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Precipitation extremes present significant risks to Midwest agriculture, water resources, and natural ecosystems. Recently, there is growing attention to the transitions of precipitation extremes, or shifts between heavy precipitation and drought, due to their profound environmental and socio‐economic impacts. Changes in Midwest precipitation extremes and transitions between extremes over the past few decades have been documented; however, their future changes are still unknown. In this study, we estimate the projected changes in transitions of precipitation extremes in the Midwest based on 17 CMIP6 models. Two Standardized Precipitation Index (SPI) based metrics, intra‐annual variability and transitions, are used to quantify the magnitude, duration, and frequency of variability and transitions between wet and dry extremes. Compared with the observation‐based precipitation datasets, the multimodel ensemble median of CMIP6 can reasonably represent the spatial patterns of SPI extremes and transitions. Climate projections show significantly intensified wet extremes across the Midwest by the end of the century, with a greater increase over the northern Midwest and the Great Lakes region. The short‐term SPI also shows intensified dry extremes over the western half of the Midwest. Consequently, there is a significant increase in the magnitude of intra‐annual variability in most areas. Projections also suggest more frequent and rapid transitions between the wet and dry extremes, especially over the Great Lakes region and northern Midwest. Seasonally, more frequent transitions from a wet spring to a dry summer (or from a dry fall to a wet winter/spring) are projected to occur; and generally, the wet and dry conditions between the transitions are projected to be more intense compared to the historical period. Furthermore, the intensified precipitation extremes and accelerated transitions are greatly alleviated under a lower emission scenario, implying that future changes in hydroclimate extremes, and impacts thereof, in the Midwest are sensitive to climate change mitigation. [ABSTRACT FROM AUTHOR]

Severe winter weather is a staple of Midwest United States (U.S.) climate and is an important natural hazard that has significant economic and environmental impacts. Winter season severity varies both spatially and temporally in the Midwest, and past studies have documented significant changes in Midwest winter characteristics, including increases in temperature and decreases in snow depth. In this study, we use the Accumulated Winter Season Severity Index (AWSSI) to characterize winter severity across the Midwest and assess its variability and change on various spatial and temporal scales during the 1951–2020 period. AWSSI is derived from daily records of snowfall (SF), snow depth (SD), maximum surface air temperature (TMAX), and minimum surface air temperature (TMIN) during the winter season. The daily total AWSSI index value reflects the sum of points accumulated from daily values of each of these four variables surpassing pre-determined thresholds, and the total season sum reflects the overall winter season severity. We find AWSSI provides a unique perspective by which to describe climatological differences in winter season character in the Midwest. Namely, over three quarters of winter season days accumulate points in the northern Midwest, compared to just over half in the southern Midwest. Meanwhile, the relative contribution of extreme winter days to total winter severity is much larger in the southern Midwest than farther north. In addition, we find overall winter season severity has significantly decreased at only a quarter of stations and the frequency of extreme winter days has significantly decreased at only a fifth of stations, despite widespread increases in winter daily maximum and minimum temperature. The results suggest Midwest U.S. winter season severity as described by the AWSSI has not significantly changed over the past 70 years. [ABSTRACT FROM AUTHOR]

The Accumulated Winter Season Severity Index (AWSSI) describes the “harshness” of the cold season. This work utilizes AWSSI to explore the interannual teleconnective influences that shaped winter severity between 1951 and 2020 across the Midwest U.S. Annual AWSSI total variability clusters into four distinct geographic areas, each shaped by different teleconnective influences. The regions are closely delineated by the main river basins and include the Ohio (Region 1), upper Mississippi and Missouri (Region 2), central Mississippi and southern Missouri (Region 3), and the Great Lakes (Region 4). Of outmost influence in shaping severe winters across the Midwest have been synoptic flow modifications associated with significant meandering and southern displacement of the Polar jet over the conterminous United States and/or the Midwest that depress near-surface air temperatures and increase the likelihood of snow fall and/or snow depth. These flows are most closely related to the combined influence of the negative (positive) phases of the NAO (PNA and PDO) in Region 1, PNA− in Region 2, NAO− in Region 3, and the negative (positive) phase of the PNA (TNH) across Region 4. Mild winters have been shaped by more zonal flows with the Polar jet positioned north of the Midwest and bringing milder temperatures, less snowfall, and reduced winter severity to the impacted areas. Such modifications have been related to TNH−, PNA+, and NAO+ across Regions 1, 2, and 3, respectively. Mild winters have been shaped by TNH− and to a limited extent NAO+, across Region 4. [ABSTRACT FROM AUTHOR]

