1. The April 18 th 2009 Extreme Rain Event Presentation by Lance Wood Image Courtesy: Galveston Daily News Friendswood, TX Taylor Lake Village, TX My house

2. The April 18 th 2009 Extreme Rain Event 19 Z

3. The April 18 th 2009 Extreme Rain Event 24 Hour Rainfall

4. 24 hr Rainfall Amounts 10 inches

5. The April 18 th 2009 Extreme Rain Event A couple quick facts about this unique April rain event : 1)Recorded the highest recorded local rainfall rates with the greatest 1 hour rainfall rate. A tipping bucket gauge on Clear Creek (@Bay Area Blvd) reported a 6.9 inch hourly rainfall rate - This was a higher rate than during TS Allison! - This was a higher rate than during TS Allison! 2)When compared to somewhat similar heavy rain events, this event stands out with its impressive 1, 3, and 6 hourly rain fall rates.

6. 18 April 2009 Synoptic Overview Upper divergence apparent at 300 mb level with SE Texas between two diverging jet streaks. 500 mb low moving over the southern plains with a shortwave trough moving across SE Texas. Deep layer moisture in the lower levels noticed at the 850 and 700 mb levels (PWATS : ~1.70” upper 25 th %). Pre-existing surface trough over the Houston area with a dry line and cold front approaching from the west.

7. 18 April 2009 Synoptic Overview courtesy of http://www.spc.noaa.gov/obswx/maps/ 12 UTC 18 April 2009 00 UTC 19 April 2009 300 mb

8. 18 April 2009 Synoptic Overview courtesy of http://www.spc.noaa.gov/obswx/maps/ 12 UTC 18 April 2009 00 UTC 19 April 2009 500 mb

9. 12 UTC 18 April 2009 00 UTC 19 April 2009 18 April 2009 Synoptic Overview courtesy of http://www.spc.noaa.gov/obswx/maps/ 850 mb

10. 18 April 2009 Synoptic Overview courtesy of http://www.hpc.ncep.noaa.gov/html/sfc_archive.shtml 18 UTC 18 Apr 2009 21 UTC 18 Apr 2009 00 UTC 19 Apr 2009 Surface plot/fronts/pressure

11. The April 18th 2009 Extreme Rain Event 0Z 568 dam thickness  1.15” PWAT (70% saturation) Within a climatologically very saturated air mass (~ 1.70”) 70% Saturation Thickness (Funk 1991)

12. L L L L Meso-Low Development 1829 Z 1902 Z 1930 Z 1956 Z

13. The April 18th 2009 Extreme Rain Event Max wind = 52 kt wind gust West Galveston Bay Wind (6 minute data) Nearly an hour of sustained gale force winds

14. The April 18th 2009 Extreme Rain Event Min pressure = 1004.7 mb West Galveston Bay Pressure (6 minute data) 11 mb drop in 4 hours 11 mb drop in 4 hours

15. Rain Gauge Limitations Tipping bucket: Can not keep pace with intense rainfall Loss of rain during bucket tips (1/2 sec) Can not be calibrated for 0.01 precision at high rainfall rates. rainfall rates. Expect a 20% loss during convective rainfall: A 1.4” error for a near 7” rainfall total A 1.4” error for a near 7” rainfall total *Greater error likely in high winds

16. Rainfall Rate Comparisons / Impacts Return Period: 10year25year50year100year500year

17. Short term Rainfall Rates ( Extremes ) Short term Rainfall Rates ( Extremes ) 0.71” 1 minute : 0.71” (Willow Spring @ Fairmont Parkway)* 2.44” 15 minute : 2.44” (Clear Creek @ I-45) 4.22” 30 minute : 4.22” (Clear Creek @ Bay Area Blvd) *{U.S. record = 1.23” Unionville, MD}

18. Thank you! Questions? Acknowledgments Mr. Patrick Blood (NWS) Mr. Jeffery Lindner (Harris County Flood Control District) Mr. Scott Overpeck (NWS) Mr. Paul Lewis (NWS) Mr. Chris McKinney (NWS)