Flooding

It looks peaceful but the force of rushing water can carry away cars (with people in them).

Floods like the Missouri and Wisconsin cases are called “Flash Floods”

The AMS glossary defines Flash Flood as “A flood that rises and falls quite rapidly with little or no advance warning, usually as the result of

intense rainfall over a relatively small area. “

Flash floods are often due to thunderstorms. The slower kind of flood is due to rivers overflowing their banks. That usually takes days to happen.

To distinguish the two types of floods, the National Weather Service also specifies that “Flash flooding occurs within 6 hours of the rain

event.”

Here’s a heavy rain event in NY in 2004.

Note how the radar echoes keep passing over the same spots in central NY. This is called “training”.

This is real training. You are in one spot and each train car passes that spot.

Thunderstorm cells sometimes act like this.

In June 2006, the echoes move north but the precipitation area is oriented from north to south

and not moving much. So central NY gets echo after echo.

This was the result - the record June 2006 flood

I-88

Delaware County

Paddling down the street in downtown Walton, NY

Flash Flooding can also happen with a single storm. This is one of the most famous cases – the Big Thompson flood of 1976.

The Big Thompson River and Canyon were named for David

Thompson, an English engineer and astronomer,

who, early in the 19th century, explored many streams in the Mid-West and mountain states

such as Colorado.

In this cross sectional view, the thunderstorm that formed at one end of the Big Thompson river

canyon is drawn with radar reflectivity. It was very intense.

Heavy rainfall for hours produced a wall of water 19 feet high in the narrow canyon. People couldn’t

get out. 144 died.

Show Big Thompson Ride.flv

In situations where training or just really heavy rain is occurring, the

NWS will issue Flash Flood warnings (in green)

Show Raging Planet