The April 18 th 2009 Extreme Rain Event

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

The April 18 th 2009 Extreme Rain Event 19 Z

The April 18 th 2009 Extreme Rain Event 24 Hour Rainfall

24 hr Rainfall Amounts 10 inches

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! 2) When compared to somewhat similar heavy rain events, this event stands out with its impressive 1, 3, and 6 hourly rain fall rates.

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.

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

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

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

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

The April 18 th 2009 Extreme Rain Event 0 Z 568 dam thickness 1. 15” PWAT (70% saturation) Within a climatologically very saturated air mass (~ 1. 70”) 70% Saturation Thickness (Funk 1991)

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

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

The April 18 th 2009 Extreme Rain Event West Galveston Bay Pressure (6 minute data) Min pressure = 1004. 7 mb 11 mb drop in 4 hours

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. Expect a 20% loss during convective rainfall: A 1. 4” error for a near 7” rainfall total *Greater error likely in high winds

Rainfall Rate Comparisons / Impacts Return Period: 10 year 25 year 50 year 100 year 500 year

Short term Rainfall Rates ( Extremes ) 1 minute : 0. 71” (Willow Spring @ Fairmont Parkway)* 15 minute : 2. 44” (Clear Creek @ I-45) 30 minute : 4. 22” (Clear Creek @ Bay Area Blvd) *{U. S. record = 1. 23” Unionville, MD}

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 Mc. Kinney (NWS)