"Tropical Storm Allison"
06 June 2001

Allison spent five days over Southeast and East Texas and dumped record amounts of rainfall across the area. Nearly 37 inches of rain was recorded at the Port of Houston during this time. Damage estimates in Harris County alone (includes the Houston Metro area) were around $5 billion. Allison will likely go down as the costliest tropical storm to ever affect the United States. Allison was even more deadly than Hurricane Alicia, which occurred in 1983 where 21 lives were lost. Allison took 24 lives in the Houston area. According to press reports, 43 people perished along Allison's path, mainly due to flooding.

June 6th and 7th
Sustained winds reached 48 mph at 9:50 am CDT on the 6th, and gusts peaked at 61 mph at 9:42 am CDT. Thereafter, the cyclone weakened to a tropical
depression and the hurricane center handed the responsibility of the advisories over to the hydrometeorological prediction center in Camp Springs, Maryland, in the form of storm summaries. This is standard procedure when a depression drifts inland.

During the night of the 6th/ 7th, heavy rains deluged northwest Jefferson and orange counties in southeast Texas when 6-10 inches of rain fell in less than five hours.
On the morning of the 7th, the subtropical high off Florida weakened and drifted to the south while a high-pressure ridge over New Mexico was intensifying. This caused the cyclone to transcribe a clockwise loop that morning, and then move to the southwest.

June 8th
As the center approached Houston for a second time on the 8th, rain intensified across southeast Texas. Conroe measured 8.23 inches during a six-hour period
that afternoon. Still heavier rains were moving across the Houston area. Over 20 inches of rain fell within a twelve-hour period. A vast area of Houston was
submerged by a second onslaught of heavy rain within six days. Allison turned out to be the flood of record for the Houston metropolitan area.

June 9th
On the 9th, Allison emerged back into the Gulf of Mexico. It became the first known tropical cyclone to make landfall in Texas, only to emerge back into the gulf.

June 11th
On the morning of the 11th, Allison was moving into the marshes/bayous of southeast Louisiana. The maximum amounts of rainfall in Louisiana occurred in the Vermilion and Atchafalaya basins of south central Louisiana, which saw heavy rains each day from the 5th through the 11th. The highest amount reported from Louisiana was 27.55 inches at salt point in St. Mary Parish.

On the 11th, the low that was once Allison development over land, which had never happened before. This was mainly due to a jet streak riding along the
subtropical jet to its north and northeast enhancing the outflow, which in turn led to a stronger surface low. By 1 am CDT on the 11th, the cyclone regained gale-force winds that were overspreading the mouth of the Mississippi river and the barrier islands of the Mississippi coast. Sustained winds increased to 45 mph, with gusts towards 60 mph in its main inflow band east of the center. As it edged farther inland that afternoon, it finally weakened and lost the eye.

June 13th and 14th
Allison tracked out of Mississippi into southern Alabama on the afternoon on the 11th. Severe weather broke out in Georgia, South Carolina, and southern North Carolina on the 13th and 14th. As the system slowed in North Carolina, flooding became a major problem.

June 16th
As the center moved north-northwest along the eastern edge of the Delmarva Peninsula, a cold front moved in from the west. This allowed moisture from Allison to pool along the front to the north of the center, setting the stage for heavy rains across the northern mid-Atlantic and southern New England. Southeast Pennsylvania
saw torrents of rain during the afternoon and evening of the 16th. Willow Grove Naval Air Station recorded 8.47 inches of rain mainly in the eight-hour period
between 2 and 10 pm EDT. Ultimately, Willow Grove received 10.16 inches while Chanfont recorded 10.17 inches for the 24-hour period ending the morning
of the 17th.

June 18th
The last of the rain associated with Allison left eastern Maine during the early morning of the 18th as the cyclone accelerated off to the east-northeast.

This case has been broken down into three sections- Texas, Georgia, and Philadelphia. The Texas floods are under allison_tx and range from 00z June 5 to 00z June 11, 2001. The Louisiana to Georgia floods are under allison_ga and range from 00z June 11 to 00z June 16, 2001. The Philadelphia floods are under allison_phi and range from 00z 16 June to 16z June 19, 2001.

Case Study 37