Preliminary Report
Tropical Storm Allison
24 June - 01 July 1989

1989 PRELIMINARY REPORTS

Tropical Storm Allison (TS)
Tropical Storm Barry (TS)
Hurricane Chantal (1)
Hurricane Dean (2)
Hurricane Erin (2)
Hurricane Felix (1)
Hurricane Gabrielle (4)
Hurricane Hugo (5)
Tropical Storm Iris (TS)
Hurricane Jerry (1)
Tropical Storm Karen (TS)

Tropical Storm Allison, although never reaching hurricane strength, proved to be a very destructive event. Allison caused $500 Million in damages during a six day period due mainly to the flooding from the torrential rains that fell along the upper Texas coast and over the western two thirds of Louisiana. The looping storm, which produced over 29 inches of rain in a few areas of central Louisiana will long be remembered as one of the wettest ever for the state of Louisiana.

 

a. Synoptic History

The formation of Tropical Storm Allison can be attributed to at least 3 meteorological phenomena: the remnants of the east Pacific Hurricane Cosme, the northern portion of a westward moving tropical wave and a strong anticyclone at 200 millibars over the Gulf of Mexico. These three factors in addition to a building ridge of high pressure over the central plains provided the environment that created Allison. A strong mid to upper level trough in th westerlies which turned Cosme to the north and spread the remnants of that storm over northern Mexico and the western Gulf of Mexico had been replaced by a large well defined 200 millibar anticyclone that covered the entire Gulf of Mexico. To the north, moderate to strong ridging occured on 22 through 24 June, and during this time the northern portion of tropical wave number eight had moved into the western Gulf of Mexico

Heavy thunderstorms began to develop over the Gulf of Mexico on 22 June and by 23 June the activity became concentrated over the northwest portion of the Gulf. Data from upper air soundings indicated that the circulation from the remnants of Cosme were just to the southwest of Brownsville, Texas, at 1200 UTC on 23 June. During the following 24 hours, a new broad weak surface circulation developed just off the upper Mexican coast.

Based upon surface observation along the coast and data from offshore oil rigs, the area of disturbed weather was upgraded to Tropical Depression Two at 1800 UTC on 24 June. The depression gradually became better organized during the next 2 days. Early on the morning of 26 June, Air Force reconnaissance aircraft detected a large area of 46 to 52 mph winds at a flight level of 1500 feet; and the depression was officially upgraded to Tropical Storm Allison at 1200 UTC 26 June. However, post analysis indicated the depression probably reached tropical storm strength near 0000 UTC on 26 June. At 0100 UTC on 26 June the ship M/T Jacinth (LAOE2), located about 100 nautical miles northeast of the estimated center of the storm, reported east southeast winds of 40 mph with gusts to 52 mph.

By 1200 UTC 26 June a frontal trough in the westerlies moving across the western United States began to erode the ridge of high pressure to the north of Allison, and the storm began to accelerate toward the north in advance of the trough. The center of Allison moved inland on the middle Texas coast near the northeast end of Matagorda Bay at 1300 UTC 26 June with a central pressure of 1002 millibars. The central pressure of the storm continued to decrease and reached an estimated minimum reading of 999 millibars at 0100 UTC 27 June while the center of Allison was located just to the west northwest of Houston, Texas. Thereafter, the storm began to weaken, was downgraded to a tropical depression by 1200 UTC 27 June, and became an extratropical low pressure center by 0000 UTC 28 June.

Allison turned toward the northeast by 0600 UTC 27 June in advance of a fast moving trough. However, the trough weakened and lost its influence on the storm. By 0000 UTC 28 June the building ridge of high pressure in the wake of a frontal trough began to drive the now extratropical low pressure center in a clockwise loop back toward the south and then toward the southwest.

The extratropical low pressure center moved toward the southwest through 0600 UTC 30 June and crossed the northerly track that Tropical Storm Allison had made 3 days earlierjust to the west of Houston, Texas. After the low completed a 360° clockwise loop over western Louisiana and eastern Texas, the ridge to the north collapsed and the low center turned back to the northeast.

