On 24 June 1998, NOGAPS was upgraded to version 4.0. The primary change was an increase in the number of vertical levels from 18 to 24.

The operational T159L24 NOGAPS (4.0) runs on a CRAY C90 and executes several times each 00-UTC and 12-UTC watch, including a six-day forecast completing approximately five and one-half hours past the synoptic time. NOGAPS currently outputs close to 25,000 gridded fields per day. Products from NOGAPS are distributed to a worldwide customer base. NOGAPS also provides essential and tailored input to many other models, including the Navy's advanced Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS), ocean wave model, sea ice model, ocean circulation model, ocean thermodynamics model, tropical cyclone model, aircraft and ship-routing programs, and application programs at both FNMOC (U.S. Navy's Fleet Numerical Meteorology and Oceanography Center) and the Air Force Weather Agency (AFWA).

Along with the GFDL tropical cyclone forecast model and the UK Met Office and Japanese global models, NOGAPS is a primary tropical cyclone forecast tool for forecasters at the Joint Typhoon Warning Center (JTWC) and the National Hurricane Center (NHC).

NOGAPS uses a multi-variate optimal interpolation (MVOI) scheme. This analysis scheme uses multivariate statistical analytical techniques, patterned after the volume method developed by Lorenc (1981) for ECMWF. First implemented operationally at FNMOC in January 1988, NOGAPS has undergone evolution to reach its current configuration. The analysis is performed on the Gaussian grid of the T159L24 global spectral model on the 16 standard pressure levels from 1000 to 10 hPa, inclusive. The maximum number of observations used per volume is 600.

In addition to conventional observations (surface, rawinsonde, pibal, and aircraft), the analysis makes heavy use of various forms of satellite-derived observations. The analysis uses derived soundings from the NOAA and DMSP polar-orbiting satellites as well as DMSP SSM/I total column precipitable water and surface wind speeds. NOGAPS was the first operational model to use SSM/I wind speeds (Goerss and Phoebus 1992). In addition to the wind observations derived from various operational processing centers for the geostationary satellites, the NOGAPS also untilizes high-density multispectral wind observations produced by the University of Wisconsin-Cooperative Institute for Meteorological Satellite Studies (CIMSS).

   The NOGAPS model time step begins as 540 seconds, but is dynamically reduced to a shorter time step if stratospheric jets go beyond a designated threshold.

   The physics package includes

        -Bulk-Richardson number-dependent vertical mixing patterned after ECMWF's vertical mixing parameterization
        -A time-implicit Louis surface flux parameterization
        -Gravity wave drag
        -Shallow cumulus mixing of moisture, temperature, and winds
        -Relaxed Arakawa-Schubert cumulus parameterization
        -Convective and stratiform cloud parameterization
        -Harshvardhan solar and longwave radiation
        -Semi-implicit treatment of gravity wave propagation and Robert time filtering