Report in Brief flash flood final
Flash Flood Forecasting
Over Complex Terrain
With An Assessment Of The
Sulphur Mountain NEXRAD In
Southern California
Next Generation Radar (NEXRAD) is used to detect wind and precipitation to aid National Weather
Service forecasters in predicting lash loods and other storms. The National Academies were asked
to assess the availability, performance, and capability of the Sulphur Mountain NEXRAD in Southern
California. The report concludes that it provides crucial radar coverage and is appropriately situated
to assist the Los Angeles-Oxnard National Weather Service Forecast Ofice in forecasting and warning
of lash loods. In general, NEXRAD technology is effective for lash lood forecasting in mountainous
terrain but can be improved.
E
very year, floods are responsible for more deaths nationwide than any other weather
phenomenon. On average in the past 30 years, loods have caused more than 120 fatalities per year;
of these, most were due to lash loods. From 1996 through 2003, the average number of lash lood
events recorded nationwide was nearly 3000 per year. Although lash lood forecasting is improving due to
better radar and satellite observations, lash loods continue to be among nature’s worst killers.
The National Weather Service is responsible for detecting hazardous weather and warning the public in
a timely, accurate, and effective way. Forecasting lash loods is especially challenging because forecasters
must predict not only the occurrence of the event but also its magnitude. The amount of precipitation and
WhAT ARE NEXRADs?
Set up in the mid-1990s, the nationwide
network of Next Generation Radars has
greatly improved precipitation analysis.
Today, there are more than 130 radars in
the network including several in southern
California. NEXRADs are active remote
sensing systems that send out pulses of
energy. If the radar energy strikes an
object, such as a rain drop, the energy
scatters and a small fraction is directed
back toward the radar detector. The travel
time, strength, and Doppler phase shift of
the pulse are used to measure the size and
motion of the rain drops which, in turn,
gives wind and rain rate information.
The map shows coverage provided by the
Sulphur Mountain NEXRAD (KVTX)
and 4 adjoining radars. Source: Witold
Krajewski
the time frame in which it occurs can transform an
ordinary rainfall event into a deadly one. The creation
and nationwide deployment of the Next Generation
Radar (NEXRAD) has been instrumental in assisting
forecasters with the detection and tracking of precipitating systems, leading to improved lash lood
forecasts and warnings.
The challenge of predicting flash floods
is particularly great in regions with complex,
or mountainous, terrain where there are sparse,
intermittent surface-level observations. The radar
beam can be partially or completely blocked in
various directions by higher terrain. Placing the
radar at a higher elevation or on a mountaintop may
alleviate this problem, but may also cause the radar
beam to overshoot lower terrain. This problem
is exacerbated by the current restriction on the
NEXRAD system limiting its minimum elevation
angles to 0.5°. As a result, the precipitation intensity
can be underestimated, or some precipitation may go
undetected altogether.
SuLphuR MOuNTAIN RADAR IS EFFECTIvE
AND AppROpRIATELy LOCATED
The Sulphur Mountain NEXRAD located north
of Los Angeles in Ventura County, California, has been
scrutinized in terms of its ability to detect precipitation
events below 1.83-km (6000-ft) altitude and assist
local National Weather Service meteorologists in lash
lood forecasting. Thus, in February 2003, Senator
Barbara Boxer of California, with support from the
National Oceanic and Atmospheric Administration
(NOAA), requested that the National Academies
assess the effectiveness of operating NEXRADs
in complex terrain—with a speciic analysis of the
Sulphur Mountain NEXRAD—to aid the NWS in
forecasting heavy precipitation events and issuing lash
lood forecasts, watches, and warnings.
After conducting a thorough analysis of the
Sulphur Mountain NEXRAD, including the coverage
provided by the radar and the performance of the Los
Angeles-Oxnard Weather Forecast Ofice (LOX WFO)
as measured by the lash lood warning statistics, the
committee found little basis for concerns regarding
the operational effectiveness of the Sulphur Mountain
radar.
