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Precipitation in California is highly variable and not well forecasted on subseasonal‐to‐seasonal (S2S) timescales. Understanding relationships between synoptic‐scale atmospheric circulation and hydrometeorological extremes could improve predictability. This work demonstrates the importance of four North Pacific circulation regimes (called the NP4 modes) in modulating precipitation, flooding, and water resources in California. Here we demonstrate how, on daily timescales, interactions between the NP4 modes drive coastal flow that can result in dry conditions from atmospheric ridging or wet conditions associated with enhanced onshore flow and atmospheric river (AR) landfalls. Seasonally, the prevalence of certain NP4 phase relationships can tip the scale toward wet or dry conditions. Relationships between El Niño Southern Oscillation (ENSO) and the NP4 are explored, and we provide insight into the poorly forecasted Western US seasonal precipitation during the "Godzilla" El Niño winter of 2016 by examining climate‐weather linkages in a historical context. Plain Language Summary: The amount of precipitation that falls over California in a given water year is highly variable, which presents challenges for water resource management. Skill of both dynamical and statistical models in forecasting precipitation is low at lead times beyond 2 weeks. Recent research has explored relationships between atmospheric circulation patterns and precipitation extremes in the western United States, with the goal of improving predictability. This work demonstrates the importance of four North Pacific circulation regimes in modulating coastal flow patterns that ultimately determines the amount and spatial distribution of precipitation over California on daily and seasonal timescales. We show how the interaction between these modes on daily timescales drives wet and dry episodes within a season. Seasonally, we show how the daily variability occurs within certain constraints established by the larger, more slowly varying climate system, including (but not limited to) El Niño‐Southern Oscillation. This work improves understanding of climate‐weather relationships, which has implications for predictability of hydrometeorological extremes over California and the western United States. Key Points: Coastal vapor transport and California precipitation are modulated by four North Pacific circulation regimes on daily to seasonal timescalesThe most damaging California floods have occurred when these modes were jointly aligned to reinforce onshore flowSeasonally, these modes are influenced by ENSO but much variability occurs within the constraints set by the larger‐scale climate system [ABSTRACT FROM AUTHOR]

Airborne lidar altimetry can measure the sea surface height (SSH) over scales ranging from hundreds of kilometers to a few meters. Here, we analyze the spectrum of SSH observations collected during an airborne lidar campaign conducted off the California coast. We show that the variance in the surface wave band can be over 20 times larger than the variance at submesoscales and that the observed SSH variability is sensitive to the directionality of surface waves. Our results support the hypothesis that there is a spectral gap between meso‐to‐submesoscale motions and small‐scale surface waves and also indicate that aliasing of surface waves into lower wavenumbers may complicate the interpretation of SSH spectra. These results highlight the importance of better understanding the contributions of different physics to the SSH variability and considering the SSH spectrum as a continuum in the context of future satellite altimetry missions. Plain Language Summary: Using unique measurements of the sea surface height (SSH) taken from an instrument onboard an airplane off the California coast, we examine how the variance of the SSH is distributed across different spatial scales, ranging from hundreds of kilometers to a few meters. We show that surface waves can be 20 times more energetic than motions with larger scales (known as submesocales) and that the direction of surface waves impacts the SSH variability depending on different sampling and averaging strategies. These results highlight the importance of better understanding the contributions of different types of motions and physics to the SSH variability. In particular, we advocate measuring scales from meters to hundreds of kilometers as a continuum to better inform future satellite altimetry missions. Key Points: We present unique broadband airborne lidar observations of sea surface height spanning scales from hundreds of kilometers to a few metersThe sea surface height variance due to surface waves can be over an order of magnitude larger than the variance at submesoscalesThe observed sea surface height is sensitive to the direction of waves which may impact the interpretation of remote sensing measurements [ABSTRACT FROM AUTHOR]

Increasing wildfire and declining snowpacks in mountain regions threaten water availability. We combine satellite‐based fire detections with snow seasonality classifications to examine fire activity in California's seasonal and ephemeral snow zones. We find a nearly tenfold increase in fire activity during 2020–2021 versus 2001–2019. Accumulation season broadband snow albedo declined 25%–71% at two burned sites (2021 and 2022) according to in‐situ data relative to un‐burned conditions, with greater declines associated with increased burn severity. By enhancing snowpack susceptibility to melt, both decreased snow albedo and canopy drove midwinter melt during a multi‐week dry spell in 2022. Despite similar meteorological conditions in December–February 2013 and 2022–linked to persistent high pressure weather regimes–minimal melt occurred in 2013. Post‐fire snowpack differences are confirmed with satellite measurements. With growing geographical overlap between wildfire and snow, our findings suggest California's snowpack is increasingly vulnerable to the compounding effects of dry spells and wildfire. Plain Language Summary: Satellite fire detections indicate substantial increases in wildfire activity in California's snow‐covered landscapes during 2020 and 2021, suggesting wildfire is increasingly altering mountain hydrology. During 2022, a multi‐week mid‐winter drought, or dry spell, occurred. A meteorologically‐similar dry spell occurred in 2013, and the 2022 event provides a test case to examine how post‐fire changes (canopy loss and deposition of burned dark material on snowpack) alter snowmelt patterns. Using field observations, weather station data, and satellite remote sensing of snow, we find large reductions in snow albedo and canopy cover drove rapid melt during the 2022 dry spell in burned areas whereas during 2013, minimal melt occurred. The societal connection between mountains and humans will be strained as mountains face increasing climate‐related stressors. Midwinter drought, snow loss, and increasing wildfire are expectations of a warming world. Addressing these challenges requires innovative water and forest management paradigms. Our findings motivate additional research into assessing and planning for post‐fire hydrologic changes in snow‐dominated landscapes as both wildfire and dry spells will increase in frequency with climate warming. Key Points: A 9.8x increase in satellite fire detections in California's snow zones in 2020–2021 versus 2001–2019 implies growing overlap in fire and snowPost‐fire accumulation season broadband snow albedo declined 25%–71%, driving fewer snow‐covered days and lower snow‐cover fractionCompared with the meteorologically similar 2013 dry spell, albedo and canopy declines led to rapid midwinter melt in 2022 [ABSTRACT FROM AUTHOR]