Nacreous Clouds: Clouds of unknown composition that have a soft, pearly luster and that form at altitudes about 25 to 30 km above the Earth's surface. They are also called "mother-of-the-pearl clouds".

Narrowband Communications:  This is the narrowband (9600 Baud Rate) communication lines that distribute the WSR-88D radar products to the various users.

National Fire Danger Rating System (NFDRS) A system that directly integrates the effects of fuels, topography, and weather into components that deal with fire occurrence and fire behavior potential.  The system uses the components to derive indices that indicate the number of fires, difficulty of containment, and finally, the total fire control job in a rating area.  The system is intended to provide guidance for short-range planning by evaluating the near upper limits of the behavior of fires that might occur in an area during the rating period.  It is not designed to serve as a direct fire behavior forecast.  NFDRS computations are based on once-daily, mid-afternoon observations (2 p.m. LST) from the Fire Weather Network which is comprised of some 1500 weather stations throughout the Conterminous United States and Alaska. These observations are reported to the Weather Information Management System (WIMS) where they are processed by NFDRS algorithms. Many of the stations are seasonal and do not report during the off season. WFAS queries WIMS each afternoon and generates maps from the day's weather observations. Each afternoon Fire Weather Forecasters from the National Weather Service also view these local observations and issue trend forecasts for fire weather forecast zones. WIMS processes these forecasts into next-day index forecasts.  On the maps, reporting station locations are indicated with a marker. Values between stations are estimated with an inverse distance-squared technique on a 10-km grid. This works pretty well in areas of relatively high station density, but has obvious shortcomings in other areas. Station location is based on the latitude/longitude cataloged by local station managers in WIMS.

National Flood Summary (FLN):  This NWS daily product contains nationwide information on current flood conditions.  It is issued by the Hydrometeorological Information Center of the Office of Hydrology.

National Climatic Data Center (NCDC)This Center is part of the Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), and the National Environmental Satellite, Data and Information Service (NESDIS).  The Center is responsible for archiving and distributing climate data.  In November 1951, the Weather Bureau, Air Force and Navy Tabulation Units in New Orleans, LA were combined and formed into the National Weather Records Center in Asheville, NC.  Authority to establish the joint Weather Records Center was granted under section 506(c) of the Federal Records Act of 1950 (Public Law 754, 81st Congress). The Center was eventually renamed the National Climatic Data Center.  The climate data that NCDC receives are from a wide variety of sources, including satellites, radar, remote sensing systems, NWS cooperative observers, aircraft, ships, radiosonde, wind profiler, rocketsonde, solar radiation networks, NWS Forecast/Warnings/Analyses Products, Military Services, Federal Aviation Administration, and the Coast Guard.  NCDC archives 99 percent of all NOAA data, including over 320 million paper records; 2.5 million microfiche records; over 500,000 tape cartridges/magnetic tapes, and has satellite weather images back to 1960.  As operator of the World Data Center-A for Meteorology, which provides for international data exchange, NCDC also collects data from around the globe. The Center has more than 150 years of data on hand with 55 gigabytes of new information added each day--that is equivalent to 18 million pages a day.  NCDC supports many forms of data and information dissemination such as paper copies of original records, publications, atlases, computer printouts, microfiche, microfilm, movie loops, photographs, magnetic tape, floppy disks, CD-ROM, electronic mail, on-line dial-up, telephone, facsimile and personal visit.  The National Archives and Records Administration has designated NCDC as the Commerce Department's only Agency Records Center.  The Center, which produces numerous climate publications and responds to requests from all over the world, provides historical perspectives on climate which are vital to studies on global climate change, the greenhouse effect, and other environmental issues. The Center stores information essential to industry, agriculture, science, agriculture, hydrology, transportation, recreation, and engineering. This information can mean tens of millions of dollars to concerned parties.  NCDC annually publishes over 1.2 million copies of climate publications that are sent to individual users and 33,000 subscribers. NCDC maintains over 500 digital data sets to respond to over 170,000 requests each year.   

