The flood-prone Saint John River (SJR, Wolastoq), which lies within a drainage basin of 55110 km2, flows a length of 673 km from its source in northern Maine, United States, to its mouth in southern New Brunswick, Canada. Major industries in the basin include forestry, agriculture, and hydroelectric power. During the 1991-2020 reference period, the SJR basin (SJRB) experienced major spring flood events in 2008, 2018, and 2019. As part of the Saint John River Experiment on Cold Season Storms, the objective of this research is to characterize and contrast these three major spring flood events. Given that the floods all occurred during spring, the hypothesis being tested is that rapid snow- melt alone is the dominant driver of flooding in the SJRB. There were commonalities and differences regarding the contributing factors of the three flood years. When averaged across the upper basin, they showed consistency in terms of positive winter and spring total precipitation anomalies, positive snow water equivalent anomalies, and steep increases in April cumulative runoff. Rain-on-snow events were a prominent feature of all three flood years. However, differences between flood years were also evident, including inconsistencies with respect to ice jams and high tides. Certain factors were present in only one or two of the three flood years, including positive total precipitation anomalies in spring, positive heavy liquid precipitation anomalies in spring, positive heavy solid precipitation anomalies in winter, and positive temperature anomalies in spring. The dominant factor contributing to peak water levels was rapid snowmelt.

Occurrences of near-0 degrees C temperatures (-2 degrees C <= T <= 2 degrees C) are common in cold regions such as Canada, and these conditions can lead to freeze-thaw events and hazardous precipitation. Many locations in the Coast Mountains within the Canadian province of British Columbia (BC) are especially prone to near-0 degrees C conditions. This study examines the factors that promote anomalously persistent near-0 degrees C conditions, often with precipitation, in Terrace, a small industrial city within the Coast Mountains of northwestern BC. The climatology of near-0 degrees C conditions and associated precipitation over the 1956-2020 period was developed using a combined network of weather stations and field data to study the atmospheric conditions and precipitation during periods of near-0 degrees C conditions. Events with continuous near-0 degrees C conditions generally had long durations, with a climatological mean of 11 h, which increases substantially if accompanied by precipitation (18 h), and even more if accompanied with freezing precipitation (38 h). The longest near-0 degrees C event lasted 233 h and was associated with long-lasting snow and rain. By combining the field data information, large-scale weather conditions and long-term climatology, several factors that contribute to near-0 degrees C conditions at Terrace were identified. These include the ocean's proximity, the surrounding topography, persistent cloudiness, and diabatic processes associated with melting and freezing, although local factors linked with topographic features are also important. Collectively, this study has characterized and improved our understanding of Terrace's near-0 degrees C conditions and its associated precipitation, and these insights can be used for improved forecasting of hazardous events in the area. [Traduit par la redaction] Les temperatures proches de 0 degrees C (-2 degrees C <= T <= 2 degrees C) sont courantes dans les regions froides comme le Canada, et ces conditions peuvent entrainer des phenomenes de gel-degel et des precipitations dangereuses. De nombreux endroits de la chaine cotiere de la province canadienne de la Colombie-Britannique sont particulierement exposes a des conditions proches de 0 degrees C. Cette etude se penche sur les facteurs qui favorisent des conditions anormalement persistantes proches de 0 degrees C, souvent accompagnees de precipitations, a Terrace, une petite ville industrielle situee dans la chaine cotiere du nord-ouest de la Colombie-Britannique. La climatologie des conditions proches de 0 degrees C et des precipitations associees sur la periode 1956-2020 a ete elaboree au moyen d'un reseau combine de stations meteorologiques et de donnees de terrain afin d'etudier les conditions atmospheriques et les precipitations pendant les periodes de conditions proches de 0 degrees C. Les evenements avec des conditions continues proches de 0 degrees C ont generalement une longue duree, avec une moyenne climatologique de 11 heures, qui augmente considerablement s'ils sont accompagnes de precipitations (18 heures), et encore plus s'ils sont accompagnes de precipitations verglacantes (38 heures). Le plus long episode de temperatures proches de 0 degrees C a dure 233 heures et a ete associe a des chutes de neige et de pluie de longue duree. En combinant les donnees de terrain, les conditions meteorologiques a grande echelle et la climatologie a long terme, plusieurs facteurs contribuant a des conditions proches de 0 degrees C a Terrace ont ete recenses. Il s'agit notamment de la proximite de l'ocean, de la topographie environnante, de la nebulosite persistante et des processus diabatiques associes a la fonte et au gel, bien que les facteurs locaux lies aux caracteristiques topographiques soient egalement importants. Cette etude a permis de caracteriser et d'ameliorer notre comprehension des conditions de Terrace proches de 0 degrees C et des precipitations associees, et ces connaissances peuvent servir a ameliorer la prevision des phenomenes meteorologiques dangereux dans la region.

