Mid-Atlantic Regional Climate Impacts Summary and Outlook: Summer 2025

Highlights

• Average temperatures for the 2025 summer season were a bit above normal (0-4 degrees) across the region. This was due to warmer than normal temperatures in late June and July followed by a cooler than normal August. For example, Dulles Airport, Virginia experienced its fourth warmest June on record, followed by its fifth warmest July, and its seventh coolest August on record.
• Precipitation varied across the watershed,some locations saw precipitation significantly above normal in June and July then dropping to drier, below normal conditions in August.
• The increased precipitation helped to nearly remove all abnormal dryness from the region by the end of July, only to have it return due to the dry August conditions.
• An overview of flood risk—covering how it is changing and what watches and warnings mean—is included in response to the numerous flood and flash flood events in the region this summer.

View all climate summaries

This summary focuses on summer weather and climate events in the Chesapeake Bay watershed and provides highlights from the greater Mid-Atlantic region. The summer season is defined as the months of June, July, and August. The MARISA region covers Maryland, Delaware, Virginia, 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 below. We refer to this region as the Mid-Atlantic region in the rest of the climate summary.

This map shows the “MARISA region”. The lightly shaded area shows the extent of the Chesapeake Bay Watershed.

Part 1: Significant Weather Events and Impacts

Increased Moisture and Rainfall

Summertime rain and thunderstorms can produce highly variable precipitation patterns, with some areas seeing little to no rainfall, but nearby areas receiving extreme amounts.¹ Rain storms this summer have had an unusually large amount of atmospheric moisture to tap into, which has caused them to produce exceptional rainfall amounts that resulted in flash flooding.² There are a few ways to measure this moisture in the atmosphere, for this climate summary, we will look at two: dew point and precipitable water.

Dew point is defined by the National Weather Service as "the temperature to which air must be cooled in order to reach saturation."³ The higher the dew point, the more moisture in the air and the more uncomfortably humid, or "sticky" the air feels.⁴ A threshold for when the dewpoint makes the air feel oppressively humid is around 70 degrees.⁵ Figure 2 shows that the region has seen more days than normal with dewpoints of 70 degrees or higher in June and July. This means that the region has seen many "sticky" feeling days and there has been a lot of moisture in the air.

SOURCE: Northeast Regional Climate Center

NOTE: The long-term average of these sites is based on the 1991-2020 time period when available, but for Harrisburg, PA, and Charlottesville, VA, the long-term average is based on the 2005-2020 time period.

In all weather station locations included in the chart, the number of days in June and July with dewpoints greater than 70 degrees fahrenheit exceeded the long term average. The greatest number of days with dewpoints greater than 70 degrees was in Norfolk, VA, with more than 55 days in June and July.

Picture wringing out all of the water in the atmosphere above a location, from the ground up to about 30,000 feet; the amount of liquid water that would result is called precipitable water.⁶ This is not equal to the amount of water that will fall as precipitation, it is a measure of the amount of water that is available in the event precipitation does develop.⁷

Precipitable water values have been unusually high multiple times this summer.⁸ For instance, on the morning of July 19, the Baltimore/Washington, D.C. National Weather Service office reported 2.08 inches of precipitable water, which was very close to a record amount for that date.⁹ Later that day, extreme rainfall amounts caused flash flooding in some Washington, D.C., suburbs (discussed below).¹⁰

Severe Weather and Flooding

During the month of June, severe thunderstorms produced a few weak tornadoes in the watershed, including an Enhanced Fujita scale 1 (EF-1) in Bedford County, Pennsylvania on June 17, and an EF-0 in Anne Arundel County, Maryland on June 18.^11,12 There were also numerous instances of damaging non-tornadic thunderstorm winds, such as a widespread event on June 19 that generated dozens of wind-related storm reports.¹³ EF-0 tornadoes have wind speeds of 65-85 mph and EF-1 tornadoes have wind speeds of 86-110 mph.¹⁴

