NOAA's National Weather Service Weather Forecast Office

 
E
 
E-3, Flood Stage Report:  A form that a Service Hydrologist/Hydrology Focal Point completes to document the dates in which forecast points are above flood stage, as well as the crest dates and stages. Discussion of the flood event must also be included in the E-5, Monthly Report of River and Flood conditions. An E-3 report is sent to Regional Headquarters, the appropriate RFC, as well as the Office of Hydrology (OH).
 
E-5, Monthly Report of River and Flood conditions:  A monthly narrative report covering flooding which occurred over the past  month. Flood stage, flood crest and dates  in which flooding occurred is covered within this report for each data point which was in flood. If the flooding involved a forecast point, an E-3 must be filled out as well. If no flooding has occurred within the past month, a climatic summary of the past month can be included as well as other interesting non-flood events, such as water supply, ice jams and the occurrence of drought. An E-5 report is sent to Regional Headquarters, the appropriate RFC, as well as the Office of Hydrology (OH).
 
E-7, Flood Damage Report:  A report to be completed anytime there is reported flood damage or loss of life as a direct result of flooding. An E-7 report is sent to Regional Headquarters, as well as the Office of Hydrology (OH).
 
E-19, Report on River Gage Station: A report to be completed every 5 years providing a complete history of a river station and all gages that have been used for public forecasts since the establishment of the station.
 
E-19a, Abridged Report on River Gage Station:  An abridged version of an E-19, an E-19a updates the E-19 as additional information, or changes occur at the station during the intervening five year period. An E-19a is to be completed anytime a significant change occurs at a forecast point. An E-19a is also used to take the place of an E-19 in documenting any gage history, or information of any non-forecast point (i.e; data point).

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.
 
Eastern North Pacific Basin:  The region north of the Equator east of 140W. 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.

Echo: Energy back scattered from a target (precipitation, clouds, etc.) and received by and displayed on a radar screen.

Echo Tops (ET): This WSR-88D radar product displays echo top heights (thousands of feet) based on the highest elevation angle at which greater than or equal to 18 dBZ reflectivities are determined.  The heights are referenced to Mean Sea Level (MSL).  A circular stair-step appearance often occurs due to echo beam top limitations.  It is used to gain a quick estimation of the most intense convection (highest tops); detect mid-level echoes before low level echoes are detected; and assist in differentiating non-precipitation echoes from real storms.

Echo Tops Contour (ETC):  This WSR-88D radar product displays a line contour image of echo tops data.  The contour interval and base contour value are selected at the WSR-88D's Principle User Processor (PUP).  It is used to view a contour image of echo tops; gain a quick estimation of the most intense convection (highest tops); detect mid-level echoes before low level echoes are detected; and assist in differentiating non-precipitation echoes from real storms.

ECMWF:  An acronym for European Centre for Medium-Range Weather Forecasting model.  See European Model.

Eddy:  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.

EHI:  An acronym for Energy Helicity Index.  See Energy Helicity Index.

EIF:  Enhanced IFLOWS Format.

EL:  An acronym for Equilibrium Level.  See 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^o to +60^o.

El Nino: 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 Nino", 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. The westerly anomalies cause the development of a Kelvin wave in the ocean which slowly propagates eastward. 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 Nino typically lasts from 12 to 18 months. See Southern Oscillation and ENSO.

El Nino Southern Oscillation (ENSO): An acronym designed to stress the special importance of the warm phase (El Nino) of the Southern Oscillation. See El Nino and Southern Oscillation.

Embankment:  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 Services:  Services provided in order to minimize the impact of a flood that is already happening. These measures are the responsibility of city, or county emergency management staff and the owners or operators of major, or critical facilities. Some examples of emergency services are flood warning and evacuation, flood response, and post flood activities.

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.  While Internet access is convenient, there are times, especially during major weather events, that access may be difficult or impossible due to server overloads.  The IWIN server has been online since September 1995, handling an average load of 1 million connections per day with peak loads of over 2 million connections per day during major weather events.

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 our own GOES 8 and GOES 10 satellites. GOES 8 is at 75 degrees West, elevation 45 degrees (from the latitude of Washington, DC). GOES 10 is at 135 degrees West. Data is uplinked to satellite from the NOAA Command and Data Acquisition (CDA) Station on Wallops Island, VA.  The NWS GOES downlink frequency used for the 9600 baud EMWIN datastream is 1690.725 MHz, 275 KHz lower than the standard WEFAX 1691.0 MHz signal. The signal is passed through a down convertor, received as if a Radio signal at 137.225 MHz for example, and then demodulated to 9600 baud.  The EMWIN data is also currently uplinked to the SBS 6 satellite by Spacecom Systems of Tulsa, OK as a public service. SBS 6 is at 74 degrees W: Ku-band, Transponder 13, FM-FM, DFSK, .5425 MHz subcarrier.

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.

