AgWeatherNet Weekly Weather Outlook Monday Update for September 21 to September 27, 2020

Significant frontal passage mid week likely to produce rain statewide

Craig Oswald, AgWeatherNet Field Meteorologist

Synopsis:

The outlook period looks to be quite active, bringing much welcomed rain throughout portions of the state all week and through the weekend. A weak upper disturbance will bring light showers to the Cascades and western Washington tonight (Monday) and into Tuesday. This quick teaser will be the precursor to a potent frontal passage that is expected to bring moderate rain showers to western Washington beginning Wednesday morning. Showers look to spread east/southeast across the state bringing mostly scattered light rain showers throughout eastern Washington, with more consistent activity throughout high terrain. Windy conditions can also be expected with the passage of this system. Two quick moving and weaker disturbances are to pass the region Friday and Saturday night, bringing more rain to western Washington and chances for rain across the majority or eastern Washington along with breezy conditions.

Discussion:

The broader synoptic set up that is to be the influencer of the weather over the course of the outlook period is a large low pressure system currently located in the Gulf of Alaska. A modest upper disturbance ejected from this system looks to move across the Pacific Northwest tonight and tomorrow (Tuesday). This will bring a few light scattered showers to western Washington with more consistent shower activity throughout the Olympics and Cascades. High elevation areas of northern Washington east of the Cascades stand a low chance to catch a light rain shower or two as well. Otherwise the majority of eastern Washington will experience an influx of mid level clouds (already well moved in at the time of this writing) but will likely remain dry.

Fast on the heels of this disturbance is the passage of a robust cold front with plenty of moisture available to produce rain showers throughout the state beginning as early as Tuesday night along the coast. Widespread light to moderate rain showers can be expected through Thursday in western Washington before slightly backing off, with scattered light showers to prevail through Thursday night. Eastern Washington can expect light rain showers by Wednesday as the front continues to move through from the west/northwest. It is expected that all areas of the Columbia Basin will see scattered light rain showers move through with more consistent shower activity throughout the northern mountain ranges, Blue Mountains of southeast Washington, and even the Palouse. Windy conditions can also be anticipated as the front progresses across the state. Gusts to 40-45 mph are likely, especially in areas such as the coast and Basin/Palouse.

As an aside, the robust low pressure system associated with the frontal passage is expected to continue to deepen thanks, in part, to its proximity to a strong jet across the north Pacific. The center of the system looks to remain in the Gulf of Alaska and not actually impact the west coast. The persistence of this low pressure system, which looks to be rejuvenated again beyond the outlook period, may be hints of the initiation of the Alaskan low that takes residence in the Gulf of Alaska through the winter storm season. This acts as the main influencer of storm tracks across the Pacific Northwest as the region continues to slowly transition toward the storm season.

The next upper disturbance looks to quickly pass across British Columbia Friday but will bring enough forcing to initialize a very modest surface depression that will impact the Pacific Northwest according to the latest model guidance. Western Washington looks to again see widespread light to moderate rain showers with the Olympics and Cascades maintaining more consistent showers. All portions of eastern Washington will also see a renewed chance for scattered light rain showers Friday and Friday night with of course high terrain areas receiving more consistent rain fall. The Palouse and portions of far eastern Washington along the Idaho border can also expected to see more scattered shower activity than what is anticipated for the Columbia Basin and South Central Washington. Breezy conditions can also be expected.

Finally, Saturday looks to see yet another upper disturbance ejected from the large circulation over the Gulf of Alaska quickly move through British Columbia and stir up a few rain showers throughout the state. These showers are anticipated to be weaker and less widespread than Friday’s event. Showers are expected to impact primarily western Washington and the Cascades with greater activity to the north. Mostly upland areas of northern Washington will also likely see light rain showers with a chance for a shower or two across low land areas of northern Washington (excluding the Basin however) and northern parts of eastern Washington by Saturday night. South central, the Columbia Basin, and southeast Washington are expected to remain dry with mostly cloudy skies. Showers will linger through Saturday night and slowly begin to trend dry through Sunday and Sunday night as the upper wave pattern shifts west, allowing for a ridge to build into the western U.S. and Canada ushering in dry weather and warmer temperatures by early next week.

All in all, rain totals by Sunday, at the end of the outlook period, look to quickly add up. Most low land areas throughout western Washington are expected to see around 2” of rain, with higher elevation areas throughout the Olympics and Cascades edging to near 5”. Low land portions of eastern Washington may find rainfall totals highly variable, especially with rain showers expected to be more scattered in nature, however, areas in south central Washington and the Columbia Basin can generally be expected to see between 0” and 0.25” by Sunday. Far eastern Washington looks to trend more toward 0.25” and 0.5” with some spots along the Idaho border possibly receiving more. The mountain ranges of northern Washington and the Blue Mountains can be anticipated to receive between 1” and 1.5” total by Sunday. While this rainy period is very much welcomed, please be cognizant that the risk of land and/or mud slides is a strong possibility, especially over fresh burn scars as alluded to in last Fridays outlook. The loss of vegetation in burned areas makes slopes much more likely to slide as root systems are not available to help hold soil in place.