At 0000 UTC 01 July both a cutoff 700 through 500 millibar low over eastern South Dakota and southwestern Minnestota and the closed 700 through 500 millibar circulation of the extratropical low pressure over northwestern Louisiana were analyzed (not shown). By 1200 UTC 01 July the 500 millibar low over Louisiana and Arkansas became an open trough, and the closed 500 millibar low center to the north (then over Iowa and northern Missouri) became the dominant center. Thereafter, only at the surface and the 850 millibar level could the remnants of Allison be tracked with any degree of certainty.

The remnants of Allison, now reinorced at the mid and upper levels of the atmosphere by a short wave trough from the westerlies, moved northeastward over northwestern Kentucky on 2 July and remained quasi-stationary near the convergence of the Kentucky/Illinois/Indiana borders until the morning of 4 July. During the 4th, a second short wave through moved into the domimant frontal trough that contained the remnants of Allison and created a second cut-off circulation center at the 700 and 500 millibar levels well to the south of the residual circulation, which remained over the Kentucky/southwestern Ohio area. However, once again the remnants of Allison at the surface and 850 millibar level remained intact and were traced southward into Alabama by the evening of 4 July. Thereafter, the remnants drifted westward and finally dissipated over northwest Louisiana and east Texas on 6 and 7 July after dropping an additional 1 to 2 inches of rain over portions of the still-flooded rivers of east Texas and western Louisiana.

b. Meteorological Statistics

Wind gusts to to tropical storm force occured in the clusters of heavy showers and thunderstorms as early as 24 June. Practically all of the strongest surface winds and peak gusts measured in Allison occured in the clusters of heavy thunderstorms that developed in the right half of the tropical cyclone's circulation. Satellite pictures the heavy thunderstorm activity that had developed by 26 and 27 June. Strongest winds and peak gusts from the offshore oil rigs occurred as these heavy thunderstorms crossed over the observation platforms. Note: The oil rigs L40 and 01T measured maximum sustained winds of 58 to 46 mph, respectively, on 26 June while in heavy precipitation. Also the coastal observation site 7R5 recorded a peak gust of 69 mph in a thunderstorm.

Strongest 1-minute winds over land ranged from 40 to 46 mph with a gust to nearly 58 mph. The Galveston weather office measured the strongest 1-minute wind of 52 mph at 1238 UTC on 26 June which was near the time Allison made landfall.

Based upon satellite imagery and surface observations from oil rigs, initial central pressure estimates on 24 June were near 1008 millibars. Air Force reconnaissance data on 25 June indicated that the central pressure decreased slightly to 1005 millibars. By the time Air Force Reconnaissance reached the center of Tropical Storm Allison on the morning of 26 June, the storm's broad center of circulation appeared to be near the middle Texas coast. Lowest pressure measurements from the aircraft indicated that the central pressure had fallen to 1002 millibars when Allison's center moved onshore near the northeast end of Matagorda Bay. The central pressure of the storm continued to decrease for an additional 12 hours after landfall and reached an estimated minimum reading of 999 millibars at 0100 UTC 27 June when the storm was centered just to the west-northwest of Houston, Texas. Thereafter, gradual weakening occured and the storm was downgraded to a depression by 1200 UTC 27 June.

1. Rainfall Data

Torrential rains accompanying Allison fell along the upper Texas coast and over the western two thirds of Louisiana. Nearly 30 inches (29.52) fell in a six day period at a few locations in north central Louisiana and amounts of 10 to 15 inches were common along the upper Texas coast. The small community of Winfield, Louisiana, had 29.52 inches of rain from 26 June through 01 July, with 17 inches falling in a three day period. Portions of Harris County, Texas, received over 18 inches.

c. Casualty and Damage Statistics

There were 11 deaths attributed to Allison; 5 in Mississippi, 3 in Louisiana and 3 in Texas.

The death toll in Tropical Storm Allison was 11. Three deaths occurred in Texas, 3 in Louisiana, and 5 in Mississippi. Two teenage boys drwoned in Beaumont after the rubber raft they were riding in capsized and they were swept down a drainage pipe by the flood runoff of Allison. Also an 18 year old boy drowned while swimming in Spring Creek in northern Harris County. The 3 deaths in Louisiana and the five in Mississippi were all by drowning.

Damage from Allison is estimated at $500 Million. Almost all damage occured from the flooding producd by the heavy rains. Estimates include $200 Million to $400 Million in damages in Texas while upwards of $100 Million was reported in Louisiana. Mississippi claimed nearly $60 Million in damages.