Weather Glossary: F's

Since 1983, the National Weather Service (NWS) has provided external user access to U.S. Government obtained or derived weather information through a collection of data communication line services called the Family of Services (FOS). All FOS data services are driven by the NWS Telecommunication Gateway computer systems located at NWS headquarters in Silver Spring, MD.

Users may obtain any of the individual services from NWS for a one-time connection charge and an annual user fee. Several private companies subscribe to the FOS and then resell the data as received and/or provide value-added information services for their customers.

1. An area from which waves are generated by a wind that is nearly constant in direction and speed.
2. The effective distance which waves have traveled in open water, from their point of origin to the point where they break.
3. The distance of the water or the homogenous type surface over which the wind blows without appreciable change in direction.

1. An official sky cover classification for aviation weather observations, descriptive of a sky cover of 1/8 to 2/8. This is applied only when obscuring phenomenon aloft are present--that is, not when obscuring phenomenon are surface-based, such as fog.
2. A National Weather Service convective precipitation descriptor for a 10% chance of measurable precipitation (0.01" / 0.25 mm). Few is used interchangeably with isolated. See Precipitation Probability (PoP).

1. a sustained wind average 15 mph (24 km/h) or greater,
2. relative humidity less than or equal to 25%, and
3. a temperature of greater than 75°F (24°C).

This image will once in a while appear on the WSR-88D reflectivity product. It shows up as a straight line along the radial pointing away from a very high reflectivity (usually over 60 dBZ) core of a thunderstorm and the radar site itself.

This occurs as a result of the radar beam hitting a hail shaft (usually containing hail of greater than 1" in diameter). The hail shaft causes the radar beam to be reflected to the ground. When the radar beam hits the wet ground, it is reflected back up into the hail shaft and eventually arrives at the radar site.

When it arrives back at the radar site, it assumes that the radar beam went out further than it actually did and it plots it along the radial beyond the high reflectivity core. This image tells that radar operator that the thunderstorm is likely producing hail of greater than 1" in diameter.

This product is issued after either a Flash Flood Watch or a Flash Flood Warning has been issued by a local National Weather Service Forecast Office. It will provide the latest information on the flash flooding situation or event.

It will also be used to remove parts of the geographical area covered by the original watch or warning when the flash flooding event is no longer a threat or has ended in a certain area. Finally, this statement can be used to terminate the original watch or warning when it is no longer valid. This is usually optional when either a watch or warning expires.

This warning signifies a short duration of intense flooding of counties, communities, streams, or urban areas with high peak rate of flow. Flash floods may result from such things as torrential downpours, dam breaks, or ice jam breaks. They are issued by the local National Weather Service Office for 4 hours or less.

Since flash flooding can occur in severe thunderstorms, this type of warning can be combined with either a Tornado Warning or a Severe Thunderstorm Warning.

This product is issued by the local National Weather Service office (NWFO) for events that have the potential for short duration (usually less than 6 hours) intense flooding of counties, communities, streams or areas for which the occurrence is neither certain nor imminent.

This watch indicates that flash flooding is a possibility in or close to the watch area. Those in the affected area are urged to be ready to act if a Flash Flood Warning is issued or flooding is observed. A Flash Flood Watch may be issued for potential flooding from either dam breaks, ice jam breaks, or torrential downpours.

1. A graph showing the number of times per year on the average, plotted as abscissa, that floods of magnitude, indicated by the ordinate, are equaled or exceeded.
2. A similar graph but with recurrence intervals of floods plotted as abscissa.

This is a long range (36-72 hours) outlook issued by a local National Weather Service Office when forecast meteorological conditions indicate that a significant heavy rainfall event may occur that would either cause flooding or aggravate an existing flooding situation.

It is issued when successive numerical guidance model runs indicate that synoptic conditions are favorable for flooding; however, either uncertainty or the time frame precludes the forecaster from issuing either a Flood Watch or Flash Flood Watch.

This product is issued after either a Flash Flood Watch or a Flash Flood Warning has been issued by a local National Weather Service Forecast Office. It will provide the latest information on the flash flooding situation or event.

It will also be used to remove parts of the geographical area covered by the original watch or warning when the flash flooding event is no longer a threat or has ended in a certain area. Finally, this statement can be used to terminate the original watch or warning when it is no longer valid. This is usually optional when either a watch or warning expires.