Flash flooding also occurred in parts of the region during June, with two particularly noteworthy events.^15,16 On June 22, a Flash Flood Emergency was issued by the National Weather Service for Chenango County, New York, where up to 6.50 inches of rain flooded roads, bridges, and homes.^17,18 On June 30, the National Weather Service declared a Flash Flood Emergency for northern Lancaster County in Pennsylvania, where there were numerous water rescues and road closures after 5 inches of rain fell.^19,20

There was more severe weather in July. For example, on July 3, a supercell with widespread winds of up to 70 mph, and locally higher gusts of up to 100 mph, caused significant tree damage from central New York into northeastern Pennsylvania.²¹ In additional, hail as large as 2 inches in diameter in Broome County, New York tore leaves off trees, damaged homes and vehicles, and accumulated on roads.^22,23

There were multiple flash flood events across the watershed during the month of July. We discuss a few of them below. We provide more information about flooding and flash flooding in Part 4.

On July 13, up to 5 inches of rain fell in a few hours in central New York, trapping people in their homes and inundating roads.^24,25 On the same day, around 3 inches of rain fell in less than an hour in central Virginia, resulting in numerous road closures and water rescues.^26,27

On July 14, parts of Pennsylvania, including Lancaster, Lycoming, and Luzerne counties saw between 3 and 8 inches of rain, causing flash flooding.^28,29 There were dozens of evacuations, numerous road closures, and multiple water rescues.^30,31 Heavy rain, with localized amounts of up to 9 inches, hit central Virginia again on July 14 into July 15.³² A Flash Flood Emergency was issued by the National Weather Service for the cities of Colonial Heights and Petersburg, where water levels on Lieutenant Run rose 10 feet in around two hours.³³ Floodwaters swamped buildings and roadways, leading to water rescues.³⁴

On July 19, up to 4.50 inches of rain fell, most of it in about two hours, leading to flash flooding in parts of the Washington, D.C., metro area.^35,36 Roads were submerged and people were trapped in buildings and vehicles, which required evacuations and water rescues.^37,38 A Flash Flood Emergency was issued by the National Weather Service for part of Montgomery County, Maryland, where Sligo Creek rose roughly 10 feet in 30 minutes.^39,40

On July 31, portions of Virginia, Maryland, and central Pennsylvania experienced heavy rainfall and flash flooding.⁴¹ Some waterways rose rapidly, such as Honeygo Run near White March, Maryland, which rose 7.29 feet in an hour.⁴² There were numerous flooded roads, stranded vehicles, and water rescues across the impacted areas.^43,44 One flood-related fatality occurred in Maryland.⁴⁵

The heavy rainfall during the month of July impacted farming operators and crop quality in Pennsylvania.⁴⁶ Rainfall runoff also contributed to elevated bacteria levels in some bodies of water, forcing their closure and/or reducing recreation opportunities.^47,48 For instance, officials in Anne Arundel County, Maryland issued an unusually high number of health advisories for bacteria-contaminated bodies of water in July, including the first-even (since 2000) for Mayo Beach Park.⁴⁹ Harmful algal blooms also affected recreation activities at some beach locations.^50,51

Severe weather was extremely limited in the watershed during the month of August.⁵²There were only a few, localized flash flood events during the month of August. For instance, vehicles got stuck on flooded roads in northeastern Pennsylvania on August 13, and again in Accomack County, Virginia on August 19.^53,54

SOURCE: Petersburgh Fire Rescue and Emergency Services

Drought

The U.S Drought Monitor from June 3 showed a pocket of moderate drought in central Maryland, and abnormal dryness in several spots including in eastern West Virginia, western and northern Virginia, parts of Maryland, and south-central Pennsylvania.⁵⁵ Enough rain fell during June to allow drought to contract in Maryland and abnormal dryness to ease in much of the watershed.⁵⁶ In fact, by mid-month, Virginia and West Virginia became free of both drought and abnormal dryness for the first time in over a year, since April 2024 and May 2024 respectively.⁵⁷ Streamflow and groundwater levels improved in most locations during the month.^58,59

Conditions continued to improve in the watershed during July. Maryland became free of drought for the first time since June 2024, while Pennsylvania became free of both drought and abnormal dryness for the first time since May 2024.^60,61 By month's end, the only dryness in the region was a small spot of abnormal dryness in central Maryland.⁶²

