Public Information Statement
Issued by NWS State College, PA
NOUS41 KCTP 101026
The week of November 7th to 11th is Winter Weather Awareness Week
Each day we will feature a different educational message concerning
the dangers and safety concerns associated with winter weather.
One of our deadliest winter weather hazards is flooding.
In Pennsylvania one usually associates snow, ice, and biting cold
with winter. But sometimes nature throws a curve at us with
unseasonable warmth, and with it, rain.
A number of different factors work together to produce floods in
When unseasonable warmth comes to the region it will often melt
much, if not all of the snow on the ground in the lower elevations.
The melting snow will saturate the ground and also begin to swell
Often the warmer air will rise over colder air trapped in the
region. As this air rises it results in clouds and rain, which will
combine with the melting snow to increase flows in the rivers. When
this melting snow combines with heavy rains it can put enough water
into the rivers to send them over their banks.
Some of our worst winter floods are caused by intense cyclones that
track from the Ohio valley northeast up the Saint Lawrence Valley
into Canada. These storms bring a lot of warm and moist air into the
region from the Gulf of Mexico and the Atlantic, bringing both mild
temperatures and heavy rain to the region.
In January 1996, unseasonably warm air and heavy rain combined to
melt a deep snowpack in Pennsylvania. The combination of melting
snow and heavy rain led to the worst flooding in Pennsylvania since
hurricane Agnes in 1972.
Another winter problem is flooding caused by ice jams on the rivers.
As river flows increase, water levels rise. Since ice that covers
the rivers is lighter than water it will tend to float. Under the
pressure it will often break into huge slabs. These slabs will then
move downstream in the current until they run into an obstruction
such as a bend, island, or wide shallow area. When this happens the
ice will often stop and pile up into a jam. When the flow of the
river is blocked by an ice jam, the water can overflow the river
banks in less than an hour as it tries to get around the ice. As the
water rises, the pressure can break the jam and release a sudden
surge of water and ice down the river.
While ice jams often form in the same spots year after year it is
nearly impossible to predict exactly when or where a jam will form,
or when one will break. Sometimes a jam that forms in early winter
will stay put most of the winter.
Severe flooding of roads and built up areas can also occur when
mounds of plowed snow and ice block and plug up the grates and storm
drains, so the water from the pavement has nowhere to go. This
standing water can cause dangerous black ice if it freezes.