Mid-Atlantic Regional Climate Impacts Summary and Outlook: Fall 2021

Highlights

• Tropical Storm Ida brought significant rainfall to the region with the greatest rainfall totals in the watershed ranging from 5 to 8 inches in parts of central and northern Maryland and central and eastern Pennsylvania.
• Temperatures were on average a bit above normal across the region for the fall season, due to a warmer September and October but a generally cooler November.
• Central and northern parts of the region experienced above-normal precipitation, while southern parts experienced below-normal precipitation.
• Abnormal dryness persisted through November in portions of Virginia and southern Maryland and moderate drought was introduced in southeastern Virginia.
• In an analysis of future differences from normal temperature, the use of the 1981–2010 "old" normal shows over a half-degree greater increase in future average annual temperature compared to the newly-released 1991–2020 normal.

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Map showing the Mid-Atlantic region shaded in blue.

This summary focuses on fall weather and climate events in the Chesapeake Bay watershed and provides highlights from the greater Mid-Atlantic region. The fall season is defined as the months of September, October, and November. The MARISA region covers Maryland, Delaware, Virginia, and Pennsylvania and the portions of New York and West Virginia that fall within the boundaries of the Chesapeake Bay watershed, as shown in Figure 1 above. We refer to this region as the Mid-Atlantic region in the rest of the climate summary.

Part 1: Significant Weather Events and Impacts

Severe Weather

On September 8 and 9, severe thunderstorms moved through the Chesapeake Bay Watershed.¹ Portions of Maryland and Virginia saw as much as three inches of rain and localized flooding.^2,3,4 Additionally, two tornadoes, an Enhanced Fujita Scale 1 (EF-1, winds 86-110 mph) and an EF-0 (winds 65-85 mph), touched down in Snyder County, Pennsylvania. Damage from these tornadoes consisted mainly of downed trees.^6,7

From September 13 to 16, several rounds of thunderstorms produced wind damage and flooding in the watershed.^7,8 Heavy rain from these storms caused flooding in parts of central Maryland and northern and eastern Virginia, resulting in road closures due to high water and multiple stranded vehicles.^9,10 Flooding along major transportation routes in Virginia, such as Interstate 395 in Arlington and Interstate 95 in Richmond, created travel difficulties for commuters.^11,12 In addition, thunderstorm winds downed trees and wires, and straight-line winds of up to 80 miles per hour (mph) were recorded in Columbia and Schuylkill counties in Pennsylvania.¹³

From September 21 to 23, a frontal system that was enhanced by lingering tropical moisture from what was once Hurricane Nicholas, dropped up to seven inches of rain on parts of Maryland and Virginia.^14,15, Flash flooding led to road closures and water rescues in areas of western and northern Virginia and central and southern Maryland.^16,17 Moderate coastal flooding occurred in Annapolis and St. Mary’s County, Maryland.¹⁸ Severe thunderstorms associated with the frontal system downed trees and wires in central Maryland and northern Virginia.^{19, 20}

On September 28, lightning struck a building in Herndon, Virginia, injuring two people.²¹ A lightning strike in the same area was suspected of producing a loud, prolonged boom that was heard across the Washington, D.C. metro area and up to 25 miles away from the strike, which is unusual.²² This lightning bolt was estimated to be 442,000 amps of current, much higher than the average 30,000 amps, which may have contributed to the widespread noise. Additionally, the atmospheric conditions were such that the sound was reflected back toward the ground, which allowed it to travel long distances.²³

From October 25 to 27, the energy from two storm systems, one moving in from the Ohio Valley and another moving up the East Coast, combined off the New England coast and led to the development of a strong nor’easter that rapidly intensified.²⁴ The northern portions of the watershed saw as much as three inches of rain, with flash flooding leading to road closures.^25,26 In the southern portions of the watershed, severe storms produced localized flash flooding and damaging wind gusts that downed trees.²⁷ Non-thunderstorm wind gusts of up to 55 mph also downed trees and wires.^28,29