This watch is issued by a local National Weather Service Office to indicate that there ia a potential of flooding in or close to the watch area. Those in the affected area are urged to be ready to act if a flood warning is issued or flooding is observed.

In flooding, the onset of flooding take place much slower (usually greater than six hours) than a flash flood. This type of flooding usually occurs with "train echoes" or slow moving thunderstorms, and can also occur with synoptic scale systems that last a relatively long period of time and encompass a large area. They are usually issued up to 12 hours prior to the possible flood event. These watches can vary in size depending on the size of the meteorological event.

The process of protecting a building from flood damage on site. Floodproofing can be divided into wet and dry floodproofing. In areas subject to slow-moving, shallow flooding, buildings can be elevated, or barriers can be constructed to block the water's approach to the building.

These techniques have the advantage of being less disruptive to the neighborhood. It must be noted that during a flood, a floodproofed building may be isolated and without utilities and therefore unusable, even though it has not been damaged.

• A part of the flood plain, otherwise levied, reserved for emergency diversion of water during floods. A part of the flood plain which, to facilitate the passage of floodwater, is kept clear of encumbrances.
• The channel of a river or stream and those parts of the flood plains adjoining the channel, which are reasonably required to carry and discharge the floodwater or floodflow of any river or stream.

A visible aggregate of minute water particle (droplets) which are based at the Earth's surface and reduces horizontal visibility to less than 5/8 statue mile (1 km), and unlike drizzle, it does not fall to the ground. It occurs most frequently in coastal regions because of the great water vapor content of the air.

However, it can occur anywhere. The rapidity with which fog can form makes it especially hazardous. It forms by any atmospheric process that does one of the following:

• Cools the air to its dew point, or
• Raises the dew point to the air temperature.

Forecasters use numerical weather models to make their forecasts. These numerical models are classified into four main classes. The first is global models, which focus on the entire northern hemisphere. The second is national models, which focus on the USA. The third is regional models. These regional models have a finer grid than national models and are run out for smaller periods of time.

The final class of models is relocatable models, which do not focus on any permanent geographical location. Relocatable models are very limited on the size of the geographical area for which they can forecast, but these models have very high resolutions, or very small forecast grid boxes.

It is when the surface air temperature is expected to be 32°F (0°C) or below over a widespread area for a climatologically significant period of time. Use of the term is usually restricted to advective situations or to occasions when wind or other conditions prevent frost.

Adjectives such as "killing", "severe", or "hard" will be used when appropriate. "Killing" may be used during the growing season when the temperature is expected to be low enough for a sufficient duration to kill all but the hardiest herbaceous crops or plants.

This occurs when rain or drizzle freezes on surfaces (such as the ground, trees, power lines, motor vehicles, streets, highways, etc.) that have a temperature of 32°F (0°C) or below.

Small accumulations of ice can cause driving and walking difficulties. Meanwhile, heavy accumulations of ice can pull down trees and utility lines. In this situation, it would be called an Ice Storm.

A transition zone between two different air masses. The temperature curve is the basic reference for locating frontal zones aloft. If the front were a sharp discontinuity, the temperature curve should show a clear-cut inversion separating the lapse rates typical of cold and warm air masses.

However, a shallow isothermal or relatively stable layer is more usual indication of a well-defined front. Frequently, the frontal boundary is so weak or distorted by other discontinuities that frontal identification is difficult.

In an ideal case, the dew point curve through the frontal zone will show an inversion or sharp change associated with that of the temperature curve. The frontal surface is considered to be located at the top of the inversion.

The formation of ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans. Frost develops under conditions similar to dew, except the temperatures of the Earth's surface and earthbound objects fall below 32°F (0°C).

As with the term "freeze", this condition is primarily significant during the growing season. If a frost period is sufficiently severe to end the growing season or delay its beginning, it is commonly referred to as a "killing frost".

Because frost is primarily an event that occurs as the result of radiational cooling, it frequently occurs with a temperature reading in the mid 30'sF (0-2°C). Frost may also form on aircraft in flight during descent from subfreezing into a warmer, moist layer below. This may cause considerable consternation if the windshield glazes over or the windows frost over.

• It is in contact with the ground, or
• A debris cloud or dust whirl is visible beneath it.