This section offers the most important data to study when predicting heavy rain. Each factor will be presented as well as its importance to heavy rain forecasting.
1. Precipitable water:
Precipitable water is the amount of water that would fall to the surface if all moisture in the atmosphere were brought down to the surface. The value can be found by either finding the data from a Skew-T or located a precipitable water chart. Precipitable water values less than 0.75 inches are considered relatively low while greater than 1.5 relatively high. The value must be compared with the climatology of a region. A vegetated region will support higher values of precipitable water than a semi-desert region. Precipitable water should not be used to predict an exact rain total since moisture convergence can enhance the amount of moisture that has the potential to fall over an area. Higher PW values are maximized when surface dewpoints are high and the atmosphere is near saturation through a large depth of the atmosphere. Often the precipitable water value will be given with a percent of climatology value. If the percent of climatology value is above 150%, this is an indication heavy prolonged rain will produce flooding in that region.
2. Moisture convergence:
Quasi-stationary trigger mechanisms can force moisture into the same area for a prolonged period of time. Examples include stalled fronts and a near stationary mid-latitude cyclone. As long as moisture is supplied into the stationary trigger mechanism it will continue to rain.
3. Stalled Trigger mechanism:
Slow movement of fronts or tropical systems that have heavy rain already associated with them will continue to produce heavy rain over the same areas. Trigger mechanisms such as fronts will begin to stall if upper level winds are nearly parallel to the surface trigger mechanism.
4. Deep level of moisture:
This is related to a high precipitable water if a deep layer of moisture exists in warm and humid air. The lifting of a deep layer of moisture has the potential to produce heavy rain. In the US, this can occur from a deep layer of Gulf moisture banking up against a slow moving trigger mechanism.
5. Saturated Soil and Snow Melt:
Once soils have exceeded their capacity to capture water, rainfall after this point will lead to runoff. Soils can be saturated from rain in a previous storm system or a prolonged rain event from one storm system. Snow melt adds to the amount of runoff. Heavy rain combined with rapid snowmelt is conducive to flash flooding.
6. Frozen soil and very dry soil:
Frozen soil acts like saturated soil. All rainfall will immediately runoff. The pore spaces in very dry soil do not allow water to penetrate at a quick rate. Very heavy rain on bone dry soil can caused flooding. This occurs with heavy convective thunderstorms over arid and semi-arid regions.
7. Weak winds aloft:
Weak winds aloft do not allow thunderstorms or fronts to move very quickly. This can produce local or widespread heavy rain.