The 500mb forecast chart is an excellent tool for mariners to estimate where to anticipate the heaviest sea and swell conditions as well as a guide to the expected surface storm tracks and intensities. The closer the height contours on the 500mb chart, the faster the upper level wind flow, the stronger the temperature contrasts and the more active is the surface weather below. The development and strengthening of surface lows and the associated bad weather most often occurs on the eastern side of 500mb troughs while surface high pressure and good weather is associated with the western side of these troughs. Without even looking at the surface pressure charts, a mariner can estimate what areas might be best to avoid.
Today’s 500 mb chart of the North Pacific is a good example.
NOAA OPC 500 mb Analysis Dec
Note the actual significant wave height analysis 
Click here to read more about using the 500 mb chart at sea.
Thank you for this.
Very usefull , thank You! Gretting from Poland Fred!