The Canadian province of British Columbia (BC) is subjected to large-scale, destructive floods. The most dramatic was a mid-November 2021 event when atmospheric rivers (ARs) linked to high-intensity storms caused heavy rainfall in southwestern BC, triggering catastrophic flooding. This study examines 37 floods from 2000 to 2021 using information from over 250 climatological stations and compares events with the mid-November 2021 flood. The dates of the floods showed a bi-modal pattern: a primary season (spring to early summer, 16 floods) and a secondary season (fall to early winter, 21 floods). Five mechanisms controlled these floods: heavy rainfall, rapid snowmelt, severe ice jam, rain-on-snow, and a mixture of snowmelt and ice jam; the mid-November 2021 flood was mainly driven by heavy rainfall. Of the 37 floods, those affected by either heavy rainfall (18 floods) or rain-on-snow (10 floods) were used to derive a relationship between the average daily precipitation amount over the duration of an event and the associated integrated water vapour transport (). Flood events showed a strong linear relationship between these variables with R2 , p < 0.05) and values of these parameters were significantly higher for the mid-November 2021 flood than for > 90% of the others although they were not the highest. The mid-November 2021 flood was also one of the four rainfall-related floods that occurred in the secondary season with > 400 kg m-1 s-1. The frequency of flood events over the last five years of the study period has slightly decreased when considering flood events with unknown insured cost. In contrast, insured costs of these events have increased, suggesting that present-day floods are becoming more impactful and may require changes to flood management strategies to reduce costs.

The southern Canadian Rockies are prone to extreme precipitation that often leads to high streamflow, deep snowpacks, and avalanche risks. Many of these precipitation events are associated with rain-snow transitions, which are highly variable in time and space due to the complex topography. A warming climate will certainly affect these extremes and the associated rain-snow transitions. The goal of this study is to investigate the characteristics and variability of rain-snow transitions aloft and how they will change in the future. Weather Research and Forecasting (WRF) simulations were conducted from 2000 to 2013 and these were repeated in a warmer pseudo-global warming (PGW) future. Rain-snow transitions occurred aloft throughout the year over the southern Canadian Rockies, but their elevations and depths were highly variable, especially across the continental divide. In PGW conditions, with future air temperatures up to 4-5 & DEG;C higher on average over the Canadian Rockies, rain-snow transitions are projected to occur more often throughout the year, except during summer. The near-0 & DEG;C conditions associated with rain-snow transitions are expected to increase in elevation by more than 500 m, resulting in more rain reaching the surface. Overall, this study illustrates the variability of rain-snow transitions, which often impact the location of the snowline. This study also demonstrates the non-uniform changes under PGW conditions, due in part to differences in the types of weather patterns that generate rain-snow transitions across the region. [Traduit par la redaction] Le sud des Rocheuses canadiennes est sujet a des precipitations extremes qui entrainent souvent des debits eleves, des accumulations de neige importantes et des risques d'avalanche. Bon nombre de ces precipitations sont associees a des transitions pluie-neige, qui sont tres variables dans le temps et l'espace en raison de la complexite de la topographie. Le rechauffement climatique aura certainement une incidence sur ces extremes et les transitions pluie-neige qui y sont associees. Cette etude vise a examiner les caracteristiques et la variabilite des transitions pluie-neige en altitude et leur evolution future. Des simulations de Weather Research and Forecasting (WRF) ont ete effectuees de 2000 a 2013 et ont ete repetees dans un avenir plus chaud de pseudo-rechauffement global (PRG). Des transitions pluie-neige se sont produites en altitude tout au long de l'annee sur le sud des Rocheuses canadiennes, mais leur altitude et leur profondeur etaient tres variables, en particulier de part et d'autre de la ligne continentale de partage des eaux. Dans les conditions de PRG, avec des temperatures de l'air futures jusqu'a 4-5 & DEG;C plus elevees en moyenne sur les Rocheuses canadiennes, les transitions pluie-neige devraient se produire plus souvent tout au long de l'annee, sauf en ete. Les conditions proches de 0 & DEG;C associees aux transitions pluie-neige devraient augmenter l'altitude de plus de 500 m, ce qui signifie que davantage de pluie atteindra la surface. Dans l'ensemble, cette etude illustre la variabilite des transitions pluie-neige, qui ont souvent une incidence sur l'emplacement de la limite de la neige. Cette etude montre egalement les changements non uniformes dans les conditions du PRG, attribuables en partie aux differences dans les types de modeles meteorologiques qui generent des transitions pluie-neige a travers la region.