With water levels already elevated from the departing nor’easter and persistent onshore winds from another storm moving into the watershed on October 29 and 30, water levels along the Chesapeake Bay and its tidal tributaries, such as the Potomac River, reached heights not seen in a decade or more.³⁰ Water levels were record or near-record high at a few gauges. This included the Patuxent River at Solomons Island, Maryland, where the stream gauge reached 4.82 feet, beating the prior record of 4.80 feet set by Hurricane Isabel on September 19, 2003.^31,30,32 The storm dropped as much as four inches of rain on the watershed, with localized flooding leading to road closures in parts of western and northern Virginia, northeastern Pennsylvania, and central New York.^33,34,35 Storm reports noted several road closures and flooding of buildings and houses.^36,37 The entire village of Lewisetta, Virginia was evacuated due to high water levels.³⁸ In addition, strong wind gusts that accompanied the storms, peaking between 40 and 55 mph, downed trees and wires.³⁹

Tropical Storms

Tropical Storm Ida formed in the Caribbean Sea on August 26, rapidly intensifying into a Category 4 hurricane over the Gulf of Mexico’s extremely warm waters and making landfall in Louisiana as a major hurricane on August 29.⁴⁰ The storm weakened as it moved inland, passing through the Chesapeake Bay Watershed on September 1 as a post-tropical cyclone.⁴¹ The storm then interacted with a stationary front, producing heavy rain. The greatest rainfall totals in the watershed ranged from 5 to 8 inches in parts of central and northern Maryland and central and eastern Pennsylvania.^42,43 Daily rainfall totals exceeded the amounts that would be considered the 100-year storm at a few locations.⁴⁴ September 1 ranked among the three all-time wettest days on record for Harrisburg and Scranton, Pennsylvania, which saw 6.64 inches and 5.09 inches of rain, respectively.⁴⁵

The remnants of Tropical Storm Fred and Tropical Storm Henri, which impacted the Mid-Atlantic on August 18-19 and 21-23 respectively, caused some locations to see above-normal rainfall in the month of August. This increased the risk of flooding from Ida.⁴⁶ Flooding occurred in parts of eastern West Virginia, Maryland, and central and eastern Pennsylvania, with several long-term stream gauges reaching water levels that were among their 10 highest on record.⁴⁷ For example, the Conestoga River at Lancaster, Pennsylvania, which has records to the early 1930s, reached 18.49 feet, its third highest crest on record.⁴⁸ Similarly, the Monocacy River at Frederick, Maryland, which also has records dating back to the early 1930s, reached 22.10 feet, its eighth highest crest.⁴⁹ The National Weather Service office in State College, Pennsylvania, issued a rare Flash Flood Emergency for parts of central Pennsylvania several times during the storm.^50,51 Numerous roads and bridges were closed due to flooding, and the flooding of homes, businesses and vehicles resulted in water rescues, including for a stranded school bus in Frederick County, Maryland.^{52,53,54,55,56} In Rockville, Maryland, a thunderstorm fed by moisture from Ida dropped as much as four inches of rain in 45 minutes, inundating an apartment complex and killing one person.^57,58

Ida’s remnants also produced multiple tornadoes, including four in the watershed.⁵⁹ The strongest tornadoes were rated as EF-2 tornadoes, with one traveling 11.5 miles from Owensville to Annapolis, Maryland, and the other traversing more than six miles through southwestern Chester County, Pennsylvania.^60,61 These two tornadoes were particularly damaging, tearing off roofs, blowing out exterior walls, ripping off siding, and downing numerous trees.^62,63 In addition, there were two EF-0 tornadoes in Maryland, one in southeastern Baltimore County and another in Dorchester County.^64,65 Damage from the Annapolis tornado is shown in Figure 2.

Historic flooding and catastrophic impacts from the remnants of Ida were experienced just outside the Chesapeake Bay Watershed.⁶⁶ In total, there were at least 50 deaths due to Ida in the Northeast, and early estimates indicated the storm caused $117 million in damage in Pennsylvania and more than $50 million in damage in New York.⁶⁷

A house in Annapolis, Maryland, is covered with a blue tarp due to damage from a tornado that was produced by the remnants of Tropical Storm Ida on September 1, 2021.

