Chesapeake Bay Climate Impacts Summary and Outlook
Mid-Atlantic Regional Climate Impacts Summary and Outlook: Summer 2020
- This summer saw a wide range of precipitation conditions; the northern portions of the Chesapeake Bay watershed had an abnormally dry summer, while the southern and eastern portions of the watershed experienced above normal precipitation.
- Precipitation in the southern and coastal portions of the Chesapeake Bay watershed ranked among some of the top five, ten, or 20 wettest summers on record for several locations.
- The Mid-Atlantic region experienced an active summer season because of near-weekly severe storm events and the arrival of Tropical Storm Isaias.
- The entire region experienced warmer-than-normal conditions; it was the hottest summer on record for Norfolk, Virginia, and Harrisburg and Williamsport, Pennsylvania.
- In each decade since 1981–1990, the region as a whole has seen an increase in the number of days with warm daily low temperatures—those above 70, 75, and 80°F.
- An analysis of future climate model data shows that the region is projected to see increases in the number of days with warm daily low temperatures by mid- to late-century.
Part 1: Significant Weather Events and Impacts
This June ranked among the 20 warmest on record for Scranton and Harrisburg, Pennsylvania; Baltimore and Salisbury, Maryland; and Washington D.C., and was among the ten warmest at Washington Dulles International Airport (Dulles Airport) and Charlottesville, Virginia.1 On June 10, Dulles Airport, Virginia, tied for its warmest June low temperature on record with a low of 74°F.2
This July set temperature records at sites across the region. It was the all-time hottest month on record for Scranton and Harrisburg, Pennsylvania; Baltimore, Maryland; and Dulles Airport, Norfolk, and Richmond, Virginia. It also ranked amongst the three hottest months on record for Binghamton, New York; Williamsport, Pennsylvania; Washington, D.C.; Salisbury, Maryland; and Lynchburg, Virginia.3
Several sites also set records for the number of 90°F days during July. Baltimore, Maryland; Scranton, Pennsylvania; Washington D.C.; and Lynchburg, Norfolk, and Richmond, Virginia recorded their greatest number of days with highs of at least 90°F for any month on record. Dulles Airport, Virginia; and Harrisburg and Williamsport, Pennsylvania recorded their second-greatest number of 90°F days for any month.4
This August ranked among the five hottest Augusts on record for Norfolk, Virginia and Scranton, Harrisburg, and Williamsport, Pennsylvania. It also ranked among the ten hottest on record for Baltimore and Salisbury, Maryland and Dulles Airport, Virginia and among the 20 hottest on record for Binghamton, New York, and Richmond and Lynchburg, Virginia.5,6
Overall, this was the hottest summer on record for Norfolk, Virginia and Harrisburg and Williamsport, Pennsylvania. This summer also ranked among the five hottest on record for Scranton, Pennsylvania; Dulles International Airport, Richmond, and Lynchburg, Virginia; Baltimore and Salisbury, Maryland; and Washington, D.C., and among the ten hottest on record for Binghamton, New York.7,8
In early June, the region was free of abnormally dry conditions and drought,9 but by late June, below-normal rainfall led to the introduction of abnormal dryness in southern Pennsylvania and on the Delmarva Peninsula.10 Conditions in much of the region became drier in July, resulting in moderate drought conditions in central Pennsylvania, central and southeastern Maryland, and northern Virginia and abnormal dryness expanded to portions of each state in the Chesapeake Bay watershed.11,12
Moisture stress affected corn and other crops in Orange County, Virginia.13 Growers in western Maryland, central Pennsylvania, and southern New York also noted impacts from dry conditions including shorter-than-usual corn and reduced hay yields.14, 15, 16, 17
During August, dry conditions eased in Virginia and Maryland but expanded in portions of central Pennsylvania and southern New York.18 A drought watch was issued for 16 counties in Pennsylvania, with voluntary water restrictions enacted by several water suppliers.19,20 Drought stress caused some leaves to change color and drop earlier than usual in parts of Pennsylvania and New York.21
This June ranked among the 20 wettest on record for Dulles Airport, Virginia, and Baltimore, Maryland.22 July was also one of the 20 wettest on records for Dulles Airport, Virginia; for other areas in the region, however, July was considerably drier. In fact, this July ranked among the top 20 driest on record for Harrisburg and Williamsport, Pennsylvania; Norfolk, Virginia; and Martinsburg, West Virginia.23 August ranked among the five wettest on record for Baltimore and Salisbury, Maryland, and Dulles Airport and Lynchburg, Virginia; among the ten wettest on record for Binghamton, New York; and among the 20 wettest on record for Washington, D.C.24,25
This summer ranked among the five wettest summers on record for Dulles Airport and Lynchburg, Virginia; among the ten wettest on record for Baltimore and Salisbury, Maryland; and Richmond, Virginia; and among the 20 wettest on record for Washington, D.C.26,27 In contrast, Williamsport, Pennsylvania had its ninth-driest summer on record.28
The region experienced a high number of days this summer with strong to severe thunderstorms, the main impact of which was downed trees caused by strong winds.29 There were also many reports of localized flash flooding across the region caused by these storms.30 A few of these storm events are discussed below.