The amount and the phase of cold-season precipitation accumulating in the upper Saint John River (SJR) basin are critical factors in determining spring runoff, ice jams, and flooding. To study the impact of winter and spring storms on the snowpack in the upper SJR basin, the Saint John River Experiment on Cold Season Storms (SAJESS) was conducted during winter–spring 2020–2021. Here, we provide an overview of the SAJESS study area, field campaign, and data collected. The upper SJR basin represents 41 % of the entire SJR watershed and encompasses parts of the US state of Maine and the Canadian provinces of Quebec and New Brunswick. In early December 2020, meteorological instruments were co-located with an Environment and Climate Change Canada station near Edmundston, New Brunswick. This included a separate weather station for measuring standard meteorological variables, an optical disdrometer, and a micro rain radar. This instrumentation was augmented during an intensive observation period that also included upper-air soundings, surface weather observations, a multi-angle snowflake camera, and macrophotography of solid hydrometeors throughout March and April 2021. During the study, the region experienced a lower-than-average snowpack that peaked at ∼ 65 cm, with a total of 287 mm of precipitation (liquid-equivalent) falling between December 2020 and April 2021, a 21 % lower amount of precipitation than the climatological normal. Observers were present for 13 storms during which they conducted 183 h of precipitation observations and took more than 4000 images of hydrometeors. The inclusion of local volunteers and schools provided an additional 1700 measurements of precipitation amounts across the area. The resulting datasets are publicly available from the Federated Research Data Repository at https://doi.org/10.20383/103.0591 (Thompson et al., 2023). We also include a synopsis of the data management plan and a brief assessment of the rewards and challenges of conducting the field campaign and utilizing community volunteers for citizen science.

Temperatures near 0°C represent a critical threshold for many environmental processes and socio‐economic activities. This study examines surface air temperatures ( T ) near 0°C (−2°C ≤ T ≤ 2°C) across much of southern Canada over a 13 year period (October 2000–September 2013). It utilized hourly data from 39 weather stations and from 4‐km resolution Weather Research and Forecasting model simulations that were both a retrospective simulation as well as a pseudo‐global warming simulation applicable near the end of the 21st century. Average annual occurrences of near‐0°C conditions increase by a relatively small amount of 5.1% from 985 hr in the current climate to 1,035 hr within the future one. Near‐0°C occurrences with precipitation vary from <5% to approximately 50% of these values. Near‐0°C occurrences are sometimes higher than values of neighboring temperatures. These near‐0°C peaks in temperature distributions can occur in both the current and future climate, in only one, or in neither. Only 4.3% of southern Canada is not associated with a near‐0°C peak and 65.8% is associated with a near‐0°C peak in both climates. It is inferred that latent heat exchanges from the melting and freezing of, for example, precipitation and the snowpack contribute significantly to some of these findings.

