500mb Geopotential Heights, Height Change and Vorticity

• Black contours indicate the geopotential height of the 500 millibar surface, in tens of meters.
  • Low geopotential height (compared to other locations at the same latitude) indicates the presence of a storm or trough at mid-troposphere levels.
  • Relatively high geopotential height indicates a ridge, and quiescent weather.
• In the forecast panels, the colored contours indicate the change in geopotential height during the 12 hours leading up to the valid time.
  • Decreasing geopotential height usually indicates an approaching or intensifying storm.
  • Increasing heights usually indicate clearing weather for the period.
• The color shading indicates vorticity at 500 millibars: Red for positive vorticity, blue for negative.
  • Positive vorticity indicates counterclockwise rotation of the winds, and/or lateral shear of the wind with stronger flow to the right of the direction of flow.
  • Negative vorticity indicates clockwise rotation of the winds, and/or lateral shear of the wind with stronger flow to the left of the direction of flow.
  • Positive (or negative in the Southern Hemisphere) vorticity at 500 millibars is associated with cyclones or storms at upper levels, and will tend to coincide with troughs in the geopotential height field.
  • Negative (positive in SH) vorticity is associated with calm weather, and will tend to coincide with ridges in the geopotential height field.

Vertical Velocity or Precipitation

• The vertical velocity at 700mb (in mb/hr) is shown instead of precipitation for the 00hr forecast.
  • Negative values indicate ascending air, and positive values denote sinking air. Ascending motion is associated with cloudiness and rain. Large negative values of vertical velocity correspond to areas of heavy rainfall if moisture is available (see description of panel 4). These areas tend to correspond with the storms in the first two panels.
• The remaining forecasts panels indicate 12 or 24 hour accumulated precipitation, measured in millimeters.
  • The total is the amount of rainfall forecast during the 12 or 24 hours immediately preceding the verification time in the lower lefthand corner of the map.
  • Comparison with the 540 thickness line in panel 2, and the 0?C isotherm in panel 4 can give a good indication of the dividing line between snow and rain.