Tropical Storm Allison developed over the northwestern Gulf of Mexico and moved inland over the upper Texas coast, producing extremely heavy rainfall and catastrophic floods in the Houston area. Allison then acquired subtropical characteristics and continued to produce heavy rainfall and flooding near its track from Louisiana eastward to North Carolina, and then northward along the U.S. east coast to Massachusetts.
Satellite images and synoptic data indicate that Allison originated from a tropical wave that moved off the west coast of Africa on 21 May. The wave tracked westward at about 15 kt across the tropical Atlantic with little associated convection before moving inland over the northern part of South America on 26 May. It emerged over the southwestern Caribbean Sea on 29 May. The wave moved westward into the eastern North Pacific Ocean on 1 June, then slowed down over the Gulf of Tehuantepec. On 2 June, the wave produced a low-level cyclonic circulation centered about 200 n mi south-southeast of Salina Cruz, Mexico.
On 3 June, low- to mid-level southwesterly flow in the southeast quadrant of the Mexican thermal low moved the broad cyclonic circulation inland over extreme southeastern Mexico and western Guatemala. The low-level circulation weakened and became ill-defined after moving inland, whereas the strong mid-level circulation remained intact and tracked northeastward over the southern Yucatan Peninsula early on the 4th. By 0000 UTC 5 June, the mid-level circulation had moved northwestward into the Bay of Campeche of the Gulf of Mexico. Deep convection developed along the western (cyclonic-shear) side of a 30 to 40 kt low-level jet whose axis extended north-northwestward from Merida, Mexico to the Texas-Louisiana border. The deep convection persisted during the night of the 5th and eventually moved north-northwestward around an upper-level cold low centered over south Texas. The difluent eastern semicircle of the upper-level low acted to enhance the development of deep convection.
By 1200 UTC on the 5th, satellite imagery and surface observations suggest that the mid-level circulation developed downward and became Tropical Storm Allison about 120 n mi south of Galveston, TX (see Figure 1 - Figure 3 and Table 1). The cold-core nature of the upper low detected in 0000 and 1200 UTC 5 June upper-air data suggests, however, that this system possessed some subtropical characteristics.
A strong pressure gradient developed between Allison's center and the Atlantic subtropical ridge, which extended westward across Florida and the southeast U.S. and into the northeast Gulf of Mexico. Allison's central pressure dropped and sustained winds increased across a large portion of the central Gulf of Mexico. Winds as high as 40 kt, with gusts to 50 kt, occurred more than 200 n mi east of the center. At 1500 UTC, a ship reported a sustained wind of 43 kt with gusts to 60 kt. (see Table 2) about 90 n mi east of the estimated center location.
Confirmation of a closed circulation came at 1800 UTC when Buoy 42019 (located about 60 n mi south of Freeport, TX) reported a light northwest wind and a pressure of 1006.6 mb just west of the surface center. At 1852 UTC, aircraft personnel on the first U.S. Air Force Reserve reconnaissance flight of the day "closed off" the low-level circulation about 80 n mi south-southwest of Galveston, TX. A combination of a 55 kt flight-level (1500 ft) wind report and the earlier ship reports indicated that the storm had strengthened. The cyclone tracked slowly north-northwestward and weakened slightly (based on offshore buoy reports) just before making landfall southwest of Galveston near Freeport, TX. Its center then tracked slowly northward across the western portions of the Houston metropolitan area during the night of the 5th and the morning of the 6th.
After moving inland, surface observations indicated that Allison quickly weakened to a tropical depression. It drifted slowly northward before becoming stationary over eastern Texas near Lufkin on 7 June. On 8 June, Tropical Depression Allison began to move slowly southward. It eventually moved back over the Gulf of Mexico around 0000 UTC on the 10th, at nearly the same location where it had made landfall as a tropical storm. Although the low-level center remained over warm water on the 10th, very dry air in the mid- and upper-levels of the troposphere overlaid the surface center. The dry air, combined with moderate upper-level westerly shear, inhibited the redevelopment of thunderstorms near the low-level center.
