The West Coast's weather patterns have been a rollercoaster ride lately, and it's all thanks to an unusually strong Pacific jet stream. This jet stream has been driving the region's changeable and unpredictable weather, from late-season troughs and low pressure systems to the potential for a heatwave in mid-June. But what makes this story even more fascinating is the interplay between the jet stream, El Niño, and the Pacific meridional mode, and the potential for extreme weather events in the Western U.S. Let's dive into the details and explore the implications of these complex weather patterns.
The Jet Stream's Role
The Pacific jet stream has been unusually strong and eastward-extended, particularly over central and southern California. This has been facilitated by the extreme warm ocean water to the southwest of California, creating a thermal differential between the subtropics and midlatitudes. In my opinion, this is a classic case of the jet stream's direct contribution to aggressive 'ridge-building' downstream. The meteorology involved spans multiple processes, from local mass accumulation near the jet exit region to diabatic heating, and even an indirect contribution from a recurving West Pacific typhoon.
The West Coast's Weather Patterns
The West Coast has been experiencing a changeable and unpredictable weather pattern, with repeated late-season troughs and low pressure systems. These systems have brought both unusually late-season showers and periods of fire weather-elevating dry north winds. The jet stream's strength and position have been key factors in driving these patterns, with the Pacific Northwest and western Canada being particularly affected.
The Potential for a Heatwave
The jet stream's role in driving the ridge-building event is particularly interesting. Once the mid-week low pressure system moves on to the east, a broad and strong ridge of high pressure is likely to build in along or just west of the West Coast from later next week and then continuing well into mid-June. This ridge will not be centered over California, but its maximum impacts will be centered farther north, in the Pacific Northwest and British Columbia.
In my opinion, this ridge will usher in a major pattern change in California, with a trend toward much warmer and drier conditions. The highest odds of truly anomalous 'heatwave' conditions will be across Washington and Oregon as well as far NorCal, but there is still some possibility of a more widespread major heat event across the entire West Coast during this period. The models can struggle with the exact placement and magnitude of ridges under such conditions, which is why I will be keeping an eye out for further intensification of the upcoming ridge/heat event.
The Impact of El Niño
The potential for extreme weather events in the Western U.S. is also linked to El Niño. The updated June ensemble predictions are indicating the potential for even more extreme outcomes than earlier forecast cycles, and I'm going to wait for the full suite of new predictions to become available before taking another deep dive. However, everything from my ENSO-focused livestream in May still very much applies.
New Research on Western U.S. Sub-Daily Precipitation Extremes
New research on Western U.S. sub-daily precipitation extremes has confirmed familiar spatiotemporal patterns and found evidence of emerging increases in the heaviest downpours. The study also found recent increases in potential atmospheric contributors to sub-daily downpours during 2000-2024 vs 1980-2024, including widespread increases in column water vapor despite mixed or even decreasing trends in surface humidity.
In my opinion, this research highlights the complex and interconnected nature of weather patterns in the Western U.S. and the potential for extreme weather events. The interplay between the jet stream, El Niño, and the Pacific meridional mode, as well as the impact of human activity on the climate, are all factors that contribute to the region's unpredictable and changeable weather.
Conclusion
In conclusion, the West Coast's weather patterns are a fascinating and complex interplay of various factors, from the jet stream to El Niño and the Pacific meridional mode. The potential for extreme weather events, such as heatwaves and heavy downpours, highlights the need for continued research and monitoring of these patterns. As a weather enthusiast, I'm excited to see how these patterns evolve and what new insights we can gain from them.