National Hurricane Center (NHC):  One of three branches of the Tropical Prediction Center (TPC).  This center maintains a continuous watch on tropical cyclones over the Atlantic, Caribbean, Gulf of Mexico, and the Eastern Pacific from 15 May through November 30. The Center prepares and distributes hurricane watches and warnings for the general public, and also prepares and distributes marine and military advisories for other users. During the "off-season" NHC provides training for U.S. emergency managers and representatives from many other countries that are affected by tropical cyclones. NHC also conducts applied research to evaluate and improve hurricane forecasting techniques, and is involved in public awareness programs.

National Weather and Crop Summary:  A product of the National Agricultural Statistics Service, Agricultural Statistics Board, and U.S. Department of Agriculture.  It contains weekly national agricultural weather summaries, including the weather's effect on crops; summaries and farm progress for 44 states and New England area.

Natural Control:  A stream gaging control which is natural to the stream channel, in contrast to an artificial control structure by man.

Navigation Methods:  Three basic methods of providing and managing inland waterways - 1) Run-of-the-River: no provision of upstream storage; 2) Slack-Water: locks and dams provide slack water or pools with adequate depth for the draft of heavy barges and area to prevent excessive velocities; 3) Canalization: in lieu of a series of dams on the river a canal with locks adjoins the river.

Navigational Telex (Navtex)One part of Global Maritime Distress and Safety System (GMDSS) for automatically disseminating safety information, including weather warnings and forecasts, in text form via medium frequency radio to mariners within 200 nautical miles of shore.

Nautical Mile: A unit of distance used in marine navigation and marine forecasts. It is equal to 1.15 statue miles. It is also the approximate length of 1 minute of latitude.

NCCF:  An acronym for the NOAA Central Computer Facility.

NCEP:  An acronym for the National Centers for Environmental Prediction.  The National Oceanic and Atmospheric Administration created the National Centers for Environmental Prediction (NCEP) to take advantage of improving technology and better serve the public and modernized National Weather Service. The NCEP's goal is to protect life and property, as well as mitigate economic loss, by providing accurate forecasts and forecast guidance products to weather service field offices.

The NCEP prepares and makes available national forecasts and outlooks of weather and climate. Meteorologists currently
generate weather forecasts for three to five days, soon to extend to seven days. Climate predictions are made for two weeks
out to a year.

Nine national centers comprise the NCEP:

     Aviation Weather Center
     Climate Prediction Center
     Environmental Modeling Center
     Hydrometeorological Prediction Center
     Marine Prediction Center
     NCEP Central Operations
     Space Environmental Center
     Storm Prediction Center
     Tropical Prediction Center

The NCEP was established in 1958 as the National Meteorological Center (NMC). Since the center's beginning, operational weather forecasting has transformed from an infant discipline into a mature science.

Neap Tide: A minimum tide occurring at the first and third quarters of the moon.

Nearshore Lake Forecast (NSH):  This National Weather Service Marine forecast is issued every 6 hours and they will usually cover a 36 hour period.  The term nearshore refers to The Great Lakes waters within 5 nautical miles of shore.  Since The Great Lakes have a large shoreline, these forecasts are broken up into numerous segments and they are issued by several National Weather Service offices in Minnesota, Michigan, Wisconsin, Illinois, Ohio, and New York.

Nearshore Waters: The waters of The Great Lakes extending out to five miles from shore.

NebulaphobiaThe fear of fog.  See Homichlophobia.

Negative-Tilt Trough:  An upper level system which is tilted to the west with increasing latitude (i.e., with an axis from southeast to northwest). A negative-tilt trough often is a sign of a developing or intensifying system.

Negative Vorticity Advection (NVA): A region of negative vorticity usually several hundred of kilometers wide on a upper level chart that moves with the general wind flow. It aids in weather prediction by showing where regions of sinking air. This is typically associated with clear skies.

Nephophobia:  The fear of clouds.

