PRECIPITATION FORECAST STRATEGY
METEOROLOGIST JEFF HABY
(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
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
for low level convergence resulting from fronts, topography, low
pressure, WAA and moisture advection.
(3) Study the
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
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.
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
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).
*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.