Meteorological history of Tropical Storm Allison

Infobox Hurricane History
Name=Tropical Storm Allison
Type=Tropical storm
Year=2001
Basin=Atl
Image location=Allison 2001 track.png

Formed=June 4, 2001
Dissipated=June 18, 2001
1-min winds=50
Pressure=1000
Areas=Texas, Gulf Coast, Southeast U.S.
Hurricane season=2001 Atlantic hurricane season Tropical Storm Allison lasted for fifteen days from June 4 through June 18, 2001, attaining tropical or subtropical storm status on three separate occasions. Allison's origins were in a tropical wave that left the coast of Africa on May 21. After moving across the Atlantic Ocean and South America, the wave entered the eastern North Pacific Ocean on June 1. A low-level circulation developed on June 2, as it was 230 miles (370 km) south-southeast of Salina Cruz, Mexico. Southerly flow forced the system northward, and the wave moved inland on June 3. It emerged into the Gulf of Mexico on the next day, and developed deep convection on its eastern side. On June 5, the system organized sufficiently to be declared Tropical Storm Allison.

The storm rapidly strengthened to its peak with winds of 60 mph (95 km/h). Allison made landfall near Freeport, Texas with winds of 50 mph (80 km/h) late on June 5. The depression drifted northward before reaching Lufkin, Texas, where it stalled due to a ridge to its north, and dropped extreme amounts of rainfall. The storm turned southward, and re-emerged into the Gulf of Mexico on June 10. It transformed into a subtropical cyclone, and moved eastward before moving ashore at Morgan City, Louisiana on June 11. Despite being over land, Allison re-intensified into a subtropical storm.

After once again weakening, the system moved east-northeast through Mississippi, Alabama, Georgia, and South Carolina before becoming nearly stationary near Wilmington, North Carolina. It drifted through the state, and accelerated to the northeast as a result of an approaching cold front. Allison re-entered the Atlantic Ocean on June 17, and for the final time reached subtropical storm status. Allison became extratropical while south of Long Island, and was absorbed by a frontal system by June 18.

Formation

A tropical wave moved off the coast of Africa on May 21, 2001. It moved westward across the Atlantic Ocean, retaining little convection on its way. After moving across South America and the southwestern Caribbean Sea, the wave entered the eastern North Pacific Ocean on June 1. A low-level circulation developed on June 2, while it was about 230 miles (370 km) south-southeast of Salina Cruz, Mexico. Southerly flow forced the system northward, and the wave moved inland on June 3. The low-level circulation dissipated, though the mid-level circulation persisted. It emerged into the Gulf of Mexico on June 4, and developed deep convection on its eastern side.cite web|author=Stacy R. Stewart|title=Tropical Storm Allison Tropical Cyclone Report|date=February 28, 2001|publisher=National Hurricane Center|accessdate=2008-08-16|url=http://www.nhc.noaa.gov/2001allison.html] Early on June 5, satellite imagery suggested that a tropical depression was forming in the northwest Gulf of Mexico, which was furthered by reports of wind gusts as high as 60 mph (95 km/h) just a few hundred feet above the surface, towards the east side of the system. [cite web|author=Stewart|title=June 5 Tropical Weather Outlook|date=June 5, 2001|publisher=National Hurricane Center|accessdate=2008-08-16|url=ftp://ftp.met.fsu.edu/pub/weather/tropical/Outlook-A/2001/Jun/2001060514.ABNT20]

At 1200 UTC on June 5, the disturbance developed a broad, low-level circulation, and was classified as Tropical Storm Allison, the first storm of the 2001 Atlantic hurricane season. Some intensification was projected, though it was expected to be hindered by cool offshore sea surface temperatures. [cite web|author=Stewart|title=Tropical Storm Allison Discussion Number 1|date=June 5, 2001|publisher=National Hurricane Center|accessdate=2008-08-16|url=http://www.nhc.noaa.gov/archive/2001/dis/al012001.discus.001.html] Due to the cold-core nature of the center, Allison initially contained subtropical characteristics. Despite this, the storm quickly strengthened to attain peak sustained winds of 60 mph (95 km/h), with tropical storm-force winds extending up to 230 miles (370 km) east of the center, and a minimum central pressure of 1000 mbar. The storm initially moved very little, and the presence of several small vortices from within the deep convection caused difficulty in determining the exact center location.cite web|author=Stewart|title=Tropical Storm Allison Discussion Number 2|date=June 5, 2001|url=http://www.nhc.noaa.gov/archive/2001/dis/al012001.discus.002.html|accessdate=2008-08-16|publisher=National Hurricane Center]

