Understanding What a 1,000-year Flood Means: The Ellicott City Tale (credit: NOAA NWS Baltimore, MD)
DISCUSSION: On Sunday, May 27th, 2018 Ellicott City in Maryland experienced a devastating flood that tore through the heart of the town. Cars were washed away by what looked like a raging river as water rushed down main street. The water rose fast and high in some locations above the first story of houses. When all was said and done, an estimated eight inches of rain fell over the city in less than 12 hours, with the majority of the rain falling over a three-hour time span. Analysis of the flood determined that Ellicott City was hit with a 1,000-year flood. However, this was not the first 1,000-year flood to hit the city. On July 30, 2016, Ellicott City experienced a similar flooding event, with approximately six inches of rain falling over a two-hour period. Like the most recent event, the 2016 flood left the city in ruins.
But how is it possible to have two 1,000-year floods less than two-years apart from each other? The answer is in what a 1,000-year flood actually means. The term itself is misleading, suggesting that such a flood should happen only once every 1000 years. However, 1,000-year flood represents a hydrological probability defining the likelihood or rarity of a flood occurring. A 1,000-year refers to a 1 in 1000 chance or a 0.1% chance of the flooding event occurring any given year. Similarly, a 100-year flood has a 1 in 100 chance or a 1% chance of occurring each year while a 10-year flood has a 1 in 10 chance or a 10% chance of occurring each year. The U.S. Geological Survey, also known as USGS, refer to this probability as the one percent Annual Exceedance Probability, or the AEP. Along rivers, gauges measure the height of the water and the quality of the waters’ flow across America. The information from the gauges is then statistically analyzed for any given location and the flood probabilities are determined. However, other factors play a role in how likely a flooding event is, which can not necessarily be included in the statistical analysis. For example, the accuracy of the incoming data and changes in how the land is used in any given year. These, combined with generally unpredictable changes in yearly weather patterns, lead to rare events, like a 1,000-year flood, happening in the same location less than two-years apart.
Information provided by the USGS and the NWS in Baltimore (LWX)
Image from the Hydrometeorological Design Studies Center ftp://hdsc.nws.noaa.gov/pub/hdsc/data/aep/201805_Ellicott_City/AEP_Ellicott_City_May2018.jpg
To learn more about other flooding-related stories and topics from around the world, be sure to click on the following link: https://www.globalweatherclimatecenter.com/flooding!
©2018 Meteorologist Sarah Trojniak
Subtropical Storm Alberto indirectly brought torrential downpours and thunderstorms to the Mid-Atlantic Region. While Alberto did not go straight over the region, a flow of moisture was moving northeast coming from Alberto. The northeast flow moved towards a stationary front that was sitting over the Tennessee Valley and the Mid-Atlantic region. These were the ingredients for the destructive flooding that occurred in Maryland, Virginia and North Carolina.
Ellicott City in Maryland was under water on Sunday, May 27, 2018 when a nearby river overflowed its banks and into the streets of local homes and businesses. Many people needed to be rescued from buildings and vehicles as the force of the water pushed vehicles down the streets. One person lost his life when he was attempting to help a person in need. The above image is from Syria, Virginia where a woman and a young girl were swept away in the flooding. The young girl was quickly found; however, the woman has yet to be found as of the time this article was posted. The image on the top used to be a bridge where people could walk, but the strength of the flooded waters pushed the bridge off and you can see it in the image on the bottom. On Route 33, in Central Virginia, a landslide covered the highway prompting several road closures and detours. If we continue south, North Carolina had its share of landslides and flooding. One landslide compromised the integrity of the Lake Tahoma Dam in North Carolina prompting evacuations. Some areas have received over 6 inches of rain and the threat of rain continues. Many roads have been closed throughout the Mid-Atlantic due to debris and flooding.
Always remember; if you see high water, turn around, don’t drown. It only takes 6 inches of water to stall a vehicle while a foot of water will float many vehicles. Once the water level reaches two feet, your vehicle will likely be carried away. Flooded waters are typically muddy so any debris lurking in the water may not be visible.
Alberto is no longer a threat, but rain will persist through the weekend. With already saturated soil, any additional rainfall will produce more flooding. After the first weekend of June dry weather will make its way back into the area.
