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Tropical Storm Allison Floods, June 5-9, 2001

SYNOPTIC SITUATION

Tropical Storm Allison formed in the northwest Gulf of Mexico on June 5th, only five days into the 2001 Hurricane Season. The system that became Allison could be traced all the way back to May 21st as a tropical wave that moved off the west coast of Africa. By the 29th, the disorganized system reached the western Caribbean Sea and then crossed into the Pacific Ocean on June 1st. The system then moved north into the Bay of Campeche late on the 4th. Organization began to take place on the 5th and by the afternoon hours, reconnaissance aircraft along with ship and buoy reports indicated that Tropical Storm Allison was born while located 80 miles south of Galveston. At her peak, Allison had tropical storm force winds up to 60 mph which were confined to offshore oil platforms and the Sea Rim State Park area in western Jefferson County. 2 to 3 foot tides along the Texas coast had little impact on the area. Allison moved inland that evening less than 12 hours after forming on the west end of Galveston Island, and during the next five days produced record rainfall that led to devastating flooding across portions of Southeast Texas, including the Houston area.

The first significant flooding occurred as Allison was a tropical storm and poised to move onshore on the 5th. A large spiral band with very heavy rainfall moved inland from Galveston to the Houston area while Allison's low level circulation center was still offshore. The rain began late in the afternoon, and by mid evening 8 to 12 inches totals were common from northern Galveston County into southern Harris County. As a result, flash flooding was observed across these areas, but no deaths or injuries occurred with this first round of storms. Heavy rain continued over inland portions of Southeast Texas overnight with 3 to 5 inch amounts common.

Allison, being steered by two subtropical ridges (one centered east of Florida and one centered west of Texas), moved inland and weakened overnight. On Wednesday the 6th, the remnants of Allison steadily moved northward into East Texas. By the evening hours, the center was near Lufkin and moving less than 5 mph. Rainfall amounts averaged 4 to 6 inches across the northern portions of our county warning area (CWA), well north of the Houston area, resulting in some minor flooding. The most significant rainfall on the day was found across east Texas and on into Louisiana.

On Thursday the 7th, the subtropical ridge off Florida weakened and drifted to the south. At the same time, the ridge west of Texas strengthened. This caused the circulation center of Allison (which was still well defined on radar) to make a clockwise loop and begin a drift to the southwest. By the time the center was around the Huntsville area (before sunrise), a band of heavy rain began to back build from Louisiana and the Beaumont area westward into Liberty County. This band produced 5 to 10 inches of rain. Another area of thunderstorms developed over the Sugarland-Stafford areas of Fort Bend and Harris Counties during the same overnight hours and produced another 8 to 12 inches of rain. No deaths or injuries were observed with these rains, but there was abundant street flooding, and water did get into some homes. During the day, moderate to heavy rains continued to wrap around the circulation center in our northern CWA resulting in additional reports of street flooding.

Friday the 8th began with another 5 to 10 inch swath of rain developing during the early morning hours along the immediate coast from around Freeport to the Beaumont area. This rain band moved onshore and weakened during the morning hours, resulting in clear skies across most of the CWA by the neginning of the afternoon. With the circulation center now between College Station and Huntsville and drifting slowly to the southwest, it became evident that daytime heating combined with a strong moist feed across the center of the CWA would set up the most significant round of rainfall so far. Rains just east of the center drifted south over Montgomery County and produced heavy rain all day long. During the mid to late afternoon hours, convection began to develop where skies had been sunny and thunderstorms began to rapidly intensify over southern Montgomery and eastern Harris counties. At this time, thunderstorms began to train and merge across the Houston metro area, and the system evolved into a powerful complex right over the most populated portion of our CWA thayt evening. This complex progressed south and east into the early morning hours of Saturday the 9th. Very heavy rainfall was observed for up to 10 hours in some locations, and rainfall rates of 4 inches or more per hour were observed throughout the night. A station in northeast Houston recorded over 26 inches of rain in almost 10 hours.

Flash flooding initiated quite rapidly during rush hour late Friday afternoon and on into the evening hours. Widespread street flooding was the initial threat, but the high rainfall amounts forced almost all the major Houston area bayou systems into severe flooding, with some to record levels. All major freeways in the Houston area were severely flooded at at least one location during this event. During this single event alone, rainfall in Harris County ranged from just 2 inches in the extreme west to in excess of 20 inches over Green's Bayou in the east. Countywide, the average rainfall was 8 inches with over two-thirds of the county receiving over 10 inches.

The center gradually made its way toward Palacios by the evening on the 9th. Before moving offshore, another round of heavy rain - between 4 and 8 inches - developed across extreme southern Harris and extreme northern Galveston Counties. Some homes along Clear Creek which had escaped significant flooding from the heavy rains on Friday night ended up flooding from these rains on Saturday. The Federal Emergency Management Agency (FEMA) declared a large part of east and southeast Texas and portions of Louisiana disaster areas.

The center finally moved off to the east on Sunday the 10th and ended the event for our area. The remnants of Allison went on to wreak havoc across several Gulf Coast states with tornadoes and heavy rainfall, and eventually made its way up the east coast of the United States and continued to produce devastating flooding. Allison will go down as the costliest tropical storm in United States history.

As of the completion of this article, damage totals are still being determined. Early estimates have total damage across Southeast Texas close to $5 billion ($4.88 billion in Harris County alone). Twenty-two deaths were caused by Allison, with each of these fatalities occurred in Harris County. By comparison, Hurricane Alicia, the last major hurricane to strike the Upper Texas Coast, caused $2 billion damage and 21 fatalities.
| Synopsis | Conclusion |


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Related Links
.NOAA's SSD Tropical Storm Allison Precipitation page
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.NOAA's News Report on Allison
.HPC's Allison Summary
.NHC's June Summary
.NHC's Advisory Archive

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