Hailstorms represent a significant natural hazard worldwide and can result in severe socioeconomic impacts. In Canada, the province of Alberta experiences the highest rate of hail occurrences causing tremendous damage to cities, properties, vehicles and crops. Cloud seeding is often adopted to reduce the hailstorm impacts; however, the efficacy of this mitigation measure is still unclear. In this study, a ten-year set of hailstorm events (2011-2020) in Alberta was analyzed using radar-based records to evaluate the potential effects of seeding. The radar completed a full volume-scan every four minutes, and a three-dimensional radar reflectivity thresholdbased method was utilized to identify hailstorms. A single 3D storm is tracked over time and the ensemble is called a "storm track" in this contribution. 176 storm tracks persisting for more than one hour were considered for further analysis. We used two radar-derived metrics as proxies for hail damage potential, namely, VILmax (Vertically Integrated Liquid calculated from the maximum reflectivity profile in the storm) and hail mean coverage area (defined as the mean area with reflectivity greater than 60 dBZ). Seeding was performed by releasing silver iodide aerosols from aircraft, and seeded scans of the storm were identified based on the location and seeding time. The efficacy of seeding was evaluated by comparing the characteristics of the storms before, during and after seeding. Results show that for nearly 60% of cases, the median values of the largest (quantile 99) VILmax and mean area corresponding to the seeded scans are lower than those of the unseeded portions. For around 8% and 20% of cases, there is no change, while for approximately 17% to 30% of cases, the values are higher. The effects of seeding are more pronounced after 30 min than at the initiation of seeding. Furthermore, statistical analyses using Mann-Whitney and Wilcoxon's tests reveal that differences in VILmax and mean area between the seeded and unseeded scan groups are statistically significant. These findings suggest that cloud seeding could potentially influence the hail damage potential of storms, particularly those with high VILmax values.

Solid precipitation falling near 0 °C, mainly snow, can adhere to surface features and produce major impacts. This study is concerned with characterizing this precipitation over the Canadian Prairie provinces of Manitoba and Saskatchewan in the current (2000–2013) and pseudo-global warming future climate, with an average 5.9 °C temperature increase, through the use of high resolution (4 km) model simulations. On average, simulations in the current climate suggest that this precipitation occurs within 11 events per year, lasting 33.6 h in total and producing 27.5 mm melted equivalent, but there are wide spatial variations that are partly due to enhancements arising from its relatively low terrain. Within the warmer climate, average values generally increase, and spatial patterns shift somewhat. This precipitation consists of four categories covering its occurrence just below and just above a wet-bulb temperature of 0 °C, and with or without liquid precipitation. It generally peaks in March or April, as well as in October, and these peaks move towards mid-winter by approximately one month within the warmer climate. Storms producing this precipitation generally produce winds with a northerly component during or shortly after the precipitation; these winds contribute to further damage. Overall, this study has determined the features of and expected changes to adhering precipitation across this region.

Freezing rain usually occurs when snow falls into an above-freezing layer aloft where it melts before reaching the sub-freezing surface as supercooled water. Although the occurrence of freezing rain is uncommon, it can produce extremely costly impacts. To date, field chemical observations of supercooled drops that form freezing rain are lacking. In this study, we collected 101 precipitation (freezing rain/rain/snow) samples on Mt. Heng in southern China for 7 years in the winters between 2015 and 2021. The winter precipitation collected at Mt. Heng was mostly acidic with pH values lower than 5. Acidity, contributed from NO3− and SO42−, was neutralized by NH4+ and Ca2+, indicating the additional contribution from organic acids. The major ions were NH4+, Ca2+, SO42− and NO3−, with concentrations of 135.7, 86.3, 72.5 and 61.8 μeq L−1, respectively. In general, the total ionic concentrations of freezing rain were, in comparison with mountain-based observations elsewhere, lower than those in cloud water but higher than those in rain. The higher concentrations of major ions in freezing rain would depress the freezing point of supercooled water drops and act to reduce the likelihood of freezing with effects increasing with concentration. This process would be countered by the presence of ice nuclei that is acting to freeze supercooled drops. These opposing processes complicate the understanding of the onset of freezing and consequently the occurrence of freezing rain. This study highlighted that additional model simulations and laboratory experiments are needed to validate the relationship between air pollution and freezing rain.