Low-Level Atmosphere Coverage Good
The ability to predict flash floods is greatly
enhanced when radars can observe the low-level
atmosphere in order to detect precipitation. To assess
the extent to which the Sulphur Mountain NEXRAD
is able to observe the low-level atmosphere, the
committee conducted detailed calculations to assess
the coverage provided by the radar. The calculations
considered both the surrounding terrain—to determine
if and where the radar beam is blocked by mountains—
and the path of the beam through the atmosphere.
The calculations show that the Sulphur Mountain
NEXRAD provides greater coverage below 6000 ft
than previously believed, making it appropriately sited
to assist Los-Angeles in forecasting and warning of
lash loods. (See Box 1)
Calculations were also completed to assess the
coverage provided by the Sulphur Mountain NEXRAD
compared to the adjoining Vandenberg Air Force Base,
Santa Ana Mountain, Edwards Air Force Base, and San
Diego NEXRADs (see map, p.1). As shown in the map,
the radars provide overlapping coverage for much of the
area, but the tan-shaded areas southwest of the Sulphur
Figure 1. This schematic
representation of a radar site
shows how a radar beam
might travel in mountainous
terrain. The greatest strength
(bright yellow) is centered
near the beam axis with the
intensity weakening further
away (light yellow to white).
Only part of the beam is
blocked by mountains, so
that wind and rain can still
be detected.
Source: Witold Krajewski,
University of Iowa
Box 1. Sulphur Mountain NEXRAD
Coverage Below 6,000 feet
To assess whether the Sulphur Mountain
NEXRAD provides adequate coverage, particularly
below 6,000 feet, the committee considered the full
3-D structure of its radar beam in calculations, not just
the beam axis as was used in prior studies. The full 3D structure includes not only the power concentrated
at the center of the beam, but also the weaker part of
the beam surrounding the center. (See Figure 7.2).
This consideration is important because:
•
•
even if part of the radar beam is intercepted
by terrain, the remaining part of the beam still
travels and can detect precipitation;
the entire 3-D structure of the radar beam can
sense precipitation, so when considering the
lower half of the beam, the radar provides
more coverage of the lower atmosphere than
previously thought.
At its elevation angle of 0.5° upward, the main
axis of the Sulphur Mountain radar beam rises above
1.83 km (6000 ft) altitude approximately 75 km away
from the radar site. However, the lower half of the
radar beam provides an additional 50 km of coverage
below 6000 ft out to approximately 125 km away
from the radar. Lowering the elevation angle would
provide even greater coverage. These coverage and
beam propagation calculations were done at every
degree around the Sulphur Mountain NEXRAD site
for radar beam elevations of 0.5°, 0.0°, and -0.5°.
These calculations are available electronically from
the National Acadmies at http://dels.nas.edu/basc/
nexradsm/radar_beam_and_terrain_viewer.html.
Mountain radar over the Paciic Ocean are covered
exclusively by the Sulphur Mountain radar. Because
storms often approach from this area, additional lowlevel coverage for these areas would be beneicial for
monitoring incoming storms and assessing their lash
lood potential before they move onshore.
Los Angeles-Oxnard Weather Forecast Office’s
Flash Floods Warning Record Excellent
The LOX WFO’s lash lood warning statistics
show that the ofice has an excellent record of issuing
lash lood warnings. When the LOX statistics are
compared to those of the other 115 WFOs throughout
the continental United States, their record of accurately
forecasting lash lood events with advance warnings,
at 79 percent, is better than the national average value
of 69 percent. The ratio of lash lood events in Los
Angeles that were forecast but failed to materialize
(called the false alarm ratio) is less than the national
average. Finally, when compared with the 2004 goals
of the National Weather Service Western Region, which
are established by considering the complex terrain in
the western United States and thus the greater tendency
for rapid onset of lash looding, LOX WFO’s average
lead time, percentage of forecast events, and false alarm
ratio all are superior. (See Figure 2)
IMpROvING NATIONWIDE FLASh FLOOD
FORECASTING
There are several ways to improve lash lood
forecasting and warning, not only in Southern
California and in other regions where NEXRADs are
sited in complex terrain, but throughout the country.
