Pennsylvania has a diverse landscape of mountains, agricultural regions, and large metropolitan areas. The Appalachian Mountains cut across the center of the state, with the Piedmont and Coastal Plain to the east and the Allegheny Plateau and Lake Erie lowlands to the west. The state is bordered by Lake Erie to the northwest and the Delaware River and Delaware Estuary to the east. The state's climate is heavily influenced by several geographic features. The Atlantic Ocean has a moderating effect on coastal areas, while Lake Erie moderates the northwestern part of the state. During much of the year, the prevailing westerly flow brings air masses from the North American interior across the entire region, with occasional episodes of bitter cold during winter. The jet stream, which is often located near or over the region during winter, brings frequent storm systems, causing cloudy skies, windy conditions, and precipitation. Winter low temperatures range from less than 20°F in northern portions of the state to almost 30°F in the southeast (1991–2020 averages). Summer high temperatures generally range from the mid-70s (°F) to the mid-80s across the state, with higher values in the most densely developed parts of Philadelphia. Annual precipitation totals range from less than 35 inches in the western valleys and north-central counties to more than 50 inches at higher mountain elevations and across eastern parts of the state. Annual snowfall in Pennsylvania ranges from under 20 inches in the southeastern lowlands to over 100 inches in the northwestern Lake Erie snowbelt and some high-elevation regions.
Time series of observed and projected temperature change (in degrees Fahrenheit) for Pennsylvania from 1900 to 2100 as described in the caption.
Y-axis values range from negative 4 to positive 20 degrees. Observed annual temperatures from 1900 to 2024 show variability and range from negative 2.9 to positive 4.0 degrees. By the end of the century, projected increases in temperature range from 2.5 to 8.8 degrees under the lower emissions pathway, from 4.6 to 10.7 degrees under the intermediate pathway, and from 8.7 to 17.5 degrees under the higher pathway.
Since the beginning of the 20th century, annual average temperatures in Pennsylvania have increased more than 2°F (Figure 1). This warming has occurred mostly in the winter and spring, while summer and fall have not warmed as much. The relatively small amount of summer warming is evident in the generally below-average occurrence of hot days since the 1960s (Figure 2a). Warm nights are rare in Pennsylvania; however, the annual number of such nights has generally been above the long-term average since the 2005–2009 period (Figure 2b). The number of very cold nights has generally been below average since the 1990s (Figure 3).
Two graphs of the observed annual number of (a) hot days and (b) warm nights for Pennsylvania from 1900 to 2024 as described in the caption.
Panel A: Hot Days
Y-axis values range from 0 to 32 days. Annual values show year-to-year variability and range from 0 to 30.3 days. Five-year values also show variability and are mostly below the long-term average of 6.7 days between 1900 and 1929, all above average between 1930 and 1959, and all near or below average since 1960 except for the 1985 to 1989 period. The 1905 to 1909 and 1975 to 1979 periods have the lowest 5-year value at 2.6 and 2.9 days, respectively, and the 1930 to 1934 period has the highest at 15.4 days.
Panel B: Warm Nights
Y-axis values range from 0 to 8 nights. Annual values show year-to-year variability and range from 0 to 6.5 nights. Five-year values also show variability and are mostly near or above the long-term average of 1.5 nights between 1900 and 1959 but are all below average between 1960 and 2004. All 5-year values since 2005 are above average. The 1960 to 1964 and 1980 to 1984 periods have the lowest 5-year values at 0.4 and 0.3 nights, respectively, and the 2010 to 2014 and 2015 to 2019 periods have the highest 5-year values at 3.5 and 3.4 nights, respectively.
Graph of the observed annual number of very cold nights for Pennsylvania from 1900 to 2024 as described in the caption.
Y-axis values range from 0 to 20 nights. Annual values show year-to-year variability and range from 0 to 17.7 nights. Five-year values also show variability and are all above the long-term average of 6.1 nights between 1900 and 1924, all below average between 1925 and 1959 except for the 1935 to 1939 period, and all near or above average between 1960 and 1989. Since 1990, five-year values are all below average. The 2020 to 2024 period has the lowest 5-year value at 1.8 nights, and the 1960 to 1964 and 1975 to 1979 periods have the highest at 9.7 and 10.6 nights, respectively.