During the month of August, factors such as drier-than-normal weather, reduced streamflow, below-normal groundwater levels, and declining soil moisture led to the introduction and expansion of abnormal dryness in several parts of the watershed, which can be seen in Figure 4.^63,64 By late August, abnormal dryness was present in central New York, northern/central Pennsylvania, eastern West Virginia, parts of Maryland, and northern/eastern Virginia.⁶⁵

SOURCE: U.S. Drought Monitor

The U.S. Drought Monitor for the Mid-Atlantic region showed abnormally dry conditions in small areas of the region in June expanding to a greater proportion of the region by August. Isolated areas of moderate drought are recorded in June and August. No areas of severe, extreme, or exceptional drought are indicated.

Wildfire

The jet stream can steer plumes of wildfire smoke from Canada toward the East Coast, which can create hazy skies and periodically affect air quality.⁶⁶ There were several instances this summer where this was the case.

At times, during the first half of June, smoke from wildfires burning in Canada moved across the watershed, producing hazy skies and, in some locations, reducing air quality.^67,68,69 Smoke from wildfires in New Jersey also affected sky cover and air quality in parts of Maryland and Delaware mid-June.⁷⁰

In mid-July, Canadian wildfire smoke once again led to hazy skies and reduced air quality in central New York.⁷¹

Reduced air quality was an issue for several days in early August in Pennsylvania and New York due to smoke from Canadian wildfires, with a few places seeing pollution levels that were unhealthy for all.^72,73,74

Tropical Systems

On July 7, the remnants of Tropical Storm Chantal moved through southeastern parts of the watershed, bringing rain and rough surf but limited impacts.^75,76

In early August, Tropical Storm Dexter moved across the Atlantic Ocean, producing elevated water levels, rough surf, and rip currents in coastal parts of the Chesapeake Bay watershed, despite being located hundreds of miles offshore.⁷⁷

From August 19 to 23, Hurricane Erin became the first major hurricane of the season, rapidly intensifying at a near-record rate from a category 1 to a category 5 in a little over 24 hours.^78,79 While the large storm stayed offshore, it produced impacts to the Mid-Atlantic coastline including rough surf, rip currents, gusty winds, and coastal flooding.^80,81,82

SOURCE: Mycoast.org

Part 2: Seasonal Temperature and Precipitation

Temperature

Figure 5 shows the summer 2025 average temperature compared with the climate normal—i.e., the average seasonal temperature from 1991 to 2020.⁸³ The figure shows that nearly the entire region experienced above normal temperatures this summer. The eastern portions of the watershed experienced temperatures that were 0–2 degrees above normal, while the western portions of the region experienced temperatures that were 2–4 degrees above normal. This is a continuation of the above normal temperatures seen in the spring 2025 season.

SOURCE: Northeast Regional Climate Center, 2025 (https://www.nrcc.cornell.edu). Used with permission.

NOTE: Normal temperature is based on the summer season's average temperature data from 1991–2020. Shades of red indicate above-normal temperatures. Shades of blue indicate below-normal temperatures. The boundaries of the Chesapeake Bay watershed are outlined in bold black. Average departure from normal temperature is based on comparing the Parameter Elevation Relationships on Independent Slopes Model (PRISM) gridded dataset from the summer season to the PRISM gridded historical dataset from 1991-2020. Additional details about these data are available at https://www.rcc-acis.org/docs_gridded.html.

While the seasonal average temperatures appeared generally a little above normal, Figure 6 illustrates the variability of daily temperatures within the summer season at six sites within the watershed. All six sites saw much higher-than-normal temperatures in late June and above normal temperatures for much of the month of July. For the month of August, there was a change in the weather patterns, and all sites saw a large number of days with below normal temperatures. This variability is also shown in the temperature rankings, which are discussed in detail below.

Source: Northeast Regional Climate Center, 2025

This figure displays average temperature departures from a baseline for six cities in the Mid-Atlantic region: Binghamton, NY; Scranton, PA; Harrisburg, PA; Martinsburg, WV; Washington, DC; and Norfolk, VA.

