1. “A Study of Causes Behind the Heavy Rainfall Amounts From Tropical Storm Allison” Paul Lewis National Weather Service WFO Houston/Galveston Contact information: Paul.Lewis@noaa.gov February 2012 National Flood Workshop

2. Overview of the 2001 TS Allison Event (courtesy of the TPC Tropical Cyclone Report) Tropical wave moved off Africa May 21 st and reached the Gulf of Tehuantepec June 1 st Remnants of the wave interacted with an upper low over South Texas June 3 rd and 4 th TS Allison formed early June 5 th, moved inland, and became a depression by that evening Allison meandered for 5 days over E/SE Texas before moving NE through the Mid Atlantic coast

3. Courtesy of the Tropical Prediction Center report on Tropical Storm Allison

4. 14-Days of Allison Rainfall from the Office of Satellite Data and Distribution http://www.ssd.noaa.gov/PS/TROP/GFX/allison3.gif Why did Allison Generate So Much Rainfall?

5. Rainfall potential is not determined by the strength of a land-falling tropical cyclone (i.e. TS, CAT 1, 2, 3, etc.) Factors impacting land-falling tropical cyclone rainfall – From http://www.hpc.ncep.noaa.gov/tropical/rain/tcrainfall.html Note slide number 20 in the “Tropical Cyclone Rainfall Slideshow” link To this list, add an abundant nearby moisture source 1.Storm Size 2.Topography 3.Wind Shear 4.Nearby Weather Factors 5.Storm Movement (or track) 6.Time of Day

6. Negative or Minor Factors to Allison’s Heavy Rainfall 1.Storm size – Allison was a rather small storm – Bigger storms mean more rainfall for a location 2.Topography – Fairly flat across Southeast Texas 3.Wind shear – Virtually non-existent for Allison in Southeast Texas 4.Nearby synoptic scale weather features – None – Synoptic scale features can lead to extratropical transition and impact the rainfall potential

7. Major Factors Pertaining to Allison’s Heavy Rainfall 5.Storm movement (or track) Under 5 mph => Weak Steering Currents Note in the following NCEP/NCAR 500 mb reanalysis maps (courtesy of Plymouth State)  Allison formed in a weakness in the subtropical ridge  After landfall, the remnants of Allison did a 5-day loop as the weakness persisted overhead  Lack of wind shear and nearby weather features

8. 12 UTC 5 June 2001

9. 12 UTC 6 June 2001

10. 12 UTC 7 June 2001

11. 12 UTC 8 June 2001

12. 12 UTC 9 June 2001

13. 12 UTC 10 June 2001

14. 6.Time of day – Rain Band and Core Rain Event Phenomena Note following combination for a land-falling TC 1.Daytime into the evening – Rain bands generate heavy rainfall 2.Nighttime – Core rains develop Rain bands dissipate or may contract to the core Core rains are not always at the storm’s center The following images are from satellite archives at WFO HGX generated by the NAWIPS Garp program Major Factors Pertaining to Allison’s Heavy Rainfall

15. Rain band developing near the coast 1115Z 07 June 2001

16. Heavy rain band along the coast 2145Z 07 June 2001

17. Core rain developing 0315Z 08 June 2001

18. Core rain north of Houston 0845Z 08 June 2001

19. Source WFO UNR – Note Max PW ~ 2.6” and +2SD ~ 2.3” for 01 June Nearby Abundant Moisture Source Max PW +2SD

20. Nearby Abundant Moisture Source The Gulf of Mexico NCEP/NCAR Reanalysis of PW (courtesy Plymouth State University) 12 UTC 07 June 2001 12 UTC 08 June 2001

21. The Setup The abundant moisture access to the nearby Gulf of Mexico Combined with the daily rain band/core rain cycle And the storm’s slow movement factor... Led to incredible 5-day rainfall totals!

22. June 5 th – Rain bands impacted E & S of Houston June 6 th and 7 th – Core rain and rain bands – Flooding from core rains between Conroe & Crockett – Flooding from rain bands from Sugar Land to Stafford June 7 th and 8 th – Rain bands at the coast – 12 inches of rainfall in Freeport and Brazoria June 8 th and 9 th – Core rain event over Houston – Greater than 26 inches over Greens Bayou Significant Rainfall Events

23. June 8 – 9, 2001

24. Harris County Flooding Flooding in Houston following core rain event of June 8 th and 9 th Downtown TunnelsI-610 Loop at Kelly

25. June 4 – 10, 2001

26. What caused the heavy rainfall in Allison? – Allison was a land-falling tropical cyclone with the following characteristics 1.Slow storm movement under 5 mph 2.Daily rain band/core rain phenomena occurred over a 5-day period 3.Abundant nearby moisture source (Gulf of Mexico) kept the system replenished Summary

27. Questions?