Guide to the Forecasts and Analyses

This guide provides a brief explanation for the weather maps displayed here. The underlying data are output from the forecast models of the U.S. National Center for Environmental Prediction (NCEP). The analysis is the initial state for the forecast model. The analysis is produced from observations at weather stations around the world, as well as ship and buoy reports at sea, reports from aircraft, radiosonde balloons, and satellite data. These observational data sets 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 initialization time. The model is then integrated forward in time to produce the forecasts. 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. Individual weather maps and the variables they contain are described below.

500mb Geopotential Height 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.

The color shaded contours indicate 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.

Sea Level Pressure and 1000-500mb Thickness

The colored contour lines 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 shaded 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 in black, 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).

Vertical Velocity or Precipitation

The vertical velocity at 700mb (in mb/hr) is shown instead of accumulated precipitation for the analysis. 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.

The remaining forecast 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. The with the 540 thickness line and the 0 °C isotherm at 850mb can give a good indication of the dividing line between snow and rain.

850mb Temperature, Humidity and Winds

Colored contour lines indicate the air temperature at the 850 millibar level, in degrees Celsius. The 0 °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 areas of large upward vertical velocity are co-located with high moisture availability, heavy rainfall will likely occur.

The streamlines indicate the wind flow. 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.

Wind Streamlines and Isotachs (and Divergence)

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.

Purple shading indicates the speed of the winds at the given pressure level, in meters per second. The 200mb level is near the core of the jet stream, so the tracks of the jet streams can be seen very clearly.

The maps for the Tropics also have areas of blue and orange shading which indicate horizontal convergence or divergence of the flow at the given pressure level. Orange and red indicate strong divergence, and light and dark blue indicate strong convergence. Low-level convergence with divergence aloft at the same location is usually associated with strong vertical velocities in the middle troposphere, and severe weather/heavy rainfall

Precipitable Water

The shaded 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.

Convective Available Potential Energy (CAPE)

Convective available potential energy 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. CAPE is drawn in red-colored contour lines overlaid on top of the preciptable water shaded contours in most panels. For the weather maps for the Tropics, CAPE is drawn as shaded contours with sea level pressre contours overlaid in black.

Low, Middle, and High Cloud Cover or Relative Humidity at 800mb, 500mb, and 300mb

Light blue shaded areas show where low level cloud cover exceeds 50%. For the analysis, cloud cover is not available so relative humidity > 70% at 800mb is used as a proxy.

Light green shaded areas show where middle level cloud cover exceeds 50%. For the analysis, cloud cover is not available so relative humidity > 70% at 500mb is used as a proxy.

Pink shaded areas show where high level cloud cover exceeds 50%. For the analysis, cloud cover is not available so relative humidity > 70% at 300mb is used as a proxy.

Temperature of the Sea Surface, and 2 Meter Air Temperature

These variables only appear in the weather maps for the Tropics. Over the ocean, red shading and contours indicate the sea surface temperature (SST), in degrees Celsius. Sea surface temperatures above 25C are shaded in tones of red. The shading over land indicates the air temperature 2 meters above the land surface.