(1) Study the 700mb vertical velocity progs on the NAM, and GFS, paying close attention to your forecast area. Take note whether the vertical velocity is upwards or downwards. Upward motion (+UVV's) are caused by low level convergence and/or upper level divergence. A UVV of 6 to 9 is moderate, 10+ is large. (2) Study the 850mb progs and see if your forecast area will be experiencing warm air advection, cold air advection or neutral advection. Low level warm air advection leads to upward vertical velocity while low level cold air advection leads to downward vertical velocity. Neutral thermal advection will neither inhibit nor enhance upward vertical velocity. Examine 1000-mb prog for low level convergence resulting from fronts, topography, low pressure, WAA and moisture advection. (3) Study the 500mb vorticity panels and see if your forecast area will be experiencing positive vorticity advection or negative vorticity advection. PVA will lead to upward vertical velocity while NVA will lead to downward vertical velocity. Small values of vorticity will neither inhibit nor enhance upward vertical velocity. (4) Study 300mb panels. See if any jet streaks will influence your forecast region. The divergence sector of a jet streak will lead to upward vertical velocities. These sectors are the right rear and left front quadrants. If the jet is in a highly curved flow, then the divergence occurs anywhere just north of the jet axis in the Northern Hemisphere. If the upper level winds are weak, this will neither inhibit nor enhance upward vertical velocity. (5) Study thermodynamic diagrams to assess the potential for convective precipitation and precipitation type. Pay attention to moist/dry layers, wind profile, and indices. (6) Ask yourself which phenomena (WAA, PVA, Streaks) are leading to upward vertical velocity and which phenomena (CAA, NVA, Streaks) are leading to downward vertical velocity. (7) Take note of the variation between the graphical forecast models on the forecasting of synoptic scale precipitation. If vertical velocities are positive, and no precipitation is progged, then there is likely not enough available moisture. Check RH panels to see if uplift is enough to saturate atmosphere. PBL dewpoints can be used to assess how much moisture can be lifted. (8) Decide whether precipitation is likely or unlikely and decide if severe weather is likely or unlikely. Decide on the following characteristics of the precipitation: *Convective or stratiform *Heavy or light *Widespread or numerous *Severe or non-severe *Long lasting or brief *Wintry or non-wintry (9) Read NWS convective discussion, zones forecast, state forecast discussion and state forecast. (10) Examine MOS precipitation data from several synoptic scale forecast models (11) Look at national and local satellite and radar data as well as a current surface map. Note the features on each of these three sources and ask yourself how they will move through time. (12) Write out your precipitation forecast, and the expected character of the precipitation if you expect precipitation to occur. Forecast should include amount, duration, type, intensity and other unique characteristics of the precipitation given in (8). NOTES: *If vertical velocity is significantly upwards and PVA and jet streaks are not present, then precipitation development is expected to be thermodynamic in origin (low level buoyancy, low level forcing) or the result of low level convergence. *If vertical velocity is significantly upwards and little WAA or low level convergence is present, then precipitation development is expected to be dynamic in origin (upper level forcing by PVA and / or jet streak). *If all three +UVV mechanisms are significantly present in the same vacinity (WAA, PVA, Streak), then expect severe storms or heavy precipitation. (must also have low level moisture, deep instability) *If none of these UVV mechanisms are present, then precipitation can only occur by a mesoscale process (outflow boundaries, sea breeze, orographic lifting, shallow fronts, air mass thermodynamic thunderstorms). *All forecasting rules of thumbs have exceptions. Add your own intuition to determine precipitation potential. |