The extended range plots actually show two parameters: 500 mb heights (in color contours) and sea level pressure (in black line contours).
Sea level pressure
The sea level pressure field can be used to find low and high pressure systems as well as the location of cold fronts. The highs and lows can be located by the white H and L symbols on the map. Cold fronts will generally start from areas of low pressure and follow the trough of low pressure south and to the west of the low. Rain and/or snow is likely in the regions directly around the low and somewhat less likely along the cold front.
500 mb heights
The 500 mb heights can be used to estimate surface temperatures since height of a pressure surface is related to the mean temperature of the air below it. Here is a rough estimate of temperatures based on color. The variability is on the order of +-10 degrees F:
Height Color Hi Temp 5940-6000 Red 90 5880-5940 Red Orange 84 5820-5880 Orange 78 5760-5820 Yellow Orange 72 5700-5760 Yellow 66 5640-5700 Greenish Yellow 60 5580-5640 Yellowsh Green 54 5520-5580 Green 48 5460-5520 Cyanish Green 42 5400-5460 Greenish Cyan 36 5340-5400 Cyan 30 5280-5340 Blueish Cyan 24 5220-5280 Blue Cyan 18 5160-5220 Cyanish Blue 12 5100-5160 Blue 4 5040-5100 Magentaish Blue -2 4980-5040 Blueish Magenta -8 4920-4980 Magenta -14 4860-4920 Medium Magenta -20 4800-4860 Dark Magenta -26Modify these by:
Deserts +20 Mid Summer +10 Tropics +5 Spring/Fall 0 Cloud cover -5 Rainy areas -10 Near Low Press -10 Wintertime -10 Snow cover -10 Arctic Winter -20Current Images
The 850 mb chart details weather conditions at the 850 mb level or around 5000 feet above sea level. The parameters plotted are temperatures in Celsius (in color contours), and winds plotted as vectors.
850 mb temperatures The temperature field shows where warm and cold air are located. Temperatures at this level do not show the diurnal temperature changes from morning low to afternoon high we see at the earth's surface. You can determine potential afternoon highs from these temperatures by adding: 15C in the summer, 12C in spring and fall, and 9C in the winter and converting to Fahrenheit. For example, if the 850 temp is 4C in summer, the afternoon temperature could reach 4+15=19C=67F. This algorithm does not work in the western third of the country due to its high altitude.
The 850 temperature is also a decent indicator of the type of precipitation. Since most precipitation forms at 5000 feet or above, a temperature of freezing (0 Celsius) or below would indicate snow whereas a temperature above freezing would indicate rain.
850 mb wind vector field
The vector field shows wind direction and speed. Often this can be used to qualitatively show areas of convergence and divergence. In the middle and upper levels of the troposphere, this can be an indicator of existing upward (from divergence) or downward (from convergence) air motion. Upward motion is often linked to precipitation and downward to clear skies.
NOTE: The wind data are only broadcast out to 3 days and for a range of latitude from 35 south to 35 north.