Upper-air observations indicated that by 0000 UTC Allison had acquired some baroclinic characteristics and became a subtropical depression. Most of the persistent deep convection was then developing a couple hundred miles east of the low-level center. A new low-level circulation developed around 0000 UTC 11 June, near the strongest thunderstorms about 150 n mi east of the original center, approximately 50 n mi south of Intracoastal City, LA. By 0200 UTC 11 June, the new center moved inland over southeast Louisiana near Morgan City.
During the early morning hours of the 11th, strong thunderstorms became organized near the low-level circulation center and surface observations indicated that Allison had become a subtropical storm by 0600 UTC. Convection wrapped all the way around the center creating an "eye-like" feature (Figure 4) near 1200 UTC. In spite of the eye-like feature apparent in radar imagery, the cyclone's radius of maximum winds (~100 n mi) was more typical of a subtropical low. Also, surrounding upper-air observations continued to show that Allison was in a weakly baroclinic environment.
The subtropical storm tracked east-northeastward across southern Mississippi. It weakened back to subtropical depression status by 0000 UTC 12 June, when the system was located over southwestern Alabama. It continued to track east-northeastward across southern Alabama, southern Georgia, and southern South Carolina before becoming stationary just north of Wilmington, NC on the 14th. The center of Allison then drifted slowly northward over eastern North Carolina and extreme southeastern Virginia on the 15th. It then moved more quickly northeastward and reached the mid-Atlantic coast on the 17th and eventually cleared the United States mainland along the Delmarva Peninsula later that day. At 1200 UTC 17 June, Allison began to interact with an approaching cold front and briefly strengthened back to a subtropical storm.
Allison merged with the cold front around 0000 UTC 18 June, then becoming an extratropical low pressure system. Shortly after becoming extratropical, the system accelerated east-northeastward before dissipating southeast of Nova Scotia around 0600 UTC 19 June.
Observations in Figure 2 and Figure 3 include satellite-based Dvorak technique intensity estimates from the NOAA Tropical Prediction Center's (TPC) Tropical Analysis and Forecast Branch (TAFB), the NOAA Satellite Analysis Branch (SAB), and the U. S. Air Force Weather Agency (AFWA), as well as data from flights of the 53rd Weather Reconnaissance Squadron of the U. S. Air Force Reserve Command.
Ship reports of tropical storm force winds associated with Allison are in Table 2, and selected surface observations from land stations, C-MAN stations, and data buoys are in Table 3. Allison's peak intensity of 50 kt at 1800 UTC 5 June was based on a 1700 UTC ship (WTEJ) observation of 48 kt and a 55 kt surface wind estimate made by reconnaissance flight crew personnel at 1852 UTC.
During and after Allison's landfall over the upper Texas coast and its southward return to the Gulf of Mexico, extremely heavy rainfall occurred over much of eastern and southeastern Texas, including the Houston metroplex, and much of southwestern Louisiana. Several locations on the east side of Houston received more than 30 inches of rainfall (Figure 5). Heavy rainfall and widespread flooding also occurred with the subtropical low pressure system as it tracked across the southeast and eastern United States (Figure 6).
Evacuations of the west end of Galveston Island were required due to coastal flooding since this area is not protected by the seawall that was built after the Great Hurricane of 1900. Storm surge of 2 to 3 feet, when combined with wave heights up to 8 feet, created considerable overwash. This resulted in significant beach erosion and damage to some of the island's roads. Additional storm surge of 1 to 2 feet occurred over southeast Louisiana on the 11th when Allison passed over that area as a subtropical storm.
A total of 23 tornadoes were confirmed from Mississippi to South Carolina from 11-16 June Specifically, 10 tornadoes were reported in South Carolina, 4 in Mississippi, 3 in Florida, 2 in both Alabama and Georgia, and 1 each in Louisiana and Virginia.