Figure 2. Flash lood warning statistics for the Los Angeles-Oxnard Weather Forecast Ofice compare favorably
with national statistics for lash lood events from 1996 and 2003. The probability of detection is the fraction of
the forecast events that occurred when forecast. The false alarm ratio is the number of events that were forecast
but did not occur. Also shown for comparison sake are the 2004 national and Western Region goals.
Source: Compiled by Paul Smith from NOAA data.
Enhancing Weather Radar Coverage
Low-level radar coverage is limited by the
Earth’s curvature and blockage due to surrounding
terrain. This is exacerbated by the current restriction
of the NEXRAD system to minimum elevation
angles of 0.5°. The use of lower, and perhaps even
negative, elevation angles would allow monitoring of
precipitation and wind at lower altitudes, providing
a more representative assessment of near-surface
rainfall. In addition, high-quality, real-time weather
radar data are becoming more widespread. Making
these data accessible to National Weather Service
forecasters would increase the area and density of
coverage of weather radar data, especially in regions of
complex terrain. To enhance weather radar coverage,
the National Weather Service should:
•
Improve nationwide NEXRAD coverage of
low-level precipitation and wind, especially for
elevated radar sites in complex terrain, through
the adoption of a modiied scan strategy that will
allow scanning at lower elevation angles.
•
Have access to all available regional real-time
weather radar data including the Federal Aviation
Administration (FAA) and Department of Defense
NEXRAD radars; FAA Terminal Doppler Weather
Radars and other surveillance radars equipped to
provide weather-echo data; local television station
Doppler radars; and operational radars from other
organizations.
•
Consider augmenting the NEXRAD network
with additional short-range radars to improve
observation of low-level meteorological
phenomena (e.g. rain, snow, wind).
Guiding Future Directions
Radars can observe the evolving weather systems
that cause flash floods as they move into areas of
concern, and future NEXRAD enhancements will
improve the quality of those observations. The
measurements of accumulating precipitation may not
provide the most effective warning capability, but new
and evolving techniques are available to improve this
process. There also are gains to be made from rapidly
evolving capabilities for better short-term and inescale forecasting, using regional and local numerical
models that ingest real-time observations. In addition
to these near-term improvements, consideration of
hydrologic factors should be an integral part of future
radar siting.
•
•
To increase the accuracy and lead time of lash
flood forecasts and warnings, the National
Weather Service should continue to implement
new technologies and techniques including (a) the
Flash Flood Monitoring and Prediction program
at all Weather Forecast Ofices, (b) polarimetric
modiications to NEXRAD, (c) data assimilation
systems that integrate radar and other operational
datasets into coupled hydrometeorological
and hydrological models, and (d) data fusion
systems.
In addition to the original NEXRAD siting
considerations, future siting of radars in complex
terrain should include detailed assessments of
coverage in areas at risk for lash looding.
Committee to Assess NEXRAD Flash Flood Forecasting Capabilities at Sulphur Mountain, California: paul
L. Smith (Chair), South Dakota School of Mines and Technology, Rapid City; Ana Barros, Duke University,
Durham, North Carolina; v. Chandrasekar, Colorado State University, Fort Collins; Greg Forbes, The Weather
Channel, Inc., Atlanta, Georgia; Eve Gruntfest, University of Colorado, Colorado Springs; Witold Krajewski,
University of Iowa, Iowa City; Thomas potter, University of Utah, Salt Lake City; Rita Roberts, National Center
for Atmospheric Research, Boulder, Colorado; Matthias Steiner, Princeton University, New Jersey; Roger Wakimoto, University of California, Los Angeles; Julie Demuth (Study Director), Board on Atmospheric Sciences
and Climate; Elizabeth Galinis (Senior Program Assistant), Board on Atmospheric Sciences and Climate.
This report brief was prepared by the National Research Council’s Board on Atmospheric
Sciencs and Climate. For more information, contact the Board at 202-334-3512 or visit
http://www.national-academies.org/basc. Flash Flood Forecasting Over Complex Terrain
is available from the National Academies Press,
500 Fifth Street, NW, Washington, DC 20001; 800-624-6242; www.nap.edu.
Permission granted to reproduce this brief in its entirety with no additions or alterations.
