|LOCAL FREAKISH WX SERIES:|
4+ INCHES OF RAIN IN AN HOUR
METEOROLOGIST JEFF HABY
A freakish weather event that can occur includes extreme rainfall on a local area. The extreme rainfall can be generated by moisture
converging into a stationary lifting mechanism, a stalled strong thunderstorm or a rain band from a tropical storm system. These
freakish events can fill up a backyard rain gauge in an hour. There can be for example a 5 inches rainfall gauge that fills up and
overflows due to a freakish rainfall event.
An example of moisture converging into a stationary lifting mechanism is when a moisture rich flow collides into a mountain range. The
steady supply of moisture and lifting can generate extreme rainfall amounts. This can occur for example along the Rocky Mountains and
mountains of the Pacific. This can generate prodigious flash floods as the water moves down mountain streams.
A strong storm with slow movement can generate prolonged heavy rain over a location. This type of heavy rain event will commonly
lead to flash floods. Typically storms have enough forward motion so that the heaviest of the rain lasts for less than half an
hour at any one location. A slow moving storm though can allow the heavy rain core to stay over a location or a solid hour or more
and this can generate the 4+ inches of rain in an hour. Training of storms can also produce very heavy local rainfall. Training
occurs when storms keep developing and dumping rain over the same region. This can occur even when the individual storms have
a progressive forward motion.
Tropical systems are also notorious for generating very high rainfall rates. The tropical storm brings in a deep layer of moisture. The
lifting along with the moisture will generate very heavy bands of precipitation. If these bands occur over the same region then
rainfall rates can exceed 4+ in an hour. As the tropical moisture collides into additional lifting mechanisms such as higher
elevations or a front then this can also cause tremendous rainfall rates.