The vertical axis ranges from -15.0 to 15.0 degrees, with increments of 10.0, 0.0, and -10.0 degrees indicated for each city. The horizontal axis shows dates in 2025, beginning May 27 and continuing through Jun 6, Jun 16, Jun 26, Jul 6, Jul 16, Jul 26, Aug 5, Aug 15, Aug 25, and Sep 4.

For each city, the chart tracks how much the actual average temperature deviates from the expected average (referred to as "Avg Temp Departure") across the listed dates. Individual lines or markers are present for each city, with temperature departures rising as high as 15.0 and dipping as low as -15.0 degrees.

This visualization enables comparison of temperature anomalies over time for Binghamton, Scranton, Harrisburg, Martinsburg, Washington, and Norfolk, highlighting periods of notable warmth or coolness relative to typical conditions.

As shown in Table 1, three sites in the Mid-Atlantic experienced average summer temperatures that ranked among its top 20 warmest on record.

Station Name	Avg. Temp (degrees F)	Normal Temp (degrees F)	Rank (warmest)
Binghamton, NY	68.5	66.9	17
Harrisburg, PA	75.4	75.0	18
Dulles Airport, VA	75.8	75.2	19

SOURCE: Northeast Regional Climate Center, 2025 (https://www.nrcc.cornell.edu). Used with permission.

Monthly Temperature Rankings

June 2025 ranked among the top 20 warmest Junes on record for 12 sites within the watershed.

The watershed experienced a heat event from June 21 to 26, with the highest daytime temperatures ranging from 90 to over 100 degrees Fahrenheit (F).⁸⁴ Salisbury, Maryland reached 100 degrees F on June 24 and 25, the sites' first time with back-to-back 100-degree days in June and tying as the site's third hottest temperature for the month.⁸⁵ There was little relief from the heat at night as low temperatures remained in the 70s and low 80s in some areas.⁸⁶ Norfolk and Richmond, Virginia, tied their records for hottest low temperature (minimum temperature) in June with 80 degrees F and 78 degrees F respectively.⁸⁷ Several locations saw low temperatures that ranked among the 10 hottest for June, in some cases on multiple days.⁸⁸ For instance, at Norfolk, Virginia, three of the 10 hottest low temperatures for June occurred during this event: 79 degrees F on June 24 tied as fourth hottest, while 80 degrees F on June 25 ranked as the hottest June 25 on record, and 79 degrees F on June 27 tied as fourth hottest.⁸⁹ Additionally, this event contributed to Dulles Airport, Virginia seeing 10 days in June with a low of at least 70 degrees F, a new record for the month.⁹⁰ The heat caused some roads to buckle and commuter trains were delayed due to "temperature-related speed restrictions".^91,92 In Maryland, there were hundreds of visits to emergency departments and urgent care clinics for heat-related illnesses, as well as seven heat-related deaths, between June 22 and 28.⁹³

July 2025 ranked among the 20 hottest Julys for most sites within the watershed.

The temperature rankings in Table 2 confirm the widespread above normal temperatures that were experienced throughout the region during June and July.

August 2025 ranked among the 20 coolest Augusts for eight sites, with Salisbury, Maryland seeing its third coolest August on record.

The full set of monthly rankings, locations, and temperatures is shown in Table 2.

June Temperature Records (warmest)
Station Name	Avg. Temp (degrees F)	Normal Temp (degrees F)	Rank (warmest)
Dulles Airport, VA	75.0	72.5	4
Richmond, VA	78.1	75.0	5
Norfolk, VA	78.8	76.7	6
Binghamton, NY	67.0	64.4	8
Baltimore, MD	76.1	73.5	9
Salisbury, MD	75.6	72.7	9
Martinsburg, WV	73.9	71.1	10
Harrisburg, PA	73.8	72.5	13
Scranton, PA	69.8	69.0	17
Lynchburg, VA	75.3	72.0	18
Washington National, DC	76.9	76.3	18
Charlottesville, VA	75.3	74.8	19