Copyright 2004 The National Academy of Sciences
Over Complex Terrain
With An Assessment Of The
Sulphur Mountain NEXRAD In
Southern California
Next Generation Radar (NEXRAD) is used to detect wind and precipitation to aid National Weather
Service forecasters in predicting lash loods and other storms. The National Academies were asked
to assess the availability, performance, and capability of the Sulphur Mountain NEXRAD in Southern
California. The report concludes that it provides crucial radar coverage and is appropriately situated
to assist the Los Angeles-Oxnard National Weather Service Forecast Ofice in forecasting and warning
of lash loods. In general, NEXRAD technology is effective for lash lood forecasting in mountainous
terrain but can be improved.
E
very year, floods are responsible for more deaths nationwide than any other weather
phenomenon. On average in the past 30 years, loods have caused more than 120 fatalities per year;
of these, most were due to lash loods. From 1996 through 2003, the average number of lash lood
events recorded nationwide was nearly 3000 per year. Although lash lood forecasting is improving due to
better radar and satellite observations, lash loods continue to be among nature’s worst killers.
The National Weather Service is responsible for detecting hazardous weather and warning the public in
a timely, accurate, and effective way. Forecasting lash loods is especially challenging because forecasters
must predict not only the occurrence of the event but also its magnitude. The amount of precipitation and
WhAT ARE NEXRADs?
Set up in the mid-1990s, the nationwide
network of Next Generation Radars has
greatly improved precipitation analysis.
Today, there are more than 130 radars in
the network including several in southern
California. NEXRADs are active remote
sensing systems that send out pulses of
energy. If the radar energy strikes an
object, such as a rain drop, the energy
scatters and a small fraction is directed
back toward the radar detector. The travel
time, strength, and Doppler phase shift of
the pulse are used to measure the size and
motion of the rain drops which, in turn,
gives wind and rain rate information.
The map shows coverage provided by the
Sulphur Mountain NEXRAD (KVTX)
and 4 adjoining radars. Source: Witold
Krajewski
the time frame in which it occurs can transform an
ordinary rainfall event into a deadly one. The creation
and nationwide deployment of the Next Generation
Radar (NEXRAD) has been instrumental in assisting
forecasters with the detection and tracking of precipitating systems, leading to improved lash lood
forecasts and warnings.
The challenge of predicting flash floods
is particularly great in regions with complex,
or mountainous, terrain where there are sparse,
intermittent surface-level observations. The radar
beam can be partially or completely blocked in
various directions by higher terrain. Placing the
radar at a higher elevation or on a mountaintop may
alleviate this problem, but may also cause the radar
beam to overshoot lower terrain. This problem
is exacerbated by the current restriction on the
NEXRAD system limiting its minimum elevation
angles to 0.5°. As a result, the precipitation intensity
can be underestimated, or some precipitation may go
undetected altogether.
SuLphuR MOuNTAIN RADAR IS EFFECTIvE
AND AppROpRIATELy LOCATED
The Sulphur Mountain NEXRAD located north
of Los Angeles in Ventura County, California, has been
scrutinized in terms of its ability to detect precipitation
events below 1.83-km (6000-ft) altitude and assist
local National Weather Service meteorologists in lash
lood forecasting. Thus, in February 2003, Senator
Barbara Boxer of California, with support from the
National Oceanic and Atmospheric Administration
(NOAA), requested that the National Academies
assess the effectiveness of operating NEXRADs
in complex terrain—with a speciic analysis of the
Sulphur Mountain NEXRAD—to aid the NWS in
forecasting heavy precipitation events and issuing lash
lood forecasts, watches, and warnings.
After conducting a thorough analysis of the
Sulphur Mountain NEXRAD, including the coverage
provided by the radar and the performance of the Los
Angeles-Oxnard Weather Forecast Ofice (LOX WFO)
as measured by the lash lood warning statistics, the
committee found little basis for concerns regarding
the operational effectiveness of the Sulphur Mountain
radar.
