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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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