What’s the difference between a flood watch and warning? Read up on key weather terms

Moose Run starts to encroach houses along Front Street in Milesburg on Wednesday, April 3, 2024.

Prolonged periods of heavy rain have prompted flood watches in the State College area through Thursday morning. Although the rain is far from over, it has already flooded roads and closed a few Centre County schools.

According to the National Weather Service, the area could receive roughly 3 inches of total rain by Friday morning. Additional flooding is possible throughout the region as heavy rain continues this week.

Understanding some of the key terms meteorologists use in their forecasts can help keep you and your family safe during severe weather. Here’s a quick guide to some of the most essential topics you might see while reading weather reports when flood risks are high.

You can join the AlertPA notification system to get updates on emergency and weather-related alerts in your area. You can also follow the NWS’s State College office on Facebook or X, formerly known as Twitter.

What’s the difference between a river flood and a flash flood?

There are important distinctions to be made between flash floods and their more common counterparts — especially since the former is often much more dangerous.

River floods are caused by gradual increases in the water levels of rivers or creeks, according to the NWS. These floods are seen seasonally as a product of general rains or through more torrential rainfall produced by tropical storms. As natural waterways exceed the capacity of their channels, water overflows the banks and spills out into nearby low-lying land. This type of flooding is seen more commonly as snow melts in the late winter and early spring or as spring rainfall gets heavy.

The NWS defines flash flooding as flooding that begins within six hours (and often within three hours) of heavy rainfall. This is often seen through extremely heavy rainfall from thunderstorms and other severe weather events that exceed the ground’s ability to absorb it.

Flash floods are thought to be more dangerous than river floods because they develop quickly, sometimes even without much notice. Densely populated areas, including urban settings, are at a higher risk for flash floods since buildings, roads, parking lots, driveways, sidewalks and more increase water runoff and reduce absorption into the ground.

What do advisories, watches and warnings mean?

Understanding these terms can help you evaluate the urgency of weather-related warnings in your area.

The NWS issues a “watch” when the risk of a hazardous weather event, such as a flood, has significantly increased but the exact timing or location remains uncertain. Watches are thought to help give residents in the area enough time to prepare for severe weather and understand the threats that might come their way.

Advisories are issued when hazardous weather events are occurring, imminent or likely, the NWS says. They are often issued for less serious conditions that might cause inconvenience or pose a small threat, such as a heat or frost advisory.

Warnings are the most serious categorization used by the NWS. They are issued when hazardous weather events or conditions that pose a threat to life and property are presently occurring, imminent or likely. People who reside in areas under a severe weather warning “need to take protective action,” the agency says.

Here are the exact definitions for flood-related advisories, watches and warnings from the NWS:

  • A flood watch is issued when conditions are favorable for flooding. It does not mean flooding will occur, but it is possible.

  • A flood advisory is issued when expected flooding is not thought to be particularly severe but may cause an inconvenience and, if caution is not exercised, “could lead to situations that may threaten life and/or property.”

  • A flood warning is issued when flooding is imminent or occurring.

  • A flash flood warning is issued when a flash flood is imminent or occurring. Those caught in a flood-prone area should move to higher ground. It is possible to experience a flash flood in an area that is not immediately receiving rain.

Staying safe during flooding

Flooding kills more people in the U.S. every year than any other severe weather-related hazard, according to the NWS. Taking proper precautions can save lives when floods occur.

More than half of all flood-related drownings occur when vehicles are driven into hazardous flood waters, according to the U.S. Centers for Disease Control and Prevention. The NWS uses the popular phrase “Turn Around, Don’t Drown” to emphasize the unsuspecting power of flood water. After all, just 6 inches of fast-moving water can knock over an adult and carry them downstream. A foot of water can sweep away a small car, while between 18 and 24 inches of fast-moving flood water can drag away most large SUVs, trucks and vans.

Moving to higher ground is key when flooding strikes. Avoid areas that are more susceptible to flooding, such as underground parking garages, basements, low water crossings and underpasses on roadways. Areas near rivers are at a higher risk of flooding during periods of heavy or prolonged rainfall.

Even if you do not live in an area thought to be at a high risk for flooding, be prepared. Roughly 94% of flood reports in Pennsylvania made to the NWS between 1993 and 2023 occurred outside of Federal Emergency Management Agency-designated flood hazard areas. According to the Pennsylvania Emergency Management Agency, where it rains, it can flood.

If you are ordered to evacuate your home, make sure to turn off your utilities, close central gas valves and gather emergency supplies, the CDC advises. Never drink flood water or use it for household purposes if your home floods.