First landfall

Later in the day, several different track forecasts arose. One scenario had the cyclone tracking westward in to Mexico. Another projected the storm moving east towards southern Louisiana. At the time, it was noted that little rain or wind persisted near the center, but rather to the north and east. [cite web|author=Lawrence|title=Tropical Storm Allison Discussion Number 3|date=June 5, 2001|publisher=National Hurricane Center|accessdate=2008-08-16|url=http://www.nhc.noaa.gov/archive/2001/dis/al012001.discus.003.html] Under the steering currents of a subtropical ridge that extended in an east–west orientation across the southeast United States, Allison weakened while nearing the Texas coastline, and struck near Freeport, Texas with 50 mph (80 km/h) winds. With a storm surge of 2–3 feet (1 meter) combined with waves on top, areas of Galveston Island experienced water 8 feet (2.5 m) in height, creating overwash along the coastline. The storm caused winds of up to 43 mph (69 km/h) at the Galveston Pier. Inland, the storm rapidly weakened, and the National Hurricane Center discontinued advisories early on June 6. [cite web|author=Pasch|title=Tropical Depression Allison Public Advisory Number 4|date=June 6, 2001|publisher=National Hurricane Center|accessdate=2008-08-16|url=http://www.nhc.noaa.gov/archive/2001/pub/al012001.public.004.html] Shortly after being downgraded to a tropical depression, surface observations showed an elongated circulation with a poorly-defined center, which had reformed closer to the deep convection. [cite web|author=Pasch|title=Tropical Depression Allison Discussion Number 4|date=June 6, 2001|publisher=National Hurricane Center|accessdate=2008-08-16|url=http://www.nhc.noaa.gov/archive/2001/ALLISON.html]

The depression drifted northward until reaching Lufkin, Texas, where it stalled due to a high pressure system to its north. While stalling over Texas, the storm dropped excessive rainfall, peaking at just over 40 inches (1,033 mm) in northwestern Jefferson County. In the Port of Houston, a total of 36.99 inches (940 mm) was reported.cite web|author=Hydrometeorological Prediction Center|year=2006|title=Rainfall totals from Tropical Storm Allison|accessdate=2006-05-17|url=http://www.hpc.ncep.noaa.gov/tropical/rain/allison2001.html] Other totals include 9.77 inches (248 mm) in Galveston and 12.13 inches (308 mm) in Jamaica Beach, Texas. The high amounts of precipitation led to horrendous flooding in and around Houston, Texas,cite web|author=Risk Management Solutions|year=2001|title=Tropical Storm Allison Event Report|accessdate=2008-08-16|url=http://www.rms.com/Publications/TS_Allison.pdf|format=PDF] with flash flooding continuing for days.cite web|author=Houston/Galveston National Weather Service (Synoptic Situation)|year=2006|title=Tropical Storm Allison Floods, June 5–9, 2001|accessdate=2006-05-17|url=http://www.srh.noaa.gov/hgx/projects/allison01/synoptic.htm]

On June 7, the subtropical ridge off Florida weakened, while the ridge west of Texas intensified. This steered Tropical Depression Allison to make a clockwise loop, and the storm began drifting to the southwest. As the center reached Huntsville, Texas, a heavy rain band began to back build from Louisiana westward into Liberty County, Texas, which had caused additional flooding. At the time, the system had a minimum central pressure of about 1004 mb and maximum sustained winds of about convert|10|mph|km/h|abbr=on. [cite web|author=Pereira|title=Storm Summary Number 12 for the Remnants of Allison|date=June 8, 2001|publisher=Hydrometeorological Prediction Center|accessdate=2008-08-16|url=http://www.hpc.ncep.noaa.gov/tropical2001/allison/06080109.html]

Gulf of Mexico, second landfall and southeast U.S.

