– Planetary Boundary Layer. The lowest level of the atmosphere where friction is an important force and vertical mixing is common.

– a soil layer below the surface of tundra regions that remains frozen permanently.

– Period

– Pressure gradient. Tightening pressure gradient indicates stronger winds.

– When two separate short waves come together to form one wave. Also, when upper and lower level features are positioned so that each provides energy to the other, it is said that the features are in phase with one another.

A warm and moist flow of air with origins from the tropics which precedes major winter storms. It can bring significant rain and snow to the Pacific Northwest.

– Probability of Precipitation. This is the chance the NWS gives to precipitation across there forecast region. It may also refer to the POP a forecast model is predicting.

(PIVA)