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.
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© 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.
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