~ Guide to the Meteograms ~

Forecast meteograms are based on the grid point nearest the location in question. The models cannot directly discern features of the terrain or atmosphere that are smaller than the grid resolution, although certain small-scale phenomena are represented indirectly by using parameterizations. Thus these models may not do a good job of forecasting very localized weather such as might be associated with mountainous terrain or sea breezes. A description of the meteorological variables in the meteograms is given below.

Tropospheric Time-Height Cross Section

Winds, temperature, and relative humidity in the lower troposphere (up to ~500 millibars) are shown in profile. The model data we receive is interpolated down to 1000 millibars, but over high terrain only data that is near or above ground level is displayed. Data below ground level has no physical meaning and is omitted.

The wind barbs indcate the direction and speed of the wind, rounded to the nearest 5 (mph or m/s). A full barb = 10, and a short barb = 5, and a pennant = 50. The speed can be found by tallying the barbs. The barbs project into the wind -- they point to the direction the wind is coming from. A stem pointing left with one full barb and one half barb indicates a wind from the west at 15.

The colored contour lines indicate the profile of temperature, in °F or °C. The contour interval is 10°F or 5°C. The freezing level is indicated by the double black line labeled FR.

The graduated green shading indicates relative humidity.

1000-500 mb Thickness

Thickness is the vertical distance between two pressure levels. In general it is true that the distance will be a function of the density of the air between the two pressure levels, which is itself directly related to the temperature of the air. Thus, thickness is a good indicator of the mean temperature in the layer of atmosphere between the two levels -- greater thickness = warmer air.

The cyan line shows the thickness of the 1000-500 millibar layer of the atmosphere. This is approximately the bottom half of the atmosphere (except over high terrain). The units are dekameters (10s of meters, 10 meters is about 33 feet). One rule of thumb is that if the thickness of this layer is less than 540 dm, that any precipitation will be in the form of snow.

Sea Level Pressure

Sea level pressure (SLP) is the surface pressure interpolated down to sea level from the altitude of the grid box of the model. This corresponds to the barometric pressure one hears reported on local radio or TV weather reports. The dark blue line indicates SLP in millibars. Valleys in SLP often indicate frontal passages, and will often coincide with pronounced changes in wind direction, temperatures, and humidity. In subtropical regions (the deserts of the Southwest, and much of the Sunbelt during summer) the SLP often oscillates daily, with a peak in the early morning and a trough during mid-afternoon.

Stability Indices

The stability indices are measures of the potential for strong or severe weather. The indices shown here are the Lifted Index (LI) and the Convective Available Potential Energy (CAPE). The lifted index, indicated by the red line, is a measure of the thunderstorm potential which accounts for low level moisture availability. LI values greater than 0 mean thunderstorms are unlikely. LI values between 0 and -2 mean thunderstorms are possible with good trigger. LI values between -3 and -5 mean thunderstorms are probable. LI values less than -5 mean a strong potential for severe thunderstorms.

The purple bars indicate the CAPE value at the surface in units of J/kg. CAPE is a measure of the buoyancy of a layer. The larger the CAPE, the greater the potential for severe weather. Any value greater than 0 J/kg indicates instability and the possibility of thunderstorms. The base line for the bar graph of CAPE is 0.

10-Meter Winds

10-Meter Winds (about 33 feet above the ground) correspond to measured winds at weather stations. The orange line indcates the wind speed (in mph or m/s). The wind barbs are as in the time-height cross-section.

2-meter Temperatures

Air temperature and dew point temperature are given at the 2-meter level (6½ feet above ground). The dew point temperature is the temperature that a sample of air would have if it was cooled (at constant pressure) until it reached saturation. The dew point temperature is an alternative way to describe the amount of moisture or humidity in the air. If the dew-point temperature is close to the air temperature, the relative humidity is high, and if the dew point is well below the air temperature, the relative humidity is low. One technique for forecasting overnight low temperature is to look at the daytime dew point: if no fronts are expected to come through, tonight's low temperature will not get much below today's dew point.

Air temperature is indicated by the solid brown line (with color shading below the line to aid interpretation). The dew point temperature is indicated by the dashed brown line. The freezing level is drawn with a dot-dashed black line. The vertical bars indicate the range of temperatures over the previous 3, 6, or 12-hour interval.

2-Meter Reletive Humidity

The green line and graduated green shading indicates the relative humidity.

Cloud Cover

This panel has a blue background to show the cloud-free areas. The panel is divided into three horizontal layers for the display of low, middle, and high cloud cover, which are drawn as white bars. If the white bar covers the full height of its layer, that is 100% cloudiness. The white bars have no gap between them to better simulate the appearance of cloudiness in the panel.

Precipitation

The bar graph at the bottom of the figure indicates predicted precipitation types and amounts. The color of the bar inidicates the type of precipitation: rain, sleet, snow, or ice pellets. Narrow red bars within the wide bars indicates the portion of precipitation likely to come from convection (showers and thunderstorms)

The precipitation amounts are in inches or millimeters. Values are accumulated precipitation over the previous forecast period, so no precip values are given at hour 00. The amounts are liquid water equivalents. For example, 1" of snow means one inch of water in the melted snow. It could mean anywhere from 3-12" of actual snow, depending on how "wet" the snowfall is. Depths of sleet and ice will more closely match their liquid water equivalents.