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National Flood Workshop

National Flood Workshop. “Precipitable Water Values Associated with Recent Flood Events in Southeast Texas” Paul Lewis. National Flood Workshop. Outline For Today Few introductory items What is PW? Heavy rainfall forecasting methods SE Texas “recent” heavy rainfall events

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National Flood Workshop

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  1. National Flood Workshop “Precipitable Water Values Associated with Recent Flood Events in Southeast Texas” Paul Lewis

  2. National Flood Workshop Outline For Today • Few introductory items • What is PW? • Heavy rainfall forecasting methods • SE Texas “recent” heavy rainfall events • A study of two events utilizing a PW statistical analysis method (from WFO Rapid City, SD)

  3. National Flood Workshop What is PW? – From the AMS “Glossary of Meteorology” http://amsglossary.allenpress.com/glossary • “The total precipitable water is that contained in a column of unit cross section extending all of the way from the earth's surface to the ‘top’ of the atmosphere” • Utilizing inches & millimeters in this presentation • “W” is total precipitable water vapor • “g” is acceleration of gravity • “x(p)” is the mixing ratio at pressure “p” • “p1” & “p2” define a pressure layer

  4. National Flood Workshop A Word of Caution • Rainfall in convective events often exceed PW • Convergence of water vapor is frequently quite large • The AMS “Glossary of Meteorology” states: “Nevertheless, there is general correlation between precipitation amounts in given storms and the precipitable water vapor of the air masses involved in those storms.”

  5. National Flood Workshop Heavy Rainfall Forecasting • “No one method can be utilized by itself without consideration of all other patterns” (T.W. Funk) • Implies that pattern and event recognition is required • For this presentation, cases with heavy rainfall • Note: High PW can occur without rainfall

  6. National Flood Workshop Using 2SD and PW MAX – An Empirical Observation • 95% of the values lie within ±2SD of the mean value (Bunkers, WFO UNR) • This implies that PW +2SD is a fairly rare event(Bunkers, WFO UNR) • Thus, PW MAX would be a very rare event See: http://www.crh.noaa.gov/unr/?n=pw

  7. National Flood Workshop Other Items to Keep in Mind • Other heavy rainfall forecasting rules of thumb could be considered • PW & 70% Thickness saturation (T.W. Funk 1991) • Flash Flood Decision Tree (HGX/SIL) • Ingredients-Based Methodology (Doswell et al 1996) • Upper air sounding data records have systematic observational errors in humidity (Wang and Zhang 2007) • Includes sensor errors and external factors, some due to human error • Sounding data time and spatial gaps can be filled with GPS data • GPS data can contain errors but can be more reliable than model data

  8. National Flood Workshop SE Texas Heavy Rainfall Events “Droughts are broken by floods” • Data selection – A headache! • Utilizing a 24-hour calendar day lower limit of 4 inches gives dozens of examples at just the official Houston observing sites since 1900

  9. 77 cases of 4-inch or greater rainfall in a 24-hour period Courtesy of the NOAA Regional Climate Centers – http://xmacis.rcc-acis.org/HGX/

  10. National Flood Workshop Recent (10-Year) SE Texas Heavy Rainfall Events • Major events do not always cover the entire CWA • Plus, events may be missed by the official observing sites • Rainfall data from just the official Houston observing site presents a large number of cases

  11. Courtesy of the NOAA Regional Climate Centers – http://xmacis.rcc-acis.org/HGX/

  12. 14 cases of 4-inch or greater rainfall Courtesy of the NOAA Regional Climate Centers – http://xmacis.rcc-acis.org/HGX/

  13. National Flood Workshop For this presentation: • June 2001 – TS Allison (no GPS data available) • Two 2009 events show GPS data importance: • April 17 • April 18

  14. National Flood Workshop Tropical Storm Allison – June 2001 Clear Creek flooding at FM-528 • Rain band & core rain events • PW statistical method corresponded well to the heaviest rainfall locations Buffalo Bayou flooding near downtown Houston

  15. National Flood Workshop PW (inches) for 04 – 10 June 2001

  16. National Flood Workshop • Houston Area Rainfall • 24 hour total Saturday 9 June • 5-day total 4 – 9 June 2001 KHGX Storm Total Precipitation Allison Event 4 – 10 June 2001