Pennsylvania experiences abundant precipitation. Statewide total annual precipitation has ranged from a low of 28.9 inches in 1930 to a high of 64.0 inches in 2018 (Figure 4). Annual precipitation has generally been above average since the 1970s, primarily due to increases in summer and fall precipitation. The number of extreme precipitation events has increased since the 1960s (Figure 5). These events can cause devastating flooding, particularly in the state's smaller streams and tributaries. One of the worst natural disasters in the history of the United States was the Great Flood of 1889, when intense rains and a catastrophic failure of the South Fork Dam upstream of Johnstown killed more than 2,000 people. Heavy rains can also cause particularly damaging floods when they combine with spring snowmelt. In 1936, rain falling on snow caused a record flood on the Monongahela River. Occasional droughts are a natural part of the state's climate. Notable periods of extreme or severe drought (as defined by the US Drought Monitor) occurred in 2002, 2016–2017, 2020, and 2024–2025, which led to impacts on agriculture, water supplies, and recreational activities.
Graph of the observed total annual precipitation for Pennsylvania from 1895 to 2024 as described in the caption.
Y-axis values range from 24 to 68 inches. Annual values show year-to-year variability and range from 28.9 to 64.0 inches. Five-year values also show variability and are mostly near or below the long-term average of 42.6 inches between 1895 and 1969 but are all near or above average since 1970 except for the 1980 to 1984 and 1985 to 1989 periods. The 1960 to 1964 and 1965 to 1969 periods have the lowest 5-year values at 37.4 and 37.6 inches, respectively, and the 2015 to 2019 period has the highest at 48.7 inches.
Graph of the observed annual number of 2-inch extreme precipitation events for Pennsylvania from 1900 to 2024 as described in the caption.
Y-axis values range from 0.0 to 3.5 events. Annual values show year-to-year variability and range from 0.1 to 3.0 events. Five-year values also show variability and are mostly below the long-term average of 0.9 events between 1900 and 1994 but are all above average since 1995. The 1960 to 1964 and 1965 to 1969 periods have the lowest 5-year values at 0.48 and 0.52 events, respectively, and the 2010 to 2014 period has the highest at 1.53 events.
Pennsylvania is affected by a number of extreme events, including floods, tropical cyclones, heat waves and cold waves, severe thunderstorms, and snow and ice storms. Lake-effect snows, caused by the warming and moistening of Arctic air masses passing over the Great Lakes, are a hazard for the northwestern part of the state, which can receive more than 100 inches of snow annually. A historic lake-effect snowstorm hit northwestern Pennsylvania in late November and early December 2024, bringing five-day totals of 2 to 5 feet; Erie had its snowiest day on record on November 29, with a total accumulation of 22.6 inches. Another winter storm hit the same region on December 23, 2022, bringing relatively light snow accumulations but wind chills as low as –40°F and gusts of 40–50 mph; a rapid freeze and blowing snow made roads treacherous during the busy Christmas travel season. Areas along the shore of Lake Erie can experience flooding and erosion when water levels are high. Wet spring conditions contributed to high Great Lakes water levels and flooding in 2017 and 2019, causing costly infrastructure damage and agricultural losses. The area to the east of the Appalachian Mountains is at risk for nor'easters, which are cold-season coastal storms that can generate a tremendous amount of precipitation (in the form of snow, sleet, or freezing rain), strong winds, coastal flooding, and damage to infrastructure. These storms are fueled by the large temperature contrast between the cold interior of the eastern United States and the warm moist air over the western Atlantic. The Blizzard of 1996, a classic nor'easter, dropped more than 30 inches of snow in several parts of the state; Philadelphia received 30.7 inches, and 27.6 inches of that fell in 24 hours, a record for the city. February 2010 brought three winter storms to the state, causing Philadelphia, Pittsburgh, and Harrisburg to have their snowiest month on record. The Blizzard of 2016 brought high winds and heavy snow; Allentown recorded more than 30 inches.
Tropical cyclones occasionally impact the state. On September 1, 2021, the remnants of Hurricane Ida spawned 5 tornadoes in southeastern Pennsylvania and dropped 5–7 inches of rain, leading to catastrophic flooding and 5 deaths. A year earlier, on August 4, 2020, Tropical Storm Isaias produced 2 tornadoes, widespread rainfall totals of 4–7 inches, and severe flash flooding, leading to two fatalities. In southeastern Pennsylvania, the Perkiomen Creek at Graterford set a record high at 19.1 feet, nearly a foot higher than its previous record set in 1935. Over a 2-week period in 2011, the remnants of both Hurricane Irene and Tropical Storm Lee drenched Pennsylvania with heavy rains and caused historic flooding. One of the worst storms to hit the state was Tropical Storm Agnes in 1972. Agnes caused torrential rainfall, dropping more than 10 inches in parts of the state and causing catastrophic flooding, with statewide damages estimated at almost $3 billion. The number of landfalling Atlantic hurricanes is not expected to rise. However, rainfall rates from such storms, as well as the proportion of time they spend as major hurricanes, are projected to increase.
