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Public Information Statement
National Weather Service State College PA
210 AM EDT Fri Mar 22 2024

...Pennsylvania Flood Awareness Week 2024...

The National Weather Service has declared March 18th through March 22nd
to be Flood Awareness Week in Pennsylvania.

Today`s topic: Flooding Causes and Severity.

There are two main ways that flooding typically develops in
Pennsylvania: Excessive rainfall and snow melt combined with rain.

Excessive rainfall occurs when too much rain falls in a specific
amount of time, overwhelming the creeks and streams and
filling the rivers. Excessive rainfall can occur in a variety of
weather scenarios. Widespread heavy rain to the north of a warm front,
slow moving thunderstorms, and tropical cyclones are the chief causes of
excessive rainfall in Pennsylvania.

The snow melt combined with rainfall scenario occurs during the winter
and early spring when there is snow on the ground. The water content
of the snow is released and runs off into the waterways when it
melts due to rapidly warming temperatures or rain falling on the
snow.

Let`s take a closer look at one of the causes of excessive
rainfall:

Tropical cyclones are prolific producers of rainfall and often
create flooding when they move inland. When it comes to tropical
cyclone, which is the generic term for a hurricane, typhoon, or
tropical storm, the wind speeds do not tell the whole story.

Intense rainfall, not directly related to the wind speed of a
tropical cyclone, often causes more damage. Since the 1970s,
inland flooding has been responsible for more than half of the
deaths associated with tropical cyclones in the United States.

Typically, greater rainfall amounts and flooding are associated
with tropical cyclones that have a slow forward speed, or stall
over an area. This allows heavy rain to persist for an
extended period of time over a location.

What types of inland flooding are caused by tropical cyclones?
Both flash flooding and longer-lasting river flooding can result.

Flash flooding occurs in creeks, streams, and urban areas
within a few minutes or hours of excessive rainfall. Streets can
become swift moving rivers and underpasses can become death traps.

River flooding occurs from prolonged heavy rains associated with
decaying hurricanes or tropical storms, and in extreme cases,
river floods can last a week or more.

How do I know how severe a flood will be? Within river flood warning
products, the National Weather Service conveys the magnitude of
observed or forecast flooding using flood severity categories.
These flood severity categories include minor flooding, moderate
flooding, and major flooding. Each category has a definition based
on property damage and public threat.

The definitions of flood severity:

Minor flooding: minimal property damage. Some public threat or
inconvenience.

Moderate flooding: some inundation of structures and roads near
streams. Some evacuations of people and transfer of property
to higher elevations are necessary.

Major flooding: extensive inundation of structures and roads.
Significant evacuations of people and transfer of property to
higher elevations.

The effects of floods vary locally. For each National Weather
Service river forecast location, flood stage, and the stage
associated with each of the National Weather Service flood
severity categories are established in cooperation with local
public officials. Increasing river levels above flood stage
constitute minor, moderate, and major flooding. Impacts vary from
one river location to another because a certain river stage (also
known as river height) in one location may have an entirely
different impact than the same level above flood stage at another
location.

Topics for the rest of flood awareness week:
Friday: Flood alerts.

$$