NOAA's Weather Prediction Center (WPC) is the agency responsible for issuing daily excessive rainfall outlooks. The outlooks use four categories – marginal, slight, moderate and high – to indicate the probability of excessive rainfall leading to rapid-onset flooding within 25 miles of a point.
When a high risk – the highest risk level that can be issued – is in place, it means severe, widespread flash floods are expected in the highlighted areas.
"Areas that don't normally experience flash flooding, could," the WPC says. "Lives and property (are) in greater danger."
High risks are only issued on about 4% of days (including tropical and non-tropical events), but this risk category accounts for 39% of flood-related fatalities and 83% of flood-related damages in the continental U.S., according to research by WPC meteorologists.
What's more, 46% of high-risk flood days have at least one fatality or injury, and 62% of such days have at least $1 million in damages, Greg Carbin, forecast operations branch chief at the WPC, noted in the research.
"Expect significant to potentially catastrophic widespread flash flooding through Monday evening, with possible impacts not seen since (Hurricane) Irene," the National Weather Service office in Burlington, Vermont, wrote in a forecast discussion.
Irene produced a wide swath of 5 to 8 inches of rain across Vermont, which led to devastating mudslides and deadly flooding.
This is not your typical flash-flood threat.
Your commute to work or school, the subway system, a parking lot, the street on which you live or even your home are all areas that could become inundated by floodwaters on a high-risk flood day. This can include areas that have never flooded in the past.
Many people underestimate the force and power of water.
"A mere 6 inches of fast-moving floodwater can knock over an adult," the National Weather Service says. "It takes just 12 inches of rushing water to carry away most cars, and just 2 feet of rushing water can carry away SUVs and trucks. It is NEVER safe to drive or walk into floodwaters."