AgWeatherNet Weekly Outlook for October 09 to October 15, 2020

Active fall weather starting this weekend, giving way to drier conditions later next week

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

Synopsis:

A strong cold front will cross Washington from Friday evening through Saturday. Anticipate a sudden change in weather with widespread rain to the entire state including modest rainfall even in the arid regions of the state. Colder northwest flow behind the front will allow for continued shower activity, primarily in mountains and areas west of the Cascades through Sunday before another frontal passage that evening brings a return to widespread shower activity. An additional frontal system will bring continued rain chances throughout the state into early next week. For Wednesday and beyond, a ridge is expected to build offshore and push the storm track northward. With this, it is expected that most of Washington will dry out.

Discussion:

An approaching storm system was producing light showers in the northwestern tip of the Olympic Peninsula Friday morning. An amplifying upper-level wave embedded in a strong jet stream will advance across the state Friday into Saturday. As it moves eastward, it will produce rain showers in Western Washington and in mountainous areas. Winds will pick up this afternoon on the coast and downwind of the Strait of Juan de Fuca, gusting up to 50 mph in exposed locations. An increase in surface winds can as be expected Friday evening downwind of the Cascade mountain gaps such as the Columbia Gorge and Kittitas Valley, though peak gusts are only expected to be around 25 mph in isolated spots.

Modest rainfall values can be expected for most of Washington, mainly Friday night into the first half of Saturday as it moves from west to east. Western Washington lowlands can expect up to an inch of rain with around a quarter inch in east side lowlands. Widespread strong winds will continue through Saturday. Gusts to 35 mph are expected in much of the Columbia Basin and adjacent valleys Saturday afternoon, with higher gusts on ridgetops and in the lee of the Cascade mountain gaps.

The front will also be bringing in a big temperature change. During this week, afternoon highs in some areas have been close to 20° above seasonal averages. The Tri-Cities even hit 90° on Tuesday, a new monthly record for that location. Things have moderated a bit since then, but through the weekend highs will be a few degrees below average statewide with snow levels dropping to around 4500 to 5000 feet. This is high enough that major travel impacts aren't anticipated, but White and Stevens Passes are likely to see snow mixing with rain and the highest points on the North Cascades Highway may have a few inches of snow Saturday night into Sunday.

Temperatures won't just be changing at the surface. The trough behind Saturday's cold front will usher in unusually cold conditions in the mid and upper levels of the atmosphere. With temperatures in the lowlands remaining in the 50s and 60s, postfrontal convective showers are possible in Western Washington through Saturday. A few of these may produce lightning, and it appears likely that the Puget Sound Convergence Zone will become active Saturday afternoon. Given surface, 850 mb, and 700 mb winds, expect this to be oriented mainly west-to-east between Shoreline/Bothell and Everett. Because of the narrow nature of the convergence zone, not everyone between the locations listed will fall under the increased shower activity once it develops.

The center axis of the trough continues eastward into Sunday positioning Washington to the north of the jet stream and under northwesterly upper level flow. Another frontal system will pass through the state on Sunday evening in association with a surface low. This should bring widespread rain to the west side of the state and the Cascades beginning primarily on Sunday afternoon. By Monday morning, the low and front will have moved eastward into the Northern Rockies, leaving behind only leftover upslope showers in the Cascades, Blues, and Okanogan.

Washington being north of the jet stream will keep afternoon high temperatures cool, with showery conditions in the mountains and Western Washington through the weekend and into Monday. The moist and active nature of the pattern will have the opposite effect on low temperatures – helping them to remain a few degrees above average for most of the state.

The third and final frontal system is expected on Tuesday as an upper level high will be centered off San Francisco with an upper level low in southwestern Nunavut. A robust moisture plume is expected to squeeze right between these two systems taking aim at Washington and bringing another round of rain. Current indications are that widespread rain is possible in Western Washington and the mountains, with some lighter showers in Eastern Washington.

Wednesday, the aforementioned upper level high is expected to build into a broader ridge that will extend well into the Gulf of Alaska. This will shift the storm track to the north, starting another period of dry conditions. Temperatures Wednesday and Thursday should still remain near to below average for highs, with lows a few degrees above average.