Low-Level Atmosphere Coverage Good
The ability to predict flash floods is greatly
enhanced when radars can observe the low-level
atmosphere in order to detect precipitation. To assess
the extent to which the Sulphur Mountain NEXRAD
is able to observe the low-level atmosphere, the
committee conducted detailed calculations to assess
the coverage provided by the radar. The calculations
considered both the surrounding terrain—to determine
if and where the radar beam is blocked by mountains—
and the path of the beam through the atmosphere.
The calculations show that the Sulphur Mountain
NEXRAD provides greater coverage below 6000 ft
than previously believed, making it appropriately sited
to assist Los-Angeles in forecasting and warning of
lash loods. (See Box 1)
Calculations were also completed to assess the
coverage provided by the Sulphur Mountain NEXRAD
compared to the adjoining Vandenberg Air Force Base,
Santa Ana Mountain, Edwards Air Force Base, and San
Diego NEXRADs (see map, p.1). As shown in the map,
the radars provide overlapping coverage for much of the
area, but the tan-shaded areas southwest of the Sulphur
Figure 1. This schematic
representation of a radar site
shows how a radar beam
might travel in mountainous
terrain. The greatest strength
(bright yellow) is centered
near the beam axis with the
intensity weakening further
away (light yellow to white).
Only part of the beam is
blocked by mountains, so
that wind and rain can still
be detected.
Source: Witold Krajewski,
University of Iowa
Box 1. Sulphur Mountain NEXRAD
Coverage Below 6,000 feet
To assess whether the Sulphur Mountain
NEXRAD provides adequate coverage, particularly
below 6,000 feet, the committee considered the full
3-D structure of its radar beam in calculations, not just
the beam axis as was used in prior studies. The full 3D structure includes not only the power concentrated
at the center of the beam, but also the weaker part of
the beam surrounding the center. (See Figure 7.2).
This consideration is important because:
•
•
even if part of the radar beam is intercepted
by terrain, the remaining part of the beam still
travels and can detect precipitation;
the entire 3-D structure of the radar beam can
sense precipitation, so when considering the
lower half of the beam, the radar provides
more coverage of the lower atmosphere than
previously thought.
At its elevation angle of 0.5° upward, the main
axis of the Sulphur Mountain radar beam rises above
1.83 km (6000 ft) altitude approximately 75 km away
from the radar site. However, the lower half of the
radar beam provides an additional 50 km of coverage
below 6000 ft out to approximately 125 km away
from the radar. Lowering the elevation angle would
provide even greater coverage. These coverage and
beam propagation calculations were done at every
degree around the Sulphur Mountain NEXRAD site
for radar beam elevations of 0.5°, 0.0°, and -0.5°.
These calculations are available electronically from
the National Acadmies at http://dels.nas.edu/basc/
nexradsm/radar_beam_and_terrain_viewer.html.
Mountain radar over the Paciic Ocean are covered
exclusively by the Sulphur Mountain radar. Because
storms often approach from this area, additional lowlevel coverage for these areas would be beneicial for
monitoring incoming storms and assessing their lash
lood potential before they move onshore.
Los Angeles-Oxnard Weather Forecast Office’s
Flash Floods Warning Record Excellent
The LOX WFO’s lash lood warning statistics
show that the ofice has an excellent record of issuing
lash lood warnings. When the LOX statistics are
compared to those of the other 115 WFOs throughout
the continental United States, their record of accurately
forecasting lash lood events with advance warnings,
at 79 percent, is better than the national average value
of 69 percent. The ratio of lash lood events in Los
Angeles that were forecast but failed to materialize
(called the false alarm ratio) is less than the national
average. Finally, when compared with the 2004 goals
of the National Weather Service Western Region, which
are established by considering the complex terrain in
the western United States and thus the greater tendency
for rapid onset of lash looding, LOX WFO’s average
lead time, percentage of forecast events, and false alarm
ratio all are superior. (See Figure 2)
IMpROvING NATIONWIDE FLASh FLOOD
FORECASTING
There are several ways to improve lash lood
forecasting and warning, not only in Southern
California and in other regions where NEXRADs are
sited in complex terrain, but throughout the country.
