The April 18th 2009 Extreme Rain EventPresentation by Lance Wood

Image Courtesy: Galveston Daily NewsImage Courtesy: Galveston Daily News

Friendswood, TXFriendswood, TX Taylor Lake Village, TXTaylor Lake Village, TX

My houseMy house

The April 18th 2009 Extreme Rain Event

19 Z

The April 18th 2009 Extreme Rain Event

24 Hour Rainfall

24 hr Rainfall Amounts

10 inches

The April 18th 2009 Extreme Rain Event

A couple quick facts about this unique April rain event :A couple quick facts about this unique April rain event :

1)1)Recorded the highest recorded local rainfall rates with the Recorded the highest recorded local rainfall rates with the greatest 1 greatest 1 hour rainfall rate.hour rainfall rate.

A tipping bucket gauge on Clear Creek (@Bay Area Blvd) reported a A tipping bucket gauge on Clear Creek (@Bay Area Blvd) reported a

6.9 inch6.9 inch hourly rainfall rate hourly rainfall rate

-- This was a higher rate than during TS Allison!This was a higher rate than during TS Allison!

2)2) When compared to somewhat similar heavy rain events, this event When compared to somewhat similar heavy rain events, this event stands out with its stands out with its impressiveimpressive 1, 3, and 6 hourly rain fall rates. 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 25th %).

• Pre-existing surface trough over the Houston area with a dry line and cold front approaching from the west.

18 April 2009 Synoptic Overviewcourtesy of http://www.spc.noaa.gov/obswx/maps/

12 UTC 18 April 2009 00 UTC 19 April 2009

300 mb

18 April 2009 Synoptic Overviewcourtesy of http://www.spc.noaa.gov/obswx/maps/

12 UTC 18 April 2009 00 UTC 19 April 2009

500 mb

12 UTC 18 April 2009 00 UTC 19 April 2009

18 April 2009 Synoptic Overviewcourtesy of http://www.spc.noaa.gov/obswx/maps/

850 mb

18 April 2009 Synoptic Overviewcourtesy 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 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)

LL LL

LL LL

Meso-Low DevelopmentMeso-Low Development

1829 Z1829 Z 1902 Z1902 Z

1930 Z1930 Z 1956 Z1956 Z

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 Nearly an hour of sustained gale force winds

The April 18th 2009 Extreme Rain Event

Min pressure = 1004.7 mb

West Galveston Bay Pressure (6 minute data)

11 mb drop in 4 hours11 mb drop in 4 hours

Rain Gauge LimitationsRain Gauge Limitations

Tipping bucket:Tipping bucket:Can not keep pace with intense rainfallCan not keep pace with intense rainfallLoss of rain during bucket tips (1/2 sec)Loss of rain during bucket tips (1/2 sec)Can not be calibrated for 0.01 precision at high Can not be calibrated for 0.01 precision at high rainfall rates.rainfall rates.

Expect a 20% loss during convective rainfall: Expect a 20% loss during convective rainfall:

A 1.4” error for a near 7” rainfall totalA 1.4” error for a near 7” rainfall total

*Greater error likely in high winds *Greater error likely in high winds

Rainfall Rate Comparisons / ImpactsRainfall Rate Comparisons / Impacts

Return Period:Return Period: 10year 25year 50year 100year 500year

Short term Rainfall RatesShort term Rainfall Rates ( ( Extremes Extremes ))

1 minute : 0.71”0.71” (Willow Spring @ Fairmont Parkway)*

15 minute : 2.44”2.44” (Clear Creek @ I-45)

30 minute : 4.22”4.22” (Clear Creek @ Bay Area Blvd)

*{U.S. record = *{U.S. record = 1.23”1.23” Unionville, MD} Unionville, MD}

Thank you! Questions?AcknowledgmentsAcknowledgments

Mr. Patrick Blood (NWS)Mr. Patrick Blood (NWS)

Mr. Jeffery Lindner (Harris County Flood Control District)Mr. Jeffery Lindner (Harris County Flood Control District)

Mr. Scott Overpeck (NWS)Mr. Scott Overpeck (NWS)

Mr. Paul Lewis (NWS)Mr. Paul Lewis (NWS)

Mr. Chris McKinney (NWS)Mr. Chris McKinney (NWS)