NAM Model Forcasts

850mb Temperature, Humidity and Winds

• Colored contours indicate the air temperature at the 850 millibar level, in degrees Celsius. The 0 degree C contour is highlighted, as this is also often used as a divider between rain and snow.
• The green shading indicates the relative humidity percentage at the 850 millibar level. High values indicate the availability of moisture. When large rates of ascent  are located with high moisture availability, heavy rainfall will likely occur.
• The barbs indicate the direction and speed of the wind, in meters per second. Each full barb indicates 10 m/s (20 knots), and each half barb 5 m/s. The direction of the wind is parallel to the shaft with the barbs pointing into the wind.
• Advection of moisture by the wind can be inferred by noticing the direction and rate at which moist areas appear to be blown. Similarly, temperature advection can be inferred by noticing whether the wind is blowing cold air toward a warm region, or warm air toward a cold region.

200mb Winds

• Purple shading indicates the speed of the winds at the 200 millibar level, in meters per second. This altitude is near the level of the core of the jet stream. So the tracks of the jet streams can be seen very clearly.
• The streamlines indicate the direction of flow of the wind, which is generally from west to east throughout most of the subtropics, mid- and high-latitudes.
• The color of the streamlines indicates a relative measure of divergence of the flow in the upper troposphere. Orange and red indicates strong divergence at upper levels, usually associated with strong vertical velocities in the middle troposphere, and severe weather/heavy rainfall.

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.

  Precipitable Water and Convective Available Potential Energy (CAPE) or Total-Totals Index (TTI)

  • The colored contours indicate total precipitable water in the atmosphere. Precipitable water is the total depth of liquid water that would result if all water vapor contained in a vertical column of air could be “wrung out”, leaving the air completely dry. It indicates the total humidity of the air above a location, and is a good indicator of the amount of moisture potentially available to supply rainfall.
  • In the analysis and forecasts for the ETA model, the yellow-brown shading indicates the amount of CAPE in the atmosphere, which is a good indicator of the potential for strong thunderstorms and severe weather. High values of CAPE indicate that most (but not necessarily all) conditions exist for strong thunderstorms.
  • For the other models the yellow-brown shading indicates the value of the TTI, which is a measure of the vertical stability of the atmosphere, and over central and eastern North America is also a good empirical indicator of the potential for severe weather.
    • TTI = TD[850]-T[500] + (T[850]-T[500])
      where T is temperature, TD is dew point (both in Celsius), and the [number] is the pressure level.
    • Values of TTI of around 40-45 indicate the potential for thunderstorms. Around 50, severe thunderstorms are possible. Around 55, storms producing tornados are possible. This rule-of-thumb does not hold over western North America and other areas where there is alot of high terrain.

Source: COLA Weather and Climate Forecasts