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Glossary: E's

Earthen (or Earthfill) Dam
An embankment dam in which more than 50% of the total volume is formed of compacted fine-grained material. A homogeneous earthen dam is constructed of similar earthen material throughout. These are the most common type of dam because their construction involves using materials in the natural state, requiring little processing.
Easterly Wave
A wavelike disturbance in the tropical easterly winds that usually moves from east to west. Such waves can grow into tropical depressions.
Eastern North Pacific Basin
The region north of the Equator east of 140°W. The National Hurricane Center in Miami, FL is responsible for tracking tropical cyclones in this region.
Ebb Current
A tidal current that is receding or declining.
Energy back scattered from a target (precipitation, clouds, etc.) and received by and displayed on a radar screen.
An acronym for European Centre for Medium-Range Weather Forecasting model. See European Model.
A small rotating area of water.
Effective Porosity
The ratio, usually expressed as a percentage, of the volume of water or other liquid which a given saturated volume of rock or soil will yield under any specified hydraulic condition, to the given volume of soil or rock.
Effective Precipitation (Rainfall)
  1. That part of the precipitation that produces runoff.
  2. A weighted average of current and antecedent precipitation that is "effective" in correlating with runoff.
  3. That part of the precipitation falling on an irrigated area that is effective in meeting the consumptive use requirements.
Effective Radar Reflectivity Factor
See Equivalent Radar Reflectivity Factor.
Effluent Seepage
Diffuse discharge of ground water to the ground surface.
Effluent Stream
Any watercourse in which all, or a portion of the water volume came from the Phreatic zone, or zone of saturation by way of groundwater flow, or baseflow.
An acronym for Energy Helicity Index. See Energy Helicity Index.
An acronym for Equilibrium Level.
Elevated Convection
A thunderstorm which occurs above a frontal inversion on the cold side of the surface cold front. Since these thunderstorms form above a very stable layer of atmosphere, surface based indices, such as the lifted index (LI), are useless in predicting their development. Severe weather is possible from elevated convection, but is less likely than it is with surface-based convection.
Elevation Angle
The vertical pointing angle of the antenna. The WSR-88D antenna can vary from -1° to +60°.
El Niño
The warm phase of the Southern Oscillation (SO). Characterized by the warming of the sea surface temperatures (SST) in the central and eastern equatorial Pacific Ocean, beginning at about Christmas time (hence the name "El Niño", which is Spanish for "Christ child"). This causes the sardine population to die off the Peru coast. The anomalously warm water also causes the deep convection to shift from its normal position near Indonesia to the east. This is also preceded and accompanied by anomalous westerly wind at low levels.

During the warm phase of the SO severe drought occurs over Indonesia and Australia. The warming of the ocean in the tropical Pacific increases the strength of the Hadley circulation (a global wind pattern) and causes the entire tropics to warm. The strengthened hemispheric north-south temperature gradient adds energy to the atmosphere. In particular, the subtropical jet is stronger and its maximum wind extends farther to the east than is normal.

This is often related to the deeper than normal Aleutian low, a split jet-level flow over the western U.S. and a trough in the southeastern U.S.. This pattern is called the "Pacific North American Teleconnection pattern". When established, it leads to warm, dry conditions over the northern U.S., particularly the Northwest, and to unusually wet conditions over the southern U.S.. The El Niño typically lasts from 12 to 18 months. See Southern Oscillation and ENSO.
El Niño Southern Oscillation (ENSO)
An acronym designed to stress the special importance of the warm phase (El Niño) of the Southern Oscillation. See El Niño and Southern Oscillation.
Fill material, usually earth or rock, placed with sloping sides and usually with length greater than height. All dams are types of embankments.
Emergency Action Plan
A predetermined plan of action to be taken to reduce the potential for property damage and loss of life in an area affected by a dam break or excessive spillway.
Emergency Managers Weather Information Network (EMWIN)
As an integral part of its mission, the NWS recognizes the need to provide the emergency management community with access to a set of NWS warnings, watches, forecasts, and other products at no recurring cost. Toward that end, the Emergency Managers Weather Information Network (EMWIN) system was developed. In partnership with the Federal Emergency Management Agency (FEMA) and other public and private organizations, EMWIN is now evolving into a fully operational and supported NWS service.

EMWIN is a suite of data access methods which make available a live stream of weather and other critical emergency information. Each method has unique advantages. EMWIN's present methods in use or under development for disseminating the basic datastream include:

  • Radio - Digital weather information is transmitted using inexpensive radio broadcast and personal computer (PC) technologies. The NWS (and other public and private agencies) transmits selected text, graphics, and imagery products as an audio signal on a dedicated VHF or UHF radio frequency. This information can be received, by anyone within the 40-50 mile broadcast area, using an inexpensive radio receiver, a demodulator, and a personal computer. EMWIN software on your PC, running under Windows, receives the signal through a serial port, stores the received weather products onto disk, and simultaneously allows you to display this information.

