NWS Burlington Flood Safety Awareness: Warm Season Flash Flooding

Welcome to this presentation on warm-season
flash flooding for the National Weather Service’s Flood Safety Awareness Week. The photo in
the background is of a flash-flood event that took place in Rutland, VT on August 21st,
2009. In this presentation we’ll first define what
a flash flood is and what’s meant by the warm season. Next we’ll go into what can cause
flash flooding, areas that can be more susceptible to flash floods; and some dangers flash floods
can pose. Finally, we’ll attempt to summarize all the topics by looking at a Flash Flood
case that took place in southern Addison County in Vermont. Flash Flooding by definition is a sudden,
rapid rise in water level which can produce potentially devastating flooding. The key
in the definition of a flash flood is the time element: the sudden, short-duration nature
of flash flooding differentiates it from a more prolonged, long-duration flood event
where water rises are slower. Note that initial flash flooding can transition to a long-duration
flood event.   
We define the “Warm Season” as spanning between April through October. So warm season flash
flooding effectively excludes ice-jam-related flash flooding which can happen during our
colder months. We’ll refer to warm season flash flooding as “flash flooding” through
the rest of this presentation. So what can produce flash flooding here in
the North Country? Very heavy rainfall associated with thunderstorms produces most of the flash
flood events that we find here in the North Country.
  Since thunderstorms tend to form in humid,
high-moisture environments, any thunderstorm can produce copious amounts of rainfall and
an associated flash flood risk. The flash flood risk can be enhanced by thunderstorms
that are stationary or slow-moving because of their potential to produce very heavy rain
over a localized area. Thunderstorms that move over the same area repeatedly (often
in sequence) also can produce flash flooding. We call this effect “training”, as shown in
the graphic. We also experience flash flooding produced by tropical cyclones. Torrential,
tropical downpours associated with heavy rainbands and/or embedded thunderstorms in the tropical
cyclone itself can cause flash flooding. Though not necessarily weather-related, the
failure of a dam will cause an immediate, sudden release of water prompting flash flood
warnings. Some areas are more vulnerable to flash flooding
than others. The reason for this variation depends on effects such as antecedent rainfall
and differing land use practices.  
Prior rainfall events can help to “prime” areas to flash flood as the ground becomes
more saturated in these locations. It would take a lower amount of rainfall over a short
period of time to produce flash flooding if soils are already saturated.
  You might not have realized that urban areas
are susceptible to flash flooding. Recall the photo at the beginning of this presentation
depicting the Rutland flash flood. It can take a lower amount of rainfall to flood an
urban area than one that has more porous soils. Since rain can’t absorb into the ground if
the ground is covered by pavement and asphalt surfaces, it goes directly to run off on the
Mountainous terrain, canyons and valley locations are also prone to flash flooding. Rainfall
runs down the mountain and pools into low-lying valleys, making it easier for flash flooding
to occur here. Orographic effects can enhance rainfall rates on the windward side of the
Flash flooding carries with it a variety of societal impacts which can range from minor
to catastrophic.  
The threat to human life is significant even in the most minor of flash flood events. Field
and culvert flooding and water on roadways can occur even in minor flash flood events.
In higher-end flash flood events, significant structural damage to infrastructure (such
as road washouts, where the pavement is eroded by floodwaters), or complete property losses
(due to water damage or water sweeping buildings away) can occur.
  Though flash flooding is potentially dangerous
at any time, it is especially so at night simply because it is difficult to see advancing
floodwaters until it is too late.  
Flash floods can catch outdoor enthusiasts off guard, including hikers and swimmers. To help tie in to what we’ve learned up to
this point about warm season flash flooding, we’ll close this presentation with a brief
overview of a flash flood case that affected Southern Addison County on August 6th, 2008. This event began around midnight on August
6th and continued through the afternoon of the 6th. Terrain influences in this part of
Addison County, which the color shading in the graphic show, helped to enhance the flash
flood threat. Some impacts from this event can be seen on the graphic as well, which
largely affected campgrounds in the central Green Mountains. This is a composite Radar image loop which
shows areas of heavy rain and thunderstorms in orange/red shading. As this loop plays,
focus your attention to the red box which corresponds roughly to the area that experienced
the flash flooding. A couple of slow-moving isolated thunderstorms developed around midnight
as the loop begins playing; another round of moderate/locally heavy rain then moved
in during the early morning hours of the 6th. Some weakening thunderstorm activity then
moved into the affected area during the early- to mid-afternoon. Estimates of 3-4″ of rain
fell in this area with anecdotal reports of up to 6″ through the entire event. Here’s some photographs of road washouts and
an aerial photo of road erosion from the affected area. Since part of this event took place
during the pre-dawn hours, anyone driving in the dark on any of these roads would have
put themselves in significant danger. Thanks for watching this presentation on Warm
Season Flash Flooding. For more information, visit on the web at www.weather.gov/Burlington,
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