Oregon is entering September under an abrupt change in weather as a succession of unusually strong storm systems descends from the Gulf of Alaska, replacing the state’s prolonged summer dryness with widespread rain, gusty winds, falling temperatures and the possibility of thunderstorms.
The most consequential period is expected Wednesday and Thursday, when a deeper low-pressure system and its associated front cross the Pacific Northwest. Western Oregon will receive the greatest rainfall, but the effects will extend from the coast and Willamette Valley into Southern Oregon, the Cascades and portions of the state east of the mountains.
For a state that has spent much of the summer contending with wildfire, smoke, parched vegetation and exceptionally dry ground, the change will be pronounced. Temperatures in some areas will fall 15 to 20 degrees below seasonal norms while substantial precipitation reaches sections of Oregon that have received little meaningful rainfall for weeks.
Northwestern Oregon will experience some of the strongest conditions. Rainfall from Wednesday morning through Friday morning has a high probability of exceeding one inch from the coast through the Cascade crest, with considerably greater amounts possible in the Coast Range, Cascades and other elevated terrain. Portions of those areas could approach or exceed two inches.
The Interstate 5 corridor will generally receive less rain than the mountains and coast, although a widespread soaking is anticipated across much of the Willamette Valley. Wednesday temperatures in the interior valleys are expected to remain largely in the 60s, a significant departure from the summer warmth that preceded the storm.
Southerly winds will strengthen as the front arrives. Inland gusts could approach 30 mph in northwestern Oregon, with stronger winds through the Columbia River Gorge and Hood River Valley. Oregon’s beaches and exposed headlands could experience gusts approaching 55 to 60 mph.
Conditions offshore will become considerably more hazardous. Gale conditions are forecast along portions of the Oregon coast as southerly winds increase and ocean seas build into the 10 to 14 foot range. Isolated storm-force gusts are possible near the coastline before winds diminish Wednesday night. Elevated seas are expected to subside more slowly and could remain hazardous into Thursday or early Friday.
Thursday introduces another element as colder air moves overhead and atmospheric instability increases. Thunderstorms could develop across western Oregon, carrying the potential for lightning, short periods of heavier rainfall, small hail and erratic wind gusts. Thunderstorm coverage is not expected to be widespread, but scattered development could produce rapidly changing conditions where individual storms form.
Southern Oregon will undergo the same broader transition, although rainfall will vary substantially according to location and elevation.
Rain is expected to spread across southwestern Oregon Wednesday and continue into Thursday before becoming more intermittent. Coastal areas and portions of Douglas and Curry counties are positioned to receive considerably more precipitation than the Rogue Valley, while the Illinois Valley and surrounding higher terrain could also receive meaningful rainfall.
In the Rogue Valley, including Grants Pass and Medford, the storm is expected to produce a noticeable period of rain without the heavier totals anticipated farther north or along the coast. Current forecasts around Medford call for rain developing Wednesday, continuing Wednesday night and diminishing Thursday. Wednesday night could produce between one-quarter and one-half inch in the Medford area, although rainfall totals will vary across Jackson and Josephine counties.
The consequences in Southern Oregon extend beyond the amount collected in rain gauges.
Road conditions will change quickly after months of dry weather. Oils and other residue accumulated on pavement during summer can rise to the surface during the first significant rainfall, temporarily making highways and city streets unusually slick. Periods of heavier rain can also reduce visibility and create standing water where drainage systems are obstructed by leaves, debris or accumulated material.
Travel through portions of Douglas County and other areas containing wildfire scars carries an additional concern. Rainfall over recently burned terrain can accelerate runoff and loosen rock, soil and vegetation. Highways 38, 42 and 138 are among the routes where rockfall, debris movement or localized drainage problems could develop. Burned areas around East Evans Creek and Fielder Mountain are also among the locations being watched as precipitation arrives.
The rain will provide meaningful assistance to wildfire suppression across western Oregon, raising humidity, wetting vegetation and reducing the immediate ability of fuels to ignite and spread. It will not, however, automatically bring Oregon’s fire season to an end. Larger fuels can retain heat and dryness beneath their surfaces, while eastern and southeastern portions of the state will receive substantially less precipitation than western Oregon.
Lightning accompanying thunderstorms could further complicate fire conditions in locations that receive limited rainfall. The broader effect should nevertheless be favorable for active fires across the wetter portions of western Oregon, particularly where sustained precipitation reaches forested terrain.
Smoke conditions may also improve in some locations. The transition toward stronger onshore and southerly flow, combined with rainfall and increasing atmospheric mixing, can disperse smoke that has lingered in valleys and along the western slopes of the Cascades. Local conditions will continue to depend upon active fires, wind direction and terrain.
East of the Cascades, the storm will be less uniform. Portions of central and northeastern Oregon can expect periods of rain, breezy conditions and isolated thunderstorms, while southeastern Oregon is likely to receive considerably lighter precipitation. Cooler temperatures will extend across a much larger portion of the state than the heaviest rainfall.
Southern Oregon could also encounter notably cold mornings. Higher elevations and areas east of the Cascades may approach frost conditions as colder air settles across the region, an unusually sharp contrast to the hot and dry conditions experienced during much of the summer.
The main storm will begin loosening its grip late Thursday and Friday, but Oregon is not expected to return immediately to sustained summer weather. Cooler-than-normal temperatures and intermittent showers remain possible into the weekend as low pressure lingers near the region.
For Josephine and Jackson counties, Wednesday and Wednesday night currently represent the principal period for widespread rainfall, followed by diminishing precipitation Thursday and lingering chances for showers later in the week. Residents traveling north toward Douglas County, the Willamette Valley or the Oregon Coast will encounter progressively wetter and, in some areas, considerably windier conditions.
The September storm marks one of Oregon’s sharpest weather transitions of the late summer season. Within a matter of days, parts of the state will move from wildfire-season dryness and summer temperatures into a distinctly cooler Pacific pattern capable of delivering substantial mountain rainfall, coastal gales, thunderstorms and the first widespread soaking of the approaching fall season.