July Temperature Rankings (warmest)
Station Name	Avg. Temp (degrees F)	Normal Temp (degrees F)	Rank (warmest)
Dulles Airport, VA	80.1	77.2	5
Martinsburg, WV	79.2	75.7	5
Harrisburg, PA	79.8	77.3	8
Williamsport, PA	76.8	73.7	9
Washington National, DC	82.4	81.0	10
Binghamton, NY	72.0	68.9	11
Lynchburg, VA	79.3	76.0	11
Salisbury, MD	79.8	77.9	12
Richmond, VA	81.2	79.4	13
Baltimore, MD	80.6	78.3	14
Norfolk, VA	82.1	81.1	15
Scranton, PA	75.2	73.7	16
Charlottesville, VA	79.4	79.0	17

August Temperature Rankings (coolest)
Station Name	Avg. Temp (degrees F)	Normal Temp (degrees F)	Rank (warmest)
Salisbury, MD	71.5	75.8	3
Lynchburg, VA	71.7	74.5	5
Baltimore, MD	72.7	76.2	6
Dulles Airport, VA	72.2	75.7	7
Richmond, VA	73.8	77.5	7
Charlottesville, VA	72.7	76.9	11
Martinsburg, WV	71.0	73.8	14
Norfolk, VA	75.7	79.2	18

SOURCE: Northeast Regional Climate Center, 2025 (https://www.nrcc.cornell.edu). Used with permission.

NOTE: These rankings are for a single site over time.

Precipitation

Figure 7 shows how the total precipitation for June 1 through August 31, 2025, differed from normal, with normal being defined as the average summer precipitation from 1991–2020. This figure shows that the region experienced a variety of precipitation conditions. Generally, more of the southern portions of the region saw above normal precipitation, with those areas experiencing 100-150% of normal precipitation. Much of the northern and easternmost portions of the region experienced below normal precipitation.

SOURCE: Northeast Regional Climate Center, 2025 (http://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. Blue 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 the Northeast Regional Climate Center's interpolated gridded precipitation data for the summer season compared with the National Centers for Environmental Information's gridded normals for the summer season, which is based on historical data from 1991-2020. Additional information about the Northeast Regional Climate Center's gridded datasets can be found at https://www.rcc-acis.org/docs_gridded.html.

Precipitation amounts, similarly to temperature, varied over the course of the summer season. This is illustrated in Figure 8 below, which shows the cumulative amount of precipitation seen in Washington, DC. The season started slowly, nearly tracking the driest season on record, then in mid-June quickly ramped up to well above normal and even briefly above the site's wettest summer on record. Then for the rest of June and into July, precipitation amounts remained well above average. Finally, the site saw barely any rainfall throughout the month of August, driving the total precipitation amount for the summer below normal levels. These trends are also illustrated in the precipitation rankings at other sites across the region (see Tables below).

SOURCE: Northeast Regional Climate Center, 2025 (http://www.nrcc.cornell.edu). Used with permission.

The cumulative rainfall figure for Washington, DC shows precipitation accumulation in inches from May 27 to September 4, 2025. The graph compares the 2025 accumulation to three benchmarks: the historical highest value from 1906, the lowest from 1966, and the normal average. The vertical axis ranges from 0 to 28 inches of precipitation. In 2025, the accumulation exceeded the previous highest recorded precipitation in June, before falling below normal levels in late July and August.

Summer 2025 did not rank among the top 20 driest or wettest summer seasons on record for any sites in the watershed (Table 3).

Station Name	Precipitation (inches)	Normal Precipitation (inches)	Rank
This summer did not rank in the wettest or driest summers for any sites within the watershed.

SOURCE: Northeast Regional Climate Center, 2025 (https://www.nrcc.cornell.edu). Used with permission.