Engineer's Level:  A telescope which is attached to a spirit-tube level, all revolving around a vertical axis and is mounted on a tripod. An Engineer's Level is used for determining the difference in elevation between two points. The telescope on the level has a vertical cross hair and a horizontal cross hair. Once the instrument is leveled, the sighting through the horizontal cross hair represent a horizontal plane of equal elevation.
 
Enhanced 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.

Engineer's Level:  A telescope which is attached to a spirit-tube level, all revolving around a vertical axis and is mounted on a tripod. An Engineer's Level is used for determining the difference in elevation between two points. The telescope on the level has a vertical cross hair and a horizontal cross hair. Once the instrument is leveled, the sighting through the horizontal cross hair represent a horizontal plane of equal elevation.
 
Ensemble Hydrologic Forecasting:  A process whereby a continuous hydrologic model is successively executed several times for the same forecast period by use of varied data input scenarios, or a perturbation of a key variable state for each model run. A common method employed to obtain a varied data input scenario is to use the historical meteorological record, with the assumption that several years of observed data covering the time period beginning on the current date and extending through the forecast period comprises a reasonable estimate of the possible range of future conditions.

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. Severe weather potential sometimes increases in this area as a result. See also exit region and left exit region.

Environmental Modeling Center (EMC, formerly the Development Division):  This is one of 9 centers that comprises the National Centers for Environmental Prediction (NCEP, formerly the National Meteorological Center).  This center improves numerical weather, marine and climate predictions at the , 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.

Eosophobia:  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.

Equilibrium Surface Discharge:  The steady rate of surface discharge which results from a long-continued, steady rate of net rainfall, with discharge rate equal to net rainfall rate. 

Equilibrium Time:  The time when flow conditions become substantially equal to those corresponding to equilibrium discharge or equilibrium drawdown..
 
Equi-Potential Line:  A line, in a field of flow, such that the total head is the same for all points on the line, and therefore the direction of flow is perpendicular to the line at all points.

Equivalent Radar Reflectivity (Ze): The concentration of uniformly distributed small (diameter 1/16 wavelength or less) water particles which would return the amount of power received. Typically expressed as: dBZ = 10 Log Ze.

Erosion:  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.

ESP:  Extended Streamflow Prediction.
 
ESPINIT:  ESP Initialization Program

Estimate:  A statement of the value of a quantity or function based on a finite number of samples.

Estuary:  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 (29 kilometers). In other words, the grid box is much smaller than in the NGM. 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 across the USA.  By the way, the name, ETA is the seventh letter of the Greek alphabet.

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.  References to it can be found in NMC (National Meteorological Center) and area forecast discussions.  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 MRF and UKMET.

EVAD:  See VAD.

Evaporation: 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.

Evaporimeter:  An instrument which measures the evaporation rate of water into the atmosphere.

Evapotranspiration:  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.  In Michigan, the criteria is a heat index of 115 degree F or higher for a period of 3 hours or more.

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 until 7 AM EST (8 AM EDT) the next day.  This includes mentioning the areas where rainfall is forecast to be locally heavy, approach or exceed flash flood guidance, or exceed 5 inches.  This product includes evaluation of initial conditions and short-term numerical model forecasts and analysis of radar and satellite data.  This product is issued 3 times a day as described below.  There is an accompanying graphic for each forecast under the AFOS identifier 94E.

1) Around 2 AM EST (3 AM EDT), a narrative is issued which describes and explains from 7 AM EST (8 AM EDT) until 7 AM EST (8 AM EDT) the next day.  Although this forecast is largely
    based on interpretation of the numerical model circulation are also considered.  This narrative may also address the potential for excessive rainfall from 2 AM EST (3 AM EDT) to 7 AM EST
   (8 AM EDT) the same day as appropriate.

2) About 9 AM EST (10 AM EDT), the previous ERD is revised to account for analysis of the 7 AM EST (8 AM EDT) upper air data as well as later information from satellite and radar.
    This revision is valid from 10:00 AM EST (11:00 AM EDT) until 7 AM EST (8 AM EDT) the next day.

3) Around 2 AM EST (3:00 AM EDT), the ERD is revised based on the latest numerical model guidance and observations from satellite, radar, and surface reports.  This product is valid from
    4 PM EST (5 PM EDT) until 7 AM EST (8 AM EDT) next day.

Unscheduled special updates (NFDQPFSRD) may be issued as needed, particularly around 9 PM EST (10 PM EDT), following careful and detailed analysis and evaluation of the 7 PM EST (8 PM EDT) upper air data.  This forecast would generally be valid for the period from 10 PM EST (11 PM EDT) to 7 AM EST (8 AM EDT).
 
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).  Although portions of this narrative will parallel the Hemispheric Map Discussion, a much greater effort is made to routinely relate the model forecasts and necessary modifications to weather forecasts, mainly in terms of temperature and precipitation.  Other significant parameters, such as wind, may be discussed when deemed reliably predictable.  This discussion serves as primary guidance to local National Weather Service offices for the preparation of their extended forecasts in the State Forecast Product and some Zone Forecast Products.

Extratropical: A term used in advisories and tropical summaries to indicate that a 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.

Eye: 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.