Stay tuned for more updates on the flooding situation in the Mid-Atlantic by clicking www.globalweatherclimatecenter.com/flooding
ⓒ 2018 Meteorologist Brandie Cantrell
DISCUSSION: This past weekend, Ellicott City, Maryland was devastated by what was referred to as a 1000-year flood. In recent years, there have been multiple storms that have produced 100-year, 500-year, and even 1000-year floods. But what do these terms actually mean and how can there be multiple of these historic floods within the given timespan?
Contrary to the name, a 1000-year flood does not necessarily mean this type of flooding will occur exactly once every 1000 years. Weather does not occur on a timed pattern so it is not possible to say that a certain severity of flooding will only occur exactly every 1000 years. What is actually meant by the name is that there is a 1 in 1000 chance of a storm producing flooding of that severity occurring within a year. It is not unheard of to have multiple historic floods within a relatively short time period. In 2017, flooding from Hurricane Harvey was reported as the third 500-year flood to affect Houston, Texas in three years. There was some confusion from the public as to how it was possible to have multiple 500-year floods less than 500 years apart from each other within one area. Since the naming of the category of these floods is based on probability, we can’t exactly say for certain there will or won’t be more than one flood of that magnitude within the 100/500/1000 etc. year timeframe.
When you flip a coin, there is a 1 in 2 chance of the coin landing on heads. Say you flip the coin twice. The only possible outcomes are the coin landing on heads twice, the coin landing on tails twice, or the coin landing on each once. Two of these scenarios do not represent this 1 in 2 chance probability. That is because the probability does not mean that for every 2 coins one of them must be heads. More accurately what this probability represents is that for a single coin toss, there is a 1 in 2 chance that the coin will land on heads. When the coin is tossed again, there is once again a 1 in 2 chance that the coin will land on heads since each coin toss is treated as an individual event. Since the 100/500/1000 year floods are based on the probability that type of flooding will occur within a given year, each year would be treated as an individual event. This means that for each year, there is a 1 in 100 chance that a 100-year flood will occur. The occurrence of that type of flooding in one year does not affect the probability of it happening again the next year, which is why it is entirely possible to have more than one flood of the same probability within that given time frame.
Ultimately, weather does not occur like clockwork. We cannot predict that a severe flood will occur exactly every 1000-years. We can, however, deduce that there is a 1 in 1000 chance that particular type of flood can occur in any given year. While the terminology may seem confusing, it is important to remember it refers to the probability of a flood, not an exact timeline of occurrence.
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©2018 Meteorologist Stephanie Edwards
Discussion: Heavy rainfall from deep convective storms fell this afternoon in Ellicott City, Maryland. Earlier this afternoon, severe weather moved through the area producing torrential downfalls. Radar estimates and actual reports of seven inches or more fell in just a few short hours. The rainfall that fell this afternoon caused the Patapsco River, to overflow their banks and reach above major flood stage. That water flowed downstream into Ellicott City, flooding the historic Main Street. As of tonight the heavy rain has moved out and the flood waters have receded.
This is not the first time Ellicott City has seen flooding like this. The geography of Ellicott City is unique in which it has streams upstream that converge to flow into the Patapsco River, making it susceptible to flooding and or flash flooding events. Two years ago, strong storms brought heavy rain that fell in a short amount of time producing flooding that destroyed businesses, homes and cars. That flash flood back in 2016 had two fatalities. A state of Emergency has been declared for the state of Maryland, as it starts to clean up and recover from another historic flood. Now the focus turns to recovery for this city once again. With the flash flood waters receding, there is still a danger being outside. Some streets and buildings are not structurally sound which can collapse, certain roadways may be partially washed away or still covered in standing water making it dangerous to drive. The National Weather Service advises people to stay informed and to avoid standing waters due to not knowing what is underneath them whether it be the condition of the road or the toxins and chemicals in the water. In the image above are more after the flood safety tips from the National Weather Service.
For more information on flash floods, how to stay safe after a flash flood and other flooding stories be sure to click here!
