Average temperatures have increased by more than 2°F since the beginning of the 20th century. The last decade was the warmest on record. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century. The intensity of extreme winter cold is projected to decrease.
Average annual precipitation has increased nearly 6 inches since the early 20th century, with the largest increases occurring in mountainous regions of the state. Increased winter and spring precipitation is projected for the 21st century, and warming will increase the proportion falling as rain.
Extreme weather events, particularly floods and severe storms, are having an increased impact on Vermont. Extreme rainfall events are projected to become more frequent and intense in the future.
Vermont’s northerly latitude and geographic location on the eastern edge of the North American continent exposes it both to the moderating and moistening influence of the Atlantic Ocean, as well as the effects of the hot and cold air masses from the interior of the continent. Its climate is characterized by cold, snowy winters and pleasantly warm summers. The polar jet stream is often located near the state giving it highly variable weather patterns, widely ranging daily and annual temperatures, and generally abundant precipitation throughout the year. Variations in climate across the state are due to changes in elevation, terrain, and proximity to Lake Champlain and the Atlantic Ocean. The western part of the state is moderated by the lake and experiences higher temperatures and a longer growing season than the more mountainous northeastern region (also referred to as the Northeast Kingdom). Southeastern Vermont, with its lower elevation and landlocked location tends to be warmer and more drought-prone than the rest of the state.
Temperatures in Vermont have increased more than 2°F since the beginning of the 20th century (Figure 1). While there is no trend in the number of hot days (maximum temperature above 90°F; Figure 2a), the annual number of warm nights (minimum temperature above 70°F) has been above average for the past 15 years (2000–2014), with a historically high peak from 2005 to 2009 (Figure 3). Both winter and summer temperatures have increased considerably in the past two decades (Figure 2c). The winter warming trend is reflected in a below-average annual number of very cold nights (minimum temperature below 0°F) since the early 1990s (Figure 2b). Higher spring and fall temperatures have resulted in corresponding changes in the length of the growing season from later “first-fall freeze” and earlier “last-spring freeze” dates. Over the past four decades, the growing season in Vermont has lengthened by an average of 4 days each decade.
Annual mean precipitation has been above average for the last several decades (Figure 4). The driest multi-year periods were in the 1910s and the early 1960s. The wettest periods have been observed since 2005 to the present. The driest 5-year period was 1961–1965 and the wettest was 2007–2011. The annual number of extreme precipitation events (number of days with greater than 2 inches) has been above the long-term average over the past two decades (1995–2014), with the highest number of events occurring during the period from 2005 to 2009 (Figure 2d). Average annual precipitation has increased nearly 6 inches since the 1960s.
Extreme weather events in Vermont can take the form of prolonged heavy snowstorms, flash floods, river floods (following snowmelt and heavy rains), severe thunderstorms, droughts, tornadoes, and temperature extremes. Some of the heaviest flooding in the state’s history has been due to tropical cyclones or their remnants. In 2011, Tropical Storm Irene transitioned into an extratropical cyclone as it moved quickly northeastward along the Vermont/New Hampshire border. Roughly 3 to 7 inches of rain fell in less than 18 hours, causing the worst flooding in Vermont since the Great Flood of November 1927. Many rivers reached stages that were second to only the 1927 flood. The flooding resulted in an estimated $733 million in damage across the state.
Severe winter storms are common in Vermont’s cold winter climate and may include snowstorms, blizzards, and icing events. In addition to ice jams and melting snowpack as winter hazards, freezing rain and frozen ground conditions can also give rise to flooding. During the first week of January 1998, a prolonged storm brought 2 to 5 inches of rain to Vermont. Particularly across the Champlain Valley and parts of northern Vermont, temperatures were below freezing for much of the storm. This resulted in the “Great Ice Storm of ‘98” where heavy ice accumulations of 1 to 2 inches caused severe damage to trees and utility lines. Total damage from the ice storm across the whole of the northeastern United States was about $2 billion ($1.4 billion in 1998 dollars)
Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century (Figure 1). Even under a pathway of lower greenhouse gas emissions, average annual temperatures are projected to most likely exceed historical record levels by the middle of the 21st century. However, there is a large range of temperature increases under both pathways, and under the lower pathway, a few projections are only slightly warmer than historical records (Figure 1). Increases in the number of hot days and decreases in the number of very cold nights are projected to accompany the overall warming.
Average annual precipitation is projected to increase in Vermont over the 21st century, particularly during winter and spring (Figure 5). Corresponding increases in temperature will increase the proportion of precipitation falling as rain rather than snow. In addition, extreme precipitation is projected to increase, potentially increasing the frequency and intensity of floods.