In the ever-changing landscape of California's weather, the month of April has been nothing short of extraordinary. What began with record rainfall in October and December, and then a rollercoaster of dryness and storms in January and February, has now transformed into a month of unusual activity and damp conditions. As we delve into the details, it becomes clear that this April is not just another month; it's a testament to the unpredictable nature of our climate. One thing that immediately stands out is the contrast between the mild temperatures and the extreme precipitation. While March was the hottest on record in many parts of California, April has brought cooler temperatures and much-needed rain. This shift is particularly interesting, as it highlights the delicate balance between the state's climate patterns. The Sierra Nevada snowpack, which was extremely low in March, is now in a virtual dead heat with 2015 for the lowest on record. This is despite the fact that April has seen much higher precipitation than the bone-dry 2015. The Upper Colorado River basin snowpack, too, has stabilized somewhat, but it remains the lowest on record for the calendar date. The unusual weather pattern in April is not just a local phenomenon; it's part of a larger trend. The subtropical jet stream, typically weak in spring, has driven a sequence of events in the subtropical central and eastern Pacific. From persistent Kona Lows in Hawaii to record-shattering rainfall and flooding, and from contributing to ridge-building during our exceptional March heat episode to now allowing for the persistence of an unusually active weather pattern, the subtropical jet stream has been a key player. This trend has implications for the entire North Pacific, with the potential for continued showers later into the spring than usual in California. What makes this particularly fascinating is the connection between the subtropical ocean warmth and the Pacific Meridional Mode (PMM). The PMM, a precursor to El Niño, has been increasingly recognized as a driver of extreme events. The current marine heatwave in the Pacific Ocean, which began in late 2025 and intensified in March and April, is a major driver of conditions from Hawaii to Mexico to California. This heatwave is not just a local phenomenon; it's a global event with far-reaching implications. The warm ocean temperatures will strongly affect California and the broader Southwestern climate in the weeks and months to come. I expect a muted 'May Gray/June Gloom' marine layer season, with less persistent and chilly fog, and potentially uncomfortably muggy days in Southern California. The exceptional warmth of the subtropical and tropical oceans will also raise the odds of 'tropical remnant events' in California, especially if a strong El Niño event develops. In short, the unusual weather pattern in April is a reflection of the complex interplay between the state's climate patterns and global trends. As we move forward, it's crucial to consider the broader implications of these events and how they might shape our future. From the Sierra Nevada snowpack to the subtropical jet stream and the Pacific Meridional Mode, there are many factors at play. As an expert, I believe that understanding these connections is essential for predicting and preparing for future climate events. Personally, I think that the active pattern in April is a reminder of the delicate balance between the state's climate patterns and global trends. It's a fascinating and complex topic, and I look forward to discussing it further in my live virtual office hour on Tuesday, April 21. Join me at noon Pacific Time to explore the active pattern and context of record Pacific warmth.
