Surface Prog Chart Help

Surface Prog Charts

Prog Charts are forecasts for surface conditions. These are generated by WPC and rendered for the web site. WPC provides an analysis updated every three hours plus 12 and 24 hour forecasts updated four times a day and a 36 and 48 hour forecast updated twice a day. In addition, medium range forecasts every day from three to seven days are also provided. These are valid for the contiguous United States.

Product Frequency Times
Current Analysis 3 hours every 3 hours about 90 minutes after valid time
12 hour Forecast 4 times daily ~0200 (valid 12Z), 0400 (18Z), 1400 (00Z), and 1630 (06Z)
24 hour Forecast 4 times daily ~0430 (valid 00Z), 0700 (06Z), 1630 (12Z), and 1930 (18Z)
36 hour Forecast Twice daily ~0730 (valid 12Z), 1930 (00Z)
48 hour Forecast Twice daily ~0730 (valid 00Z), 1930 (12Z)
3, 4, 5, 6 and 7 day Forecasts Once daily ~0300 (valid 12Z). The 3 day forecast is actually a 3 1/2 day forecast

Plot Legend

Surface Fronts and Boundaries

Cold Front - a zone separating two air masses, of which the cooler, denser mass is advancing and replacing the warmer.
Warm Front - a transition zone between a mass of warm air and the cold air it is replacing.
Stationary Front - a front between warm and cold air masses that is moving very slowly or not at all.
Occluded Front - a composite of two fronts, formed as a cold front overtakes a warm or quasi-stationary front. Two types of occlusions can form depending on the relative coldness of the air behind the cold front to the air ahead of the warm or stationary front. A cold occlusion results when the coldest air is behind the cold front and a warm occlusion results when the coldest air is ahead of the warm front.
Trough - an elongated area of relatively low atmospheric pressure; the opposite of a ridge. On WPC's surface analyses, this feature is also used to depict outflow boundaries.
Squall Line - a line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms.
Dry Line - a boundary separating moist and dry air masses. It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west).
Tropical Wave - a trough or cyclonic curvature maximum in the trade wind easterlies.

Precipitation

Areas of precipitation expected at the valid time of the forecast are outlined in green. Shading within these lines, or lack of shading, indicates the expected coverage (not intensity) of precipitation.

Broken light rain, coverage = 50-100%
Scattered light snow, coverage = 30-50%
Dashed line separates areas or snow from other precipitation types such as rain, freezing rain and sleet.

Below are symbols found on our short range forecasts that represent categories (and in some cases intensities) of precipitation. In forecast areas where the form of the precipitation is expected to vary, two symbols will be depicted and separated by a slash (/). For instance, rain showers and thundershowers are often combined in regions where convection is forecast.

Rain (light, moderage, heavy)
Snow (light, moderage, heavy)
Thunder (with rain, with snow, no precip)
Shower (rain, snow)
Drizzle
Freezing (rain, snow)
Ice pellets or sleet