On June 3, a line of intense thunderstorms advanced across Pennsylvania and into New Jersey. Wind gusts of up to 93 miles per hour (mph) caused widespread damage, downing trees and power lines, knocking out power to more than 500,000 customers, and contributing to four deaths.31 The storm complex’s severity and speed led the National Weather Service to classify it as a derecho.32
On June 17, heavy rain led to flash flooding in parts of Virginia. Botetourt County saw up to 7 inches of rain with these storms, leading to closed roads and some evacuations.33
Just over a week later, on June 25, thunderstorm winds downed trees in eastern West Virginia, northern Virginia, and central and southern Maryland. Storm reports from Montgomery County, Maryland noted uprooted trees, closed roads, damage to homes, and a wind gust of 70 mph.34
On July 5, strong thunderstorm winds in Anne Arundel County, Maryland downed a large tree. The tree fell on a garage, injuring 19 people.35
The next day, on July 6, straight-line winds on Maryland’s Eastern Shore snapped and downed trees; the debris caused several road closures.36 Meanwhile, in Baltimore, Maryland, the roof of the Martin State Regional Airport helicopter hangar was damaged by storms.37 Localized flooding rains in southern and central Maryland led to road closures and several water rescues.38,39 A lightning strike killed two people and injured two others in Bradford County, Pennsylvania.40
On July 21, severe thunderstorms blew roofs from buildings in Franklin County, Pennsylvania.41
On July 22, severe storms downed numerous trees across northern Virginia and central and southern Maryland, as well as hundreds of trees in Washington, D.C.42 As much as 3 inches of rain fell, leading to flash flooding in Fairfax County, Virginia, where several roads were closed, and in Baltimore, Maryland, where floodwaters left a bus stranded and moved several cars.43 The storm also left thousands without power, with more than 18,000 outages reported.44
Two days later, on July 24, there was more flash flooding in Baltimore, Maryland, which flooded roads, stranded multiple vehicles, and resulted in several water rescues.45,46 Northeast of the city, Minebank Run, near Glen Arm, Maryland, rose 6 feet in 30 minutes.47
On August 1, a tornado touched down in Botetourt County, Virginia.48
On August 6–7, just days after Tropical Storm Isaias moved through the region, portions of Virginia, Maryland, and Pennsylvania saw more heavy rain, flash flooding, damaging winds, and tornadoes. The Baltimore-Washington National Weather Service issued a rare flash flood emergency for parts of Loudoun County, Virginia, which saw up to 6 inches of rain and rapid rises of waterways.49,50 For example, Limestone Branch, a creek near Leesburg, Virginia, rose 7.5 feet in one hour, and the South Fork Catoctin Creek near Waterford, Virginia rose 7 feet in less than an hour.51,52 As a result, there were multiple water rescues and closed roads in Loudoun County, Virginia.53 Over the same period, two EF-0 tornadoes (on the Enhanced Fujita scale) and strong straight-line winds snapped and uprooted trees in Augusta County and Albemarle County in western Virginia.54 Straight-line winds of up to 95 mph downed multiple trees and led to a partial roof collapse in Adams County, Pennsylvania.55
August 12–13 brought additional heavy rain and flooding to northern Virginia and central and southern Maryland. There were several closed roads and vehicles trapped in floodwaters.56,57,58 In Prince William County, Virginia, South Fork Quantico Creek near Independent Hill rose to a record high of 12.04 feet.59