  17. National Flood Workshop 17 & 18 April 2009 Events • PW from CRP (2SD=1.75) and LCH (2SD=1.85) • GPS data available • Note. . . • PW at upper air sites low (limitations in time/space) • GPS data filled in the time and space gaps • PW Statistical Method indicated good flood potential

  18. 7:15 AM CDT 17 April 2009 mm: 35 37 39 41 43454749 51 53 55 inches: 1.38 1.46 1.54 1.61 1.691.771.851.93 2.01 2.09 2.17

  19. 10:15 AM CDT 17 April 2009 mm: 35 37 39 41 43454749 51 53 55 inches: 1.38 1.46 1.54 1.61 1.691.771.851.93 2.01 2.09 2.17

  20. 1:15 PM CDT 17 April 2009 mm: 35 37 39 41 43454749 51 53 55 inches: 1.38 1.46 1.54 1.61 1.691.771.851.93 2.01 2.09 2.17

  21. 4:15 PM CDT 17 April 2009 mm: 35 37 39 41 43454749 51 53 55 inches: 1.38 1.46 1.54 1.61 1.691.771.851.93 2.01 2.09 2.17

  22. 7:15 PM CDT 17 April 2009 mm: 35 37 39 41 43454749 51 53 55 inches: 1.38 1.46 1.54 1.61 1.691.771.851.93 2.01 2.09 2.17

  23. 7:15 AM CDT 18 April 2009 mm: 35 37 39 41 43454749 51 53 55 inches: 1.38 1.46 1.54 1.61 1.691.771.851.93 2.01 2.09 2.17

  24. 10:15 AM CDT 18 April 2009 mm: 35 37 39 41 43454749 51 53 55 inches: 1.38 1.46 1.54 1.61 1.691.771.851.93 2.01 2.09 2.17

  25. 1:15 PM CDT 18 April 2009 mm: 35 37 39 41 43454749 51 53 55 inches: 1.38 1.46 1.54 1.61 1.691.771.851.93 2.01 2.09 2.17

  26. 3:15 PM CDT 18 April 2009 mm: 35 37 39 41 43454749 51 53 55 inches: 1.38 1.46 1.54 1.61 1.691.771.851.93 2.01 2.09 2.17

  27. 3:45 PM CDT 18 April 2009 mm: 35 37 39 41 43454749 51 53 55 inches: 1.38 1.46 1.54 1.61 1.691.771.851.93 2.01 2.09 2.17

  28. 4:15 PM CDT 18 April 2009 mm: 35 37 39 41 43454749 51 53 55 inches: 1.38 1.46 1.54 1.61 1.691.771.851.93 2.01 2.09 2.17

  29. 7:15 PM CDT 18 April 2009 mm: 35 37 39 41 43454749 51 53 55 inches: 1.38 1.46 1.54 1.61 1.691.771.851.93 2.01 2.09 2.17

  30. National Flood Workshop 17 & 18 April 2009

  31. Observed Rainfall

  32. Observed Rainfall My home

  33. National Flood Workshop Conclusion • The PW Statistical Method can give a general idea of the potential for a heavy rainfall event • However, remember that pattern and event recognition is required • A PW value close to 2SD above normal is a fairly rare event • A PW value near the PW MAX is a very rare event • The 12Z and 00Z upper air sounding network is limited • GPS data can be utilized to fill in the PW picture both on the temporal and spatial scales

  34. National Flood Workshop Research and Data Sources • GPS data – http://www.unidata.ucar.edu/data/suominet/gif/ • PW study graphs – http://www.crh.noaa.gov/unr/?n=pw • Rainfall data • http://xmacis.rcc-acis.org/HGX/ • http://hurricane.ncdc.noaa.gov/pls/plhas/has.dsselect • Radiosonde data • Wang, J., and Zhang, L., 2007: “Systematic Errors in Global Radiosonde Precipitable Water Data from Comparisons with Ground-Based GPS Measurements,” Journal of Climate, Volume 21, pp 2218–2238 • Upper air data • http://vortex.plymouth.edu/get_raob-u.html • http://weather.uwyo.edu/upperair/sounding.html • Flash Flood Studies • Doswell, C.A., Brooks, H.E., and Maddox, R.A., 1996: “Flash Flood Forecasting: An Ingredients-Based Methodology,” Weather and Forecasting, Volume 11, pp 560–581 • Funk, T. W., 1991: “Forecasting Techniques,” Weather and Forecasting, Volume 6, pp 548–564

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