Thunderstorms are a common occurrence during the warmer months, and the most severe of these can occasionally cause extensive damages and loss of life. A derecho (a widespread, long-lived straight-line windstorm associated with a series of severe thunderstorms) moved rapidly across Pennsylvania on June 3, 2020, producing heavy rain, frequent cloud-to-ground lightning, and wind gusts higher than 80 mph, leading to 4 deaths. On May 31, 1985, a storm caused 21 tornadoes—including the only F5 in the state's history—across the northern and western counties, killing more than 60 people and destroying more than 1,000 homes.
Under the intermediate and higher emissions pathways, historically unprecedented warming is projected during this century (Figure 1). Even under the lower emissions pathway, most projections show annual average temperatures exceeding historical levels by the middle of the century. However, projections for all three pathways show a large range of potential temperature increases, and some projections for the lower pathway are comparable to recent observed values. Extreme heat is a particular concern for Philadelphia and other urban areas, because heat absorbed and emitted by buildings and pavement, as well as other aspects of urban environments, makes cities even warmer than the surrounding countryside, a phenomenon known as the urban heat island effect. High temperatures combined with high humidity can create dangerous heat index values. Pennsylvania has experienced several heat waves in recent years. One of the worst peaked on July 22, 2011, when most of the state experienced high temperatures above 95°F and many locations experienced temperatures above 100°F. On that date, Reading reported an all-time high of 106°F, while Scranton reported its highest minimum temperature of 80°F on July 23. July 2020 was the hottest month on record for Scranton and Harrisburg; Scranton recorded its highest number of 90°F days for any month (16). July 2020 was also the hottest month on record for the state. During July 2022, Philadelphia set a record for consecutive number of nights (10) with temperatures of 75°F or higher. While heat waves are projected to become more intense, cold waves are projected to become less intense.
Under the intermediate emissions pathway, statistically significant increases in total winter precipitation are projected for most of Pennsylvania but are less certain elsewhere (Figure 6). Statistically significant increases are also projected in annual precipitation (across the state) and spring precipitation (for northern and western portions of the state). There is uncertainty in projections for summer and fall precipitation. In addition, extreme precipitation is projected to increase, elevating the risk for more frequent and intense floods. Heavier winter and spring precipitation and higher temperatures that lead to earlier snowmelt increase the risk of springtime flooding, which could pose a threat to Pennsylvania's agricultural industry by delaying planting and reducing crop yields.
Map of the contiguous United States showing the projected changes in total winter precipitation by the middle of this century as described in the caption. The state of Pennsylvania is outlined in black.
Legend values range from less than negative 10 to greater than positive 15 percent. Note: unless otherwise specified, projected values are not statistically significant.
Pennsylvania
The majority of Pennsylvania is projected to see statistically significant increases in total winter precipitation of 5 to 15 percent.
Contiguous US
Projected increases in total winter precipitation are as follows: up to 5 percent across a narrow swath running westward from western Georgia to southern California; 5 to 10 percent in the Southeast, Mid-Atlantic, southern Midwest, portions of the Great Plains, and most of the West; and 10 to 15 percent in the majority of the Northeast and upper Midwest as well as portions of the northern Great Plains and Intermountain West, with statistically significant increases in the 10 to 15 percent range for the Northeast, the northernmost regions of the upper Midwest, and portions of the northern Great Plains.
Projected decreases are as follows: up to 5 percent for southern regions of the Southwest and northern and central Texas; 5 to 10 percent for southern New Mexico and western Texas; and 5 to 10 percent or greater for southwestern Texas.
Naturally occurring droughts are projected to increase in intensity due to higher temperatures and the associated increase in evaporation rates and soil moisture loss.
Rising sea levels pose increasing risks to the coastal communities, infrastructure, and ecosystems along the Delaware Estuary in Pennsylvania. On average, the state experienced about 8 inches of sea level rise between 1970 and 2020—compared to 6 inches for the contiguous US as a whole—and projections under the Intermediate sea level scenario show additional increases of about 11 inches by 2050 and 1.7 feet by 2070 (Figure 7). Across the five scenarios shown in Figure 7, the projected increases from 2020 to 2070 range from about 1 to 3.5 feet. Even if storm patterns remain the same, sea level rise will increase the frequency, extent, and severity of coastal flooding.
Two time series of observed and projected relative sea level change for (a) the contiguous US and (b) Pennsylvania from 1970 to 2070 as described in the caption.
Y-axis values range from negative 1 to positive 4 feet for both graphs. Legends in both graphs denote sources of historical data and scenarios for projections. Line colors for historical data are black for tide gauge data and orange for satellite data. Line and shading colors for projections based on five scenarios are as follows: light blue, Low; medium blue, Intermediate-Low; dark blue, Intermediate; yellow, Intermediate-High; and dark orange, High.
Panel A: Contiguous US
Tide gauge and satellite data generally show increases but with year-to-year variability. Projected data show increases under all scenarios. Tide gauge values range from a minimum of negative 0.3 feet in 1976 to a maximum of positive 0.4 feet in 2023. Satellite values range from a minimum of 0.0 feet in 1994 to a maximum of 0.4 feet in 2023. Projected sea level median values (lines) for the Low scenario range from 0.3 feet in 2020 to 1.1 feet in 2070, compared to 0.4 feet in 2020 to 3.1 feet in 2070 for the High scenario. Likely ranges (shading) become wider over the period shown. The range of likely values in 2070 for each of the five scenarios are as follows: Low scenario, from 0.9 to 1.4 feet; Intermediate-Low, 1.2 to 1.7 feet; Intermediate, 1.5 to 2.2 feet; Intermediate-High, 1.8 to 3.1 feet; and High, 2.4 to 3.8 feet.
Panel B: Pennsylvania
All historical and future scenario lines show increases higher than those in the contiguous US graph. Tide gauge values range from a minimum of negative 0.5 feet in 1980 to a maximum of positive 0.7 feet in 2024. Satellite values range from a minimum of 0.0 feet in 1994 to a maximum of 0.5 feet in 2024. Projected sea level median values (lines) for the Low scenario range from 0.4 feet in 2020 to 1.4 feet in 2070, compared to 0.4 feet in 2020 to 3.3 feet in 2070 for the High scenario. Likely ranges (shading) become wider over the period shown. The range of likely values in 2070 for each of the five scenarios are as follows: Low scenario, from 1.1 to 1.8 feet; Intermediate-Low, 1.4 to 2.1 feet; Intermediate, 1.7 to 2.5 feet; Intermediate-High, 2.0 to 3.3 feet; and High, 2.5 to 3.9 feet.
Sea level rise has caused an increase in high-tide floods associated with nuisance-level impacts, which are occurring more frequently, even on sunny days. High-tide flooding occurs when one or more factors—sea level rise, winds, changes in ocean currents, full or new moons—push water levels above the normal high-tide mark. These events can damage infrastructure, cause road closures, and overwhelm storm drains. High-tide flooding has increased in all US coastal areas, with more rapid increases along the East and Gulf Coasts, and is likely to occur more frequently in Pennsylvania as global and local sea levels continue to rise. The number of high-tide flood days has increased in Philadelphia, with the greatest number occurring in 2023. Under both the Intermediate-Low and Intermediate scenarios, high-tide floods are projected to occur much more frequently in future years (Figure 8).
Graph of the observed and projected annual number of high-tide flood days at Philadelphia, Pennsylvania, from 1920 to 2100 (top), including a magnified view of the observed data for the period 1920 to 2024 (bottom), as described in the caption.
A legend shows annual observations (orange bars) and projected 19-year averages (two-colored bars) for the Intermediate-Low (medium blue) and Intermediate (dark blue) sea level scenarios.
Bottom
Y-axis labels for the magnified view range from 0 to 15 days per year. Following the first recorded event in 1933, high-tide flooding was sporadic until the early 1970s, often with gaps of 1 to 3 (and as many as 10) years between events and annual values mostly below 4 days. Between the 1970s and early 2000s, high-tide flood days increased in frequency and number. Starting in 2002, high-tide flood days occurred every year, except for 2016, with a record high of 17 days in 2023.
Top
Y-axis labels range from 0 to 350 days, with a horizontal dashed gray line indicating 365 days. Philadelphia is projected to experience 55 to 65 high-tide flood days per year by 2050 under the Intermediate-Low and Intermediate scenarios, respectively, and 285 to 360 days by 2100.
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