The forecast models can be used to not only predict precipitation, but also to give the forecaster an idea of the mechanisms in the troposphere that are causing the precipitation. A good strategy for determining the lifting mechanisms in the atmosphere is to first look at the 700-mb upward vertical velocity product. Once you have determined whether the air will be rising or not, next look at the RH plot to see how high or low the relative humidity is. A high RH indicates the air is near saturation (low vapor deficit). If air rises in an environment with low vapor deficits, clouds and precipitation are favorable. Next, determine what mechanisms are causing the air to rise. The 1000-mb prog can be used to locate convergence along frontal boundaries, low pressure, topography, etc. The 850 mb prog as well as the thickness prog can be used to determine if low level WAA is causing rising air. The 700 mb and 500 mb progs can be used to locate shortwaves (shortwaves generate positive vorticity and cool the middle levels of the atmosphere). The 500-mb prog can be used to locate PVA (PVA causes upper level divergence). The 300-mb prog can be used to locate jet streaks (divergence occurs in the right rear and left front quadrant). A forecaster also looks for mechanisms that generate downward vertical velocity at each level in the atmosphere. Downward vertical motion mechanisms include low-level divergence, low-level CAA, ridging, NVA, left rear/right front of jet streak, and downsloping wind. Often upward and downward vertical motion mechanisms will cancel each other out (i.e. low level CAA with PVA aloft). If the UVV mechanism overpowers the DVV mechanism, then motion will be upward but decreased in magnitude. The more and greater UVV mechanisms that are stacked over a region, the higher the UVV will be. After looking at the models you should be able to answer if vertical motion is up/down/neutral and the mechanisms causing the vertical motion. This type of model interpretation is often included in NWS forecast discussions. |