HyperFACETS Storylines

The storyline concept is a cornerstone of our project, providing context for model analysis, stakeholder engagement, and scientific exploration. Storylines enable framing of metrics and process understanding in a manner that is meaningful across sectors, and representative of particularly impactful conditions. As described by Shepherd et al. (2018), storylines are a physically self-consistent unfolding of past events, or of plausible future events or pathways. In essence, storylines are a way of framing a problem in terms of three factors: A geographic region, an event, and a set of process drivers for that event. Storylines will consist of a recreation of a past event or period, and a future analogue that may arise in light of non-stationarity. Extreme events (i.e., droughts, floods, and extreme winds) are natural candidates, particularly those that have historically affected stakeholder and policymaker decisions. The selected storylines are impactful examples drawn from geographic regions that have been a focus of Hyperion and FACETS and which depend on processes that are relevant to our research.

Hydroclimatic Priming of the 2018 California Wildfires: Following an aberrant 5-year drought stretching from 2011-2016 and subsequent abrupt shift to very wet conditions during 2017, California experienced one of its most devastating years of wildfire in 2018. This year recorded both the largest fire (the Mendocino Fire Complex) and deadliest fire (the Camp Fire) on record. Increase in fuel from trees and favorable meteorological and climatic conditions, including high temperatures, dry soils, and the gusty and dry Diablo winds, have been attributed to the extreme wildfire season. Future changes in climate and land use land cover may fundamentally alter the fire regimes in California, with subsequent influence on the hydrologic impacts of wildfires. For example, warmer temperatures and heat-related droughts have been projected to become more common in the future. Earlier snowmelt and potentially reduced snowpack in a warmer climate may exacerbate agricultural (soil moisture) and hydrological (streamflow) drought to create more favorable conditions for wildfires. Analyses supported by FACETS and Hyperion found a robust increase in subseasonal precipitation variability during winter in North America in the future, with particularly large changes over California, manifesting in an intensified swing between wet and dry extremes. The teams also found a robust change in precipitation seasonality over California in the future that features a sharpening precipitation seasonal cycle, with increased winter precipitation and reduced fall and spring precipitation. Fall/spring drying has already been detected in the observational record and the Coupled Model Intercomparison Project (CMIP5) models show a "time of emergence" for the seasonality change of around 2022. We noted that increasing land-sea temperature contrast, and hence moisture contrast, is a key mechanism for the fall/spring drying. Meanwhile, the eastward extension of the jet stream is contributing to the increased winter precipitation. The drying in fall and spring, which coincides with the seasonality of the Diablo winds, may have important implications for prolonging the fire season in the future. (Lead: Ruby Leung)

Sequential AR events in California based on the 1861-62 flood: The 1861-62 winter was associated with intense precipitation in Northern California over a period of 43 days, capped by a warm storm event that produced historical flooding of the Sacramento Valley. The precipitation that drove this megaflood event was delivered by what is known as the "ARkStorm," a sequence of series of particularly intense atmospheric rivers (ARs). ARs are responsible for delivering huge amounts of precipitation over California during the cold-season, and are often associated with floods over this region. Under warmer climates, thermodynamic effects are projected to intensify ARs -- a warmer atmosphere can hold more water vapor that translates into increased large-scale precipitation. Dynamically downscaled simulations of individual future extreme ARs over California, conducted through the Hyperion Project, support this notion. Initial results indicate that by late-century, California may experience extreme ARs without precedence of the recent past. In a much warmer climate, precipitation from these storms will be mostly delivered as rain instead of snow, and potentially make landfall during snow melt events. Together, these processes conspire to produce a "double whammy" of extreme runoff, with the potential to overwhelm dams and levies, and inundate towns and farms. (Lead: Alex Hall)

Hurricane Irma: In 2017, multiple Atlantic hurricanes caused vast destruction along U.S. coastlines through different combinations of wind, inland flooding, and storm surge. Forming from a tropical wave originating over Africa, Hurricane Irma intensified quickly to major hurricane strength over the North Atlantic ocean. As the storm tracked westward into the Caribbean, favorable conditions enabled the storm to develop into a powerful Category 5 storm with a maximum intensity of sustained winds of 80 m/s. After direct landfalls on various Caribbean islands, Hurricane Irma's path steered toward the Gulf and Florida coast, making landfall in Southern Florida as a Category 4 hurricane, before weakening inland of the Florida peninsula. Irma's intensity, size and track proved to be a devastating combination for the region, leading to 10 direct storm-related fatalities and $50 billion in damage in the U.S. Hurricane Irma is an example of the diversity of impacts coastal storms can inflict on U.S. coastlines. Understanding how warming may influence these impacts is important in the decision context. Though the impact of a hurricane depends in part on the peak wind speed of the storm, the size and structure of the wind field are also crucial determinants of the spatial and temporal distribution of these hazards. Irma was a large storm, meaning its wind field, including the extent of hurricane force winds, extended far from the storms center, further worsening its destructive potential. Heavy rains of over 12 inches in large portions of Florida caused extensive urban flooding, as well as fluvial flooding in Irma's aftermath. (Lead: Kevin Reed)

Repeated passages of MCSs over the Southern Great Plains: The U.S. Southern Great Plains (SGP) experienced record-breaking flood events in May 2015. Precipitation started to spike in the beginning of May and persisted for the rest of the month with a series of MCSs passing through the region. The repeated passages of MCSs and soil saturation from rainfall of the previous month culminated in the worst statewide flooding on record in Texas. A semi-stationary trough over the western U.S. that funneled Gulf of Mexico moisture into the SGP supported the MCSs and exacerbated the extreme rain. The May 2015 floods in the SGP exemplify a "perfect" alignment of seasonality (spring jet stream) and interannual variation (a strong El Nino) for inducing and sustaining MCSs, with an additional push from the warming climate that increased moisture content and modulated quasi-stationary Rossby waves that drive subseasonal wet-dry alternations.

