We've guessed that you're not using a client that supports colored tables and have tried to compensate. Strong troughs are typically preceded by stormy weather and colder air at the surface. The trough axis is denoted by the purple line.
Temperatures decrease with color from light blue to purple.
A trough is located over the eastern United States and is indicated by the dip in the geopotential height field . This is the upper level extension of a surface low pressure center , which is why troughs are also called upper level lows. Notice the relatively cold temperatures associated with the trough. This is caused by the southward transport of colder air in the lower troposphere. Credits and Acknowledgments for WW2010.