The Canadian Rockies are a triple-continental divide, whose high mountains are drained by major snow-fed and rain-fed rivers flowing to the Pacific, Atlantic and Arctic Oceans. The objective of the April-June 2019 Storms and Precipitation Across the continental Divide Experiment (SPADE) was to determine the atmospheric processes producing precipitation on the eastern and western sides of the Canadian Rockies during springtime, a period when upslope events of variable phase dominate precipitation on the eastern slopes. To do so, three observing sites across the divide were instrumented with advanced meteorological sensors. During the 13 observed events, the western side recorded only 25% of the eastern side’s precipitation accumulation, rainfall occurred rather than snowfall, and skies were mainly clear. Moisture sources and amounts varied markedly between events. An atmospheric river landfall in California led to moisture flowing persistently northward and producing the longest duration of precipitation on both sides of the divide. Moisture from the continental interior always produced precipitation on the eastern side but only in specific conditions on the western side. Mainly slow-falling ice crystals, sometimes rimed, formed at higher elevations on the eastern side (> 3 km MSL), were lifted, and subsequently drifted westward over the divide during non-convective storms to produce rain at the surface on the western side. Overall, precipitation generally crossed the divide in the Canadian Rockies during specific spring-storm atmospheric conditions although amounts at the surface varied with elevation, condensate type, and local and large-scale flow fields.

A devastating storm struck southern Manitoba, Canada on 10-13 October 2019, producing a large region of mainly sticky and wet snow. Accumulations reached 75 cm, wind gusts exceeded 100 km h(-1), and surface temperature (T) remained near 0 degrees C (-1 degrees C <= T <= 1 degrees C) for up to 88 h. It produced the largest October snowfall and was the earliest to produce at least 20 cm since 1872 in Winnipeg. These factors led to unparalleled damage and power restoration challenges for Manitoba Hydro and, with leaves still largely on vegetation, the most damaging storm to Winnipeg's trees ever recorded. The storm's track was uncommon, and produced elevated convection related to buoyancy-driven instability and conditional symmetric instability (CSI), with a moist absolutely unstable layer (MAUL) near 500 hPa. Instabilities were released via lift through lower-tropospheric warm advection and frontogenesis, differential cyclonic vorticity advection, and jet streak dynamics. Precipitation bands, elevated convection, and lake effect snow bands enhanced local snowfall. Snow adhering to structures was not always wet but, when present, it sometimes occurred because of incomplete freezing of particles partially melted aloft in a near-surface (<100 m deep) inversion. Although other storms over the historical record have produced a similar combination of severe precipitation, temperature and wind conditions, none have done this for such a long period.

