AMBER Areal Mean Basin Estimated Rainfall COMAP Symposium 00-3 Robert S. Davis, Pittsburgh WFOPresented by Tom Filiaggi
AMBER Overview • All watersheds in county warning area are defined to a Minimum Basin Area (MBA) of 2 mi2 to show spatial rainfall distribution. • The WSR-88D polar grid (1o by 1km) is mapped to the defined watersheds. • Average Basin Rainfall (ABR) is computed for each watershed every volume scan. • An hourly ABR rate is determined from the single volume scan (5-6 min.) of ABR.
National Basin Delineation • 30Sep1999 – NWS agrees to fund the AMBER National Basin Delineation project by NSSL for all of the United States. • ArcInfo used to define the watershed boundaries. • USGS 1 arc second (30 m) DEM data supplied by EROS Data Center. • www.nssl.noaa.gov/teams/western/basins
AMBER Watershed Division • All watersheds in the county warning area are divided into hydrologic segments no larger than 10 mi2. • A Minimum Basin Area of 2 mi2 is used in the initial NSSL stream creation procedure. • Each forecast office may modify the NSSL stream database (subdivide or aggregate).
AMBER 10 mi2 Watersheds • Operational 01May1985 • Basins defined on “Fishing stream map of Pennsylvania. • Color Graphics display of WSR-57 rainfall estimates RADAP-II data (2o x 1 nm grid) • 36 county map backgrounds with overlay of defined streams watersheds. • No ABR computation.
DHR WSR-88D Bin AssignmentPolar grid 1o by 1km • DHR grid is mapped to all defined watershed segments. This allows ABR computation in 5-6 minute time steps. • DHR grid is mapped to all defined Urban Areas to compute ABR urban rainfall. • Single 1km DHR bins are assigned to location of all rain gages, allowing gage comparison with radar estimated rainfall.
Pine Creek Aftermath • RADAP-II provided excellent rainfall estimates for bucket survey gages and for ABR in watersheds down to 3mi2 . • Disaster survey team recommended that a 1-hour flash flood guidance (FFG) be initiated. • Analysis of flash flood watersheds needed to be reduced from 10 mi2 to at least 3mi2.
AMBER 3 mi2 Watersheds • Operational 01May1990 (RADAP-II data) • MBR and ABR computations. • 01June1993 WSR-88D Installed. • Operational 01May1996 (WSR-88D data) • Watershed analysis accomplished on 7.5 minute topographic maps and MBA reduced to 3 mi2. • ABR computations only.
Flash Flood Threat and ABR • Severity of flash flooding is directly related to the observed ABR in a watershed. • The time period of the ABR determines the ABR rate. The higher the ABR rate, the greater the risk of serious flash flooding. • ABR varies as function of watershed area. As watershed area increases, ABR and ABR rate tend to decrease.
FF Scale for Flash FloodingABR in inches • FF0 ABR = FFG to < FFG + 0.5 • FF1 ABR = FFG + 0.5 to < FFG + 1.5 • FF2 ABR = FFG + 1.5 to < FFG + 2.5 • FF3 ABR = FFG + 2.5 to < FFG + 3.5 • FF4 ABR = FFG + 3.5 to < FFG + 4.5 • FF5 ABR = FFG + 4.5 to < FFG + 5.5 • FF6 ABR = FFG + 5.5 or higher.
ABR Rate • Based on 5-6 minute observed ABR. • Hourly ABR Rate = ABR x 3600/vsec, where vsec= the number of seconds in the volume scan. • ABR Rate is used to identify watersheds currently receiving the heaviest rainfall, before the flash flood producing accumulation occurs.
AMBER Z/R Gage Comparison • The single 1km DHR bin that contains the rain gage is used to compute the radar rainfall estimate. • Tropical vs. Standard Z/R usage can be estimated using 15 minute gage data. • Gage comparisons should be done to verify validity of the WSR-88D rainfall estimates.
Pitcairn, PA Flash Flood 01-02July1997
Thompson Run Flooding • Three feet of water was reported on Route 22 near the outflow of Thompson Run into Aber’s Creek at 2320UTC. • Route 22 is the main parkway through the eastern suburbs of Pittsburgh. • The road was closed for almost an hour due to the high water.
Pitcairn Flash Flooding • About 10 million dollars of flood damage occurred in the city of Pitcairn. • Residents in Pitcairn reported that Dirty Camp Run was up near bankfull before the heavy rain began in town. • The peak of the flooding in Pitcairn occurred from 01July/2330 UTC to 02July/0030 UTC.
Tropical Z/R Cases • Standard Z/R: Z = 300R1.4 • Tropical Z/R: Z = 250R1.2 • Maximum Rate Cap 5.0 in/hr? • Tropical Storm not required!!!
Conclusions • Small watersheds definition is the key to detection of flash flooding. • ABR Rate gives early warning for flash flood potential. • Time duration of rainfall display impacts forecaster perception of flash flood threat. • User selectable time window needed.
AMBER Summary • ABR provides basin specific rainfall to determine flash flood risk (ABR-FFG) • ABR rate focuses forecaster attention on critical streams before flooding occurs. • History of ABR helpful in adjusting FFG for multiple rainfall events. • Basin specific ABR allows forecaster to mention “threat area” in warning text.
AMBER Data Sources • ArcView and the spatial analyst extension. www.esri.com • USGS Data (3 arc second DEM data) edcwww.cr.usgs.gov • EPA Data (RF3 data files) www.epa.gov/OST/BASINS/gisdata.html • Ami Arthur firstname.lastname@example.org • Paul.Jendrowski@noaa.gov (SOO KHNL)
buff.ppt 13Jul96 epgh99.ppt 18May99 pitcairn.ppt 01Jul97 zion.ppt 27Jul98 ohio98.ppt 26Jun98 dallas95.ppt 05May95 ftc97.ppt 28Jul97 kc1998.ppt 04Oct98 Buffalo Creek, CO Pittsburgh, PA Pittsburgh, PA Cedar City, UT Eastern Ohio Dallas, TX Fort Collins, CO Kansas City, MO AMBER Case Studiesftp 220.127.116.11 public