The National Weather Service issues many hydrology related products. Some are to warn you of potential hazards; others are information statements to keep you up to date on changing weather and river situations. Below are the most commonly issued products along with a brief explanation of what they mean to you!
"Take Action!" when these products are issued
Warnings or Advisories are issued when confidence is greater than 80% that an event will occur. Depending on the type of event, these may be issued anywhere from hours before an event to days in advance of an event.
A Flash Flood Warning is issued to inform the public, emergency management and other cooperating agencies that flash flooding is in progress, imminent, or highly likely. Flash Flood Warnings are urgent messages as dangerous flooding can develop very rapidly, with a serious threat to life and/or property. Flash Flood Warnings are usually issued minutes to hours in advance of the onset of flooding. Flash Flood Warnings are changing to an Impact-Based format to improve public response. Read the factsheet.
A Flood Warning is issued to inform the public of flooding that poses a serious threat to life and/or property. A Flood Warning may be issued hours to days in advance of the onset of flooding based on forecast conditions. Floods occurring along a river usually contain river stage (level) forecasts.
A Flood Advisory is issued when a flood event warrants notification but is less urgent than a warning. . Advisories are issued for conditions that could cause a significant inconvenience, and if caution is not exercised, could lead to situations that may threaten life and/or property.
Coastal/Lakeshore Hazard Message products provide the public with detailed information on significant coastal/lakeshore events. Coastal/Lakeshore events impact land-based and near shore interests along much of the United States coastline. This product can be issued as a watch, warning or advisory and follows the same "Be Aware, Be Prepared, Take Action" definitions as with other NWS WWA products. A Watch is issued when flooding with significant impacts is possible. Warnings are issued when flooding posing a serious threat to life and property is occurring, imminent or highly likely.
Special Weather Statements provide the public with information concerning ongoing or imminent weather hazards, which require a heightened level of awareness or action, but do not rise to the level of watch, warning or advisory.
"Be Prepared" when these products are issued
Watch products associated with flooding are typically issued within a day or more of the event. The confidence level for an event to occur when a Watch is issued is 50-80%. When a Watch is issued, you should begin to gather more information about the situation and determine what actions you will need to take should a warning be issued.
A Flash Flood Watch is issued to indicate current or developing conditions that are favorable for flash flooding. The occurrence is neither certain nor imminent. A watch is typically issued within several hours to days ahead of the onset of possible flash flooding.
A Flood Watch is issued to indicate current or developing conditions that are favorable for flooding. The occurrence is neither certain nor imminent. A watch is typically issued within several hours to days ahead of the onset of possible flooding. In situations where a river or stream is expected to be the main source of the flooding, forecast confidence may allow for a Flood Watch to be issued several days in advance.
"Be Aware" when these products are issued
Outlook products are issued days, weeks or even months in advance of the event. When an Outlook is issued, you should take note. This serves as a "heads-up" that you may be impacted and gives you a general timeline of when the event could occur.
Two types:
1. Short-term (1 to 7 days) Hydrologic Outlooks can be issued to alert the public of the potential for flooding in the near-term such as when heavy rainfall is forecast that could result in flooding or aggravate an existing flood if it occurs.
2. Long-term (weeks to months) Hydrologic Outlooks may also provide river or reservoir level and/or flow information. This information could be used for water supply concerns or projection of snowmelt flooding.
The Hazardous Weather Outlook is a single source of information regarding expected hazardous weather through seven days. It can include information on severe storms, heavy rain, flooding, tropical storms, winter weather, high winds, fire weather and marine hazards.
The ERO is an outlook product for the entire CONUS, graphically illustrating the chances of excessive rainfall leading to flash flooding for the next five days. It is issued by the NWS Weather Prediction Center.
Products to keep you informed
The products listed below are produced to keep the public and NWS partners up to date on the latest forecast information.
The Area Forecast Discussion (AFD) is a semi-technical product that is primarily used as a means to explain the scientific rationale behind a forecast. It is also used to summarize watches, warnings and/or advisories that are in effect. For significant hydrologic events such as floods, a special "Hydrology" section may be included in the AFD
Hydrometeorological discussions provide a hydrology-oriented overview of the current and expected hydrometeorological situation across a River Forecast Center's (RFC) area of responsibility.
River Forecast products issued by River Forecast Centers (RFC) contain stage and/or flow forecasts for specific river/stream locations based on existing and forecast hydrometeorological conditions. The contents of these products are used by Weather Forecast Offices (WFO) to prepare WWA products (Watches, Warnings and Advisories) as well as other river-specific products available to the public
The Hydrologic (River) Statement is issued to communicate notable hydrologic conditions or forecasts which do not involve flooding, such as rising river levels which will stay within the river bank or minor ice jams that are not expected to cause significant impacts.
The River Recreation Statement informs river users of current and forecast river and lake conditions. These statements are especially useful for planning purposes.
A Quantitative Precipitation Forecast (QPF) is a precipitation forecast over a given area and period of time. It is incorporated into river forecasts to help predict future flood potential or river rises to other critical levels. The NWS Weather Prediction Center provides QPF national guidance.
The Extreme Precipitation Monitor compares the Quantitative Precipitation Forecast from the NWS Weather Prediction Center with a precipitation climatology based on historical data (from NOAA Atlas 14 and Atlas 2). This provides an estimate of how unusual that amount of rainfall would be.
Mesoscale Precipitation Discussions (MPD) depict areas where there is a threat of flash flooding from heavy rainfall events in the next 6 hours, with a graphic indicating the area of concern and significant meteorological features, a plain-language text summary, and a longer technical discussion.
A National Weather Service Public Information Statement has two purposes:
It may highlight a current or expected nonhazardous event of general interest to the public that can usually be covered with a single message (e.g., unusual atmospheric phenomena such as sun dogs, halos, rainbows, aurora borealis, lenticular clouds, and stories about a long-term dry/cold/wet/warm spell).
Public Information Statements are also used to highlight educational information and activities, such as storm safety rules, awareness activities, storm drills, etc. Additionally, information regarding service changes, service limitations, interruptions due to reduced or lost power or equipment outages, or special information clarifying interpretation of NWS data may also be announced using this product.
