A Pacific marine heatwave and El Niño are building at the same time in the Pacific Ocean, and Andrew Leising says the heat anomaly is already pushing surface temperatures 3-4C above average. California is heading into autumn with a second ocean warming pattern layered on top of a marine heatwave that has been in place since December.
Leising, a research oceanographer at Noaa, described the Pacific heatwave as "It's a really bright red eyeball in the Pacific," and added, "It's huge, it stretches across the whole [ocean]." The heatwave formed in May 2025, giving California and the US West Coast an unusually warm ocean surface before the strongest flood and snowfall season arrives.
California Coastal Water Levels
Patrick Barnard, research director of the Center for Coastal Climate Resilience at the University of California, Santa Cruz, said California coastal water levels were already 15cm above mean sea level and expected to rise to at least 15-30cm higher than normal during this El Niño. Every 5-10cm of mean sea level rise doubles flooding frequency for most coastal communities along the US West Coast, so the shift is not just a number on a tide gauge; it is a direct change in how often water reaches streets, properties and low-lying ground.
That is why California is bracing for more storm surges and flooding this autumn, with the highest sea level impacts expected to peak this winter. Warmer ocean surfaces can evaporate more heat and moisture into the air, which helps feed heavier rain when atmospheric rivers move inland.
Oregon State University And The Pacific Northwest
Larry O'Neill, professor at Oregon State University and director of the Oregon Climate Service, said the last huge Pacific marine heatwave, the Blob, changed snowfall patterns in the Pacific Northwest in 2015. He said, "It may have modified the jet stream and sent storms further north than usual. The storms we did get were much warmer than usual, so the precipitation that would usually come as snow came as rain at higher elevations," and added, "It could take multiple winters for us to recover."
The combined impact of El Niño and marine heatwaves is expected to reduce snowfall in the US and Canada. That is especially important in the Pacific Northwest, where reduced snowfall is landing on top of drought conditions rather than a wet reserve going into winter.
Northern California To Utah
The warming pattern reaches beyond the coast. Dry conditions and above-average warming in the north-west US increase wildfire risk across northern California, eastern Oregon, southern Idaho and Utah this autumn, while the same circulation pattern can shift storm tracks and leave some places warmer and snow-poor.
El Niño occurs when sea surface temperatures in the Pacific Ocean are 0.5C higher than normal for several consecutive months, and a very strong or super El Niño occurs when the increase is 2C. The source says El Niño can intensify Pacific warming, but the balance between the heatwave and El Niño in driving the expected impacts is not quantified, which leaves the practical takeaway simple for coastal and inland communities: higher water, more flood-prone storms and less snow are all on the table as this autumn turns into winter.
