Happy Monday and welcome to another edition of Beyond The Forecast!
The end of this July brought intense flooding to the country. From record rainfall in St. Louis causing flooding to other floods closer to home causing catastrophic damage, there are plenty of recent events to remind us just how powerful floods are.
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According to the National Weather Service, about 75% of all presidential disaster declarations involve flooding in some way. That can vary from river flooding due to snow melt to localized flooding from heavy rain all the way up to storm surges from hurricanes.
It’s important to stay aware when floods develop because they are not only common but also very dangerous for people and property.
The NWS has four different categories of information regarding floods, which include:
- Flood Advisory
- Flood Watch
- Flood Warning
- Flash Flood Warning
During an Advisory or Watch notice, conditions are favorable for flooding with a coming rain event. The Watch is issued if the conditions are forecast to be bad enough to warrant a Warning when they arrive, and the Advisory is used if the event won’t quite reach the Warning threshold but could still cause issues in a community. That could be storm drains getting full or low-lying roads flooding.
Be careful if either is issued since the upcoming weather in both can be hazardous if you aren’t careful.
Flood Warnings are issued when flooding is just about to happen or already is. Flood warnings can last much longer than other types of warnings. To illustrate, Severe Thunderstorm Warnings often last for about 30 minutes to an hour while Flood Warnings can stay in one place for some bodies of water for days or weeks.
Flash Flood Warnings are similar to a standard Flood Warning in that they are issued for a current or incoming flood, but flash floods develop in a shorter timeframe of minutes or hours. Flood Warnings tend to be issued along existing bodies of water whereas Flash Flood Warnings can be issued anywhere as water levels rise. Some spots in the Southwest of the U.S. that stay dry through most of the year can quickly flood from heavy seasonal rains and urban areas where water has a hard time dissipating. This can also result in flash floods.
The NWS and other agencies monitor river gauges all throughout the country.
One of those gauges is on the Roanoke River. This gauge monitors the height of the river above its bed and is useful in combination with other data like recent rainfall or snowmelt to determine how high the river will rise in the coming days.
When flooding occurs there are four different categories that the river can fall into such as:
- The Action Stage
- The Flood Stage
- The Moderate Flood Stage
- The Major Flood Stage
Each of these corresponds to a particular river height for each gauge along the length of the river. At the particular gauge in Roanoke the Action Stage is 6 feet and the Major Flood Stage stands at 16 feet.
Although Action Stage doesn’t actually contain the word “flood,” it’s very useful for emergency planners, the NWS and other stakeholders. Taking action when a river is below a flood stage can prevent damage before a flood even happens.
While the Roanoke River along with the Dan, James and New Rivers are well monitored, that doesn’t mean rivers alone have gauges. Creeks like Peak Creek in Pulaski and Cub Creek in Phenix have gauges to determine any changes in water flow, and where Smith Mountain Lake meets the Roanoke River there is another.
For some perspective on the rivers in our region, the Roanoke River gauge in Roanoke’s highest crest was in 1985 at more than 23 feet, and the almost 16-foot crest in May of 2020 ranks within the top 10.
The New River’s Major Flood Stage in Radford is 20 feet, which the river reached 9 times on record. The highest of those crests was all the way in 1940 when the river surged to more than 35 feet. That gauge has only crested above 20 feet three times in the last 20 years.
The Dan River gauge in Danville reaches the Major Flood Stage at 32 feet, but the highest crest on record stayed below that margin: in 2018 the river got just above 30 feet. That’s not to say that it can’t cause damage: reaching either the Flood or Moderate Flood Stage still means the river is carrying a volume of water well above average.
Rainfall is limited to afternoon showers most of this week, so flooding is not as much of a concern as it was at the end of July. Instead, temperatures will climb back into the 90s for highs so be sure to stay hydrated. You can download our weather app for information on storms as they develop and get Meteorologist Chris Michaels’ latest updates online.
You can always get specific forecast details for your zone, whether it’s the Roanoke Valley, Lynchburg area, the New River Valley or elsewhere around Southwest and Central Virginia, anytime at WSLS.com/weather. Know your zone!
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-- Marshall Downing