  • Internet - The Interactive Weather Information Network (IWIN) page uses HTML formatting and additional hyperlinks to an EMWIN server that ingests the data. Access to this data, as a linked series of clickable screens, is provided to clients operating web browsers such as Internet Explorer or Netscape. Graphics or text-only access is provided. FTP access is also available. The IWIN server has been online since September 1995.

  • Satellite - Satellite broadcast makes the datastream available nationwide, but not to provide detailed support (i.e. funding, manpower, or equipment) for state and local efforts to redistribute the datastream after downlink. The NWS broadcasts EMWIN on GOES 10 and GOES 12 satellites.

Note: The above methods are intended to provide data to users who currently have none or who can afford very little. Be aware that there are other methods available, at higher cost to the end-user, including various commercial weather distribution systems. EMWIN is a supplement to other NWS dissemination services, which include: NOAA Weather Radio (NWR), NOAA Weather Wire System (NWWS), Family of Services (FOS), NOAAPORT, and NEXRAD Information Dissemination Service (NIDS).
Energy Dissipater
A structure which slows fast-moving spillway flows in order to prevent erosion of the stream channel.
Energy Helicity Index (EHI)
An index that incorporates vertical shear and instability, designed for the purpose of forecasting supercell thunderstorms. It is related directly to storm-relative helicity in the lowest 2 km (SRH, in m2/s2) and CAPE (in j/kg) as follows: EHI = (CAPE x SRH)/160,000. Thus, higher values indicate unstable conditions and/or strong vertical shear. Since both parameters are important for severe weather development, higher values generally indicate a greater potential for severe weather. Values of 1 or more are said to indicate a heightened threat of tornadoes; values of 5 or more are rarely observed, and are said to indicate potential for violent tornadoes. However, there are no magic numbers or critical threshold values to confirm or predict the occurrence of tornadoes of a particular intensity.
Enhanced Fujita Scale (EF)
EF Scale Class Wind Speed Description
F5 Violent > 200 mph > 322 km/h Incredible
F4 Violent 166-200 mph 267-322 km/h Devastating
F3 Strong 136-165 mph 218-266 km/h Severe
F2 Strong 111-135 mph 178-217 km/h Significant
F1 Weak 86-110 mph 138-177 km/h Moderate
F0 Weak 65-85 mph 105-137 km/h Gale
The EF Scale takes into account more variables than the original F Scale did when assigning a wind speed rating to a tornado. The EF Scale incorporates 28 damage indicators (DIs) such as building type, structures, and trees. For each damage indicator, there are 8 degrees of damage (DOD) ranging from the beginning of visible damage to complete destruction of the damage indicator. The original F Scale did not take these details into account.
Enhanced V (Flying V)
A pattern seen on satellite infrared photographs of thunderstorms, in which a thunderstorm anvil exhibits a V-shaped region of colder cloud tops extending downwind from the thunderstorm core. The enhanced V indicates a very strong updraft, and therefore a higher potential for severe weather. Enhanced V should not be confused with V notch, which is a radar signature.
Enhanced Wording
An option used by the Storm Prediction Center (SPC) in Norman, Oklahoma in tornado and severe thunderstorm watches when the potential for strong/violent tornadoes, or unusually widespread damaging straight-line winds, is high. The statement "THIS IS A PARTICULARLY DANGEROUS SITUATION WITH THE POSSIBILITY OF VERY DAMAGING TORNADOES" appears in tornado watches with enhanced wording. Severe thunderstorm watches may include the statement "THIS IS A PARTICULARLY DANGEROUS SITUATION WITH THE POSSIBILITY OF EXTREMELY DAMAGING WINDS," usually when a derecho event is occurring or forecast to occur. See PDS Watch.
Entrance Region
The region upstream from a wind speed maximum in a jet stream (jet max), in which air is approaching (entering) the region of maximum winds, and therefore is accelerating. This acceleration results in a vertical circulation that creates divergence in the upper-level winds in the right half of the entrance region (as would be viewed looking along the direction of flow). This divergence results in upward motion of air in the right rear quadrant (or right entrance region) of the jet max.
Environmental Modeling Center (EMC)
This is one of 9 centers that comprises the National Centers for Environmental Prediction (NCEP). This center improves numerical weather, marine and climate predictions, through a broad program of research in data assimilation and modeling. In support of the NCEP operational forecasting mission, the EMC develops, improves and monitors data assimilation systems and models of the atmosphere, ocean and coupled system, using advanced methods developed internally as well as cooperatively with scientists from Universities, NOAA Laboratories and other government agencies, and the international scientific community.
The fear of dawn or daylight.
Equilibrium Drawdown
The ultimate, constant drawdown for a steady rate of pumped discharge.
Equilibrium Level (EL)
It is the height in the upper troposphere where a parcel of saturated air, rising because of its positive buoyancy, becomes equal in temperature to the surrounding environment. Beyond this point, the parcel become colder than its environment. As a result, it will be heavier than the surrounding air and it will begin to fall. Under the right conditions, severe thunderstorm tops can overshoot the EL by a considerable distance without reaching the tropopause. Conversely, non-severe thunderstorm tops can rise above the tropopause without overshooting the EL. Consequently, the EL provides more meaningful information than the tropopause for evaluating the strength of convective updrafts.