Late on June 9 and early on June 10, Allison's remnants once again reached the Gulf of Mexico and emerged over open waters. [cite web|author=Pereira|title=Storm Summary Number 19 for the Remnants of Allison|date=June 8, 2001|publisher=Hydrometeorological Prediction Center|accessdate=2008-08-16|url=http://www.hpc.ncep.noaa.gov/tropical2001/allison/06100103.html] The low once again became nearly stationary about 60 mi (100 km) south of Galveston, and despite more favorable upper-level winds, it showed no signs of redevelopment. [cite web|author=Stewart|title=June 10 Tropical Weather Outlook|date=June 10, 2001|publisher=National Hurricane Center|accessdate=2008-08-16|url=ftp://ftp.met.fsu.edu/pub/weather/tropical/Outlook-A/2001/Jun/2001061008.ABNT20] Due to dry air and moderate westerly wind shear, the storm transformed into a subtropical cyclone. While the subtropical depression moved eastward, a new low level circulation redeveloped to the east, and Allison quickly made landfall on Morgan City, Louisiana on June 11. At around the same time, the surface center reformed to the east-northeast of its previous location, aligning with the mid-level circulation. [cite web|author=Clark|title=Storm Summary Number 23 for the Remnants of Tropical Storm Allison|date=June 10, 2001|publisher=Hydrometeorological Prediction Center|accessdate=2008-08-16|url=http://www.hpc.ncep.noaa.gov/tropical2001/allison/06110103.html] Strong thunderstorms redeveloped over the circulation, and Allison strengthened into a subtropical storm over southeastern Louisiana. The storm intensified further to attain sustained winds of 45 mph (70 km/h) and a minimum barometric pressure of about 1000 mb near Mclain, Mississippi, accompanied by a well-defined eye-like feature. [cite web|author=Roth & Mcelroy|title=Storm Summary Number 25 for the Former T.S. Allison|date=June 11, 2001|publisher=Hydrometeorological Prediction Center|accessdate=2008-08-16|url=http://www.hpc.ncep.noaa.gov/tropical2001/allison/06110115.html] In Mississippi, the cyclone produced heavy rainfall of over 10 inches (255 mm) in one night.cite web|author=National Climatic Data Center|year=2001|title=Event Report for Mississippi|accessdate=2006-06-06|url=http://www4.ncdc.noaa.gov/cgi-win/wwcgi.dll?wwevent~ShowEvent~436002]

The storm was officially downgraded to a subtropical depression at 0000 UTC on June 12. Somewhat accelerating, the depression tracked to the east-northeast through Mississippi, Alabama, Georgia, and South Carolina before becoming nearly stationary near Wilmington, North Carolina. Over Georgia, the storm dropped heavy rainfall of 10 inches (255 mm) in 24 hours in locations. The deluge caused rivers to crest past their banks, including the Oconee River at Milledgeville which peaked at 33.7 feet (10.3 m).cite web|author=National Climatic Data Center|year=2001|title=Event Report for Georgia|accessdate=2006-06-08|url=http://www4.ncdc.noaa.gov/cgi-win/wwcgi.dll?wwevent~ShowEvent~424563] The depression drifted through North Carolina and sped to the northeast for a time in response to an approaching cold front. [cite web|author=Avila|title=June 14 Tropical Weather Outlook|date=June 14, 2001|publisher=National Hurricane Center|accessdate=2008-08-16|url=ftp://ftp.met.fsu.edu/pub/weather/tropical/Outlook-A/2001/Jun/2001061415.ABNT20] Though satellite and radar imagery show the system was well-organized, the system slowed and moved erratically for a period of time, [cite web|author=Beven|title=June 14 Tropical Weather Outlook|date=June 14, 2001|publisher=National Hurricane Center|accessdate=2008-08-16|url=ftp://ftp.met.fsu.edu/pub/weather/tropical/Outlook-A/2001/Jun/2001061421.ABNT20] executing what appeared to be a small counterclockwise loop. [cite web|author=Ziegenfelder & Mcelroy|title=Storm Summary Number 38 for the Remnants of T.S. Allison|date=June 14, 2001|publisher=Hydrometeorological Prediction Center|accessdate=2008-08-16|url=http://www.hpc.ncep.noaa.gov/tropical2001/allison/06140121.html]

