““A Study of Causes Behind the A Study of Causes Behind the Heavy Rainfall Amounts From Heavy Rainfall Amounts From

Tropical Storm Allison”Tropical Storm Allison”

Paul Lewis

National Weather ServiceWFO Houston/Galveston

Contact information: Paul.Lewis@noaa.gov

February 2012February 2012National Flood WorkshopNational Flood Workshop

Overview of the 2001 TS Allison Event(courtesy of the TPC Tropical Cyclone Report)

• Tropical wave moved off Africa May 21st and reached the Gulf of Tehuantepec June 1st

• Remnants of the wave interacted with an upper low over South Texas June 3rd and 4th

• TS Allison formed early June 5th, 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

Courtesy of the Tropical Prediction Center report on Tropical Storm Allison

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?

Why did Allison Generate So Much Rainfall?

• 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

Negative or Minor Factors to Allison’s Heavy Rainfall1. 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

Major Factors Pertainingto 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

12 UTC 5 June 2001

12 UTC 6 June 2001

12 UTC 7 June 2001

12 UTC 8 June 2001

12 UTC 9 June 2001

12 UTC 10 June 2001

6. Time of day – Rain Band and Core Rain Event Phenomena

• Note following combination for a land-falling TC1. Daytime into the evening – Rain bands generate

heavy rainfall2. 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 Pertainingto Allison’s Heavy Rainfall

Rain band developing near the coast 1115Z 07 June 2001

Heavy rain band along the coast 2145Z 07 June 2001

Core rain developing 0315Z 08 June 2001

Core rain north of Houston 0845Z 08 June 2001

Source WFO UNR – Note Max PW ~ 2.6” and +2SD ~ 2.3” for 01 June

Nearby Abundant Moisture Source

Max PW

+2SD

Nearby Abundant Moisture Source

The Gulf of MexicoNCEP/NCAR Reanalysis of PW (courtesy Plymouth State University)

12 UTC 07 June 2001 12 UTC 08 June 2001

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!

• June 5th – Rain bands impacted E & S of Houston

• June 6th and 7th – Core rain and rain bands– Flooding from core rains between Conroe & Crockett– Flooding from rain bands from Sugar Land to Stafford

• June 7th and 8th – Rain bands at the coast– 12 inches of rainfall in Freeport and Brazoria

• June 8th and 9th – Core rain event over Houston– Greater than 26 inches over Greens Bayou

Significant Rainfall Events

June 8 – 9, 2001

Harris County FloodingHarris County Flooding

Flooding in Houston following core rain event of June 8th and 9th

Downtown Tunnels I-610 Loop at Kelly

June 4 – 10, 2001

• 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

Questions?