Figure 2. Flash lood warning statistics for the Los Angeles-Oxnard Weather Forecast Ofice compare favorably
with national statistics for lash lood events from 1996 and 2003. The probability of detection is the fraction of
the forecast events that occurred when forecast. The false alarm ratio is the number of events that were forecast
but did not occur. Also shown for comparison sake are the 2004 national and Western Region goals.
Source: Compiled by Paul Smith from NOAA data.
Enhancing Weather Radar Coverage
Low-level radar coverage is limited by the
Earth’s curvature and blockage due to surrounding
terrain. This is exacerbated by the current restriction
of the NEXRAD system to minimum elevation
angles of 0.5°. The use of lower, and perhaps even
negative, elevation angles would allow monitoring of
precipitation and wind at lower altitudes, providing
a more representative assessment of near-surface
rainfall. In addition, high-quality, real-time weather
radar data are becoming more widespread. Making
these data accessible to National Weather Service
forecasters would increase the area and density of
coverage of weather radar data, especially in regions of
complex terrain. To enhance weather radar coverage,
the National Weather Service should:
•
Improve nationwide NEXRAD coverage of
low-level precipitation and wind, especially for
elevated radar sites in complex terrain, through
the adoption of a modiied scan strategy that will
allow scanning at lower elevation angles.
•
Have access to all available regional real-time
weather radar data including the Federal Aviation
Administration (FAA) and Department of Defense
NEXRAD radars; FAA Terminal Doppler Weather
Radars and other surveillance radars equipped to
provide weather-echo data; local television station
Doppler radars; and operational radars from other
organizations.
•
Consider augmenting the NEXRAD network
with additional short-range radars to improve
observation of low-level meteorological
phenomena (e.g. rain, snow, wind).
Guiding Future Directions
Radars can observe the evolving weather systems
that cause flash floods as they move into areas of
concern, and future NEXRAD enhancements will
improve the quality of those observations. The
measurements of accumulating precipitation may not
provide the most effective warning capability, but new
and evolving techniques are available to improve this
process. There also are gains to be made from rapidly
evolving capabilities for better short-term and inescale forecasting, using regional and local numerical
models that ingest real-time observations. In addition
to these near-term improvements, consideration of
hydrologic factors should be an integral part of future
radar siting.
•
•
To increase the accuracy and lead time of lash
flood forecasts and warnings, the National
Weather Service should continue to implement
new technologies and techniques including (a) the
Flash Flood Monitoring and Prediction program
at all Weather Forecast Ofices, (b) polarimetric
modiications to NEXRAD, (c) data assimilation
systems that integrate radar and other operational
datasets into coupled hydrometeorological
and hydrological models, and (d) data fusion
systems.
In addition to the original NEXRAD siting
considerations, future siting of radars in complex
terrain should include detailed assessments of
coverage in areas at risk for lash looding.
Committee to Assess NEXRAD Flash Flood Forecasting Capabilities at Sulphur Mountain, California: paul
L. Smith (Chair), South Dakota School of Mines and Technology, Rapid City; Ana Barros, Duke University,
Durham, North Carolina; v. Chandrasekar, Colorado State University, Fort Collins; Greg Forbes, The Weather
Channel, Inc., Atlanta, Georgia; Eve Gruntfest, University of Colorado, Colorado Springs; Witold Krajewski,
University of Iowa, Iowa City; Thomas potter, University of Utah, Salt Lake City; Rita Roberts, National Center
for Atmospheric Research, Boulder, Colorado; Matthias Steiner, Princeton University, New Jersey; Roger Wakimoto, University of California, Los Angeles; Julie Demuth (Study Director), Board on Atmospheric Sciences
and Climate; Elizabeth Galinis (Senior Program Assistant), Board on Atmospheric Sciences and Climate.
This report brief was prepared by the National Research Council’s Board on Atmospheric
Sciencs and Climate. For more information, contact the Board at 202-334-3512 or visit
http://www.national-academies.org/basc. Flash Flood Forecasting Over Complex Terrain
is available from the National Academies Press,
500 Fifth Street, NW, Washington, DC 20001; 800-624-6242; www.nap.edu.
Permission granted to reproduce this brief in its entirety with no additions or alterations.
Copyright 2004 The National Academy of Sciences