Why is late July so dry in the Pacific Northwest? And a mixed forecast for this weekend.

July 15, 2022·10 min·1 clip

Why is the Pacific Northwest drier than Arizona in late July, and what unique weather patterns cause this extreme dryness?

Cliff Mass, a professor of atmospheric sciences, explains the Pacific Northwest's dry late-July pattern and provides the weekend forecast. The episode blends a specific weather prediction with a broader climatological lesson. Mass addresses why the region experiences its driest conditions while other parts of the country receive summer rain. He forecasts a cool, cloudy weekend with possible showers for western Washington, while eastern Washington will see sun and highs in the mid to upper 90s. A weak frontal system may bring light rain to the north coast on Friday. An upper-level trough on Saturday and Sunday will maintain cooler temperatures, around 70 degrees, west of the Cascades. The long-term forecast predicts a return to sun and dry conditions with highs near 80 degrees for the following week. Mass notes the climatological probability of measurable rain in late July is below 10% in Seattle and around 5% in Spokane. He states the Pacific Northwest is drier than Arizona or New Mexico in midsummer, with only Southern California being drier. A key insight is that mid-latitude precipitation comes from either frontal systems or convective thunderstorms. The eastern U.S. maintains year-round precipitation by receiving winter storms and summer thunderstorms fueled by Gulf moisture. In contrast, the Northwest's winter precipitation from Pacific storms ceases in summer as the jet stream moves northward. The region lacks compensatory summer thunderstorms due to cool, stable air from the Pacific Ocean. Mass explains that ocean temperatures around 50 degrees Fahrenheit inhibit the surface heating and moisture necessary for thunderstorm development. This "marine effect" is most pronounced west of the Cascade Crest, creating a pronounced summer dry season. The episode's tone is educational and explanatory, delivered in a clear, lecture-style format. It is a straightforward presentation of meteorological concepts paired with immediate forecast details. Listeners interested in Pacific Northwest weather patterns and regional climatology will find this episode valuable. Those seeking a broad national forecast or narrative-driven weather stories might prefer to skip it.

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