© 2018 Meteorologist Shannon Scully
Identifying Warm, Moist Flow for the Ongoing Severe Weather Threat. (credit: UW-Madison CIMSS MIMIC)
DISCUSSION: As the Central United States continues to be under a larger-scale severe weather threat overnight tonight and into the day on Wednesday and Thursday, there is also a substantial threat for flooding and even potential flash flooding. The reason for this is due to the fact that the moisture track which is ending up northward into the southern and central Mississippi River Valley is tracking out of the Gulf of Mexico. This can be seen in the CIMSS-MIMIC Total Precipitable Water imagery courtesy of the University of Wisconsin animated imagery attached above. Whenever there are situations characterized by warm, moist flow emerging out of the Gulf of Mexico, this greatly favors heavier rainfall events due to the fact there is also substantial persistence to these moisture plumes.
The core of this particular moisture plume can be identified via the yellow to orange shaded colors emerging from the Gulf of Mexico. This yellow to orange coloring represents the core of the deepest moisture within this particular Gulf of Mexico moisture plume which was coming ashore during the overnight hours from Monday night (04/30/2018) to Tuesday morning (05/01/2018). Thus, per the track of this moisture plume via the animated CIMSS-MIMIC imagery attached above, the core of this moisture plume was ideally oriented for heavy rainfall across the southern to central Mississippi River valley. Hence, it is always critical to respect the potential for heavy rainfall events coincident with severe weather events during the Spring to Summer time-frame across the Central United States.
To learn more about other high-impact flooding or flooding-related events from around the world, be sure to click here!
DISCUSSION: Portions of the mid-west and mid-Atlantic experienced massive flooding during the latter part of this past February. Residents in the Mid-Ohio Valley were urged to evacuate as well as prep their homes and businesses for the Ohio River’s impending invasion. Residents along the Monongahela River in West Virginia were also urged to prepare for flash flooding and possible property damage.
Flooding is among our planet’s most common – and most destructive – natural disaster. While certain areas are more prone to flooding, wherever rain falls is vulnerable. The most common flooding occurs when water inundates land that’s normally dry, usually due to river or stream beds overflowing their banks. Ice jams, excessive rain, or failed levees/dams can also irritate a river, causing it to spill out over the adjacent land (floodplain). Coastal flooding can occur due to a large storm system off the coast or even a tsunami, both of which causes sea water to rush inland.
Most floods typically take hours, sometimes days, to develop which gives residents and city management teams plenty of time to prep or evacuate. Others occur rapidly, with little to no warning. These are called “flash floods” and can be exceptionally dangerous, instantly transforming a calm creek into a massive wall of water, taking everything it its path downstream.
Experts classify floods according to their likelihood of occurring in a certain time frame. For example, “a hundred-year flood” is a large, destructive flood that in theory should be expected once every century. In all reality, this classification simply means there is a 1% chance that a flood to that magnitude could occur in any given year. Throughout the recent decades, these massive “hundred-year floods” have been occurring worldwide with unprecedented regularity.
Water, especially when moving, has awesomely destructive power. When a river spills water over its banks or the sea moves inland, poorly equipped structures cannot withstand the strength of the rushing water. Homes, bridges, cars, and other debris can be picked up and carried off. Moving water also has an erosive force, able to drag dirt from under a building’s foundation, ultimately causing it collapse.
In the United States, where severe weather prediction is advanced, floods cause upwards of $6 billion in damage and kill 140 a year. A 2007 report filed by the Organization for Economic Cooperation and Development discovered that coastal flooding alone causes nearly $3 trillion in damage worldwide.
When the floodwaters recede, it’s often that affected areas are covered in silt and mud. Occasionally the water and newly transformed landscape can be contaminated with hazardous materials such as pesticides, fuel, raw sewage, and other debris. Another potential danger is mold blooms quickly overtaking water-soaked structures. Residents of flooded areas can be left without basic necessities such as power and clean drinking water, possibly leading to outbreaks of waterborne diseases.
Flooding, especially in river floodplains, is natural and has been occurring for millions of years. Fertile floodplains like the Mississippi Valley in the Midwest, the Nile in Egypt, and the Tigris in the Middle East have been a life source for millions of years thanks to annual flooding leaving behind a plethora of nutrient-rich silt deposits behind.
Most of the destruction that comes from flooding can be attributed to society’s desire to live near coastlines and river valleys. It’s a common practice for city governments to mandate residents in flood-prone areas to purchase flood insurance and build flood-resistant structures. Massive efforts to manipulate and redirect inevitable floods have resulted in brilliant engineering efforts, including the extensive levee system in New Orleans as well as effective dikes and dams in Holland.