SOURCE: NOAA.gov

Drought

An area of abnormal dryness in western Virginia contracted but persisted during September.⁶⁸ In early October, abnormal dryness (category D0) was introduced in southeastern Virginia and on the Delmarva Peninsula due to short-term precipitation deficits.^69,70

During October, abnormal dryness persisted in part of western Virginia and expanded on the Delmarva Peninsula.⁷¹ Additionally, dryness lingered in southeastern Virginia, introducing a small area of moderate drought (category D1) in the Hampton Roads area.^72,73 Continued below-normal precipitation in November and declining soil-moisture led to the widespread expansion of abnormal dryness in Virginia, extending into parts of southern and southeastern Maryland.⁷⁴ Moderate drought also expanded in southeastern Virginia.⁷⁵

Part 2: Seasonal Temperature and Precipitation

Temperature

Figure 3 shows the September through November 2021 average temperature compared with the climate normal—i.e., the average temperature from 1991 to 2020.⁷⁶ The figure shows that temperatures were above normal across the region, with much of the Mid-Atlantic experiencing temperatures 0–2 degrees Fahrenheit (F) above normal and a few areas experiencing temperatures that were 2–4 degrees Fahrenheit above normal temperatures. These differences from normal temperatures are similar to those seen in the summer 2021 season (June-August), when temperatures averaged 1–2 degrees Fahrenheit above normal in the region.

SOURCE: Northeast Regional Climate Center, 2021 (https://www.nrcc.cornell.edu). Used with permission.

NOTE: Normal temperature is based on the fall season’s average temperature data from 1991–2020. Yellow, orange, and red indicate above-normal temperatures. Blue indicates below-normal temperatures. The boundaries of the Chesapeake Bay watershed are outlined in bold black. Average departure from normal temperature is based on a station’s normal temperature for summer compared with the same station’s fall 2021 average temperature. Station-level departures from normal are spatially interpolated across the region. Both are produced by the Northeast Regional Climate Center. These can be found at https://www.rcc-acis.org/docs_gridded.html.

Figure 4 illustrates the fall season’s variability in daily average temperature departures from normal at Dulles Airport in Virginia. While the average seasonal temperatures for the site (seen in Figure 3) were within 4 degrees of normal, the daily temperatures tell a different story. The site at Dulles Airport showed large departures above normal (5–15 degrees F) in October and departures below normal (greater than 5 degrees F) in November. Additionally, Dulles Airport, experienced its 11th warmest September on record, its warmest October on record, and its 19th coolest November on record. These rankings are included in Table 2 below.

SOURCE: Northeast Regional Climate Center, 2021 (http://www.nrcc.cornell.edu). Used with permission.

Twelve sites in the Mid-Atlantic experienced average seasonal fall temperatures that ranked among their top 20 on record. The locations and ranks are reported in Table 1.

The first fall frost (minimum temperature of 32 degrees Fahrenheit or lower) did not occur until early November for several sites such as Binghamton, New York, Scranton and Williamsport, Pennsylvania; and Dulles Airport, Virginia. This was as much as 3 weeks later than usual and ranked among the 10 latest first fall frosts for these sites.⁷⁷

Station Name	Avg. Temp (degrees F)	Normal Temp (degrees F)	Rank (warmest)
Dulles Airport, VA	59.3	57.1	5
Harrisburg, PA	58.5	56.2	6
Scranton, PA	55.2	53.5	7
Washington National, D.C.	62.5	61.0	7
Baltimore, MD	60.5	57.8	8
Richmond, VA	62.1	60.2	10
Charlottesville, VA	61.1	59.6	10
Lynchburg, VA	60.1	57.1	11
Salisbury, MD	61.0	58.8	11
Williamsport, PA	55.4	53.2	12
Binghamton, NY	50.8	48.9	12
Martinsburg, WV	56.8	55.5	20

SOURCE: Northeast Regional Climate Center, 2021 (http://www.nrcc.cornell.edu). Used with permission.

Monthly Temperature Rankings

In September and October, sites across the Mid-Atlantic experienced average monthly temperatures that ranked among their top 20 on record. Locations and ranks are reported in Table 2. In addition, Washington Dulles International Airport (Dulles Airport), Virginia, and Harrisburg, Williamsport, and Scranton, Pennsylvania, experienced their highest average temperatures on record for the month of October.

In November, no sites experienced average monthly temperatures that ranked amongst their top 20 on record. In fact, only one site, Dulles Airport, Virginia, experienced a ranked temperature month, and it was their 19th coolest on record (Table 2). This was a significant change as the same site experienced its warmest October on record.

