The solid white contours represent pressure contours (isobars) in millibars. The isobars have an interval of 4 millibars. The wind speed is directly related to the distance between the isobars. The closer they are together, the stronger the pressure gradient, and the stronger the wind.
In the background, infrared satellite data shows the cloud patterns over North America. The brightness of the cloud images is inversely proportional to the temperature of cloud tops, therefore the deep clouds with high (and thus cold) cloud tops typically indicating areas of intense rain and/or hail associated with deep convection appear brightest on this image. However high cirrus clouds will often also appear very bright, but these clouds do not produce precipitation. Low and high pressure systems can also be located from the map above. Low pressure systems are located in the regions of the lowest pressure, while high pressure systems are located in the regions of highest pressure.
For more information on how pressure affects the weather go to our forces and winds module.