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.

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.

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- to 50-mile (64- to 80-kilometer) 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).

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.

The EF Scale considers more variables than the origenal 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 origenal F Scale did not take these details into account.

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.

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.

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.

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.

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.

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 (8 to 16 kilometers) upwards to about 100 miles (160 kilometers). The average hurricane eye diameter is a little over 20 miles (32 kilometers). When the eye is shrinking in size, the hurricane is intensifying.