Scientists, meteorologists, and engineering are constantly working towards bettering our forecasting and infrastructure. Highly advanced computer modeling now allows disaster authorities and forecasters to predict with high accuracy as to where floods will occur and just how severe they could be.
To learn more about other interesting global flooding stories and/or topics, be sure to click on the following link: www.globalweatherclimatecenter.com/flooding.
© 2018 Meteorologist Ash Bray
DISCUSSION: There is little to no argument across the global scientific community that the Earth has for quite some time been and still is continuing to experience net planetary warming. One of the premiere concerns in the presence of an increasingly warmer planet are the prospects of heavier rainfall events occurring with a greater frequency and intensity both across the contiguous United States and all over the world for that matter. This a direct result of the fact that as the Earth continues to experience a net global heating trend, this consequentially increases the average global concentration of atmospheric water vapor. As this gradual average increase in atmospheric water vapor content occurs, this increases the propensity of heavy rainfall event potential by way of there being more atmospheric water vapor available in the lower to middle parts of the atmosphere for developing storm systems (i.e., whether they be in the form of thunderstorms or much larger-scale extra-tropical cyclones).
Hence, in the presence of a warming planet, the threat for a greater frequency of heavy rainfall events has many regional, national, and international scientists (and specifically hydrologists) rather concerned. This is because an occurrence of heavy rainfall events with a greater average frequency also adds the possibility of their being a greater propensity for river and stream flooding events which consistently have the potential to inundate towns and cities both near and far from the given waterway. To better understand the reason for the major concerns tied to the percentage increase in heavier precipitation events over the 55 + years, there are direct excerpts from the actual article written by the Climate Central team.
"A warmer world supercharges the water cycle, leading to heavier precipitation. To start, a warmer atmosphere creates more evaporation — for every 1°F of warming, the saturation level is increased by about four percent. With more water evaporating into the atmosphere, there is more available to condense into precipitation, and it’s coming down in bigger downpours...............Even in the absence of urban development where there is more rain runoff, heavy rain will lead to more flooding. This means extreme flooding will become more common, resulting in more property damage. Over time, flood maps may have to be redrawn, which will have an impact on property values and insurance rates. Damage from the 2016 Louisiana floods was $10.4 billion and totaled $125 billion from Harvey."
To read more about this particular story, click on the following link.
To learn more about other high-impact flooding and flooding-related stories from around the world, be sure to click here!
© 2018 Meteorologist Jordan Rabinowitz
Atmospheric river event brings flooding, landslides to Southern California (Photo Credit: Matt Roberts)
DISCUSSION: On January 8-9, California received a massive amount of rain across much of the state. However, the rain did not bring all good news as the National Weather Service had issued flood warnings as well as flash flood warnings across much of the Los Angeles and Santa Barbara areas. In addition, the National Weather Service had advised that there would be a slight chance of weak waterspouts in the Pacific Ocean near Los Angeles. Northern California was not hit as bad as the storm brought weaker than expected winds to the San Francisco Bay Area. Flooding was minimal in the San Francisco Bay Area as it was mainly minor road flooding with a small closure on California Highway 121 due to a risk of landslides.
It was a different story in Southern California on the morning of January 9th, as a flash flood was observed on the Ventura River near where the Thomas fire was occurring. In addition, mudslides have been reported across Southern California resulting in multiple deaths as well as having portions of U.S. Highway 101 closed. Much of the mudslides have been reported in the Santa Barbara area especially in the higher elevations due to a large amount of rain.
The heavy rain across the state especially in Santa Barbara was due to an atmospheric river event. An atmospheric river event is when moisture was drawn from the warmer portions of the Pacific Ocean near Hawaii and is transported to the coast of California. This atmospheric river event had very high moisture content in the atmosphere up to about 13,000 feet. In addition, the heavy rain in the mountains are also due to the rain shadow effect as moisture condenses as it climbs over mountains and precipitates as the atmosphere cannot hold as much water as the temperature decreases. Forecast models had indicated a very strong upward velocity at both 700 and 850 millibar pressure levels (9500 ft and 5000 ft respectively) in the Santa Barbara mountains during the time of the heaviest rain.
