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NCEP - National Centres for Environmental Prediction; the modernized version of NMC.

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.

NEXRAD - NEXt-Generation Weather RADar. Technologically-advanced weather radar being 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.

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.

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

NOAA - National Oceanographic and Atmospheric Administration.

Nocturnal - Related to night-time, or occurring at night.

Nowcast - A short-term weather forecast, generally out to six hours or less.

NSSFC - National Severe Storms Forecast Centre, in Kansas City MO; now known as SPC.

NSSL - National Severe Storms Laboratory, in Norman OK. (Sometimes pronounced NES-sel.)

NWP - Numerical Weather Prediction.
 
NWS - National Weather Service.