Flash flooding is a serious risk for personal safety and property that is often caused by convective storms. Factors that influence the likelihood and severity of flash flood are not purely meteorological, but also hydrological. For example, storms may cause much more problems when the soil is already saturated, the terrain is sloped or water levels are already high (Doswell et al., 1996).

The amount of rain that falls at a certain location is the product of

the average intensity

the duration.

It is therefore useful to look under which circumstances those two factors are biggest in order to characterize the situations that may lead to excessive rainfall accumulations.

The average intensity is related to the amount of water vapour that is condensed in a convective updraft and the speed with which the condensation occurs. Hence, both strong instability that leads to strong updrafts and a high amount of water vapour available for condensation are important for the precipitation rate. Convection that is sustained from a deep, moist boundary layer will likely produce storms with heavier precipitation than a shallower or drier one.

Fig. 2.1. From Doswell et al. (1996).

The total duration of rainfall may especially be very large when multiple convective cells track over the same area. However, a squall line that propagates perpendicular to its convective line will probably not cause much of a flash flood threat (see fig. 2.1.). The movement of a convective system is determined by two components: advection and propagation.

Fig. 2.2. The speed and direction of movement of an MCS can be estimated using the technique developed by Corfidi et al. (1996).

Corfidi et al. (1996) have found that the advection of convective systems can often be approximated by the 0-6 km mean wind, while their propagation is opposite to the low-level flow, e.g. at 850 hPa. The sum of those two vectors gives an estimate for the propagation vector of an MCS. In a formula:

v_MCS = v_mean(850-300hPa) - v_850hPa

Fig. 2.3. Illustration of the MCS motion resulting from the motion of convective cells by advection by the mean cloud-layer wind and propagation of the system (here, to the south).

In the most common situation of MCS occurrence, where winds veer and increase with height, this method predicts a movement that is slower and to the right of the mean wind in the cloud layer (see fig. 2.3). Using this technique of MCS propagation, it is possible to find out if the MCS will move very slowly in places, which may cause a very long duration of precipitation and hence a chance of flooding.

The above method does not seem to work very well for bow-echoes that tend to move quicker downshear than other MCS's. A major reason is that its motion is strongly influenced by the movement of the cold pool. Heavy rainfall is usually much less of a problem with such systems because of their higher velocities, while severe winds occur very often. A better estimate for the movement of downshear or forward-propagating MCS's is (Corfidi, 2003):

v_MCS= v_mean(850-300hPa) - (v_850 hPa - v_mean(850-300hPa))