In addition, on 17 June, several sailboats participating in a sailing race from Annapolis, MD to Newport, RI reported sustained winds from the northwest as high as 48 kt and gusts as high as 68 kt at the mouth of Delaware Bay and just off Cape May, NJ. However, the accuracy of these unofficial observations is in doubt since they were not consistent with surrounding official observations. The maximum sustained wind observed was only 31 kt, at Buoy 44009 located just east of the mouth of Delaware Bay. The highest official gust observed during the passage of Subtropical Storm Allison off the U. S. east coast was 36 kt at Atlantic City, NJ. The local pressure gradient along the Delaware and New Jersey coasts also did not support winds as high as those indicated by the sailing vessels. Interestingly enough, though, the brisk winds associated with the subtropical storm resulted in the previous race course record being broken by 5 hours.
Allison's heavy rains produced catastrophic flooding over portions of the upper Texas coastal area, and significant flooding along the remainder of its track. The American Insurance Group reported an insured property loss estimate of $1.22 billion. The TPC standard procedure is to double the insured loss. This results in a final total loss estimate of $2.44 billion. However, damage estimates reported by the Federal Emergency Management Agency (FEMA) and other state emergency management agencies are near $5 billion, with approximately $4.8 billion in the Houston metropolitan area alone. These damage estimates in the Houston area include: $2.04 billion to public facilities (especially the Texas Medical Center), $1.76 billion to residential properties, $1.08 billion to businesses. More than 14,000 homes were destroyed or received major damage, and nearly an additional 34,000 homes incurred at least minor damage. Some of the damage estimates from emergency management agencies may include costs and expenses not directly associated with the flood-related damage.
Forty-one deaths are directly related to the heavy rain, flooding, tornadoes, and high surf generated by Tropical Storm Allison and its remnant subtropical circulation. The death toll by state is as follows: Texas 23, Florida 8, Pennsylvania 7, Louisiana 1, Mississippi 1, and Virginia 1. Twenty-seven of these deaths were due to drowning in freshwater flooding. Early morning on the 11th in Zachary, LA, a tornado (intensity unknown) knocked down trees onto a pickup truck, killing the male driver. Later that morning, an F1 tornado cut a 4 mile long path across George County, MS. It damaged several manufactured homes and completely destroyed a mobile home (tiedowns were pulled out of the ground and the home turned over and fell apart as it rolled), severely injuring a female occupant. There were also 9 indirect deaths in North Carolina as a result of traffic accidents occurring on wet roads. These damage and direct death toll estimates make Allison the deadliest and most costly tropical or subtropical storm on record in the United States.
No meaningful track and wind forecast statistics are available due to the limited period for which the National Hurricane Center (NHC) issued forecasts. Since the system reached storm strength quickly, there was little warning lead time. A tropical storm warning was issued at 1900 UTC 05 June from Sargent, Texas eastward along the Gulf of Mexico coast to Morgan City, Louisiana. This was less than 3 hours before tropical storm force winds were reported along the upper Texas coast.A subsequent analysis of satellite and surface observation data indicate that Allison was likely a tropical storm by 1200 UTC 5 June. Determination of a closed surface low pressure system was delayed by at least 6 hours as a result of Buoy 42002 not reporting due to its previous collision with a ship. In addition, Dvorak satellite classifications were not representative of the true intensity of the hybrid system.
After Tropical Depression Allison emerged over the northern Gulf of Mexico, the TAFB issued a gale warning for the northeastern Gulf of Mexico about three hours prior to Allison reaching subtropical storm status. The sustained gale force winds were primarily confined to a thunderstorm squall line in the eastern portion of the low pressure system. The Marine Prediction Center (MPC) in Washington, D.C. also issued gale warnings for portions of the mid-Atlantic offshore waters on 14 and 15 June when Allison was expected to move off the North Carolina and Virginia coasts and possibly reach subtropical storm strength.
Consistent with operational protocol, responsibility for Allison was transferred from the TPC/NHC to the Hydrometeorological Prediction Center (HPC) in Washington, D.C. when the cyclone initially moved inland, weakened to a tropical depression, and coastal warnings were discontinued. By then, the primary threat had become mainly fresh water flooding due to the heavy rainfall.
