Charles A. Doswell III, Harold E. Brooks, and Robert A. Maddox

Charles A. Doswell III, Harold E. Brooks, and Robert A. Maddox

Flash floods account for the greatest number of fatalities among convective storm-related events but it still remains difficult to forecast and warn for flash floods. The primary challenge in predicting flash flooding is that it goes beyond just predicting the occurrence of rain but the amount of rain. Hydrology also affects the prediction of flash flooding. An ingredients-based approach to forecasting creates a universally relevant framework that doesn t rely on statistical relationships but instead on physical processes.

The heaviest precipitation occurs where the rainfall rate is the highest for the longest period of time. (C. F. Chappell) This statement can be converted to the equation P RD where P is the total precipitation, R is the average rainfall rate and D is the duration. The authors of this paper decline to give exact numbers for what would be considered a high rainfall rate or a long duration as it would vary depending on the hydrometeorological situation.

In a general sense, the instantaneous rainfall rate is proportional to the magnitude of the vertical moisture flux. The moisture flux is the inflow of moist air, represented by wq where w is the ascent rate and q is the mixing ratio of the rising air. Not all water vapor that enters a cloud falls out as precipitation this relationship is quantified as the Precipitation Efficiency (E). E = m p /m i where m p is the mass of water falling as precipitation and m i is this influx of water vapor mass into the cloud. The instantaneous rainfall rate can now be expressed as R = Ewq. The calculation of the instantaneous rainfall rate itself is not important since all the amounts are averaged over the lifetime of the storm and therefore the exact calculation would be after the fact Look for where there is potential for a high rainfall rate, which would be where at least one of the factors (E, w, or q) is large and the rest at least moderate. The potential for a high rainfall rate increases as E, w, and q increase.

While flash flooding can occur in non-convective storms, flash flooding associated with convective storms is far more common. Deep moist convection is primarily a warm season event which allows for higher moisture content in the air and buoyant instability promotes strong upward motion. This combination can lead to high rain rates. Precipitation efficiency is not as important, unless unusually low, due to the higher values for ascent rate and mixing ratio. Ingredients for buoyancy and deep moist convection: The environmental lapse rate is conditionally unstable There is sufficient moisture that the rising parcel s associated moist adiabat has a level of free convection There is some process to lift the parcel to its LFC.

High rainfall rates alone cannot produce flash floods; it must be accompanied by a long duration over which rain is falling on the same area. This is most commonly achieved by quasi-stationary convective systems. Along with system size and system movement speed, the system motion vector (C s ) can greatly change the amount of precipitation a point on the ground receives. Instead of system size consider the length of the system along the system motion vector from where the point intercepts the storm to where it will pass out the other side (L s ). The Duration can now be expressed as D = L s ( C s ) -1.

Consider the following examples: a) Motion normal to storm line b) Motion more parallel to storm line c) Similar to b but with an area of trailing stratiform precipitation d) Train effect

Since C c, convective cell movement, is related to V m, the mean wind through some deep tropospheric layer, slow system movement could result from weak winds. A slow moving storm can still occur despite strong winds if cell movement is cancelled by the propagation effect resulting in slow system movement.

Multicell convection: Most convection is multicellular, even storms that may appear to have a single cell in radar images. Since most storms fall in the multicell category the importance isn t so much the number of cells but the degree of organization Supercell convection: Supercells tend to have strong updrafts as well as significant low level moisture which have potential for heavy rainfall rates but there are two factors of typical supercells that can reduce this potential. First is the loaded gun sounding often associated with supercells. The loaded gun sounding has a dry lower troposphere which can result in evaporation, reducing the Precipitation efficiency. Second, supercells tend to have high wind speeds aloft which cause fast system movement.

Squall lines (radar): A linear structure is probably the most common structure for flash flood producing storms. The train effect of storms developing and following one after another over the same spot is a characteristic of a linear structure and often a feature of flash flood events Mesoscale convective systems: MCSs tend to have a more or less linear structure as seen on a radar image. They also tend to have a trailing area of stratiform precipitation that extends the duration of rainfall. There is also a special type of evolution contributed to by MCSs where, over a period of days MCSs continually form and pass over the same area. This evolution can lead to the combination of flash floods and river floods.

