Maps : Weather Underground

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Important Notes: We here present the results from the forecast models of the U.S. National Center for Environmental Prediction (NCEP) under Grid Analysis and Display System (GRADS). The following content is not meant to be a thorough and complete description, but to provide an overview of the forecasts and their presentation.

More About The Maps
Map Fields	Description	Usefulness
300 mb	The 300 mb charts show the 300 mb geopotential heights (contours, 60 m interval), isotachs (color fill, 10 knots), and wind (vectors, m/s).	Indication of jet streams and diffluence: the left front quadrant, and the right rear quadrant of the upper-level jet stream are favored severe weather locations.
500 mb	The 500 mb charts show the 500 mb geopotential heights (contours, 60 m interval), absolute vorticity (color fill, 2x10^-5/s interval), and wind (vectors, m/s).	Indication of upper-level disturbances (i.e., shortwaves or PVA), height falls, diffluence, and cold pockets. A well defined westerly jet with sufficient low-level moisture aids the development of squall line(s).
700 mb	The 700 mb charts show the 700 mb geopotential heights (contours, 30 m interval), vertical velocity (color fill, -ub/s interval), and wind (vectors, m/s).	Indication of dry air intrusions(advection) and upper-level vertical motion.
850 mb	The 850 mb charts show geopotential heights(contours, 30 m interval), temperatures (color fill, 2 Celsius interval), and wind (vectors, m/s).	Indication of low-level jet streams, temperature advection and convergence. The majority of severe weather is located along and to the left of the 850mb jet stream.
Relative Humidity	The 850-500 mb mean relative humidity charts show lifted index (contours, 2 interval) and integrated relative humidity from 850 to 500 mb (color fill, 10% interval).	Lifted Index(negative value) is used to determine the severity of thunderstorm activity(-2 weak, -4 moderate and -6 strong).
2m Above Ground Level	The 2m Above Ground charts show near suface weather conditions with dewpoints (contours, 5 Celsius interval, bold orange=20, bold white=15, bold red=0, bold magenta=-15, bold gray=-30), temperatures (color fill, 4 Celsius interval), and wind (vectors, m/s).	Look for low-level convergence. Dew point >15 degrees Celsius: risk of moderate to strong storm development.
Mean Sea Level	The MSL charts show mean sea-level pressure (blue contours, 4 mb interval), 1000 to 500 mb thickness (yellow contours, 60m interval), and accumulated precipitation (color fill, where NAM shows the next 3 hours and GFS shows the previous 12 hours).	The 5400 line (highlighted) is a good indication of the division between rain(<5400) and snow(>5400) for low terrain.
Meteorological Terminology
Absolute Vorticity	A measure of air rotation in a horizontal plane relative to a non-rotating reference frame. Positive (or negative in the Southern Hemisphere) vorticity can be correlated with surface low pressure development and upward vertical motion (in areas of PVA where mostly likely conincide with troughs in the geopotential height field).
Advection	Transport of an atmospheric property by the wind.
Lifted Index	A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability - the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms. However there are no "magic numbers" or threshold LI values below which severe weather becomes imminent.
Positive Vorticity Advection (PVA)	An advection of higher values of vorticity into an area, which often is associated with upward motion (lifting) of the air. PVA typically is found in advance of disturbances aloft (i.e., shortwaves), and is a property which often enhances the potential for thunderstorm development.
Relative Humidity	The ratio of actual vapor pressure to the saturation vapor pressure at a specific temperature. An example would be the formation of fog or clouds where RH reaches 100%. However, sea fog formation can be as low as 97%.
Thickness	The vertical distance between isobaric surfaces at a given location. It may be shown that the thickness is directly proportional to the average temperature of the layer defined by the isobaric surfaces. Maps of thickness are thus maps of average temperature.
Vertical Velocity	The rate of upward or downward motion of air passing through a given pressure level.

A portion of the glossary is provided by The National Weather Service (NWS).