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©2018 Meteorologist JP Kalb
DISCUSSION: A major storm is expected to hit overnight tonight through Thursday night across the San Francisco Bay Area. The storm is expected to bring heavy rains in mountainous regions especially across the North Bay as well as the Santa Cruz Mountains. Flood concerns are very high in the North Bay due to the amount of precipitation being forecasted. In addition, the South Bay’s Coyote Creek will also be monitored due to the creek flooding in February which caused heavy damage in parts of San Jose.
The North Bay is expected to receive between 2 and 3 inches of rain overnight from the storm. The event is bringing flood concerns along the Napa and Russian Rivers especially in cities such as Napa, Santa Rosa, Petaluma, and Rohnert Park. These floods along the rivers are a common occurrence during heavy rain events in the Bay Area. In addition to concern of river flooding, flooding is also a concern especially in the areas that were devastated by the multiple wildfires that were burning almost a month ago. These areas are suspect to flooding due to soil conditions as well as a lack of plant life to take some of the water. The National Weather Service office in Monterey has issued a Flash Flood watch for much of the North Bay due to the concerns especially in the burnt areas.
In addition, the South Bay’s Coyote Creek will be monitored during the situation especially in San Jose. However, the South Bay is not expected to receive as much rain in the flooding area where forecast rainfall is expected to between 1 and 2 inches in the Hamilton Range mountains and an inch or less forecasted in the valleys. However, those living along Coyote Creek are bracing for a possible flood such as placing sandbags just in case more rain occurs than expected.
The storm is expected to bring heavy rain due to an Atmospheric River. An Atmospheric River is an event where moisture is drawn in from the warmer parts of the Pacific especially near Hawaii and makes its way to California when there is a deep trough in the Pacific Northwest. In addition, vertical velocity with respect to pressure are forecasted to be at about 0.1 Pa/s (pascals per second) or higher in an upward motion. This upward motion makes the atmosphere more saturated at a higher level to the point of complete saturation. At which point, the atmosphere is unable to hold any more water which is then released as rain.
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©2017 Meteorologist JP Kalb
Atmospheric River brings Historic Flooding to Juneau, AK (Photo Credit: Aaron Jacobs/National Weather Service)
A man stands outside his property near Jordan Creek on October 27, 2017
During the week of October 23rd, hints of a strong atmospheric river (a narrow band of concentrated moisture in the atmosphere) were present in the extended forecast for southeast Alaska, as moisture from super typhoon Lan was expected to push through. Typhoon Lan formed in the northwest Pacific on October 14th, affecting the Philippines, Japan, and South Korea and causing several deaths along its track. By October 23rd, Lan was showing signs of dissipation. The remnant moisture fueled the atmospheric river, bringing heavy flooding to Alaska.
On Wednesday, October 25th, an areal flood watch had been issued for the northern half of southeast Alaska due to projected small stream rises. As rain began on Thursday, a flood advisory was in effect, where rivers reached bankfull and high freezing levels (>6000 ft) increased the threat for runoff and higher elevation snowmelt. Early Friday morning, heavy rains and strong winds near 40 mph continued to impact the Skagway and downtown Juneau areas. The 48-hour rainfall totals for Thursday and Friday ranged from 3.78” at Yakutat Airport, 4” at the Glacier Bay Fuel Dock in Gustavus and at the National Weather Service office in Juneau, 4.5” for downtown Juneau, 6” at Sawmill Creek in Sitka, and 8” in Pelican.
By 10 a.m. on Friday, October 27th, a flood warning was issued for Juneau Creek near Juneau. Jordan Creek usually reaches action stage at 9.2 ft, flood stage at 9.7 ft, and moderate flood stage at 10.5 ft. It reached a record crest of 11.0 ft by 2:30 p.m. local time on Friday. The last historic crest of Jordan Creek occurred on September 9, 2016 at 10.18 ft. As a result, several residential homes, office buildings, and roads near Jordan Creek (and also Salmon Creek in Gustavus) were inundated by a foot of water. A mudslide occurred near the Mt. Roberts Tramway system in Juneau, closing roads and delaying traffic.
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© 2017 Meteorologist Sharon Sullivan