Squall lines (satellite): What can appear as a squall line on a satellite is not an indication of a linear structure on a radar. These satellite viewed linear structures tend to be synoptic scale fronts and are not known to commonly produce flash floods. Non-convective precipitation systems: Flash floods produced by non-convective systems while not as common, do occur. Usually the upward motion for this type of storm is forced rather than freely buoyant. Forced uplift usually results in a lower, warmer system which can affect how it is viewed in a satellite or radar image. This can make it difficult to forecast rainfall amounts.

There are some synoptic scale processes that affect the possibility for flash flooding. Short wave troughs are known to often coincide with deep moist convection. There is also a connection with flash flooding near a 500mb ridge axis. This connection may be due to the suppressing effect of the anticyclone on deep moist convection. This suppression allows moisture to return at low levels and accumulate. Convection on the margins of the suppressed area can now draw on this accumulated moisture.

Mesoscale processes contribute the lift to initiate convection. Mesoscale convective system propagation can prolong the duration of rainfall. MCS s tend to have a deep convective part as well as a stratiform precipitation part which causes a prolonged duration. MCS s also produce outflow boundaries which can initiate new convection.

Storm outflow can change the evolution of the system. If the downdraft evaporates a lot of condensate, the air will cool which will result in negative buoyancy. High evaporation is associated with a low precipitation efficiency. This suggests that a warmer outflow would have a higher precipitation efficiency. A warmer outflow would also move more slowly away from its source which would cause new cells to form closer to their predecessor.

Even though the low level flow is weak; the advection of warm, moist air from the gulf slowly moves north and north-eastward into Iowa. 500 hpa, 0000 UTC: a) 7 September 1989 b) 8 September 1989 Heights (solid lines, interval 60 dam), temperature (dashed lines, interval 5 C), short-wave trough axes (thick-dashed lines)

It is easier to see the movement of moist air in this image. It is an isentropic analysis on the 306K surface. Pressure (solid lines, 40 hpa interval) mixing ratio (dashed lines, 2 g kg-1 interval) a) 7 September 1989 0000 UTC b) 8 September 1989 0000 UTC

The thermodynamic profiles for a) 7 September and b) 8 September at 0000 UTC You can see the inversion on the Sep 7 sounding becomes much higher and weaker on the Sep 8 sounding. There is also a thicker layer of increased low level moisture on the Sep 8 sounding. Notice the wind profile remains weak.

High lapse rates over southern Nevada coupled with substantial low level moisture indicates that a large CAPE is possible which indicates substantial vertical motion. Vertical motion and low level moisture lead to the possibility of high rainfall rates. 700 500-hPa temperature difference (solid lines, interval 2 C) and the 850-hPa mixing ratio (dashed lines, interval 2 g kg-1). 1200 UTC 10 August 1981.

The hodograph shows the general flow of southeasterly wind. The storm cells moved relatively slowly since the wind speed remained below 15 m s-1 for the most part. The pressure levels displayed along the hodograph are in hpa This flash flood event killed 500 cattle at the Hidden Valley Ranch and caused approximately $3 million in damages.

The lifting mechanism for this flash flood event was a mountain range along the Kenai peninsula with peaks of about 1500m. A strong synoptic system approaching from the west was forced up the slope. The slow moving system caused the upslope flow to persist for 48hrs. Moderate rainfall rates were sustained for 2 days.

In order to have flash flooding it is necessary for heavy precipitation to fall in an area with the proper hydrological setup (which has not been dealt with in this paper). Heavy precipitation requires sustained high rainfall rates which result from slow moving, rain producing systems. There are different ways in which these ingredients can come together but by returning our attention to the basic underlying physical processes it is possible to see the combination of ingredients that may not be superficially visible. Ingredients based forecasting is not limited to flash flooding and the authors of this paper suggest strongly that such an approach should be used for all forecasting events.