Wearing away of the lands by running water, glaciers, winds, and waves, can be subdivided into three process: Corrasion, Corrosion, and Transportation. Weathering, although sometimes included here, is a distant process which does not imply removal of any material.
The thin zone along a coastline where freshwater systems and rivers meet and mix with a salty ocean (such as a bay, mouth of a river, salt marsh, lagoon).
Esturine waters
Deepwater tidal habitats and tidal wetlands that are usually enclosed by land but have access to the ocean and are at least occasionally diluted by freshwater runoff from the land (such as bays, mouths of rivers, salt marshes, lagoons).
Esturine Zone
The area near the coastline that consists of estuaries and coastal saltwater wetlands.
ETA Model
One of the operational numerical forecast models run at NCEP. The ETA is run twice daily, with forecast output out to 48 hours. The ETA model is a newer model, which is very similar to the NGM model and forecasts the same atmospheric variables. The main difference is the ETA model has better resolution. This allows the ETA to give a more detailed forecast over the USA. The ETA model was named after the ETA coordinate system, which is a mathematical coordinate system that takes into account topographical features such as mountains. As a result of using this coordinate system and the higher resolution, the ETA model has a much more accurate picture of the terrain.
European Model
One of medium-range (3 to 7 days) forecast models that forecasters use to write their extended forecasts. It has a resolution of 75 kilometers and covers the entire northern hemisphere. This model comes from European Centre for Medium-Range Weather Forecasts (ECMWF) which is an international organization supported by 18 European Member States. The Centre has three working languages - English, French and German. See GFS and UKMET.
See VAD.
A process by which liquid changes into a gas or vapor.
Evaporation Pan
A pan used to hold water during observations for the determination of the quantity of evaporation at a given location. Such pans are of varying sizes and shapes, the most commonly used being circular or square.
Evaporation Rate
The quantity of water, expressed in terms of depth of liquid water, which is evaporated from a given surface per unit of time. It is usually expressed in inches depth, per day, month, or year.
An instrument which measures the evaporation rate of water into the atmosphere.
Combination of evaporation from free water surfaces and transpiration of water from plant surfaces to the atmosphere.
Excessive Heat Warning
This product is issued by the National Weather Service when excessive heat is life threatening. The criteria for this warning varies from state to state.
Excess Rain
Effective rainfall in excess of infiltration capacity.
Excessive Rainfall Discussion (ERD)
This message discusses the potential for excessive rainfall in the contiguous United States. This includes mentioning the areas where rainfall is forecast to be locally heavy, approach or exceed flash flood guidance, or exceed 5". This product includes evaluation of initial conditions and short-term numerical model forecasts and analysis of radar and satellite data.
Exclusive Flood Control Storage Capacity
The space in a reservoir reserved for the sole purpose of regulating flood inflows to abate flood damage.
Exit Region
The region downstream from a wind speed maximum in a jet stream (jet max), in which air is moving away from the region of maximum winds, and therefore is decelerating. This deceleration results in divergence in the upper-level winds in the left half of the exit region (as would be viewed looking along the direction of flow). This divergence results in upward motion of air in the left front quadrant (or left exit region) of the jet max. Severe weather potential sometimes increases in this area as a result. See also entrance region and right entrance region.
Explosive Deepening
A decrease in the minimum sea-level pressure of a tropical cyclone of 2.5 mb/hr for at least 12 hours or 5 mb/hr for at least six hours.
Extended Forecast Discussion (EPD)
This discussion is issued once a day around 2 PM EST (3 PM EDT) and is primarily intended to provide insight into guidance forecasts for the 3- to 5-day forecast period. The geographic focus of this discussion is on the United States (including Alaska and Hawaii).
A term used in advisories and tropical summaries to indicate that a tropical cyclone has lost its "tropical" characteristics. The term implies both poleward displacement of the cyclone and the conversion of the cyclone's primary energy source from the release of latent heat of condensation to baroclinic (the temperature contrast between warm and cold air masses) processes. It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force.
The relatively calm center in a hurricane that is more than one half surrounded by wall cloud. The winds are light, the skies are partly cloudy or even clear (the skies are usually free of rain) and radar depicts it as an echo-free area within the eye wall. The hurricane eye typically forms when the maximum sustained tangential wind speeds exceeds about 78 miles an hour. The eye diameter, as depicted by radar, ranges typically from as small as 5 to 10 miles upwards to about 100 miles. The average hurricane eye diameter is a little over 20 miles. When the eye is shrinking in size, the hurricane is intensifying.
Eye Wall
It is an organized band of cumuliform clouds that immediately surrounds the center (eye) of a hurricane. The fiercest winds and most intense rainfall typically occur near the eye wall. VIP levels 3 or greater are typical. Eye wall and wall cloud are used synonymously, but it should not be confused with a wall cloud of thunderstorm.


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