In the future, the May 2015 floods may unfold differently with further changes of the climate system. For example, the projected, seasonally-dependent response of the Hadley circulation and subtropical highs to warming would strengthen the low-level jet (LLJ) more in spring than summer, which may enhance spring MCS precipitation in the future. A strengthening of the LLJ and enhancement of MCS lifetime and extreme precipitation have already been observed in the past decades. On the contrary, a poleward shift of the mid-latitude jet stream and expansion of the North American monsoon anticyclone may lead to summer drying in the SGP in the future. Understanding the mechanistically robust local and remote factors that contribute to the spatiotemporal clustering of MCSs conducive to flooding is critical for projecting future changes in flood potential in the SGP. Strong winds that often accompany MCSs are another important but poorly-understood feature of these events, with potential impacts on regional energy production. (Lead: Ruby Leung)

Image credit: Photo by Felix Mittermeier on Unsplash.

Rain-on-snow flooding in the Susquehanna: In late January 1996, the Susquehanna River basin experienced some of its highest streamflows in recorded history. This unique occurrence was driven by an extratropical cyclone (ETC) advecting anomalous warmth and producing heavy rainfall, which rapidly ablated a deep antecedent snowpack. This "rain-on-snow" event caused catastrophic flash flooding over a wide region, with extreme runoff volumes being exacerbated by ice jams resulting from previous cold and snowy weather. More than 150 deaths and $4.9 billion (inflation adjusted) in damages were reported, making this one of the costliest wintertime weather events in U.S. history.

Recent work projects significant shifts in precipitation frequency, rates, and phase (i.e., rain versus snow) associated with ETCs (e.g., atmospheric rivers, nor'easters, etc.) over the U.S., implying significant hydrological challenges associated with infrastructure designed around 20th century climate. While rain-on-snow has begun to be explored in a broader climate context in recent years, gaps still exist in our process-level understanding of the interplay between snowpack, runoff, precipitation, and other land-atmosphere variables during these events. For instance, the vast majority of previous research has been limited to mountainous regions, and does not consider ephemeral snowpack more common to regions such as the northeastern U.S. (Lead: Colin Zarzycki)

Wind storms in the U.S. Northeast: In recent decades there have been examples of extreme wind storms in the U.S. northeast, including two particularly intense nor'easters that occurred during 2000 (one peaked on 21 January 2000 and another on 18 December 2000 (selected based on data from the NOAA storm event reports). Since the aerodynamic force exerted by the wind scales with the square of the wind speed, thus wind extremes (including gusts) are an important natural hazard. Over the Northeastern U.S. (which is frequented by intense cyclones), the sources of wind extremes are intense mid-latitude cyclones such as Alberta Clippers and nor'easters coupled with other weather extremes (e.g., blizzards); these storms tend to impact densely populated areas. These storms are also sensitive to changes in ice cover over the Great Lakes, but changes to wind extremes in light of reduced ice cover remains a topic of future study. (Lead: Sara Pryor)

The 1960s northeastern U.S. drought: The 1960s drought in the U.S. Northeast, occurring approximately over the period 1962-1966, had pronounced implications for water management practices in this region. From 1962 to 1966, 34 out of the 46 months had below average precipitation, and by April 1966 the water shortage in this region was over 36 inches, almost equaling one year's rainfall. Tree-ring reconstructions showed that this drought had been the most severe of the last few centuries -- although other droughts had occurred over longer periods. An analysis of the synoptic-scale patterns present at the time of the drought suggested connections with the North Pacific Mode and the North Atlantic Oscillation.

Notably, the 1960s drought period was associated with below average temperatures -- increased temperatures associated with regional climate change and shifts in precipitation patterns have potentially modified the probability or character of an analogous drought event. Further, changes in land use and water demand in the region have affected the infrastructural and socioeconomic needs of this region. These reported changes suggest that existing policies based on this earlier drought may now be insufficient for future planning. (Lead: Paul Ullrich)

Image credit: Quabbin Reservoir in central Massachusetts on Oct 13, 1966, courtesy of Massachusetts DCR Archives. More info here.

Colorado's Spring Miracles: Historically dry winter seasons have sometimes been saved in the Spring by extraordinarily intense precipitation over short periods of around a month. A famous example of these events include the Miracle May of 2015, occurring after a prolonged and sever drought water year -- when Colorado water supply was down to dangerously low levels prompting concerns from dam operators. In Lake Mead the water level had fallen to lows not seen since the Hoover Dam was built in the 1930s. Officials had prepared for drastic water cutbacks to stakeholders and water users. But a rainy May 2015 drove a dramatic increase in streamflow and filled most dams. Other famous spring miracles include Miracle March 2003 and Miracle March 1991.

These miraculous months are inevitably connected to questions of climate variability and predictability. With increases in variability expected through the remainder of the 21st century, there is a growing need to understand the frequency and character of events of this nature. (Lead: Simon Wang)

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