September Temperature Rankings
Station Name	Avg. Temp (degrees F)	Normal Temp (degrees F)	Rank (warmest)
Dulles Airport, VA	70.0	68.6	11
Harrisburg, PA	69.8	67.9	14
Scranton, PA	65.7	64.6	18
Salisbury, MD	71.7	69.7	20
October Temperature Rankings
Station Name	Avg. Temp (degrees F)	Normal Temp (degrees F)	Rank (warmest)
Dulles Airport, VA	63.6	56.6	1
Harrisburg, PA	62.4	55.8	1
Scranton, PA	59.4	53.2	1
Williamsport, PA	59.4	53.0	1
Martinsburg, WV	60.7	55.2	2
Washington National, DC	66.3	60.8	2
Baltimore, MD	64.7	57.4	3
Binghamton, NY	54.9	48.8	3
Charlottesville, VA	65.2	59.3	3
Lynchburg, VA	64.1	57.0	4
Richmond, VA	65.9	60.0	4
Salisbury, MD	64.8	58.5	4
Norfolk, VA	66.6	63.7	10
November Temperature Rankings
Station Name	Avg. Temp (degrees F)	Normal Temp (degrees F)	Rank (coolest)
Dulles Airport, VA	44.1	46.0	19

Source: Northeast Regional Climate Center, 2021 (http://www.nrcc.cornell.edu). Used with permission.

Precipitation

Precipitation departures from normal for September 1 through November 30, 2021 are shown in Figure 5. Departures from normal indicate where this fall season’s average daily precipitation was above or below the climate normal—i.e., the average precipitation from 1991 to 2020. Figure 5 shows that central and northern parts of the region experienced significantly above-normal precipitation, while southern parts experienced below-normal precipitation. The wettest portions of the region were in northern Maryland, southeast West Virginia, central Pennsylvania and southern New York, where they received up to 200 percent of normal precipitation. This is likely due in part to increased precipitation as a result of remnants of Hurricane Ida that passed through the region in early September. In contrast, much of Virginia experienced below normal precipitation (25-75 percent). This is a significant change from the summer 2021 season, which had smaller differences from normal precipitation levels across the region.

SOURCE: Northeast Regional Climate Center, 2021 (https://www.nrcc.cornell.edu). Used with permission.

NOTE: Normal seasonal precipitation is based on precipitation data from 1991–2020. Brown shades indicate below normal seasonal precipitation. Green shades indicate above normal seasonal precipitation. The boundaries of the Chesapeake Bay watershed are outlined in bold black. Average departures from normal precipitation are based on a station’s normal precipitation for fall compared with the same station’s fall 2021 average amount of precipitation. Station-level departures from normal are spatially interpolated across the region. Both are produced by the Northeast Regional Climate Center. These can be found at https://www.rcc-acis.org/docs_gridded.html.

Four sites in the Mid-Atlantic region saw fall precipitation amounts that ranked in their top 20 on record. The locations and ranks are provided in Table 3.

Station Name	Precipitation (inches)	Normal Precip (inches)	Rank (wettest)
Scranton, PA	17.12	10.71	4
Harrisburg, PA	15.07	11.61	8
Williamsport, PA	16.05	11.71	9
Binghamton, NY	13.05	10.88	12

SOURCE: Northeast Regional Climate Center, 2021 (http://www.nrcc.cornell.edu). Used with permission.

Monthly Precipitation Rankings

In September and October, sites in the Mid-Atlantic saw monthly precipitation totals that ranked amongst the top 20 on record. Additionally, 5 sites had Novembers that ranked in their top 20 driest on record. Locations and ranks for these records are reported in Table 4.

September Temperature Rankings (wettest)
Station Name	Precipitation (inches)	Normal Precip (inches)	Rank (wettest)
Scranton, PA	10.49	4.15	2
Harrisburg, PA	11.40	4.83	3
Williamsport, PA	8.51	4.76	7
Martinsburg, WV	6.73	4.03	11
Dulles Airport, VA	4.72	3.94	16
Binghamton, NY	4.42	4.01	17
October Precipitation Rankings (wettest)
Station Name	Precipitation (inches)	Normal Precip (inches)	Rank (wettest)
Binghamton, NY	6.27	3.76	9
Dulles Airport, VA	4.28	3.65	17
Richmond, VA	5.61	3.39	19
November Precipitation Rankings (driest)
Station Name	Precipitation (inches)	Normal Precip (inches)	Rank (driest)
Dulles Airport, VA	0.91	3.13	6
Charlottesville, VA	0.82	2.73	13
Richmond, VA	0.70	3.06	13
Lynchburg, VA	0.95	3.39	15
Salisbury, MD	0.94	3.16	18

Source: Northeast Regional Climate Center, 2021 (http://www.nrcc.cornell.edu). Used with permission.

