The Warm Layer Aloft. Below is a chart from the 00z NAM12 of 800mb Temperature and Wind valid 12z, 15z, 18z, and 21z tomorrow. 800mb is about the level where the warm layer aloft (WLA) maximum temperature is occurring. As a general rule Snow is favored where WLA < +0.5C. Sleet is favored warmer than this up to about +3C. Freezing rain is favored generally where WLA maximum temperature is greater than +3C.
