Ingredients necessary for flash flooding : 

 

Flash flooding is most often caused either by intense slow moving isolated convective storms, by intense rainfall producing Mesoscale Convective Systems (MCSs) or by sustained large-scale or mesoscale lift of humid air over a given region or watershed. 

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Independent of the cause, certain atmospheric ingredients need to be present in order to foster intense rainfall rates susceptible to produce flash flooding : 

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• Moisture - A high atmospheric water content  - Warm and moist airmasses that exhibit high relative humidities throughout their tropospheric profiles are more apt to produce intense rainfall rates given the presence of sufficient lift. More specifically, elevated values of both Total Precipitable Water (TPW) and Integrated Water Vapor Transport (IVT) are often pertinent indicators that heavy rainfall is likely and that flash flooding is a possibility. Maritime tropical airmasses are most often the airmasses that exhibit these temperature and moisture characteristics. They tend to increase the probability of flash-flooding rains, especially when atmospheric rivers coupled with low-level jets within warm sectors are associated with them.

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• A deep Warm Cloud Layer (WCL) - The greater the altitude/depth between the cloud base and the freezing level, the thicker the warm cloud layer. This increases precipitation efficiency since rain droplets will grow rapidly via the collision-coalescence process in warm-cloud layers. Additionally, a deep warm-cloud layer implies an elevated freezing level which often allows complete melting of hail/graupel within the downdraft which can increase precipitation rates and favor wet microbursts. Studies have shown that warm cloud layer depths between 3 to 4 km are often associated with flash flooding in the US.

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• Sustained large-scale or mesoscale lift over the same region - Whether this lift is induced by large-scale processes originating from vorticity/temperature advection or forced by a low-level frontal/convergence zone or topographical feature, the moment it is sustained and operating over the same region, flash flooding can rapidly become a possibility given a sufficient moisture content of the airmass. If the precipitation is convective in nature, back-building thunderstorms creating training of storms over the same region over an extended period of time can quickly lead to flash-flooding as well.​

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• Relatively weak vertical wind shear - Weak vertical wind shear tends to increase the probabilty of flash flooding with rainfall producing systems in two ways. First by favoring slow-moving storms that affect a given region during a longer period of time. Second, by limiting the mixing of dry external air into the rainfall producing system, thereby limiting dry air entrainment.  This tends to shield the storms and prevents excessive evaporation of the moisture. An exception to this is when low-level jets advect atmospheric rivers within warm sectors which are then forced upwards by low-level convergence boundaries (ex: by topography, surface fronts)

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In a nowcasting environment, additional clues that can alert forecasters on the likelihood of impending heavy rainfall rates susceptible to produce flash flooding, include :

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• Approach of an atmospheric river on satellite-derived TPW displays coupled with a low-level jet (LLJ) - Atmospheric rivers are low-level sub-tropical atmospheric moisture filaments that advect from sub-tropical oceanic basins into the mid-latitudes. They are most often found within warm sectors of extratropical cyclones and often accelerated northward by low-level jets associated with warm conveyor belts. You can best visualize them in real-time with satellite-derived Total Precipitable Water (TPW) imagery products such as the ones found on the MIMIC-TPW website provided by the University of Wisconsin-Madison. You can also compare this real-time satellite-derived TPW imagery with NWP forecasts of TPW to see how well any particular numerical model is forecasting the strength and position of these atmospheric rivers. A related NWP field to TPW is Integrated Water Vapor Transport (IVT) which helps better quantify how much tropical moisture is actually being transported by the forecasted atmospheric rivers. IVT is often expressed in kg m-1 s-1. 

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• Imminent or ongoing low-level convergence of a low-level jet containing an atmospheric river with a topographical barrier or with a low-level frontal zone (especially if within a maritime tropical (mT) airmass) - When tropical moisture-laden atmospheric rivers are accelerated by low-level jets and impinge on convergence boundaries (topography or low-level frontal/convergence zones), the resultant mesoscale lift created can rapidly induce extremely heavy rainfall rates over the same region. If the low-level jet shows spatial and temporal persistence and operates within an unstable airmass, embedded thunderstorm can rapidly aggravate an already critical situation and greatly increase the flash flooding risk. 

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• Slow-moving convection within a very moist tropospheric profile When slow-moving radar thunderstorm echoes are active over a given region in a weakly sheared environment, these pulse storms will have more time to dump important amounts of rain over a localized area. This is especially the case if TPW values are particularly high and if warm-cloud layers are particularly deep, which tends to increase the occurrence of wet microbursts capable of producing extreme rainfall rates. In these circumstances, the underlying watersheds will typically funnel extreme water loads into the main river channel increasing the flash flooding risk.

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• Training of convective radar echoes over the same region during a prolonged period of time When radar echoes are observed to transit or "train" over the same regions repeatedly, this usually has a similar effect as what a stationnary radar echo can produce in terms of rainfall rates. This is particularly true if the radar echoes are convective in nature. This "training of echoes" should be closely monitored for possible flash flood warning issuance.

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• Back-building of thunderstorms favoring training of radar echoes When thunderstorm radar echoes tend to "back-build" with regards to their original location, this can amplify the training of thunderstorm cells over the same regions. Back-building can be amplified if facilated by a topographical feature, in which case a V-shaped thunderstorm cluster can appear on satellite imagery. These back-building thunderstorm clusters can be very persistant if the mesoscale lift generating them is long-lasting, as is the case when stationnary low-level jets impinge perpendicularly on a topographical barrier in an unstable airmass.

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