Part 3: Winter 2021-2022 Outlook

Temperature and Precipitation

As of November 18, 2021 the NOAA Climate Prediction Center forecasts a 40-50-percent chance of above normal temperatures for the Mid-Atlantic region for December, January and February 2021–2022. This indicates that the forecast is leaning towards having a warmer than normal winter season.⁷⁸ The precipitation forecast shows an equal chance of wetter than, drier than, or near-normal conditions for the eastern half of the region for the same period. Western Pennsylvania, southern New York and a small portion of western Maryland have a 33-40 percent chance of a wetter than normal winter season.⁷⁹

Drought Incidence

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 November 18, 2021 the Outlook indicates that drought is expected to persist in small portions of south-central and southeastern Virginia.⁸⁰

Climate Circulation Patterns

NOAA’s Climate Prediction Center, which monitors the likelihood of occurrence of El Niño and La Niña climate phenomena, has a La Niña advisory active as of November 11, 2021 with a 90-percent chance to continue through winter 2021–2022.⁸¹ La Niña episodes have generally resulted in average to slightly above average temperatures and below normal rainfall and precipitation during December, January, and February in the Mid-Atlantic region.⁸² However, other regional climate dynamics and natural climate variability also influence winter weather in the Mid-Atlantic.

Part 4: Future Differences from “Normal” Temperature

Climate normals are a standardized measure of typical climate conditions, such as average annual temperature and precipitation. The National Oceanic and Atmospheric Administration’s (NOAA) National Centers for Environmental Information (NCEI) calculates these “normals” for the continental United States based on observed conditions from weather stations over a 30-year period.⁸³ Climate normals were first calculated for the 1901-1930 time period and are updated each decade.⁸⁴ The latest climate normals were calculated over the period of 1991-2020 and were released to the public in May 2021.

By standardizing the time period and datasets used to produce climate normals, climate normals provide a consistent and comparable frame of reference to evaluate how different current weather conditions or future weather and climate forecasts may be from the past.⁸⁵ However, because these normals are updated every ten years, as climate conditions change, the definition of what it means to be “normal” is also changing in time. While these changes are shifting our baseline for scientific analysis, this regular updating of what is considered to be normal is something that humans do as well. A recent study showed that people adjust what they think is normal in terms of weekly temperatures on approximately a 5-year timescale.⁸⁶ This finding is significant as it indicates that changing temperatures over time due to climate change may be normalized in public thought and experience.

Figure 6 shows how each climate normal since 1901–1930 compares with the average temperature from the 20th century (1901-2000). Due to the effects of climate change, U.S. “normals” have been getting steadily warmer over time with a majority of the warming occurring since 1981, as shown in Figure 6.⁸⁷ In fact, since the 1901-1930 time period, the continguous United States has experienced 1.7 degrees-Fahrenheit of warming on average.⁸⁸ Therefore, as we base our definitions of “normal” on warmer and warmer temperatures, the changes that are projected to occur due to climate change may appear more “normal” while still representing a large departure from temperatures experienced just a few decades ago.

Annual U.S. temperature compared to the 20th-century average for each U.S. Climate Normals period from 1091-1930 (upper left) to 1991-2020 (lower right).

Source: NOAA NCEI; https://www.noaa.gov/news/new-us-climate-normals-are-here-what-do-they-tell-us-about-climate-change)

The interactive data tool in Figure 7 explores how the changing definition of a climate normal affects our interpretation of future climate changes. Figure 7 shows how future projected average annual temperatures from multi-decadal periods compare to normal. This figure includes the last two climate normals (1981–2010 and 1991–2020) and illustrates the percent change from the “normal” time period to the future time period. The absolute difference in temperatures can be seen for individual locations in the tooltip graphics.