Positive numbers are pressure falls Pressure falls are very important in short term forecasting. Radiosondes are only sent up twice a day, thus everything else must come from profilers, satellite, and surface data.
Pressure falls are a result of increased divergence aloft.
The pressure falls are a sign of what is going on in the atmosphere above the surface. Without weather balloon data during the day, pressure falls can be a good tool for figuring out what is going on overhead. Strong pressure falls ahead of the center of low pressure show essentially where the center of the low will be moving. Pressure rises should be shown in the wake of the low pressure center after it moves on. Here is a late December example .