Guide to the Forecasts and Analyses

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We describe and display the results from the forecast models of the U.S. National Center for Environmental Prediction (NCEP), but similar procedures and products are common to forecasting centers in other countries. This guide is not meant to be a thorough and complete description, but to give an overview of the forecasts and their presentation.

There are three views for the forecasts and analyses - one centered over North America (for the regional and global models), a view of the entire Northern Hemisphere centered over the Atlantic Ocean (for the global model only), and a similar vie for the Southern Hemisphere. For each model there is an analysis, and then a number of forecasts at regular intervals. The regional model forecasts are only displayed out to 48 hours, but the forecasts from global models continue longer. We now produce six different panels for each period, as opposed to the four we were producing previously, and which NCEP continues to produce. The contents and meaning of the six panels is described below in detail.

The analyses represent the initial state for the integration of the various forecast models. The analyses are produced from observations at weather stations around the world, as well as ship and buoy reports at sea, reports from aircraft, radiosonde balloons, and even satellite data. These data are merged after quality control procedures have been applied. Even with all of the data sources, there are still tremendous gaps in coverage over remote areas. An optimal interpolation (OI) procedure is performed using the previous model forecasts to fill these gaps and create a complete picture of the state of the atmosphere at the forecast time T=0. The various models are then integrated forward in time to produce the forecasts which are displayed here.

At the bottom of each map is a bar telling the date and time for which the analysis or forecast is valid, the number of hours after the analysis for which the forecast is valid, the fields displayed, and their units. The six types of maps are described below.

Panel 1

• 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.

Panel 2

• The colored contours indicate sea level pressure in millibars. High pressure is red, low pressure in green or blue. Only the last 2 digits shown -- sea level pressure is usually around 1000 millibars, so add 1000 to values in the range of 00-50, and add 900 to values in the range of 50-98.
  • Low sea level pressure indicates cyclones or storms near the surface of the earth. High sea level pressure indicates calm weather.
• The black contours indicate the vertical distance, or thickness, between the 1000 millibar surface and the 500 millibar surface, measured in tens of meters.
  • Since air behaves nearly as an ideal gas, and vertical distance is proportional to volume over a specified surface area, the thickness between two pressure levels is proportional to the mean temperature of the air between those levels. Thus, low values of thickness mean relatively cold air.
  • The 540 line is highlighted, since this line is often used as a rule of thumb to indicate the division between rain and snow for low terrain. When there is precipitation where the thickness is below 540dam, it is generally snow. If the thickness is above 540dam, it is usually rain (or sleet if the air next to the surface is below freezing).

Panel 3

• The colored contours indicate vertical velocity of the wind at the 700 millibar level, in millibars per hour (since pressure decreases with height, negative values indicate ascending air, and positive values denote sinking).
  • 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 green shading in the forecasts 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 0ºC isotherm in panel 4 can give a good indication of the dividing line between snow and rain.

Panel 4

• Colored contours indicate the air temperature at the 850 millibar level, in degrees Celsius. The 0ºC 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 (in panel 3) 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, 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.

Panel 5

• 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.

Panel 6

• 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.