Operationally, HPC continued to issue products through the remainder of Allison's track. Frequent coordination calls occurred between HPC, TPC, and National Weather Service (NWS) forecast offices during this period of highly-unusual cyclone activity where the system was a tropical depression, subtropical depression/storm, and then an extratropical cyclone. This included the short (12 to 18 hour) period when gale force winds were observed in association with the second center. While this post-storm analysis shows Allison then as a subtropical system (and, hence, technically the forecast responsibility of the TPC/NHC), the decision for the HPC to retain forecast responsibility is supported by the (1) need for consistency in service source, (2) uncertainty in storm type, (3) short duration of gale-force winds, and (4) center remaining mostly over land.
Some of the data in this report was furnished by the National Climatic Data Center in Asheville, NC, various state emergency management agencies, FEMA, National Weather Service Offices in Houston, Lake Charles, New Orleans (Slidell), Mobile, Tallahassee, Jacksonville, Atlanta, Columbia, Charleston, Newport/Morehead City, Raleigh/Durham, Wakefield, Wilmington, Baltimore/Washington, and Mount Holly (Philadelphia), and by the HPC.
|Lat. (°N)||Lon. (°W)|
|05 / 1200||27.5||95.0||1007||40||tropical storm|
|05 / 1800||28.5||95.3||1002||50||"|
|06 / 0000||29.3||95.3||1003||45||"|
|06 / 0600||30.1||95.2||1006||30||tropical depression|
|06 / 1200||31.0||95.2||1005||20||"|
|06 / 1800||31.5||95.0||1005||20||"|
|07 / 0000||31.6||95.0||1005||20||"|
|07 / 0600||31.8||94.9||1006||20||"|
|07 / 1200||31.4||94.9||1006||20||"|
|07 / 1800||31.1||95.0||1007||15||"|
|08 / 0000||30.9||95.6||1002||15||"|
|08 / 0600||30.7||96.1||1004||20||"|
|08 / 1200||30.4||96.2||1007||20||"|
|08 / 1800||30.2||96.1||1007||20||"|
|09 / 0000||29.9||95.9||1007||20||"|
|09 / 0600||29.6||95.8||1007||20||"|
|09 / 1200||29.3||95.8||1007||20||"|
|09 / 1800||28.9||95.6||1008||20||"|
|10 / 0000||28.6||95.2||1008||20||subtropical depression|
|10 / 0600||28.6||94.7||1007||25||"|
|10 / 1200||28.7||94.3||1006||25||"|
|10 / 1800||28.8||93.5||1006||30||"|
|11 / 0000||29.1||92.3||1005||30||subtropical depression (New center)|
|11 / 0600||30.0||90.5||1003||35||subtropical storm|
|11 / 1200||30.7||89.4||1000||40||"|
|11 / 1800||31.0||88.4||1002||35||"|
|12 / 0000||31.3||87.4||1004||25||subtropical depression|
|12 / 0600||31.8||86.1||1005||20||"|
|12 / 1200||32.4||84.7||1006||20||"|
|12 / 1800||32.8||83.6||1006||20||"|
|13 / 0000||33.1||82.6||1006||20||"|
|13 / 0600||33.4||81.6||1004||25||"|
|13 / 1200||33.7||80.7||1005||25||"|
|13 / 1800||34.0||79.6||1006||25||"|
|14 / 0000||34.3||78.5||1006||25||"|
|14 / 0600||34.6||77.9||1006||25||"|
|14 / 1200||34.7||77.7||1007||25||"|
|14 / 1800||34.6||77.6||1008||25||"|
|15 / 0000||34.6||77.2||1008||25||"|
