points about Next Week's Weather

South Central Washington
Weekend
  • Wind: Generally S to SW. Locally breezy Fri night.
  • Precipitation: Isolated showers Fri night, otherwise dry.
  • High Temps: Upper 70s Fri-Sun. Cooler in dense smoke.
  • Low Temps: Mid 50s Fri night, near 50 Sat night, mid 40s Sun night.
  • Sky Cover: Smoky Fri, mostly cloudy Fri night, clear to partly cloudy Sat-Sun.
Next Week
  • Wind: W to SW Mon & Tue, S Wed and Thu. Locally breezy Mon, Wed, & Thu.
  • Precipitation: Rain showers possible Wed night into Thu, otherwise dry.
  • High Temps: Upper 70s Mon-Tue, near 80 Wed-Thu.
  • Low Temps: Low 50s Mon night-Tue night, mid 50s Wed night-Thu night.
  • Sky Cover: Clear to partly cloudy Mon-Wed, partly to mostly cloudy Thu.

North Central Washington
Weekend
  • Wind: Generally W to SW. Locally breezy Fri.
  • Precipitation: Isolated showers Fri, rain likely Fri night, possible left over showers Sat. Otherwise dry.
  • High Temps: Low 80s Fri, mid 70s Sat & Sun. Cooler in dense smoke and higher terrain.
  • Low Temps: Upper 50s Fri & Sat night, mid 50s Sun night. Cooler in higher terrain.
  • Sky Cover: Partly to mostly cloudy through Sat morning, then clear to partly cloudy.
Next Week
  • Wind: W to SW Mon & Tue, S Wed and Thu. Locally breezy Mon, Wed, & Thu, particularly in Basin locations.
  • Precipitation: Isolated mountain showers Mon-Thu. Lowland showers possible Wed night & Thu.
  • High Temps: Mid 70s Mon-Thu. Cooler in higher terrain.
  • Low Temps: Mid 50s Mon & Tue night. Upper 50s Wed & Thu night. Cooler in higher terrain.
  • Sky Cover: Clear to partly cloudy Mon-Tue, partly cloudy to overcast Wed-Thu.

Eastern Washington
Weekend
  • Wind: Generally S to SW. Locally breezy Sat night.
  • Precipitation: Scattered rain showers Fri night-Sat night.
  • High Temps: Upper 70s Fri, low 70s Sat & Sun. Warmer in the Lewis & Clark Valley, cooler in dense smoke.
  • Low Temps: Near 50 Fri & Sat night, mid 40s Sun night. Warmer in the Lewis & Clark Valley.
  • Sky Cover: Areas of smoke Fri. Mostly cloudy to overcast through Sat night. Clear to partly cloudy Sun.
Next Week
  • Wind: W to SW Mon & Tue, S Wed and Thu. Locally breezy Mon, Wed, & Thu.
  • Precipitation: Chance of rain showers Wed Night-Thu, otherwise dry.
  • High Temps: Mid 70s Mon & Tue, upper 70s Wed & Thu. Warmer in the Lewis & Clark Valley.
  • Low Temps: Upper 40s Mon & Tue night, low 50s Wed & Thu night. Warmer in the Lewis & Clark Valley.
  • Sky Cover: Clear to partly cloudy Mon & Tue, partly cloudy to overcast Wed & Thu.

Western Washington
Weekend
  • Wind: Generally S Fri & Sat, becoming W Sun.
  • Precipitation: Rain likely Fri & Sat, tapering off Sat night. A few lingering mountain showers Sun.
  • High Temps: Mid 60s Fri & Sat, near 70 Sun. Cooler in higher terrain and on the coast.
  • Low Temps: Near 60 Fri & Sat night, upper 50s Sun night.
  • Sky Cover: Mostly cloudy to overcast.
Next Week
  • Wind: Light & variable Mon, W Tue, S and breezy Wed & Thu.
  • Precipitation: Rain showers possible Mon & Tue, rain likely Wed & Thu.
  • High Temps: Near 70 Mon-Thu. Cooler in higher terrain and on the coast.
  • Low Temps: Upper 50s Mon-Tue night, near 60 Wed-Thu night. Cooler in higher terrain.
  • Sky Cover: Mostly cloudy to overcast.

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