Portions of central and southeastern Virginia experienced extreme rainfall on August 15. Rain totals exceeded 5 inches in several counties, with Chesterfield County Virginia reporting as much as 11.19 inches of rain.60,61 Dozens of roads were closed across the affected counties, and in Chesterfield County Virginia, several homes downstream of the Falling Creek Dam were evacuated because of flood risk.62,63
On July 9, Tropical Storm Fay, the earliest F-named storm on record, formed in the Atlantic basin.64 Fay made landfall in New Jersey on July 10 and dropped up to 6 inches of rain on the Delmarva Peninsula, where localized flooding led to road closures.65
Tropical Storm Isaias, the earliest I-named storm on record, moved through the Mid-Atlantic region on August 3 and 4. Isaias produced multiple tornadoes, damaging winds, and flooding rains.66 Figure 1 shows some of the tornado damage in Maryland. The greatest rain totals ranged from 4–9 inches, which resulted in significant flooding, particularly in Maryland.67 The St. Clement Creek near Clements, Maryland reached a record high of 9.09 feet, which was more than a foot higher than the previous record-high level.68 Nearby, the St. Mary’s River at Great Mills, Maryland reached its sixth-highest water level on record.69 In St. Mary’s County, Maryland, where both waterways are located, 11 roads were washed out and closed indefinitely, and in neighboring Calvert County, Maryland, several bridges were damaged and there were at least three water rescues.70 Flooding also led to road closures in northern Maryland.71
Isaias also produced ten tornadoes in Maryland, seven tornadoes in Virginia, and three tornadoes in Delaware, the strongest of which was rated EF-2.72,73,74,75 One tornado in Delaware was on the ground for a record-setting 35.5 miles.76 The tornadoes downed hundreds of trees, overturned vehicles, and caused significant damage to buildings, including at least 12 homes in northern Delaware that were declared uninhabitable.77,78 In Delaware, preliminary damage estimates due to Isaias exceeded $20 million.79
Isaias’ highest wind gusts ranged from 60–75 mph, causing numerous downed trees, some of which fell on vehicles and buildings.80,81 There also were widespread power outages in the region. More than 300,000 customers in the Wakefield Virginia National Weather Service area were without power.82
A discussion of the 2020 Atlantic hurricane season is included in Part 3.
Part 2: Seasonal Temperature and Precipitation
Figure 2 shows the June–August 2020 average daily temperature compared with the climate normal—i.e., the average daily temperature from 1981 to 2010.83 The figure shows that the majority of the Mid-Atlantic region experienced temperatures well above normal during the first two months of the summer. In fact, most of the region experienced temperatures that were at least 2°F above normal. Only a few small areas had temperatures within a degree of normal. The southwestern portion of the region experienced temperatures closest to, though predominantly still above, normal.
Precipitation departures from normal for June 1–August 31, 2020 are shown in Figure 3. Departures from normal indicate where this summer season’s average daily rainfall was above or below the average daily summer season precipitation from 1981 to 2010. Figure 3 shows that the Mid-Atlantic region experienced a range of precipitation conditions, with a notable difference between the northern and southern portions of the region. Pennsylvania and the northernmost portion of the Chesapeake Bay watershed in New York were particularly dry, receiving less than 75 percent of normal precipitation. Portions of Maryland and Virginia experienced the wettest conditions, with large areas receiving over 150 percent of normal precipitation and some locations seeing more than 200 percent of normal precipitation. The wettest conditions occurred in coastal Virginia and Maryland.