NESDIS (National Environmental Satellite, Data, and Information Service):  NESDIS collects, processes, stores, analyzes, and disseminates various types of hydrologic, meteorologic, and oceanic data. NESDIS is also responsible for the development of analytical and descriptive products so as to meet the needs of it’s users.

Net Rainfall:  The portion of rainfall which reaches a stream channel or the concentration point as direct surface flow.
NEXRAD:  An acronym that stands for NEXt generation of weather RADar.  It is a technologically-advanced weather radar deployed to replace WSR-57 and WSR-74 units. NEXRAD is a high-resolution Doppler radar with increased emphasis on automation, including use of algorithms and automated volume scans. NEXRAD units are known as WSR-88D.

NEXRAD Base Data:  Those digital fields of reflectivity, mean radial velocity and spectrum width data in spherical coordinates provided at the finest resolution available.

NGM:  Nested Grid Model; one of the operational forecast models run at NCEP. The NGM is run twice daily, with forecast output out to 48 hours. The NGM is a short range model that forecasts variables such as temperature at various levels of the atmosphere, amount of precipitation, position of upper level troughs and ridges, and the position of surface high and low pressure areas.  In the nested grid model and others like it, the atmosphere is divided into squares, or a grid, for various levels of the atmosphere.  Grids with smaller squares are "nested" inside larger ones for areas forecasters are particularly interested in, such as North America.  The smaller the grids, the more calculations that have to be made and the bigger the computer needed. The resolution of the NGM is about 80 kilometers.  The NGM produces forecasts every 6 hours from 0 hours to 48 hours into the future.

Nimbostratus (Ns): A dark, gray cloud characterized by more or less continuously falling precipitation. It is not accompanied by lightning, thunder, or hail. They normally occur between 6,500 and 23,000 feet above the ground.

NMC National Meteorological Center, with headquarters near Washington D.C.; now known as NCEP.

NOAA:  An acronym for National Oceanographic and Atmospheric Administration.  NOAA warns of dangerous weather, charts our seas and skies, guides our use and protection of ocean and coastal resources, and conducts research to improve our understanding and stewardship of the environment which sustains us all.  NOAA is an organization of the Department of Commerce.  NOAA is composed of the National Ocean Service, National Weather Service, National Marine Fisheries Service, National Environmental Satellite Data, and nformation Service, and Office of Oceanic and Atmospheric Research.

NOAAPORT Broadcast System:  This provides a one-way broadcast communication of NOAA environmental data and information in near-real time to NOAA and external users. This broadcast service is implemented by a commercial provider of satellite communications utilizing C-band.  Weather data is collected by GOES satellite enviromental sensors and NWS observing systems, and processed to create products. The products are fed to the AWIPS Network Control Facility (NCF) which routes the products to the appropriate NOAAPORT channel for uplink and broadcast.

NOAA Weather RadioIt is the voice of the National Weather Service.  NOAA Weather Radio broadcasts National Weather Service warnings, watches, forecasts and other hazard information 24 hours a day.  It is provided as a public service by the Department of Commerce's National Oceanic and Atmospheric Administration.  The NOAA Weather Radio network has more than 480 stations in the 50 states and near adjacent coastal waters, Puerto Rico, the U.S. Virgin Islands and U.S. Pacific Territories.

NOAA Weather Wire Service (NWWS)The NOAA Weather Wire Service is the primary telecommunications network for NWS forecasts, warnings and other products to the mass media (newspapers, radio stations, TV, etc.) and emergency management agencies.  The NWWS is a satellite communications system that transmits NWS products directly from NWS offices to external users.  The NWWS satellite communications system is operated by GTE Corp., under contract to the NWS. The system uses satellite transmitting (i.e. "uplink") equipment at more than 58 major NWS forecast offices throughout the continental U.S., Alaska, Hawaii and Puerto Rico.  Each uplink site transmits NWS-generated weather information products to GTE's master facility in Mountain View, CA, which are then re-broadcasted via satellite to more than 1,500 users. Users then have access to this broadcast data stream of NWS products.  More than 6,400 individual products per day are transmitted. To use the system, users must obtain the equipment needed to receive the data broadcast from the system contractor.  As the NWS modernizes, it anticipates little change to this satellite system. NWS may reconfigure the number and location of its satellite uplink sites, but these changes should be transparent to subscribers.  The NWS expects, however, that modernizing operations will be matched with a significant growth in the total number of weather products available to subscribers. As part of the transition to modernized operations, the NWS is investigating the feasibility of using the NWWS to transmit forecasts and warnings in graphical form to users for direct display on computer terminals.

