AgWeatherNet Weekly Outlook for September 4 to September 10, 2020

Record heat followed by a strong cold front and dangerous wildfire conditions

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

Synopsis:

A strong upper level ridge centered offshore will be the primary influence on weather conditions across the region for the coming week, but the effects of this ridge will be quite different on either side of the Cascades. In the east, the hottest conditions are expected to come today and tomorrow (Friday and Saturday). West of the crest, the warmest days will be early next week as a strong cold front coming in from Alberta will reverse the wind flow from onshore to offshore. Intermittently breezy to windy conditions will enhance fire danger and create blowing dust in some parts of the state and easterly gap winds through the Cascades. Little to no precipitation is expected throughout Washington.

Discussion:

Meteorological fall began September 1, but a building ridge offshore will keep summer-like conditions in place across Washington for quite some time. East of the Cascades, peak high temperatures are expected to come Friday and Saturday when some parts of the Columbia Basin may reach 100°, possibly breaking records. Flow will be onshore through the weekend, keeping highs somewhat subdued in Western Washington. Highs in Seattle look to be in the mid-80s Friday and mid-70s Saturday, with warmer conditions along I-5 to the south and cooler conditions to the north and along the coast.

The ridge backs off somewhat Saturday and Sunday, so while temperatures will be unseasonably warm, they won't be extreme. Through the weekend, breezy conditions are possible in the lee of the Cascades which will increase fire danger, particularly close to the mountains. A big change comes Monday when a strong cold front shifts the wind direction from onshore (coming off the Pacific) to offshore (blowing toward the Pacific). This is the same cold front that is forecast to drop highs in Denver from near 90° to near 40° with a chance of snow, but it won't be as extreme in the Pacific Northwest.

Much of the moisture from this frontal passage will be wrung out as it crosses the Rockies, but breezy to windy conditions are expected in Eastern Washington as well as in westside locations prone to strong easterly gap winds. This includes Enumclaw, the Snoqualmie Valley and the west end of the Columbia Gorge. Fire danger will increase significantly by Monday night. Any new wildfires are at risk for extreme behavior and existing wildfires will contend with the added twist of wind direction, which may send ongoing fires heading into previously unburnt vegetation. Blowing dust will also be an issue in the Columbia Basin.

Much of the work week will be hot in Western Washington, especially for this time of year. Highs well into the 90s can be expected Tuesday through Thursday for the I-5 corridor. Somewhat cooler temperatures will be seen close to the water. Additional record highs are likely to fall in this timeframe on the west side. Model guidance suggests that isolated instances of 90° highs may extend all the way to Abbotsford, and some coastal parts of the coast may hit 80°.

Since the moisture from the front will be lost by the time it reaches Washington, dew points will drop behind it allowing for some wide diurnal temperature swings. This is especially the case Monday and Tuesday. Some of the more sheltered valleys north of Spokane may experience their first frost of the season, but freezing temperatures shouldn't be widespread. Highs will bottom out around 80° in the warmest parts of the Basin, with mid 70s forecast for most low elevation locations. The ridge will rebuild following the cold front, allowing afternoon highs to rebound in Eastern Washington. While they should be above average for this time of year, they aren't expected to be as hot as the western half of the state.

Despite appearing as though the ridge will continue into the following weekend, a more zonal pattern is possible. This would moderate temperatures in Western Washington, but the presence of the ridge may keep temperatures above average so even though we are in meteorological fall it will likely be a while before we start experiencing autumn weather.


8 to 14 Day Outlook, September 11 to September 17, 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: N wind Fri. SW wind with gusts to 20 mph Sat. Light and variable Sun.
  • Precipitation: No precipitation expected.
  • High Temps: Near 100 Fri, upper 90s Sat, mid 90s Sun.
  • Low Temps: Mid 60s Fri night, near 60 Sat night, low 60s Sun night.
  • Sky Cover: Smoky conditions possible, otherwise sunny to partly cloudy.
Next Week
  • Wind: NE wind. Breezy Monday with gusts to 40 mph.
  • Precipitation: No precipitation expected.
  • High Temps: Low 80s Mon, near 80 Tue, mid 80s Wed, near 90 Thu.
  • Low Temps: Low 50s Mon night through Thu night.
  • Sky Cover: Sunny to partly cloudy.

North Central Washington
Weekend
  • Wind: Light and variable Fri, NW wind Sat & Sun with gusts to 25 mph Sat.
  • Precipitation: No precipitation expected.
  • High Temps: Upper 90s Fri, mid 90s Sat, low 90s Sun. Cooler in higher terrain.
  • Low Temps: Near 70 Fri night through Sun night. Cooler in higher terrain.
  • Sky Cover: Smoky conditions possible, otherwise sunny to partly cloudy.
Next Week
  • Wind: NE wind. Breezy Mon with gusts to 40 mph.
  • Precipitation: No precipitation expected.
  • High Temps: Mid 80s Mon, near 80 Tue, upper 80s Wed, low 90s Thu. Cooler in higher terrain.
  • Low Temps: Low to mid 50s Mon night through Thu night. Cooler in higher terrain with isolated frost possible in sheltered mid and high elevation valleys.
  • Sky Cover: Sunny to partly cloudy.

Eastern Washington
Weekend
  • Wind: Light and variable Fri. SW wind Sat & Sun.
  • Precipitation: No precipitation expected.
  • High Temps: Mid 90s Fri, low 90s Sat, near 90 Sun. Warmer in the Lewis & Clark Valley.
  • Low Temps: Near 60 Fri night, upper 50s Sat & Sun night. Warmer in the Lewis & Clark Valley.
  • Sky Cover: Smoky conditions possible, otherwise sunny to partly cloudy.
Next Week
  • Wind: NE wind with breezy conditions Mon. Gusts to 40 mph.
  • Precipitation: No precipitation expected.
  • High Temps: Mid 70s Mon & Tue, mid 80s Wed, upper 80s Thu. Warmer in the Lewis & Clark Valley.
  • Low Temps: Mid 40s Mon night, low 40s Tue night, near 50 Wed night, low 50s Thu night.
  • Sky Cover: Sunny to partly cloudy.

Western Washington
Weekend
  • Wind: Generally W wind with localized variations due to topography. Breezy along the Strait of Juan de Fuca.
  • Precipitation: No precipitation expected.
  • High Temps: Upper 70s to upper 80s Fri, 70s Sat, mid 70s to mid 80s Sun. Cooler in higher terrain and along the coast.
  • Low Temps: Mid to upper 50s Fri night, upper 50s to low 60s Sat and Sun night. Cooler in higher terrain.
  • Sky Cover: Mostly cloudy through Saturday, then sunny to partly cloudy.
Next Week
  • Wind: Generally E wind with localized variations due to topography. Intermittently breezy in areas, especially valleys that drain the Cascades and in the Strait of Juan de Fuca.
  • Precipitation: No precipitation expected.
  • High Temps: Mainly 80s Mon & Tue, upper 80s to mid 90s Wed & Thu. Warmer near Portland. Cooler in higher terrain and along the coast.
  • Low Temps: Mid 50s to mid 60s Mon night through Thu night. Cooler in higher terrain.
  • Sky Cover: Sunny to partly cloudy.

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