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Charles A. Doswell III, Harold E. Brooks,
and Robert A. Maddox
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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.
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“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
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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.
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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 = mp/mi where mp is the mass of water falling as precipitation and mi
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.
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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.
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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 (Cs) 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 (Ls). The Duration can now
be expressed as
D = Ls(|Cs|)-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
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Since Cc, convective cell movement, is related to
Vm, 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
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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.