The weather patterns across the northeastern Pacific region have been nothing short of extraordinary this April, and it seems this unusual activity is set to continue. What makes this particularly fascinating is the extreme variability we're witnessing, with temperatures and precipitation levels swinging wildly from month to month.
In my opinion, this year's weather has been a rollercoaster ride, starting with record rainfall in October-December, followed by extreme dryness and then the hottest March on record in many areas. Now, April has brought a return to active and damp conditions, especially in the northern and Sierra regions, with the potential for one of the top three wettest Aprils ever recorded in some locations.
Despite the increased precipitation, the Sierra Nevada snowpack remains at a record low, similar to the bone-dry conditions experienced in 2015. This highlights the complexity of weather systems and the challenges of predicting and managing water resources.
Unusually Active April
The current weather pattern is characterized by a series of late-season storms, with another notable system expected this week. This 'bowling ball' low-pressure system will bring widespread precipitation, thunderstorms, and mountain snow to northern and central California. While this system may not be as dramatic as the previous week's storm, it is part of an unusual sequence of robust late-season storms in the region.
What many people don't realize is that these weather events are influenced by a strong subtropical jet stream, which is typically weaker by spring. This year, however, the STJ has been a persistent feature, driving a sequence of events in the subtropical Pacific, from record-breaking rainfall in Hawaii to the exceptional heatwave in March.
Record Oceanic Warmth
One of the key drivers of these unusual weather patterns is the record-breaking marine heatwave currently unfolding in the Pacific Ocean. A vast region between Hawaii, Mexico, and California is experiencing record sea surface temperatures, which is having a significant impact on local climates.
From my perspective, this oceanic warmth will likely lead to a less persistent and chilly 'May Gray/June Gloom' marine layer season, potentially resulting in uncomfortably humid conditions in Southern California. Furthermore, the warm oceans will increase the odds of tropical remnant events and the potential for East Pacific hurricanes later in the year, especially if a strong El Niño develops.
Looking Ahead
The seasonal predictions for late April and early May are uncertain, given the extreme variability and blocking patterns. However, model ensembles suggest that while temperatures will rise, continued (albeit lesser) April and May showers are possible. This could result in an unusual spring and summer, with elevated convective activity and potentially humid heatwaves in California and the Southwest.
Personally, I find it intriguing how these weather patterns are interconnected, with the Pacific Meridional Mode (PMM) potentially playing a causative role in El Niño development. It raises a deeper question about the long-term impacts of these extreme weather events and how they may shape future climate trends.
Join me on Tuesday, April 21, for a live virtual office hour where we can discuss these active patterns and the context of record Pacific warmth in more detail.
