“ “ A Study of Causes Behind A Study of Causes Behind the Heavy Rainfall Amounts the Heavy Rainfall Amounts From Tropical Storm From Tropical Storm Allison” Allison” Paul Lewis National Weather Service WFO Houston/Galveston Contact information: [email protected]February 2012 February 2012 National Flood Workshop National Flood Workshop
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“A Study of Causes Behind the Heavy Rainfall Amounts From Tropical Storm Allison” Paul Lewis National Weather Service WFO Houston/Galveston Contact information:
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““A Study of Causes Behind the A Study of Causes Behind the Heavy Rainfall Amounts From Heavy Rainfall Amounts From
– 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