Monthly Precipitation Rankings

June 2025 ranked as one of the top 20 wettest Junes on record for three sites within the watershed including Williamsport, Pennsylvania. Wet conditions in Pennsylvania led to reduced grain quality, slowed harvest, and allowed fungal diseases to take hold in some crops.^94,95

This July ranked as one of the top 20 wettest Julys on record for three sites in the watershed. Binghamton, New York saw 2.53 inches of rain on July 13, making it the site's third-wettest July day on record.⁹⁶ Looking at impacts in the watershed, in Pennsylvania, heavy rainfall continued to impact farming operations and crop quality during July.⁹⁷ Rainfall runoff also contributed to elevated bacteria levels in some bodies of water, forcing their closure and/or reducing recreation opportunities.^98,99 For instance, Anne Arundel County, Maryland officials issued an unusually high number of health advisories for bacteria-contaminated bodies of water this July, including the first-ever (since 2000) for Mayo Beach Park.¹⁰⁰ Harmful algal blooms also affected recreational activities at some locations.^101,102

Washington, D.C., has its driest August since records began in 1871, seeing only 0.20 inches of rainfall.¹⁰³

The full set of monthly rankings, locations, and amounts of precipitation is shown in Table 4.

June Precipitation Rankings (wettest)
Station Name	Precipitation (inches)	Normal Precipitation (inches)	Rank (wettest)
Dulles Airport, VA	6.93	4.30	7
Martinsburg, WV	6.29	3.85	14
Williamsport, PA	5.61	3.85	14

July Precipitation Rankings (wettest)
Station Name	Precipitation (inches)	Normal Precipitation (inches)	Rank (wettest)
Lynchburg, VA	8.60	4.19	6
Richmond, VA	10.66	4.37	8
Binghamton, NY	4.97	3.80	19

August Precipitation Rankings (driest)
Station Name	Precipitation (inches)	Normal Precipitation (inches)	Rank (driest)
Washington, DC	0.20	3.25	1
Dulles Airport, VA	0.85	3.53	4
Martinsburg, WV	1.04	3.02	8
Salisbury, MD	1.71	5.27	9
Charlottesville, VA	1.02	3.87	10
Lynchburg, VA	1.17	3.22	10
Norfolk, VA	1.60	5.88	11
Williamsport, PA	1.40	4.17	11
Scranton, PA	1.67	3.85	16
Harrisburg, PA	1.50	3.77	17
Binghamton, NY	2.46	4.10	20

SOURCE: Northeast Regional Climate Center, 2025 (https://www.nrcc.cornell.edu). Used with permission.

NOTE: These rankings are for a single site over time.

Part 3: Fall 2025 Outlook

Temperature and Precipitation

As of September 18, the NOAA Climate Prediction Center forecasts a 40 to 50-percent chance of above normal temperatures for the entire Mid-Atlantic.¹⁰⁴ The precipitation forecast shows that the majority of the region has an equal chance of having below-normal, near-normal, and above-normal precipitation for September, October, and November, with the exception of southwestern Virginia, which is leaning below normal precipitation.¹⁰⁵

Drought Incidence

The U.S. Seasonal Drought Outlook forecasts how drought might change across the United States and categorizes areas by whether drought could develop or become more or less intense. As of September 18, the Outlook shows that drought persistence is likely in northern and central Virginia, most of West Virginia, western edge of Pennsylvania for the September 18 through December 31, 2025, time period.¹⁰⁶

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 watch active as of September 11, 2025.¹⁰⁷ ENSO-neutral conditions started in March and continued through August.¹⁰⁸ The likelihood of La Niña conditions are expected to increase into the fall and early winter seasons.¹⁰⁹

ENSO conditions are one of the factors taken into account in NOAA's long-term forecasts and seasonal outlooks such as the one included in this climate summary.¹¹⁰ However, other regional climate dynamics and natural climate variability also influence weather in the Mid-Atlantic. Additional information on La Niña and El Niño is available from the Pacific Marine Environmental Laboratory (La Niña, El Niño).

