In order for precipitation to occur, air must rise and have moisture. The rate and total distance air rises along with the amount of moisture (primarily in the low levels of the atmosphere) determines IF precipitation will occur and HOW MUCH precipitation will occur. On the synoptic scale forecast models, UVV's are used to assess the rate at which air will rise over a region. UVV's are shown at the 700-millibar level on the synoptic scale forecast models. The units of UVV are centimeters per second (this is not convective motion such as a thunderstorm but rather large scale synoptic uplift or descent). The following link shows this chart (links to other levels in the atmosphere are found at the top of the frame once you get to the 700 mb chart) http://weather.unisys.com/nam/700.php UVV's are colored on the panel. Significant upward motion occurs when the UVV value is +6 or greater. The 700-millibar UVV chart is a great starting point for making a precipitation forecast. Assess if the UVV's are positive or negative over the forecast region. Next, it is your job as a forecaster to determine the processes in the atmosphere that are causing a positive UVV or negative UVV value over the forecast region. The following contribute to positive UVV: low level warm air advection (use 850 mb model chart to assess this); low level convergence (frontal boundaries); positive vorticity advection (the 500 mb model chart shows vorticity values); divergence created by a jet streak, and convergence into low pressure. Negative UVV is caused by: low level CAA, negative vorticity advection, subsidence over a mountain range, upper level convergence (LR, RF quads of jet streak), high pressure subsidence. After examining the UVV and finding the processes that are causing the air to rise or sink, you are in a position to describe why the air will rise or sink over the forecast region. Often, some mechanisms over the forecast region will cause air to rise while others will cause air to sink (i.e. low level CAA with PVA aloft), The UVV value can help the forecaster access which forcing mechanisms will be strongest. If low level CAA is causing a greater sinking motion than the PVA is causing a rising motion, then no precipitation will result. Once a forecaster determines if the air will rise or sink and what will cause it the rise or sink, the next step is the examine available moisture. The amount of moisture available for precipitation through positive UVV can be assessed by using the pres/thik/prec. chart. The address below will bring you to this chart. http://weather.unisys.com/nam/pres.php This chart shows regions favorable for synoptic scale precipitation. It uses UVV information and moisture information to calculate rainfall. This chart should not be used for exact rainfalls on a local scale since it is using a synoptic scale average. It is a good idea also to compare several models for predicted rainfall. The chart CAN give you an idea of the precipitation coverage, duration, and intensity averaged over a large-scale region (i.e. half the size of a state). These two model charts (700 mb UVV and surface precipitation) can be used to make a fairly reliable synoptic scale precipitation forecast. After examining each of the model panels (sfc, 850, 700, 500, and 300), you will know if rising air is likely, the precipitation pattern caused by the combination of moisture and UVV, and the mechanisms in the atmosphere that cause the air to rise. This is a part of the technique that goes into writing a NWS state forecast discussion. |