Western Oregon's Climate: Mediterranean (Cool Wet Winters, Dry Warm Summers)

Western Oregon (Willamette Valley, Coast Range, coastal strip) has a Mediterranean climate (Köppen Csb)—rare outside the Mediterranean Basin.

Seasonal Pattern

• Winter (Nov–Mar): Cool (8–12°C / 46–54°F highs), wet (70% of annual rain), cloudy. Snow rare at low elevations.
• Spring (Apr–Jun): Gradual drying, wildflower blooms, variable temps.
• Summer (Jul–Sep): Warm (25–30°C / 77–86°F), dry (<5% annual rain), low humidity, long days. Occasional heat domes >38°C.
• Autumn (Oct): Rapid transition, first rains, color change.

Key Data (Portland, Representative)

Ecological & Agricultural Impact

• Native: Douglas fir, western redcedar, Oregon white oak, camas prairies.
• Crops: Wine grapes (Pinot Noir), hazelnuts (99% US), berries, grass seed, nursery stock.
• Fire season: Aug–Sep (increasing due to climate change).

Contrast: Eastern Oregon

Rain shadow of Cascades → semi-arid (BSk), cold winters, hot summers, sagebrush steppe.