Atlantic Hurricane Outlook

As of September 3, 2025, researchers at Colorado State University (CSU) updated their forecast for the Atlantic hurricane season.¹¹¹ They continue to forecast an above average season but have less confidence in their outlook than normal.¹¹² They revised their estimates from their spring forecast to 16 named storms (down from 17) and eight hurricanes (down from 9) with three of those becoming major hurricanes (down from 4).¹¹³ They also forecast a 24% chance of at least one major hurricane making landfall on the U.S. East Coast (down from 26%).¹¹⁴ This is slightly above the average of 21% from the 1880-2020 timeframe.¹¹⁵

On August 7, 2025, NOAA's Climate Prediction Center (CPC) also updated their forecast for the Atlantic hurricane season to a 50% chance of an above-normal season.¹¹⁶ NOAA slightly revised the number of forecasted storms to 13-18 named storms (down from 13-19), out of which 5-9 could become hurricanes (down from 6-10), and 2-5 (down from 3-5) of those could become major hurricanes, which means that they would be category 3, 4, or 5 and have winds of at least 111 mph.¹¹⁷ These numbers are inclusive of any storms that have already occurred this season. A normal Atlantic hurricane season is defined as having 14 named storms, seven hurricanes, and three major hurricanes.¹¹⁸

Part 4: A Primer on Flood Risk

Overview

Flooding is a significant and recurring hazard in the Mid-Atlantic region, affecting communities, infrastructure, and public safety. While flooding can occur at any time of year, this primer focuses on summer flooding, which is often driven by heavy rainfall, thunderstorms, and tropical systems. Understanding the causes, risks, and warning systems associated with flooding is essential for public agencies responsible for stormwater management, emergency management, public works, and infrastructure resilience.

What is Flooding?

Flooding occurs when water overflows onto land that is typically dry. There are several types of flooding:¹¹⁹

• River Flooding or Fluvial Flooding: When rivers or creeks rise above their banks due to excessive rainfall or upstream runoff.¹²⁰
• Urban/Street Flooding: When stormwater overwhelms drainage systems, causing water to flow over streets and into properties.
• Coastal Flooding: When ocean water inundates coastal areas, often due to storm surge or high tides.¹²¹
• Flash Flooding: A rapid and extreme flow of high water into a normally dry area, or a sudden rise in a stream or creek, typically within six hours of a heavy rain event. Flash floods are especially dangerous due to their speed and unpredictability.¹²²

Floods can cause loss of life, damage or destroy homes, businesses, and infrastructure, and disrupt essential services. They can devastate local economies and pose public health risks by contaminating water supplies, spreading disease, and increasing mold and vector-borne illnesses.¹²³ Flooding is also one of the deadliest weather-related hazards in the U.S.¹²⁴ While we mention coastal flooding as a form of flooding above, it is not the focus of our discussion below.

What Causes Flooding?

Flooding is influenced by a combination of natural and human factors:

• Heavy Rainfall: Intense or prolonged precipitation can overwhelm natural and engineered drainage systems. The frequency of extreme precipitation events has increased in the Mid-Atlantic, raising flood risk.
• Antecedent Conditions: The state of the environment before a rain event matters. Saturated soils or already high river levels mean less capacity to absorb new rainfall, increasing flood potential.
• Topography: Areas surrounded by hills or mountains receive runoff from higher elevations, making them more prone to flooding. Downstream locations or river confluences are also at higher risk.
• Human Engineering: Urbanization increases impervious surfaces (roads, parking lots), leading to more runoff. Dams, levees, and managed lakes can mitigate or exacerbate flooding, depending on their condition and management. Failures of these structures can cause catastrophic floods. Coastal barriers may reduce, but not eliminate, coastal flood risk.

Conceptualizing Flood Risk

Flood risk is typically understood as a function of three factors:¹²⁵

1. Hazard: The likelihood and intensity of a flood event (e.g., how much rain falls, how quickly).
2. Exposure: The presence of people, property, and infrastructure in flood-prone areas.
3. Vulnerability: The degree to which a system, community or asset is likely to experience negative consequences (e.g., damage, injury, etc.) due to exposure to a hazard.

The concept of flood risk allows us to understand both the likelihood and consequences of a flood for a particular location, community or infrastructure system. It is also helpful for prioritizing and selecting risk reduction measures aimed at lowering exposure (e.g., zoning, buyouts), reducing vulnerability (e.g., floodproofing, emergency planning), or reducing the hazard (e.g., stormwater management, upstream retention).^126,127

How is Flood Risk Changing?