Noctilucent Clouds: Wavy, thin, bluish-white clouds that are best seen at twilight in polar latitudes. They form at altitudes about 80 to 90 km above the Earth's surface.

NoctiphobiaThe fear of the night.

Nocturnal:  Related to nighttime, or occurring at night.

Nocturnal Inversion: see Radiational Inversion

Nocturnal Jet: This wind speed maximum occurs just above the nocturnal inversion at night. It is typically found in the south central United States during the late spring and summer months. It is important in the development of Mesoscale Convective Complex (MCC) or Mesoscale Convective Systems (MCS).

NOHRSC (The National Operational Hydrologic Remote Sensing Center):  An organization under the National Weather Service Office of Hydrology (OH) that mainly deals with snow mapping.

Nor'easter: A strong low pressure system that affects the Mid Atlantic and New England States. It can form over land or over the coastal waters. It usually produces heavy snows, flooding rains, strong northeast winds, coastal flooding, and beach erosion.

Normal Water Surface Elevation (Normal Pool Level):  The lowest crest level of overflow on a reservoir with a fixed overflow level (spillway crest elevation). For a reservoir whose outflow is controlled wholly or partly by movable gates, siphons, or other means, it is the maximum level to which water may rise under normal operating conditions, exclusive of any provision for flood surcharge.

Normal Year:  A year during which the precipitation or stream flow approximates the average for a long period of record.

Notch Width:  The 3 dB band width of a rejection filter.
Nowcast:  A short-term weather forecast, generally out to six hours or less.  This is also called a Short Term Forecast.

NVA:  An acronym for Negative Vorticity Advection.  See Negative Vorticity Advection.

NSSFC:  National Severe Storms Forecast Center, in Norman, OK; now known as SPC.

NSSL (National Severe Storms Laboratory) :  This is one of NOAA's internationally known Environmental Research Laboratories, leading the way in investigations of all aspects of severe weather. Headquartered in Norman OK with staff in Colorado, Nevada, Washington, Utah, and Wisconsin, the people of NSSL, in partnership with the National Weather Service, are dedicated to improving severe weather warnings and forecasts in order to save lives and reduce property damage.

NumerousA National Weather Service convective precipitation descriptor for a 60 or 70 percent chance of measurable precipitation (0.01 inch).  See Precipitation Probability (PoP).

NWP:  Numerical Weather Prediction.

NWS:  National Weather Service.

NWSH:  The National Weather Service Headquarters.

NWSOAn accronym for National Weather Service Office.  This local NWS office is responsible for issuing advisories, warnings, statements, aviation forecasts, marine forecasts, Area Forecast Discussions, and Short Term Forecasts for its county warning area.
NWSRFS V5.0, or Version 5 (National Weather Service River Forecast Model Version 5)The system of data entry, data preprocessing, and forecast programs which are used by RFCs. To make river forecasts, RFCs run NWSRFS V5.0 on a mainframe computer in NWSH through Remote Job Entry, or locally via Government Development Platforms, GDPs.

Nyquist Frequency:  The highest frequency that can be determined in data that have been discretely sampled. For data sampled at frequency f, this frequency is ( f / 2 ).  Doppler radar sampling frequency (rate) is equal to the pulse repetition frequency (PRF).

Nyquist Velocity or Interval:  The maximum unambiguous velocity that can be measured by a Doppler radar.  

USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.