1. Training
2. Local
3. Magnetic vs. True Wind

Winds reported in the METAR are in TRUE heading while all flying is done using MAGNETIC heading. FS9 and XPlane do this conversion on the fly. In the Seattle ARTCC, the difference between TRUE and MAGNETIC heading varies between 15 and 19 degrees East. This could significantly affect the handling of a small aircraft when attempting a cross-wind landing. 

MAGNETIC heading should be used for determining the active runway. Also, winds, when given to the pilot, should be given using MAGNETIC HEADING. The conversion for the entire ARTCC is achieved by SUBTRACTING 20 from the TRUE heading. The magnetic variance changes annually, but to make it a little bit easier, just subtract 20.

So, given the following METAR for Seattle: 

KSEA 1556Z 013 01007 

Winds should be reported to the pilot as: Wind 350 at 07 

Please note that the "$winds" alias in VRC will report winds in TRUE heading and should not be used.