Atlantic Ocean and demise

The storm began tracking in a generally northeasterly direction, and crossed into the southern Delmarva Peninsula on June 16. [cite web|author=Mcelroy|title=Storm Summary Number 46 for the remnants of Allison|date=June 16, 2001|publisher=Hydrometeorological Prediction Center|accessdate=2008-08-16|url=http://www.hpc.ncep.noaa.gov/tropical2001/allison/06160121.html] In Delaware, moderate rainfall occurred, peaking at 4.2 inches (106 mm) in Greenwood.cite web|author=National Climatic Data Center|year=2001|title=Event Record Details for Tropical Storm Allison in Delaware|accessdate=2006-05-26|url=http://www4.ncdc.noaa.gov/cgi-win/wwcgi.dll?wwevent~ShowEvent~423153] The subtropical remnants reached the Atlantic on June 17, and while located east of Atlantic City, New Jersey, winds began to restrengthen, and heavy rains to the north of the circulation. The low was interacting with a frontal boundary, and started merging with it, as it accelerated to the northeast at convert|13|mph|km/h|abbr=on. [cite web|author=Mcelroy|title=Storm Summary Number 49 for the Remnants of Allison|date=June 17, 2001|publisher=Hydrometeorological Prediction Center|accessdate=2008-08-16|url=http://www.hpc.ncep.noaa.gov/tropical2001/allison/06170115.html]

The remnants of Allison briefly reintensified to a subtropical storm through baroclinic processes, though it became extratropical while south of Long Island. By later on June 17, the low was situated off the coast of Rhode Island, spreading a swath of precipitation over New England. [cite web|author=Clark|title=Storm Summary Number 51 for the Remnants of Allison|date=June 17, 2001|publisher=Hydrometeorological Prediction Center|accessdate=2008-08-16|url=http://www.hpc.ncep.noaa.gov/tropical2001/allison/06180103.html] Up to 7.1 inches (180 mm) of rain fell in North Smithfield, Rhode Island.cite web|author=National Climatic Data Center|year=2001|title=Event Record Details for Tropical Storm Allison in Rhode Island|accessdate=2006-05-26|url=http://www4.ncdc.noaa.gov/cgi-win/wwcgi.dll?wwevent~ShowEvent~446298] The remnants of the tropical storm were then absorbed by the frontal boundary by June 18, and eventually passed south of Cape Race, Newfoundland on June 20, where the extratropical cyclone dissipated.

ee also

* List of tropical cyclones
* List of Atlantic hurricanes
* List of wettest tropical cyclones in the United States

References

External links

* [http://www.nhc.noaa.gov/2001allison.html National Hurricane Center Tropical Cyclone Report on Allison]
* [http://www.nhc.noaa.gov/archive/2001/ALLISON_text.html US National Hurricane Center Tropical Storm Allison advisory archive]
* [http://www.hpc.ncep.noaa.gov/tropical2001/allison/allisonss.html HPC Storm Summary/Advisory Archive for Allison]
* [http://www.hpc.ncep.noaa.gov/tropical2001/allison/06190109.html HPC Final Storm Summary/Wrap-Up of Allison]
* [http://www.hpc.ncep.noaa.gov/tropical/rain/allison2001.html HPC Rainfall Summary for Allison]
* [http://www.nws.noaa.gov/om/assessments/pdfs/allison.pdf NWS Service Assessment] (PDF)
* [http://www.tsarp.org/ Tropical Storm Allison Recovery Project] Community resources and continuing research of the continued flood threat to Houston
* [http://www.srh.noaa.gov/hgx/projects/allison01/synopsis.htm Houston/Galveston National Weather Service Report]