This is a set of contour plots using data from the NAM model. The output fields are consistent with other models so the models can be compared. This model gives forecast information out to 48 hours and plots are sorted 48 hour 4 panel plots where each plot has a panel for the 12, 24, 36 and 48 hour forecast and individual forecast times from initialization to 60 hours in 6 hour increments. These plots are updated once every 12 hours at roughly 11:15 EST.
Sample 700 mb Plot
The 700 mb chart shows the forecasted vertical velocities (colors), heights (solid white lines) and wind vectors.
700 mb vertical velocity field
The vertical velocity field shows the vertical motion in -mb/sec through the 700 mb surface. Positive values (greens, yellows and reds) are upward motion and negative values (blues and magentas) are downward motion. The unit of mb/sec is roughly equal to a cm/sec so it is easy to see that vertical motion is roughly two orders of magnitude smaller than horizontal motion. This comes from the fact that our atmosphere is generally stable and that upward motion is generally an artifact of precipitation releasing heat energy. In other words, moderate to strong positive areas (>5) reflect areas where there is precipitation or soon will be. Strongly positive areas (>10) generally indicate areas of potential thunderstorms. Negative areas show descending air which are generatlly associated to areas of fair weather and often clear skies. If precipitation is occurring in these areas it will end soon. Sometimes descending air will trap cloudiness below it and result in a low overcast that can linger for days.
700 mb height field The height field works very similar to the sea level pressure field. Lows and highs can found and compared to sea level locations. Strength of winds are again related to the packing of the height contours.
700 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.