Academic research has established a clear link between climate change and changes in extreme precipitation patterns, which in turn affect flood risk in the Mid-Atlantic and across the United States.^128,129 Warmer atmospheric temperatures increase the capacity of air to hold moisture, leading to more intense and frequent heavy rainfall events.¹³⁰ Studies show that the Northeast, including the Mid-Atlantic, has experienced a significant increase in the frequency and intensity of extreme precipitation over recent decades.¹³¹

Changes in extreme precipitation can heighten flood risk by increasing the likelihood and severity of flood events in areas exposed to flood hazards. However, flood risk is influenced not only by climatic factors but also by land surface dynamics—such as soil saturation, land use, topography, and the extent of impervious surfaces. Consequently, flood risk reflects both changing climate and the physiographic characteristics of the landscape, highlighting the need for integrated flood risk management approaches—including forecasting and warning systems—that account for both meteorological and land surface factors.

Flood Forecasting and Warning Systems

The National Weather Service (NWS) and the Middle Atlantic River Forecast Center (MARFC) monitor conditions and issue alerts to help agencies and the public prepare for and respond to flooding.¹³² Understanding these alerts is critical for timely action. The following definitions are from the NWS:

• Flood Watch: Be Prepared. Issued when conditions are favorable for flooding. Flooding is possible, but not certain or imminent.
  • Flash Flood Watch: Issued when conditions are favorable for flash flooding due to non-convective causes (e.g., dam or levee failure, ice jam), and/or flash flooding and debris flows caused by excessive rainfall on burn scars or in debris flow- and landslide-prone areas.¹³³
• Flood Advisory: Be Aware. Issued when flooding is expected to be less severe but may still cause inconvenience or minor impacts. Caution is advised.
• Flood Warning: Take Action! Issued when flooding is imminent or already occurring. Indicates a serious threat to life or property, often with river stage forecasts.
• Flash Flood Warning: Take Action! Issued when a flash flood is imminent or occurring. Immediate action is necessary to protect life and property.

For more details, see: NWS Flood Safety

SOURCE: National Weather Service, https://www.weather.gov/mob/severe_flood

• Flood Watch
• A Flood Watch is issued when flooding is possible. Stay tuned to trusted news sources and be ready to seek higher ground.
• Be Prepared

• Flood Warning
• A Flood Warning is issued when flooding is happening or about to happen. Move to higher ground immediately! Never drive or walk through floodwaters.
• Take Action!

In 2019, the NWS added "threat tags" to their flash flood warnings. Flash flood warnings can have the following "threat tags":

• "Base – used most of the time, when flash flood impact is possible
• Considerable – Used rarely, when indications of flash flooding capable of unusual severity or impact are imminent or ongoing and urgent action is needed to protect lives and property.
• Catastrophic – Used exceedingly rarely, when a flash flood threat to life and catastrophic damage is occurring or is imminent, and floodwaters have risen or will rise to levels rarely if ever seen."¹³⁴

Flash flood warnings must have either a "considerable" or "catastrophic" tag in order to trigger wireless emergency alerts. These tags are an effort to create more "impact-based warnings" and reduce the number of overall warnings that people receive on their phones.¹³⁵ Flash Flood Emergency alerts are triggered with a "catastrophic" tag and Flash Flood Warning alerts are issued when they have a "considerable" tag.¹³⁶

SOURCE: National Weather Service, https://www.weather.gov/mob/severe_flood

Flash Flood WARNING: This alert is issued when dangerous flash flooding is happening or will happen soon. Some roads will be flooded, and homes and businesses may flood. The guidance is to move to higher ground and never drive through flooded roads.

Flash Flood EMERGENCY: This alert is issued for exceedingly rare situations when there is a severe threat to human life and catastrophic damage from a flash flood is happening or will happen soon. This is described as a life-threatening situation. Individuals are strongly advised not to attempt travel unless fleeing an area subject to flooding or under an evacuation order.

Resources for Preparedness

• Middle Atlantic River Forecast Center (MARFC)
• NWS Flood Safety
• NWS Flood Infographics
• NRCC's Resources for Hazard Mitigation Planning
• Climate and Hazard Mitigation Planning (CHaMP) Tool