Part 3: Fall 2020 Outlook
Temperature and Precipitation
As of August 20, 2020, the NOAA Climate Prediction Center forecasts a 50–60-percent chance of above-normal temperatures for fall for the majority of the Chesapeake Bay watershed and Mid-Atlantic region. The precipitation forecast shows an equal chance of precipitation above, at, or below normal for September through November 2020 for the majority of the Mid-Atlantic region and a 33-40-percent change of wetter than normal conditions in the southeastern corner of the Chesapeake Bay Watershed.84
The U.S. Seasonal Drought Outlook identifies how drought might change across the United States and categorizes areas by whether drought could develop or become more or less intense. As of August 20, 2020, the outlook indicates that drought removal is likely in western Pennsylvania and a portion of southern New York and shows no tendency toward drought for the rest of the Mid-Atlantic region.85
Atlantic Hurricane Season
NOAA’s Climate Prediction Center, which issues and updates hurricane outlooks for the Atlantic before and throughout the hurricane season, updated its outlook on August 6, 2020 to “extremely active”.86 NOAA announced one of the “most active seasonal forecasts … in its 22-year history of hurricane outlooks.”87 This update forecasts an 85 percent chance of an above-normal 2020 Atlantic hurricane season, with 19–25 named storms, 7–11 hurricanes, and three to six major hurricanes.88 According to NOAA, “An average hurricane season produces 12 named storms, of which 6 become hurricanes, including 3 major hurricanes.”89 As of August 5, researchers at Colorado State University (CSU), a prominent university for hurricane forecasting, adjusted their forecast to call for an “extremely active” 2020 Atlantic hurricane season.90 CSU’s forecast anticipates 24 named storms and 12 hurricanes for 2020, with a 74-percent chance of at least one major hurricane (category 3–5) making landfall on the eastern U.S. coastline.91 As of September 3, 2020, the Atlantic basin had experienced 15 named storms, five hurricanes, and one major hurricane.92 In an average (1981–2010) hurricane season, the Atlantic basin sees 5.6 named storms, 2.3 hurricanes, and 1 major hurricane by September 3.93
Part 4: Warm Daily Low Temperatures
Extreme heat and extreme heat events, such as heat waves, are key climate variables that communities should monitor and plan for to mitigate the effects of these events on human health, agriculture, ecosystems and the built environment. Extreme heat events and prolonged exposure to heat can lead to a wide range of heat stress conditions and illnesses or even death, particularly for vulnerable populations.94 These events can also strain the power grid, resulting in power outages,95 reduce crop yields,96 damage the health of livestock,97 and stress ecosystems,98 among other effects. In large urban centers, both the effects on human health and the magnitude of extreme heat may be greater because of the urban heat island effect.
Although there is a variety of metrics used to quantify extreme heat and heat waves, these generally rely on daily maximum and minimum temperatures and sometimes include humidity. Daily maximum temperatures were included in the summer 2019 version of this climate summary. The incidence of unusually warm daily minimum temperatures is particularly problematic because they provide less time for human, built, and environmental systems to recover from daily maximum temperatures.99 Further, daily minimum temperatures are anticipated to warm at a faster rate than daily maximum temperatures.100 This section presents an analysis of changes in the number of days with warm daily minimum temperatures.
The following figures provide detail on how the average annual number of days with warm daily low temperatures has changed over the last several decades (Figure 4) and could change in the future (Figure 5). Specifically, these figures show decadal and multi-decadal changes in the number of days with daily low temperatures above 70°F, 75°F and 80°F. These interactive tools can be used to generate local and regional insights into how daily lows have shifted in the historical record and might continue to shift in the future.
Figure 4. Historical Average Annual Number of Days with Warm Low Temperatures
- In each decade since 1981–1990, the region as a whole has seen an increase in the number of days with warm daily low temperatures.
- The southern portion of the Mid-Atlantic region experiences a higher number of days with warm daily low temperatures each decade.
- The far-northern portions of region and high elevation locations, such as parts of West Virginia and northwestern Virginia, tend to see either very few or zero average annual number of days with warm daily low temperatures.
How to Use the Tool
Selecting Time Periods and Temperature Thresholds
Use the Time Period slider bar to adjust the decade used to calculate the average annual number of days above the selected temperature threshold. The daily low temperature threshold can also be selected by using the slider titled Temperature Threshold.
Viewing Variability Within a Location
Hover or tap over a point of interest. A window will pop up that displays the number of days above the selected temperature threshold in each decade. You can also use the Filter by State and Select by County filters to the right of the map to zoom into a location of interest.
Figure 5. Future Projected Average Annual Number of Days with Warm Low Temperatures
- Similar to the trends in daily lows over the last several decades, the region is projected to see increases in the number of days with warm low temperatures.
- The largest increases in the average annual number of days with warm daily low temperatures are projected for a daily low temperature threshold of 70°F. On average, the region as a whole could see 53 days above this threshold by late-century in a high-emissions future. The region currently experiences eight days above this threshold.
- Under both future emissions scenarios, the southeastern portion of the Mid-Atlantic region is projected to experience the greatest increases in the number of days with warm daily lows.
