According to the BMM, windshear warning from the GPWS is a function of rate of change of headwind and amount of updraft/downdraft. There's a picture of a graph in the manual but I'll try using less than 1000 words - x-axis is increasing/decreasing headwind in knots per second, y-axis is updraft/downdraft in fpm. Draw a line passing thru the points 1500fpm updraft and 2 kts per second increasing headwind, anything to the right of this line is the windshear caution area and there appears to be no cockpit indication for this. Draw another line parallel to the first passing thru 1500fpm downdraft and 2 kts per second decreasing headwind, anything to the left of this line is the windshear warning area and gives you the "WINDSHEAR WINDSHEAR WINDSHEAR" aural and red "WINDSHEAR" visual indication.
Cheers