The interior of western Canada, like many similar cold mid- to high-latitude regions worldwide, is undergoing extensive and rapid climate and environmental change, which may accelerate in the coming decades. Understanding and predicting changes in coupled climate–land–hydrological systems are crucial to society, yet limited by lack of understanding of changes in cold region process responses and interactions, along with their representation in most current generation land surface and hydrological models. It is essential to consider the underlying processes and base predictive models on the proper physics, especially under conditions of non-stationarity where the past is no longer a reliable guide to the future and system trajectories can be unexpected. These challenges were forefront in the recently completed Changing Cold Regions Network (CCRN), which assembled and focused a wide range of multi-disciplinary expertise to improve the understanding, diagnosis, and prediction of change over the cold interior of western Canada. CCRN advanced knowledge of fundamental cold region ecological and hydrological processes through observation and experimentation across a network of highly instrumented research basins and other sites. Significant efforts were made to improve the functionality and process representation, based on this improved understanding, within the fine-scale Cold Regions Hydrological Modelling (CRHM) platform and the large-scale Modélisation Environmentale Communautaire (MEC) – Surface and Hydrology (MESH) model. These models were, and continue to be, applied under past and projected future climates, and under current and expected future land and vegetation cover configurations to diagnose historical change and predict possible future hydrological responses. This second of two articles synthesizes the nature and understanding of cold region processes and Earth system responses to future climate, as advanced by CCRN. These include changing precipitation and moisture feedbacks to the atmosphere; altered snow regimes, changing balance of snowfall and rainfall, and glacier loss; vegetation responses to climate and the loss of ecosystem resilience to wildfire and disturbance; thawing permafrost and its influence on landscapes and hydrology; groundwater storage and cycling, and its connections to surface water; and stream and river discharge as influenced by the various drivers of hydrological change. Collective insights, expert elicitation, and model application are used to provide a synthesis of this change over the CCRN region for the late-21st century.

Freezing precipitation, in the form of freezing rain, freezing drizzle, and/or wet snow, can damage transportation networks, infrastructure, and vegetation. Ten events with freezing precipitation (including freezing rain and wet snow) over the province of Manitoba, Canada were examined using surface observational datasets, reanalysis products and 4-km resolution Weather Research and Forecasting (WRF) products that were both a retrospective control (CTRL) simulation as well as a pseudo-global warming (PGW) simulation. All events tracked to the south and/or east of Manitoba and most (8 of 10) events were associated with a consistent large scale pattern of extratropical cyclone with 500 hPa trough, low surface pressure center nearby, and an atmospheric river. Local factors, such as the 400 m elevated terrain of Riding Mountain, influenced 2 events mainly by altering surface temperature to be favorable for freezing precipitation. These events in the PGW simulation occurred 40-120 km farther north on average, with freezing rain generally being enhanced and wet snow generally being reduced, although wet snow was introduced into events which originally only had freezing rain. This study further showed that power lines aligned west to east, perpendicular to the strongest winds, are most susceptible to the consequences of icing and accretion within the current climate as well as the thermodynamically forced future one.

Global Water Future’s Storms and Precipitation Across the continental Divide Experiment (SPADE) was initiated to enhance our knowledge of the contribution of different moisture flows on precipitation across the Canadian Rockies. SPADE installed instrumentation on both sides of the continental divide to gather automated and manual observations during an intensive field campaign from 24 April to 26 June 2019. Various meteorological instruments were deployed including a two Doppler LiDARs, three vertically pointing micro rain radars and three optical disdrometers, alongside human observers during precipitation events. Detailed meteorological data such as air temperature, relative humidity, 3D wind fields, vertical profiles of radar reflectivity and Doppler velocity, precipitation and its type, and snow microphotography images were collected. This dataset will serve as a baseline for future work on atmospheric conditions over major orographic features by comparing the varying conditions on either side of a large topographic feature.

The 0 ∘C temperature threshold is critical for many meteorological and hydrological processes driven by melting and freezing in the atmosphere, surface, and sub-surface and by the associated precipitation varying between rain, freezing rain, wet snow, and snow. This threshold is especially important in cold regions such as Canada, because it is linked with freeze–thaw, snowmelt, and permafrost. This study develops a Canada-wide perspective on near-0 ∘C conditions using hourly surface temperature and precipitation type observations from 92 climate stations for the period from 1981 to 2011. In addition, nine stations from various climatic regions are selected for further analysis. Near-0 ∘C conditions are defined as periods when the surface temperature is between −2 and 2 ∘C. Near-0 ∘C conditions occur often across all regions of the country, although the annual number of days and hours and the duration of these events varies dramatically. Various types of precipitation (e.g., rain, freezing rain, wet snow, and ice pellets) sometimes occur with these temperatures. Near-0 ∘C conditions and the reported precipitation type occurrences tend to be higher in Atlantic Canada, although high values also occur in other regions. Trends of most temperature-based and precipitation-based indicators show little or no change despite a systematic warming in annual surface temperatures over Canada. Over the annual cycle, near-0 ∘C temperatures and precipitation often exhibit a pattern: short durations occur around summer, driven by the diurnal cycle, and a tendency toward longer durations around winter, associated with storms. There is also a tendency for near-0 ∘C surface temperatures to occur more often than expected relative to other temperature windows at some stations due, at least in part, to diabatic cooling and heating that take place with melting and freezing, respectively, in the atmosphere and at the surface.

