Flooding - Causes and Severity

There are two main ways that flooding typically develops in Pennsylvania.
First...excessive rainfall.
Too much rain will fall in a specific amount of time...overwhelming the creeks and streams...and filling the rivers. The causes for this excessive rain vary greatly. 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.
Second...snow melt combined with rainfall.
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...or both.
Let us take a closer look at tropical cyclones:
Tropical cyclones are prolific producers of rainfall...and often create flooding when they move inland. When it comes to tropical cyclones...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.
What do I need to know about inland flooding from tropical cyclones?
Inland freshwater floods accounted for more than half of U.S. tropical cyclone deaths over the past 30 years.
Rainfall is typically heavier with slower-moving storms...as slower-moving tropical cyclones allow 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 flood warning products...the NWS 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...or transfer of property to higher elevations are necessary.
Major flooding: extensive inundation of structures and roads. Significant evacuations of people and...or transfer of property to higher elevations.
The effects of floods vary locally. For each NWS river forecast location...flood stage...and the stage associated with each of the NWS 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.
You are encouraged to use the regular tone-alert tests on the NOAA weather radio transmitters today to practice flood safety drills and procedures at your home or business.
TOPICS FOR THE REST OF FLOOD SAFETY AWARENESS WEEK:
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