Chesapeake Bay Climate Impacts Summary and Outlook

Chesapeake Bay Watershed Climate Impacts Summary and Outlook: Summer 2019

Highlights

  • High intensity rainfall events occurred throughout the Chesapeake Bay watershed, resulting in two rare National Weather Service Flash Flood Emergencies.
  • Northern Virginia, central Pennsylvania and eastern Maryland experienced less precipitation than normal—with some areas seeing as little as 50 percent of the 1981-2010 average.
  • Western Maryland and Eastern Virginia experienced precipitation up to 150 percent of normal.
  • Temperature was above normal for much of the region throughout the summer months, and July ranked as one of the ten all-time warmest months on record for several locations. This contributed to this summer ranking among the 10 warmest summers on record for Baltimore, MD; Harrisburg, PA; Dulles Airport, VA; and Washington, D.C.
  • The Atlantic hurricane season is forecasted by NOAA to be above normal.

Part 1: Significant Weather Events

Flash Flooding

Several flash flooding incidents occurred between late June and early July. On June 29-30, parts of western Maryland and eastern West Virginia received up to 5 inches of rain, the majority of which fell in just a few hours.1 Due to the intensity of precipitation, Baltimore-Washington National Weather Service issued a rare Flash Flood Emergency for a portion of Grant County, WV.2 This included the town of Bayard, WV, which recorded 4.71 inches of rain, making June 30 the site’s second wettest day since records began in 1902.3

On July 8, another significant flash flood event occurred, which affected portions of Maryland, Virginia, and West Virginia. Ronald Reagan Washington National Airport near Washington D.C. received 3.30 inches of rain (nearly an entire average July’s worth of rain) in an hour, 4 which ranks as the greatest one-hour rainfall total for the district since at least 1948.5 For the D.C. area, rainfall events of this magnitude have a 1% chance of happening in a given year.6 The largest 24-hour rainfall totals that occurred during this flash flood event across Maryland, Virginia, and West Virginia approached 5 inches.7

During the July 8 event, the Baltimore-Washington National Weather Service declared its second Flash Flood Emergency for the season.8 This was the first Flash Flood Emergency declared for the D.C. metropolitan area since the National Weather Service created the designation in 2011.9 Waterways rose quickly during the heavy rain. Data show that both Four Mile Run in Arlington, VA, and Cameron Run in Alexandria, VA, rose more than 11 feet in an hour.10 Floodwaters stranded morning commuters, inundated portions of public transportation routes, and resulted in several water rescues.11

On August 6, around 4 inches of rain fell in an hour in Baltimore, MD, flooding roads and vehicles.12 This was a 500-year storm event, which means that this intensity and duration of rainfall has a 0.2% chance of occurring in a given year.13 The following day, on August 7, Norfolk, VA received 2 inches of rain in 31 minutes, which caused some roads to become impassable due to flooding.14

Motorists are stranded on a flooded section of Canal Road in Washington, D.C. during a heavy rainstorm, Monday, July 8, 2019.

Photo by Dave Dildine / WTOP

Extreme Heat

The Chesapeake Bay region experienced a series of record setting heat waves this summer. The most intense of these heat waves of was from July 19 to 21. The National Weather Service issued Excessive Heat Warnings and Heat Advisories for much of the region during this time period. Daily high temperatures reached 100°F; and when combined with high humidity levels, the heat index reached 110°F in some areas.15 In addition, daily low temperatures were also unusually warm. On July 20, Washington, D.C., had a low of 81°F, which ranked as the city’s 10th warmest daily low temperature for July on record (since 1872). In Harrisburg, PA, a daily low of 79°F tied for the city’s eighth all-time warmest minimum temperature on record (since 1888).16

Overall, July ranked as one of the ten all-time warmest months on record for several locations, including Harrisburg and Scranton, PA; Baltimore, MD; and Dulles Airport, VA.17 The number of days during July with a high temperature of at least 90°F ranked among the ten all-time greatest for any month on record at several locations.18 For example, Baltimore had 21 days with a temperature of at least 90°F during July, which ties as its fourth greatest number of days since records began in 1872.19 In addition, the average minimum temperature for July was the all-time warmest for any month on record in Harrisburg, PA.20 Scranton, PA had 10 days in July with a minimum temperature of 70°F or higher, the greatest number for any month on record since 1901.21

Severe Weather

Despite the unusually active spring months for severe weather and tornadoes, the summer tornado count was below average. In fact, there was only one tornado in the watershed: an EF-1 in northern Maryland on June 13.22 In June and July, severe thunderstorms, some of which carried wind gusts of up to 81 mph and hail as large as 2 inches, downed numerous trees and wires, as well as damaged several buildings in Central Pennsylvania.23

Part 2: Seasonal Temperature and Precipitation

Temperature

An analysis of Summer 2019 average temperature, shown in Figure 1, compared to the climate normal, defined as the average temperature from 1981 to 2010, indicates above-average departures from normal temperature for most of the Chesapeake Bay watershed. Of these, some of the greatest departures from normal (at least 3°F) were seen in the greater Baltimore metropolitan region and in eastern Maryland. This summer ranks among the top 10 warmest summers on record for Baltimore, MD; Harrisburg, PA; Dulles Airport, VA; and Washington, D.C.24 For more data on long-term trends in temperature, see Part 4, below.