Important Points About This Week's Weather

South Central Washington
Remainder of the week
  • Wind: S/SW and light Tue. Becoming breezier Wed. Windy Thu with gusts to 40-45 mph, especially through the Basin and Palouse. Trending back to breezy Friday.
  • Precipitation: Scattered light rain showers Wed while trending dry Wed night. Another, albeit lesser round of scattered light rain showers late Fri through Fri night.
  • High Temps: Upper 70s Tue, upper 70s Wed, upper 60s Thu, upper 60s Fri.
  • Low Temps: Upper 40s Mon night, low 50s Tue night, mid 50s Wed night, upper 40s Thu night.
  • Sky Cover: Partly cloudy Mon night and clearing to sunny Tue. Transiting to mostly to partly cloudy for the remainder of the week starting Tue night.

Weekend
  • Wind: Remaining S/SW and breezy with gusts to around 30 mph. Trending light and more variable Sun night.
  • Precipitation: Dry conditions expected.
  • High Temps: Low 70s Sat, upper 70s Sun.
  • Low Temps: Upper 40s Fri night, low 50s Sat night, low 50s Sun night.
  • Sky Cover: Partly cloudy.

North Central Washington
Remainder of the week
  • Wind: Generally S and light yet variable Tue. Becoming breezier Wed. Turning SW and windy Thus with gusts to 40-45 mph, particularly through the Basin. Generally SW and breezy Fri.
  • Precipitation: Low chance for a high elevation rain shower Mon night. Light rain showers Wed while trending dry Wed night. More scattered in nature across the Basin. More consistent shower activity through high terrain with a few showers likely to linger through Thu. Another round of showers Fri through Fri night.
  • High Temps: Upper 70s Tue, upper 60s Wed, low 70s Thu, upper 60s Fri. Cooler in high terrain.
  • Low Temps: Low 50s Mon night, low 50s Tue night, low 50s Wed night, mid 40s Thu night. Cooler in high terrain.
  • Sky Cover: Mostly cloudy through the week.

Weekend
  • Wind: Remaining SW and breezy with gusts to around 30 mph, particularly through the Basin. Trending light and more variable Sun night.
  • Precipitation: A few lingering rain showers Saturday. Mostly high terrain scattered light rain showers to move back in Sat night with a chance for a stray shower or two throughout low land areas. Trending dry through Sunday. Dry conditions expected across the Basin.
  • High Temps: Near 70 Sat, upper 70s Sun. Cooler in high terrain.
  • Low Temps: Mid 40s Fri night, near 50 Sat night, low 50s Sun night. Cooler in high terrain.
  • Sky Cover: Partly to mostly cloudy.

Eastern Washington
Remainder of the week
  • Wind: S/SW and light Tue. Becoming breezier Wed. Windy Thu with gusts to 40-45 mph, especially through the Palouse. Trending back to breezy Friday.
  • Precipitation: Scattered light rain showers late Wed while trending dry Wed night with a few showers lingering near the Idaho border Thu morning. More consistent shower activity through the Blue Mtns. Another round of scattered light rain showers late Fri through Fri night.
  • High Temps: Low 70s Tue, mid 70s Wed, mid 60s Thu, low 60s Fri. Cooler through the Blues.
  • Low Temps: Mid 40s Mon night, mid 40s Tue night, mid 40s Wed night, low 40s Thu night. Cooler through the Blues.
  • Sky Cover: Partly to mostly cloudy through the week.

Weekend
  • Wind: Remaining S/SW and breezy with gusts to around 30 mph. Trending light and more variable Sun night.
  • Precipitation: A few scattered showers lingering through Sat, especially in the Blue Mtns. and northern portions of the region. Scattered rain showers to move back in Sat night through northern portions of the region while remaining/trending dry elsewhere. Any showers to trend dry through Sun.
  • High Temps: Upper 60s Sat, mid 70s Sun. Cooler through the Blues.
  • Low Temps: Low 40s Fri night, mid 40s Sat night, mid 40s Sun night. Cooler through the Blues.
  • Sky Cover: Partly to mostly cloudy.

Western Washington
Remainder of the week
  • Wind: Generally S and light yet variable Tue. Becoming breezier Wed breezy conditions and gusts near 30 mph through the week. Gusty at times near the coast with gusts to 40-45 mph.
  • Precipitation: Continued scattered light rain showers with more consistent shower activity through the Cascades and Olympics. Widespread light and moderate showers begin moving in Tue night and remain through Wed night. Trending lighter and more scattered in nature Thu. Renewed surge of light to moderate rain showers Fri with greater activity through the mountains.
  • High Temps: Near 70 Tue, mid 60s Wed, mid 60s Thu, mid 60s Fri. Much cooler through the Cascades and Olympics.
  • Low Temps: Low 50s Mon night, mid 50s Tue night, mid 50s Wed night, low 50s Thu night. Much cooler through the Cascades and Olympics.
  • Sky Cover: Mostly cloudy to overcast. Areas of fog expected.

Weekend
  • Wind: S/SW and breezy with gusts to around 30 mph. Trending light and more variable Sun night.
  • Precipitation: Shower activity lessening Sat but increasing again Sat night, primarily through the northern half of the region. Showers to slowly dissipate through Sunday and eventually trend dry through Sun night. Greater activity through the Cascades and Olympics.
  • High Temps: Upper 60s Sat, low 70s Sun.
  • Low Temps: Near 50 Fri night, low 50s Sat night, low 50s Sun night. Much cooler through the Cascades and Olympics.
  • Sky Cover: Mostly cloudy to overcast, becoming partly cloudy Sunday.

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