|15 / 0600||34.9||77.0||1008||25||"|
|15 / 1200||35.5||76.9||1008||25||"|
|15 / 1800||35.9||76.8||1009||25||"|
|16 / 0000||36.3||76.6||1007||25||"|
|16 / 0600||36.6||76.2||1007||25||"|
|16 / 1200||36.8||75.9||1007||25||"|
|16 / 1800||37.2||75.5||1006||25||"|
|17 / 0000||37.8||75.4||1006||25||"|
|17 / 0600||38.6||74.5||1005||30||"|
|17 / 1200||39.3||73.4||1004||40||subtropical storm|
|17 / 1800||40.0||72.1||1005||35||"|
|18 / 0000||40.6||70.8||1006||30||extratropical low|
|18 / 0600||41.3||69.4||1008||30||"|
|18 / 1200||42.0||67.4||1009||30||"|
|18 / 1800||42.7||64.6||1011||25||"|
|19 / 0000||43.5||61.0||1012||20||"|
|19 / 0600||dissipated|
|05 / 2100||28.9||95.3||1003||45||landfall near Freeport, TX|
|11 / 0200||29.6||91.6||1004||30||landfall near Morgan City, LA|
|11 / 1200||30.7||89.4||1000||40||minimum pressure|
|Ship Name or Call Sign||Date/Time (UTC)||Lat. (°N)||Lon. (°W)||Wind dir/speed (deg/kt)||Pressure (mb)|
|KDGR||05 / 1200||25.0||89.8||100 / 34||1008.2|
|WTEJ||05 / 1500||27.8||92.8||110 / 43G60||1010.7|
|WTEJ||05 / 1600||27.8||92.7||120 / 40||1011.9|
|WTEJ||05 / 1700||27.8||92.6||120 / 48||1011.1|
|WTEJ||05 / 1800||27.7||92.5||120 / 44||1010.1|
|KDGR||05 / 1800||24.5||88.1||110 / 34||1009.4|
|WTEJ||05 / 1900||27.6||92.5||120 / 46||1009.9|
|WTEJ||05 / 2000||27.6||92.4||120 / 40||1010.2|
|WTEJ||05 / 2100||27.5||92.3||150 / 44||1010.5|
|WTEJ||05 / 2200||27.4||92.2||140 / 40||1010.0|
|WTEJ||05 / 2300||27.4||92.1||140 / 38||1009.9|
|Maximum Surface Wind Speed|
|Dover AFB (KDOV)||3.94|
|Barataria Bay (USGS)||06/0545||37|
|Door Point (USGS)||11/0400||36|
|Grand Pass (USGS)||11/0400||38|
|Lake Charles (KLCH)||06/0912||1008.5||05/1906||27||31|
|NE Bay Gardene (USGS)||11/0354||46|
|NOLa Moisant (KMSY)||11/0739||1003.7||24||29|
|NOLa Lakefront (KNEW)||11/0753||1003.7||11/0854||33||38|
|Salt Point (KP92)||06/1021||1009.8||05/1305||35||27.55|
|Slidell (KASD )||11/0905||1002.0||19||28|
|KBIX (Keesler AFB)||11/1053||1005.3||11/1040||34||55|
|Atlantic City (KACY)||17/1241||1008.8||17/1246||28||36|
|Atlantic City (KACY)||17/1313||21||36|
|Atlantic City (KACY)||17/1431||26||34|
|Willow Grove (KNXX)||10.16|
|Brays Bayou (Stella Link)||19.72|
|Brays Bayou (Lawndale)||21.46|
|Chigger Creek (Windsong)||18.78|
|Clear Creek (Tele. Rd.)||20.04|
|College Station (KCLL)||06/1014||1006.8||3.49|
|Cypress Creek (at Grant)||21.41|
|Coward Creek (at Baker)||27.95|
|Eagle Point (port)||06/0230||1005.4||05/2130||37||42|
|Ellington Field (KEFD)||06/0250||1004.4||06/0130||20||44|
|Furr H.S. (Harris Co.)||35.14|
|Galveston Bay, N. Jetty||06/0012||1002.9||05/2116||35||42||1.8|
|Galveston Pleasure Pier||06/0130||1003.7||05/2118||38||45||2.1|
|Garners Bayou (B'tway 8)||24.61|
|Greens Bayou (US 59)||35.76|
|Harris Co. Museum N.S.||20.83|
|Heights (Harris Co.)||32.00|
|Houston Hobby (KHOU)||06/0253||1004.4||05/2005||27||18.77|
|Houston Hobby (KHOU)||05/2254||33|
