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National Weather Service Glossary

This glossary contains information on more than 2000 terms, phrases and abbreviations used by the NWS. Many of these terms and abbreviations are used by NWS forecasters to communicate between each other and have been in use for many years and before many NWS products were directly available to the public. It is the purpose of this glossary to aid the general public in better understanding NWS products.

You can either type in the word you are looking for in the box below or browse by letter.

Here are the results that start with a number
1-2-3 Rule
A means of avoiding winds associated with a tropical cyclone by taking into account the forecast track error of the National Weather Service over a 10 year period which is approximately 100 nm in 24 hours, 200 nm for 48 hours and 300 nm in 72 hours. The forecast track error is added to the 34 knot wind radii to compute the danger area. The wind radii may be found within Tropical Cyclone Forecast Advisory (TCM) forecasts.
100-year Flood
A statistic that indicates the magnitude of flood which can be expected to occur on average with a frequency of once every 100 years at a given point or reach on a river. The 100-year flood is usually developed from a statistical distribution that is based on historical floods. This is also called a base flood.
100-year Flood Plain
The flood plain that would be inundated in the event of a 100-year flood.
500 hPa
Pressure surface (geopotential height) in the troposphere equivalent to about 18,000 feet above sea level. Level of the atmosphere at which half the mass of the atmosphere lies above and half below, as measured in pressure units. This area is important for understanding surface weather, upper air storms tend to be steered in the direction of the winds at this level and are highly correlated with surface weather.
500 mb
Pressure surface (geopotential height) in the troposphere equivalent to about 18,000 feet above sea level. Level of the atmosphere at which half the mass of the atmosphere lies above and half below, as measured in pressure units. This area is important for understanding surface weather, upper air storms tend to be steered in the direction of the winds at this level and are highly correlated with surface weather.
88D
Doppler Radar currently used nationwide by the National Weather Service.