Large-area, long-duration droughts are among Canada’s costliest natural disasters. A particularly vulnerable region includes the Canadian Prairies where droughts have, and are projected to continue to have, major impacts. However, individual droughts often differ in their stages such as onset, growth, persistence, retreat, and duration. Using the Standardized Precipitation Evapotranspiration Index, this study assesses historical and projected future changes to the stages and other characteristics of severe drought occurrence across the agricultural region of the Canadian Prairies. Ten severe droughts occurred during the 1900–2014 period with each having unique temporal and spatial characteristics. Projected changes from 29 global climate models (GCMs) with three representative concentration pathways reveal an increase in severe drought occurrence, particularly toward the end of this century with a high emissions scenario. For the most part, the overall duration and intensity of future severe drought conditions is projected to increase mainly due to longer persistence stages, while growth and retreat stages are generally shorter. Considerable variability exists among individual GCM projections, including their ability to simulate observed severe drought characteristics. This study has increased understanding in potential future changes to a little studied aspect of droughts, namely, their stages and associated characteristics. This knowledge can aid in developing future adaptation strategies.

The interior of western Canada, up to and including the Arctic, has experienced rapid change in its climate, hydrology, cryosphere, and ecosystems, and this is expected to continue. Although there is general consensus that warming will occur in the future, many critical issues remain. In this first of two articles, attention is placed on atmospheric-related issues that range from large scales down to individual precipitation events. Each of these is considered in terms of expected change organized by season and utilizing mainly “business-as-usual” climate scenario information. Large-scale atmospheric circulations affecting this region are projected to shift differently in each season, with conditions that are conducive to the development of hydroclimate extremes in the domain becoming substantially more intense and frequent after the mid-century. When coupled with warming temperatures, changes in the large-scale atmospheric drivers lead to enhancements of numerous water-related and temperature-related extremes. These include winter snowstorms, freezing rain, drought, forest fires, as well as atmospheric forcing of spring floods, although not necessarily summer convection. Collective insights of these atmospheric findings are summarized in a consistent, connected physical framework.

Abstract. The 0 °C temperature threshold is critical to many meteorological and hydrological processes driven by melting and freezing in the atmosphere, surface and sub-surface and by the associated precipitation varying between rain, freezing rain, wet snow and snow. This threshold, linked with freeze-thaw, is especially important in cold regions such as Canada. This study develops a Canada-wide perspective on near 0 °C conditions with a particular focus on the occurrence of its associated precipitation. Since this analysis requires hourly values of surface temperature and precipitation type observations, it was limited to 92 stations over the 1981–2011 period. In addition, nine stations representative of various climatic regions are selected for further analysis. Near 0 °C conditions are defined as periods when the surface temperature is between −2 °C and 2 °C. Near 0 °C conditions occur often across all regions of the country although the annual number of days and hours and the duration of these events varies dramatically. Various forms of precipitation (including rain, freezing rain, wet snow and ice pellets) are sometimes linked with these temperatures with highest fractions tending to occur in Atlantic Canada. Trends of most temperature-based and precipitation-based indicators show little or no change despite a systematic warming in annual temperatures. Over the annual cycle, near 0 °C temperatures and precipitation often exhibit a pattern with short durations near summer driven by the diurnal cycle, while longer durations tend to occur more towards winter associated with storms. There is also a tendency for near 0 °C temperatures to occur more often than expected relative to other temperature windows; due at least in part to diabatic cooling and heating occurring with melting and freezing, respectively, in the atmosphere and at the surface.