8 to 14 Day Outlook, October 16 to October 22, 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/S wind. Breezy Sat PM and night, gusts to 35 mph.
  • Precipitation: Rain beginning as early as Fri night. Shifting from widespread Sat AM to scattered Sat PM. Isolated showers possible Sun, becoming scattered Sun night.
  • High Temps: Upper 70s Fri, mid 60s Sat & Sun.
  • Low Temps: Mid 50s Fri night, mid 40s Sat night & Sun night.
  • Sky Cover: Increasing clouds Fri, mostly cloudy to overcast Sat, partly to mostly cloudy Sun.
Next Week
  • Wind: SE/S wind. Breezy conditions possible Mon & Tue.
  • Precipitation: Rain showers possible Tue, otherwise dry.
  • High Temps: Mid 60s Mon-Wed, low 60s Thu.
  • Low Temps: Mid 40s Mon-Tue night, low 40s Wed-Thu night.
  • Sky Cover: Clear to partly cloudy Mon, mostly cloudy to overcast Tue, clear to partly cloudy Wed-Thu.

North Central Washington
Weekend
  • Wind: SW/S wind. Breezy Sat PM and night, gusts to 35 mph.
  • Precipitation: Rain beginning as early as Fri night. Shifting from widespread Sat AM to scattered Sat PM. Showers possible Sun & Sun night.
  • High Temps: Mid 70s Fri, mid 60s Sat, low 60s Sun. Cooler in higher terrain.
  • Low Temps: Mid 50s Fri night, mid 40s Sat night & Sun night. Cooler in higher terrain.
  • Sky Cover: Increasing clouds Fri, mostly cloudy to overcast Sat & Sun.
Next Week
  • Wind: SE/S wind. Breezy conditions possible Mon & Tue.
  • Precipitation: Mountain showers through Wed, with lowland showers possible Tue.
  • High Temps: Mid 60s Mon, low 60s Tue, mid 60s Wed-Thu. Cooler in higher terrain.
  • Low Temps: Near 50 Mon night & Tue night, upper 40s Wed night & Thu night. Cooler in higher terrain.
  • Sky Cover: Partly to mostly cloudy Mon-Wed, clear to partly cloudy Thu.

Eastern Washington
Weekend
  • Wind: SW/S wind. Breezy Sat PM and night, gusts to 35 mph.
  • Precipitation: Rain beginning as early as Fri night. Shifting from widespread Sat AM to scattered Sat PM. Showers possible Sun.
  • High Temps: Near 70 Fri, near 60 Sat, upper 50s Sun. Warmer in the Lewis & Clark Valley.
  • Low Temps: Near 50 Fri night, near 40 Sat night & Sun night. Warmer in the Lewis & Clark Valley.
  • Sky Cover: Clear to partly cloudy Fri, mostly cloudy to overcast Sat & Sun.
Next Week
  • Wind: SE/S wind. Breezy conditions possible Mon & Tue.
  • Precipitation: Rain showers Mon AM with a stray shower or two Mon PM, scattered showers Tue.
  • High Temps: Near 60 Mon-Thu. Warmer in the Lewis & Clark Valley.
  • Low Temps: Near 40 Mon night & Tue night, mid 30s Wed night & Thu night.
  • Sky Cover: Partly to mostly cloudy Mon-Wed, clear to partly cloudy Thu.

Western Washington
Weekend
  • Wind: Windy with gusts to 50 mph on the Coast, in the San Juans, and in the greater Bellingham area. Breezy Sat. Wind generally W with localized variability due to terrain.
  • Precipitation: Coastal rain Fri, widespread rain Fri night-Sun night.
  • High Temps: Mid 60s Fri, mid 50s Sat, upper 50s Sun. Cooler in higher terrain.
  • Low Temps: Mid 50s Fri night, low 50s Sat night, mid 50s Sun night. Cooler in higher terrain.
  • Sky Cover: Overcast Fri-Sun.
Next Week
  • Wind: Generally W/SW Mon-Tue, N/NE Wed-Thu. Breezy conditions possible Tue.
  • Precipitation: Mountain showers Mon, widespread rain Tue, lingering showers possible Wed.
  • High Temps: Near 60 Mon-Thu. Cooler in higher terrain.
  • Low Temps: Low 50s Mon night, near 50 Tue night, upper 40s Wed night & Thu night.
  • Sky Cover: Mostly cloudy to overcast Mon-Thu.

Like us on Facebook at https://www.facebook.com/AgWeatherNet
Follow us on Twitter at https://www.twitter.com/AgWeatherNet
Washington State University