A vast marine heat wave now stretches across roughly 5,000 miles of the Pacific, from Micronesia to waters off California. Scientists say the anomaly could influence heat, humidity, wildfire conditions, and tropical storm activity in the West over the coming months.
The scale of the event has drawn attention because sea surface temperatures within the hot spot are running as much as 6 to 8 degrees above average in some areas. That warmth is developing alongside a transition toward El Niño and after an unusually warm, low-snow year in parts of the western United States.
This matters because the ocean is not acting in isolation. The pattern described by climate scientists and fire forecasters points to a summer in which ocean temperatures, atmospheric moisture, and dry land conditions may interact in ways that affect daily weather as well as seasonal hazards.
The picture is not uniform, and the effects are not expected to arrive all at once. In the near term, parts of the West are likely to see unsettled weather and variable temperatures, while later in summer the focus may shift toward warm nights, humidity, thunderstorms, and a higher risk of fire in some areas.
Warm Pacific Waters Are Already Influencing Western Weather
According to climate scientist Daniel Swain, the marine heat wave is an “exceptional event” that is breaking records and could expand to cover the entire Pacific coast of North America by late summer. He said the event could produce a summer in California and the Southwest unlike those seen there in quite some time.
The immediate effects are expected to be mixed. Swain wrote that the marine heat wave is helping enhance the subtropical jet stream, which should keep conditions unsettled in the West over the coming weeks and bring some moisture into the dry Intermountain West. His April 19 analysis also noted that California had turned wetter in April after an extremely hot March, even as snowpack remained exceptionally low.
Later in the season, the same warm waters may push overnight temperatures higher and reduce relief from daytime heat. Swain also said the marine layer along the California coast could be weaker than usual, with less persistent fog and stratus and warmer coastal conditions.
Humidity is another concern. The Washington Post reported that warmer ocean waters increase evaporation, which can raise atmospheric moisture levels, especially along the coast. The same report said the marine heat wave and a developing El Niño could later combine to support more summer humidity and thunderstorms across the West.
Fire and Hurricane Risks May Rise as Dry Land Meets Warm Ocean
The land surface is already under stress in many areas. According to the National Interagency Fire Center’s April through July outlook, more than 56 percent of the United States was in drought by late March, and snowpack across much of the West was far below normal. The agency said above-normal significant fire potential is forecast in June and July for parts of the Southwest, northern California, the Inland Northwest, and several interior western regions.
That forecast fits with concerns that more thunderstorms may not necessarily reduce fire danger. The Washington Post noted that increased monsoonal thunderstorm activity could enhance fire risks because dry lightning can ignite wildfires.
The marine heat wave may also affect the eastern Pacific hurricane season. The same report said the unusually warm waters could significantly increase the odds of an active season, with impacts extending west toward Hawaii and raising the chances that tropical storm remnants could reach California, as Hurricane Hilary did in 2023.
Ocean ecosystems are also under pressure. According to NOAA scientist Dillon Amaya, possible impacts in Southern California include fish migration, kelp forest degradation, whale entanglements, harmful algal blooms, and seabird mortality.
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