ABSTRACT Regions of strong land-atmosphere coupling are often depicted as static in time. However, the mechanisms through which the land surface impacts atmospheric conditions vary on sub-daily to multi-decadal timescales. Therefore, characterizing the long-term variability of land-atmosphere interactions may provide a means of predicting when surface-induced extreme temperature events will be more or less likely to occur. We evaluate the coupling strength between soil moisture, as represented by in situ observations and 1-month Standardized Precipitation Index ( SPI), and subsequent monthly maximum temperature ( TMAX) over the contiguous United States. We find that the utility of SPI as a proxy for soil moisture anomalies is limited to the summer, as the correlations between SPI and TMAX are not significantly related in the other seasons. We examine the variability in summer SPI-TMAX coupling in four regions of the United States. In general, we find the strongest relationships between SPI and TMAX in the Southern Great Plains. However, our results demonstrate that the coupling strength varies considerably over time in most regions of the United States. The long-term variability in SPI-TMAX coupling strength is strongly related to the Pacific Decadal Oscillation in the northwest and midwest United States, and the Atlantic Multidecadal Oscillation in the southeast United States The results of this study suggest that land-atmosphere coupling in the contiguous United States is modulated by multi-decadal oscillations in Pacific and Atlantic sea surface temperatures. [ABSTRACT FROM AUTHOR]

ABSTRACT East China ( EC) has experienced a number of severe droughts in recent decades. Understanding the characteristics of droughts and their persistence will provide operational guidelines for water resource management and agricultural production. This study uses a logistic regression model to measure the probability of drought occurrence in the current season given the previous season's Standardized Precipitation Index ( SPI) and Southern Oscillation Index ( SOI) as well as drought persistence. Results reveal large spatial and seasonal variations in the relationship between the previous season's SPI and the drought occurrence probability in a given season. The drought persistence averaged over the entire study area for all the four seasons is approximately 34% with large variations from season to season and from region to region. The East and Northeast regions have the largest summer drought persistence (∼40%) and lowest fall drought persistence (∼28%). The spatial pattern in winter and spring drought persistence is dissimilar with stronger winter and weaker spring drought persistence in the Southwest and Northeast relative to other regions. Logistic regression analysis indicates a stronger negative relationship in summer-to-fall (or between fall drought occurrence and summer SPI) than other inter-season relationships. Most of the stations with significant relationships in summer-to-fall lie in the East and the southern part of the North region. Significant negative relationships in winter-spring and fall-winter are primarily located on the western portion of the study area. A significant negative spring-summer relationship exists in the North region. Further, spring drought occurrence has a statistically significant positive relationship with winter SOI in the Southeast. Fall drought occurrence and summer SOI are negatively correlated in the northwest portion of EC (i.e. the border of North and Southwest regions). Overall, this study demonstrates that the impact of previous season SPI and SOI on current season drought varies substantially from region to region and from season to season. This study also shows stronger drought persistence in summer than in other seasons. In other words, the probability of fall drought occurrence is closely related to summer moisture conditions in the EC. [ABSTRACT FROM AUTHOR]

ABSTRACT Understanding flash floods in watersheds of the New York City water supply system ( NYCWSS) is important, as turbidity associated with flooding degrades water quality in the unfiltered portions of this water supply system. We examined synoptic-scale atmospheric conditions most frequently associated with flash flooding in this region. Flash floods between 1987 and 2013 were identified in two small watersheds of the NYCWSS using USGS 15-min discharge data at the Esopus Creek near Allaben, NY and Neversink River at Claryville, NY gauges, both located in the Catskill Mountains. A total of 25 flash floods were detected in these watersheds and there were 17 separate flash flood days. The Spatial Synoptic Typer Tools 4.0 were used to characterize the synoptic-scale atmospheric patterns influencing the study area based on NCEP/ NCAR 500-mb geopotential height reanalysis data. Through this procedure, 17 unique synoptic patterns were identified. Three of these types were found to be strongly associated with flash flooding events. Composites of these types show southwesterly flow which suggests advection of moisture from the Gulf of Mexico and Atlantic Ocean. The flash flood days were compared to the National Weather Service flash flood warnings. The flash flood warnings issued for Ulster County compared to the flash floods in the study watersheds highlight the highly localized nature of flash flooding in this region. [ABSTRACT FROM AUTHOR]

ABSTRACT Thirty years (1954-1983) of soil moisture data from 60 sites in Iowa are used to identify the dominant modes of spatial and temporal variability. Both soil moisture and precipitation exhibit a northwest to southeast gradient across Iowa, with wetter conditions in the southeast and drier conditions in the northwest. Climate and soil characteristics are responsible for the spatial variations in soil moisture. There is a pronounced annual soil moisture cycle that has three distinct phases: a moist phase (April-June), a drying phase (June-September), and a recharge phase (September-November). Spring soil moisture is strongly controlled by the amount of recharge during the previous fall. There is no evidence of a long-term trend in soil moisture; however there is significant interannual variability and about 20% of the variance is attributable to El Niño-Southern Oscillation (ENSO). The warm (cold) phase of ENSO is associated with above (below) average soil moisture in Iowa. The multi-decadal soil moisture record is used to analyse drought frequency and magnitude in Iowa, and to place the 2012 Midwest drought in historical context. In general, Iowa drought is more common during the latter part of the warm season (August through November), but tends to be more severe during the early part of the warm season (April-July). We find that the 2012 drought was unusual because it persisted and intensified throughout the growing season. [ABSTRACT FROM AUTHOR]