NAVGEM (Navy Global Environmental Model) is a global numerical weather prediction computer model run by NOAA. It replaced NOGAPS as the prime model in the middle of February 2013 at the FNMOC Weather model synoptic site. NOGAPS: Global weather forecast model from the "Fleet Numerical Meteorology and Oceanography Center" (USA)
2 times per day, from 10:00 and 23:00 GMT
Greenwich Mean Time:
12:00 GMT = 13:00 BST
Geopotential in 500 hPa (solid, black lines) and Vorticity advection in 105/(s*6h) (colored lines)
The two types of vorticity advection are positive (PVA) and negative vorticity
The closed circles in the figure show the 500 hPa absolute vorticity
lines, the others the 500 hPa height lines. When an air parcel is moving from
an area higher vorticity to an area lower vorticity this is called: PVA
(red color). The other way around is called: NVA (blue color). PVA is
associated with upper-air divergence, i.e. upward vertical motion. NVA
is associated with down ward vertical motion. Therefore, PVA at 500
hPa is strongest above a surface low, while NVA at 500 hPa is strongest
above a surface high.
In operational meteorology Vorticity advection maps are used to identify areas
with vertical air motion to see where clouds, precipitation or clear conditions
are likely to occur. Keep in mind, however, that PVA is not the same as upward
vertical motion. Here temperature advection is important too.
Numerical weather prediction uses current weather conditions as input into mathematical models of the atmosphere to predict the weather. Although the first efforts to accomplish this were done in the 1920s, it wasn't until the advent of the computer and computer simulation that it was feasible to do in real-time. Manipulating the huge datasets and performing the complex calculations necessary to do this on a resolution fine enough to make the results useful requires the use of some of the most powerful supercomputers in the world. A number of forecast models, both global and regional in scale, are run to help create forecasts for nations worldwide. Use of model ensemble forecasts helps to define the forecast uncertainty and extend weather forecasting farther into the future than would otherwise be possible.
Wikipedia, Numerical weather prediction, http://en.wikipedia.org/wiki/Numerical_weather_prediction
(as of Feb. 9, 2010, 20:50 GMT).