Figure 1. June 1, 2019–August 31, 2019 Departure from Normal Temperature (°F)

Note: Normal temperature is based on summer seasonal average temperature data from 1981-2010. Red indicates above-average temperature.

Source: Northeast Regional Climate Center, 2019.

Precipitation

From June 1 through August 31, 2019, precipitation departures from normal precipitation show that the region experienced precipitation that was both less than and greater than historical averages, as shown in Figure 2. Northern Virginia, central Pennsylvania and eastern Maryland experienced the least amount of normal precipitation—up to 50 percent of the 1981-2010 average. Western Maryland and Eastern Virginia, for example, experienced precipitation up to 150 percent of normal. This summer ranked among the five wettest summers on record for Scranton and Williamsport, PA.25

Figure 2. June 1, 2019–August 31, 2019 Percent of Normal Precipitation

Note: Normal seasonal precipitation is based on precipitation data from 1981-2010. Oranges and reds indicate below-average seasonal precipitation.

Source: Northeast Regional Climate Center, 2019.

Part 3: Fall 2019 Outlook

Temperature and Precipitation

As of August 15, 2019, NOAA’s Climate Prediction Center forecasts for a 33–50% chance of precipitation above normal for September-November 2019 in the central and southern Mid-Atlantic region.26 They show a 40-60% chance of temperatures above normal for the majority of the Chesapeake Bay Watershed. For more information on long-term trends and averages in high temperature, see Part 4, below.

Drought Incidence

The U.S. Seasonal Drought Outlook predicts how drought may change across the United States, categorizing areas by whether drought could develop or become more or less intense. As of August 15, 2019, the outlook indicates no likelihood of drought for the Chesapeake Bay Watershed.27 The Chesapeake Bay region has experienced severe to extreme droughts in the past, most notably in the mid-1980s, the late 1990s and the 2000s.28

Atlantic Hurricane Outlook

NOAA’s Climate Prediction Center, which issues a hurricane outlook for the Atlantic throughout the hurricane season, is forecasting the season to be above normal with 10-17 named storms, 5–9 hurricanes and 2–4 major hurricanes.29 Researchers at Colorado State University (CSU) have predicted a near average 2019 hurricane season in the Atlantic. As of August 5, 2019, CSU’s total seasonal forecast calls for 14 named storms and 7 hurricanes for 2019, with a 53 percent chance of at least one major hurricane (category 3–5) making landfall on the eastern U.S. coastline.30

Part 4: Annual Extreme Temperature Projections

The following interactive figure provides detail on how annual extreme high temperature could change into the future (Figure 3).

Future Change in High Temperature Extremes

Areas of the Mid-Atlantic experienced one of the hottest summers on record this year, with several locations experiencing nearly half of summer days with temperatures above 90 degrees. For example, Baltimore, MD saw 45 days with temperatures above 90 degrees. This interactive visualization shows how we anticipate the number of extreme heat days will change in the future.

Key Findings

  • Across the Chesapeake Bay watershed, all future time periods show increases in seasonal total precipitation in the average annual number of days above extreme temperature thresholds.
  • By mid- to late-century (2041–2070), southern Maryland and eastern Virginia could see 80 or more days of days with temperatures above 90°F in a low emissions future and 90 or more days of days with temperatures above 90°F in a high emissions future.

How to Use the Tool

Selecting time periods and temperature thresholds: To view average annual number of days above various thresholds and periods of time, select 30-year time increments on the right side of the map, as well as three different temperature thresholds and two future greenhouse gas emission scenarios. Hover over a location on the map to see a pop-up window of average annual days above a given threshold for that location.

Technical Notes

LOCA or Localized Constructed Analogs is a downscaled climate data product available at 1/16th degree (6 km) resolution over the continental United States. LOCA datasets include the 32 climate models available in the CMIP5 archive, for two future greenhouse gas concentration trajectories—a low emissions future, RCP 4.5,31 and a high emissions future, RCP 8.5.32 For this study, we utilized LOCA data over the Chesapeake Bay watershed from 1981–2100 (or 2099 for some models). Access LOCA datasets and learn more about the methodology.

Extreme high temperature was determined for each year by summing all extreme temperature days (above 90°F, 95°F, 100°F) that occurred in each year. We calculated extreme high temperature days for each year and the averaged values across 30-year periods—1981–2010, 2011–2040, 2041–2070, 2071–2100 for LOCA data for RCP 4.533 and RCP 8.5.34 To average across climate models for each grid cell in the LOCA dataset, we employed a weighted average provided by the Northeast Regional Climate Center. The LOCA datasets were masked to the boundaries of the Chesapeake Bay watershed before calculating extreme high temperature days.