- In a high-emissions future, parts of western Pennsylvania could see more than 40 days per year with warm daily lows.
- Future projections indicate that only a few regions might experience significant changes in the number of days with lows above 80°F. Included in this group are Baltimore, Maryland, and Washington, D.C.
How to Use the Tool
Selecting Time Periods and Future-Emissions Scenarios
Use the slider to the right of the maps to adjust the 30-year period used to calculate the average annual number of days with temperatures above the selected temperature threshold. Users can also select the future-emissions scenario (Low or High Emissions).
Viewing Variability Within a Location
Hover or tap over a point of interest. A window will pop up that displays the average annual number of days with low temperatures above the selected threshold. You can also use the Filter by State and Select by County filters to the right of the map to zoom into a location of interest.
The Mid-Atlantic Regional Integrated Sciences and Assessments (MARISA) Seasonal Climate Impacts Summary and Outlook is a quarterly series produced by the MARISA program, a collaboration funded by NOAA through the RAND Corporation and researchers at Pennsylvania State University, Johns Hopkins University, Cornell University, and the Virginia Institute of Marine Science. This series draws information from regional climate centers, news and weather information, and regional-specific climate data sets for the benefit of policymakers, practitioners, residents, and community leaders in the Mid-Atlantic region. Projections of weather and climate variability and change in the Mid-Atlantic region come from the best available scientific information. For any questions or comments, please contact Krista Romita Grocholski at Krista_Romita_Grocholski@rand.org.
This edition of the MARISA Seasonal Climate Impacts Summary and Outlook was authored by Michelle E. Miro (RAND Corporation), Krista Romita Grocholski (RAND Corporation), Jessica Spaccio (Cornell University), Samantha Borisoff (Cornell University), Arthur T. DeGaetano (Cornell University), and Jordan R. Fischbach (RAND Corporation).
The terms abnormally dry conditions and drought come from the U.S. Drought Monitor, which defines drought as “a moisture deficit bad enough to have social, environmental or economic effects.” Abnormally dry conditions are not drought, but the term describes conditions “that could turn into drought or are recovering from drought but are not yet back to normal” (https://droughtmonitor.unl.edu/About/AbouttheData/DroughtClassification.aspx). Return to text ⤴
https://www.post-gazette.com/news/environment/2020/08/21/Sixteen-counties-under-drought-watch/stories/202008210132#:~:text=Under%20the%20watch%2C%20residents%20of,to%2060%20gallons%20a%20day Return to text ⤴
https://cumberlink.com/news/state-and-regional/penn-state-professor-says-growing-drought-causing-trees-to-drop-leaves-means-poor-foliage-display/article_bfec34d3-7013-5734-85db-58d0a7dfff92.html Return to text ⤴
https://www.ncdc.noaa.gov/stormevents/ It should be noted that there is a lag of a few months before flood events are included in the National Centers for Environmental Information Storm Events Database, so there is a chance that not all of the events reported in this climate summary will be included in the database by the time of publication. Return to text ⤴
According to the National Weather Service, a derecho "(pronounced similar to 'deh-REY-cho') is a widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms”. https://www.weather.gov/lmk/derecho; https://www.washingtonpost.com/weather/2020/06/03/philadelphia-derecho/ Return to text ⤴
https://www.wsls.com/news/local/2020/06/17/we-barely-made-it-to-the-car-flooding-leads-to-evacuations-in-botetourt-county/; http://mesonet.agron.iastate.edu/wx/afos/p.php?pil=PNSRNK&e=202006180137 Return to text ⤴
Climate normals, as defined by the National Oceanic and Atmospheric Administration (NOAA), are “three-decade averages of climatological variables including temperature and precipitation.” The latest climate normal released by NOAA is the 1981–2010 average. See https://www.ncdc.noaa.gov/data-access/land-based-station-data/land-based-datasets/climate-normals#:~:text=Climate%20Normals%20are%20three%2Ddecade,variables%20including%20temperature%20and%20precipitation. Return to text ⤴
The Mid-Atlantic has experienced severe to extreme droughts in the past, most notably in the mid-1980s, the late 1990s, and the 2000s https://www.cpc.ncep.noaa.gov/products/expert_assessment/season_drought.png; https://onlinelibrary.wiley.com/doi/pdf/10.1111/1752-1688.12600 Return to text ⤴