|Houston Int'cntl (KIAH)||06/0413||1003.0||06/0313||21||28||16.48|
|Hunting Bayou (Houston)||35.75|
|League City (NWS HGX)||06/0300||1005.6||05/2100||25||19.41|
|Little Vince (at Jackson)||22.40|
|Morgans Point ( port)||06/0300||1004.6||06/0318||27||36||1.2|
|Port of Houston||36.99|
|Sugarland City Hall||15.31|
|Vince Bayou (W. Ellaine)||25.31|
|White Oak Bayou (Ella)||18.19|
|Wallops Island (KWAL)||17/0154||1006.3||16/2103||16||23|
|ALSN6 (40.5N 73.8W)||17/1800||1010.8||17/1650||28||30|
|BURL1 (28.9N 89.4W)||11/1100||1004.5||11/1110||41|
|BURL1 (28.9N 89.4W)||11/1121||49|
|BURL1 (28.9N 89.4W)||11/1200||1004.5|
|CHLV2 (36.9N 75.7W)||16/1000||1007.5||16/1610||33||34|
|CLKN7 (34.6N 76.5W)||14/0800||1009.1||14/0600||53|
|CSBF1 (29.7N 85.4W)||12/0442||35|
|DPIA1 (30.3N 88.1W)||11/1600||1007.5||11/1420||31||44|
|DSLN7 (35.2N 75.3W)||14/0900||1013.0||14/1400||36|
|GDIL1 (29.3N 90.0W)||11/0900||1005.1||11/0850||34||45|
|SRST2 (29.7N 94.1W)||05/1450||41||53|
|TPLM2 (38.9N 76.4W)||16/2100||1011.0|
|42007 (30.1N 88.8W)||11/1120||34|
|42007 (30.1N 88.8W)||11/1108||49|
|42019 (27.9N 95.4W)||05/1300||27||33|
|42035 (29.3N 94.4W)||05/1500||31||41|
|42040 (29.2N 88.2W)||11/1100||1008.4||11/1600||33|
|42040 (29.2N 88.2W)||11/1541||41|
|44004 (38.5N 70.5W)||17/2100||1011.7||17/2310||29||33|
|44008 (40.5N 69.4W)||18/0400||1007.0|
|44009 (38.5N 74.7W)||17/1000||1004.4||11/1330||31||33|
|44011 (41.1N 66.6W)||18/1000||1008.8|
|44014 (36.6N 74.8W)||16/0900||1009.1|
|44025 (40.3N 70.2W)||17/1800||1008.3||17/1800||27||33|
|Oil Rig Platforms|
|K7B5 (28.1N 93.2W)||05/1400||40||50|
|K3B6 (28.0N 92.8W)||10/1445||42|
|K7R8 (28.3N 92.0W)||10/1542||35|
Figure 1: Best track positions for Tropical Storm Allison, 5-17 June 2001.
Figure 2: Best track maximum sustained surface wind speed curve for Tropical Storm Allison, 5-17 June 2001. Aircraft observations have been adjusted for elevation using an 85% reduction factor for observations at 1500 ft. Wind curve is based primarily on surface observations after 1200 UTC 6 June.
Figure 3: Best track minimum central pressure curve for Tropical Storm Allison, 5-17 June 2001. Pressure curve is based on surface observations after 0600 UTC 6 June. Estimates during the extratropical stage are based on analyses from the NOAA Marine Prediction Center.
Figure 4: 1227 UTC 11 June 2001 radar reflectivity image from Mobile, AL. Note the eye-like feature over southern Mississippi and the squall line to the east. The intensity was near 40 kt at this time.
Figure 5: Storm total rainfall (in) for the Houston metropolitan area (data provided by the Harris Co., TX Office of Emergency Management).
Figure 6: Storm total rainfall for Tropical Storm Allison during the period 4-18 June 2001. Rainfall data were obtained from the standard rainfall reporting network. This map does not include some isolated amounts, which were up to nearly 37 inches.