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The MARISA Seasonal Climate Impacts Summary and Outlook is a quarterly series produced by the Mid-Atlantic Regional Integrated Sciences and Assessments (MARISA) program, a collaboration funded by NOAA through RAND 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 datasets for the benefit of policymakers, practitioners, residents, and community leaders in the Chesapeake Bay Watershed. Projections of weather and climate variability and change in the Chesapeake Bay Watershed come from the best available scientific information. For any questions or comments, please contact Krista Romita Grocholski at Krista_Romita_Grocholski@rand.org.

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Footnotes

  1. https://water.weather.gov/precip/index.php Return to text ⤴

  2. https://twitter.com/NWS_BaltWash/status/1145234509022896134?s=20 Return to text ⤴

  3. http://www.nrcc.cornell.edu/services/blog/2019/07/01/index.html Return to text ⤴

  4. https://www.washingtonpost.com/weather/2019/07/08/washington-dc-flash-flood-how-why-area-was-deluged-by-months-worth-rain-an-hour-monday/ Return to text ⤴

  5. https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=va Return to text ⤴

  6. https://hdsc.nws.noaa.gov/hdsc/pfds/ Return to text ⤴

  7. http://www.nrcc.cornell.edu/services/blog/2019/07/10/20190710_lwx_rain.png Return to text ⤴

  8. https://twitter.com/NWS_BaltWash/status/1148222752593256448?s=20 Return to text ⤴

  9. https://www.arlnow.com/2019/07/08/flash-flood-warning-issued-for-arlington-3/ Return to text ⤴

  10. https://water.weather.gov/ahps2/index.php?wfo=lwx Return to text ⤴

  11. https://weather.com/safety/floods/news/2019-07-08-washington-maryland-virginia-flooding-leads-to-water-rescues Return to text ⤴

  12. https://www.washingtonpost.com/weather/2019/08/07/how-stalled-storm-over-baltimore-unleashed-flooding-rain-mph-winds/ Return to text ⤴

  13. https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=md Return to text ⤴

  14. https://www.weather.gov/akq/August_7_2019_Severe Return to text ⤴

  15. http://www.nrcc.cornell.edu/services/blog/2019/07/22/20190722_heat_index.png Return to text ⤴

  16. http://www.nrcc.cornell.edu/services/blog/2019/07/22/index.html Return to text ⤴

  17. http://www.nrcc.cornell.edu/services/blog/2019/08/01/index.htmlReturn to text ⤴

  18. http://www.nrcc.cornell.edu/services/blog/2019/08/01/index.html Return to text ⤴

  19. http://www.nrcc.cornell.edu/services/blog/2019/08/01/index.html Return to text ⤴

  20. http://www.nrcc.cornell.edu/services/blog/2019/08/01/index.html Return to text ⤴

  21. http://www.nrcc.cornell.edu/services/blog/2019/08/01/index.html Return to text ⤴

  22. https://www.spc.noaa.gov/wcm/permonth_by_state/; https://www.weather.gov/phi/EventReview20190613 Return to text ⤴

  23. https://www.pennlive.com/news/2019/07/severe-storm-blows-down-trees-damages-homes-closes-roads-in-central-pa.html; https://www.spc.noaa.gov/climo/reports/190722_rpts.html Return to text ⤴

  24. http://www.nrcc.cornell.edu/services/blog/2019/09/03/index.html Return to text ⤴

  25. http://www.nrcc.cornell.edu/services/blog/2019/09/03/index.html Return to text ⤴

  26. https://www.cpc.ncep.noaa.gov/products/predictions/long_range/seasonal.php?lead=1 Return to text ⤴

  27. https://www.cpc.ncep.noaa.gov/products/expert_assessment/season_drought.png Return to text ⤴

  28. https://onlinelibrary.wiley.com/doi/pdf/10.1111/1752-1688.12600 Return to text ⤴

  29. https://www.noaa.gov/media-release/noaa-increases-chance-for-above-normal-hurricane-season Return to text ⤴

  30. https://tropical.colostate.edu/media/sites/111/2019/08/2019-08.pdf Return to text ⤴

  31. More information on RCP 4.5 can be found in: Thomson, A.M., Calvin, K.V., Smith, S.J. et al. Climatic Change (2011) 109: 77. https://doi.org/10.1007/s10584-011-0151-4 Return to text ⤴

  32. More information on RCP 8.5 can be found in: Riahi, K., Rao, S., Krey, V. et al. Climatic Change (2011) 109: 33. https://doi.org/10.1007/s10584-011-0149-y Return to text ⤴

  33. More information on RCP 4.5 can be found in: Thomson, A.M., Calvin, K.V., Smith, S.J. et al. Climatic Change (2011) 109: 77. https://doi.org/10.1007/s10584-011-0151-4 Return to text ⤴

  34. More information on RCP 8.5 can be found in: Riahi, K., Rao, S., Krey, V. et al. Climatic Change (2011) 109: 33. https://doi.org/10.1007/s10584-011-0149-y Return to text ⤴

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