This evenings topic deals with flooding.
While hurricanes and tornadoes often make more headlines
than any other weather event...Flooding can take the blame for the most weather related
fatalities. On average, nearly 150 people in the United States lose their lives each year
due to floods...while lightning is a distant second with half as many deaths. We have been
lucky in recent years across New England in that flooding has been more of a problem for
property rather than lives...but nonetheless that threat will always be present.
Flooding for New England can occur at any time of year. During the Spring and Summer months, all areas experience the threat of heavy rain from thunderstorms. Slow moving thunderstorms and thunderstorms that train ( repeatedly move across the same area) ...often result in flooding. The training of thunderstorms is most common along stationary fronts. In 1997, a series of thunderstorms dropped over 7 inches of rain on Hillsborough county in less than one hour. This resulted in many washed out roads, stranded motorists, and small stream and river flooding.
One to three inches of rain in a short span of time can
raise smaller creeks and streams to near bank-full...with urban areas experiencing
flooding from poor drainage. In urban areas where land is converted from fields and
woodlands to roads and parking lots, the surface loses its ability to absorb rainfall.
During periods of urban flooding, streets can become swift moving rivers, while basements
can become death traps as they fill with water. Urbanization increases runoff some 2 to 6
times over what would occur on natural terrain.
The Summer and early Fall months are also favorable months for excessive rain from the remains of tropical systems. Some of the worst flooding in recent times have come from such events.
Possibly the largest contributors to flooding in New England is from winter snowmelt...combined with heavy Spring rains.
A pair of spring storms occurring in March and April 1987 combined
with snowmelt to produce record or major flooding in Maine, New Hampshire, and
Massachusetts. Fortunately, the heaviest rains for the two storms did not fall over the
same area or else more hazardous flooding could have occurred. The highest rainfall
accumulations recorded were 8.30 inches at Pinkham Notch, New Hampshire, and 7.3 inches at
Blanchard Maine. Total damages in Maine from this first storm were estimated at 74.5
million dollars. Amazingly, only one flood related death was known to occur.
The second storm, following only a few days after the rains from the first storm ended, missed Maine, but delivered tremendous amounts of rain to New Hampshire and Massachusetts. Generally 4 to 7 inch rains occurred over much of this area. Amounts over 8 inches were recorded at stations in northwest Massachusetts as well as along the northeast Massachusetts coast. This rainfall, combined with the already high river conditions caused by the previous storms, produced major flooding in both the Connecticut and Merrimack River basins. In addition, several of the Army Corps of Engineers dams recorded record pool levels, including uncontrolled spillway flooding at several locations.
True Flash Flooding is rare in New England, but as mentioned...urban drainage and small stream flooding can occur at virtually anytime of year. This type of flooding is usually short lived, but can cause serious problems in the metropolitan areas, as the terrain is relatively flat with a lot of bridges and viaducts. A rough guide of an inch of rain in an hour can be used to predict significant problems in urban areas.
Even if you do not have a rain gauge...using a can or bucket can give you a good idea of the amount of rain that has fallen. As long as the container has straight vertical sides on it...the rain inside can be measured with the use of a ruler.
If you receive an inch or more of rain in three hours time
or less...get your report into the National Weather Service. We are also looking for
reports of creeks and streams that are close to or out of their banks...roads that are
flooded...and flooded basements. Remember that if a SKYWARN net is not in progress...you
can make your report by phone or start a net yourself.
Remember not to attempt to drive over a flooded road. The depth of the water is not always obvious. The road bed may be washed out under the water, and you could be stranded or trapped. It is also much easier for your vehicle to be swept away in flood waters...as just two feet of water can carry away most vehicles. Nearly half of all flash flood fatalities are auto related.
This script, as well as all others we've covered so far can be found at http://www.qsl.net/kb1dfe on the SKYWARN page.