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), 1300 (00Z), and 1430 (06Z)|
|24 hour Forecast||4 times daily||~0430 (valid 00Z), 0700 (06Z), 1330 (12Z), and 1930 (18Z)|
|36 hour Forecast||Twice daily||~0730 (valid 12Z), 1930 (00Z)|
|48 hour Forecast||Twice daily||~0730 (valid 00Z), 1930 (12Z)|
|60 hour Forecast||Twice daily||~0730 (valid 00Z), 1930 (12Z)|
|3, 4, 5, 6 and 7 day Forecasts||Once daily||~1400 (valid 12Z). The 3 day forecast is actually a 3 1/2 day forecast|
Here is a sample surface prog chart.
|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.|
Areas of precipitation expected at the valid time of the forecast are shaded in colors depending on the type and likelihood of precipitation. The areas are derived from the NDFD grid for weather.
The National Digital Forecast Database (NDFD) contains a mosaic of digital forecasts from the National Weather Service (NWS) field offices working in collaboration with NCEP.
Here are the precipitation types: