VERMONT

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.

   

Figure 1

Figure 1: Observed and projected changes (compared to the 1901–1960 average) in near-surface air temperature for Vermont. Observed data are for 1900–2014. Projected changes for 2006–2100 are from global climate models for two possible futures: one in which greenhouse gas emissions continue to increase (higher emissions) and another in which greenhouse gas emissions increase at a slower rate (lower emissions). Temperatures in Vermont (orange line) have risen by more than 2°F since the beginning of the 20th century. Shading indicates the range of annual temperatures from the set of models. Observed temperatures are generally within the envelope of model simulations of the historical period (grey shading). Historically unprecedented warming is projected to continue through the 21st century. Less warming is expected under a lower emissions future (the coldest years being about 2°F warmer than the historical average; green shading) and more warming under a higher emissions future (the hottest years being about 12°F warmer than the hottest year in the historical record; red shading). Source: CICS-NC and NOAA NCEI.

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.

Resources

1. Kunkel, K.E, L.E. Stevens, S.E. Stevens, L. Sun, E. Janssen, D. Wuebbles, K.T. Redmond, and J.G. Dobson, 2013: Regional Climate Trends and Scenarios for the U.S. National Climate Assessment. Part 6. Climate of the Northwest U.S., NOAA Technical Report NESDIS 142-6, 75 pp. [URL]
2. NOAA, 2008. Eastern U.S. flooding and ice storm - Ice Storm of 1998, published August 20, 2008, retrieved May 25, 2015, National Oceanic and Atmospheric Administration. [URL]
3. NOAA, 2012: Service assessment: Hurricane Irene, August 21-30, 2011, 129pp, National Oceanic and Atmospheric Administration. [URL]
4. NOAA, 2016: U.S. Billion-Dollar Weather and Climate Disasters, National Oceanic and Atmospheric Administration National Centers for Environmental Information [URL]
5. NOAA, cited 2015: Climate of Vermont, National Oceanic and Atmospheric Administration. [URL]
6. Northeast Regional Climate Center Data Resources, cited 2015: SC ACIS Version 2. [URL]
7. The Vermont Agency of Natural Resources, 2011: Resilience: A report on the health of Vermont’s environment, 28pp. [URL]
8. Vermont Climate Assessment, cited 2015: VT’s changing climate. [URL]
9. Vermont State Climate Office, cited 2015: Climate anomalies and extreme weather events. [URL]
10. Vermont State Climate Office, cited 2015: Climate resources: Climate change. [URL]
11. Vermont State Climate Office, cited 2015: Vermont climate. [URL]
12. Vose, R.S., D.R. Easterling, K.E. Kunkel, A.N. LeGrande, and M.F. Wehner, 2017: Temperature changes in the United States. In: Climate Science Special Report: Fourth National Climate Assessment, Volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 267-301.