Event Summary
     National Weather Service, Raleigh NC

Remnants of Tropical Storm Allison

  • Very Detailed Summary of Allison from NCEP

  • Brief Summary -

    Allison was the first tropical cyclone of the Atlantic Hurricane season. Allison's origin can be traced back to a tropical wave moving across the Caribbean in late May. The disturbance moved into central America and remained relatively stationary for several days before moving into the southwestern Gulf of Mexico in early June.

    The system drifted northward and at 2pm CDT on June 5th, the system was named Tropical Storm Allison with maximum sustained winds of 60 mph. Allison made landfall on the east end of Galveston Island, TX, late on the afternoon of June 5th. The storm moved slowly north and at 5am CDT on June 6th, Allison was downgraded to a Tropical Depression and was located about 30 miles north of Houston, TX.

    The system moved west and then southwest on June 7th and 8th and then looped toward the east across the southeast Texas coast on June 9th (see map.) The remnants of Allison dumped a second round of very heavy rain across southeastern Texas as the system moved east and northeast across the Texas and Louisiana coast on June 8th and 9th. Devastating flooding was reported across the Houston Texas area. The remnants of Allison moved slowly northeast across the southern Gulf coast on June 11th and 12th. By the afternoon of June 12th, the remnants of Allison were centered west of Augusta, GA (see radar image at 2pm on June 12th.)

    The first round of heavy rain affected North Carolina during the afternoon of June 13th (see radar image at 2pm on June 13th.) The remnants of Allison were centered near Newport, NC on the afternoon of June 14th (see radar image at 2pm on June 14th.) The system moved slowly north during the 15th of June and by the afternoon of June 16th the center was located over the eastern shore of Virginia (see radar images at 2pm on June 15th and 2pm on June 16th.)

    During four consecutive days (from the 13th through 16th of June), spiral bands associated with the storm produced very heavy rain across much of North Carolina. The rain was generally most widespread and heaviest during the afternoon and evening hours. On the 15th of June, between 4 and 8 inches of rain fell across portions of the northern coastal plain resulting in flash flooding.

    The storm began to accelerate northeast on the 17th and 18th of June. The center was located over Delaware bay on the morning of the 17th (see radar image at 8am on June 17th) and then southeast of Cape Cod on June 18th.

    Tropical Storm Allison Track

    Tropical Storm Allison Track with 18Z Approximate Positions

  • Meteorological Discussion - Allison's Heavy Rainfall

    Radar returns, rainfall reports, and satellite imagery indicated Allison exemplified a rather typical night-time convection pattern for a warm-core tropical system as it moved across Texas and Louisiana. During the late night hours the convection with Allison, located close to its circulation center, increased. This exhibition of increased rainfall rates at night, accompanied by increasing cold cloud tops has historically been linked to so-called cloud-top cooling. Tropical systems are warm core. Indeed, the 500 mb temperature associated with Allison's landfall in southeast Texas was minus 5 degrees C. Diurnal cooling at night in the upper regions of the system (hence, cloud- topped) in such a deeply saturated system is thought to be enough to increase lift and enhance nocturnal rainfall rates

    As the remnants of Allison began to move slowly east-northeastward into Georgia, the repeating pattern of enhanced rainfall at night was significantly altered. The system appeared to undergo a change from its classical tropical nature into a modified hybrid mid-latitude system. Instead of a well defined heavy rainfall core near the system's circulation, an increasing number of outer bands began to appear.

    By the time Allison's remnants reached South Carolina, another diurnal pattern emerged. Indeed, the heaviest rains across the Carolinas was due to convection that would initially develop around 15Z to the northeast and north of the circulation center. As this convection rotated into the northwest quadrant of the system's circulation, it would intensify during the 18 - 21Z period. Now absent was the nocturnal flare ups of the most heavy rain at night. This pattern typified the system's behavior as it tracked through the Carolinas (see radar images at 5am on June 14th and 2pm on June 14th.) Meanwhile the intensification of convection in the form of outer bands during the afternoon period was also seen. There were reports across upstate of South Carolina of large hail and damaging thunderstorm winds with some of the convection.

    As the system became more hybrid than tropical, the associated instabilities increased. The result was an increase in convection resulting from the heat of the day rather than cooling at night. It is possible that the initiation of the convection around 15Z each day to the north and northeast of the system's circulation was due in part to low-level speed confluence. As winds least affected by friction moved onshore, they were slowed (see radar image at 11am on June 13th.) The resulting convergence of air was enough in the very moist and unstable air to produce banded convection.

    A mid-latitude 500 mb short-wave interacted with the remnants of Allison across central North Carolina during the afternoon and evening of 16 June. (see radar image at 8pm on June 16th.) This too helped to increase the rainfall rates in the area.

    The above speculations stemmed principally from a analysis of radar loops as Allison tracked across the southeast United States and from discussions with National Weather Service meteorologists who worked the event. This discussion suggests that the system was no longer a typical warm core system as it impacted the Carolinas. The decrease in the nocturnal flare up of heavy rain near the system's core, the appearance of an increasing number of outer bands well removed from the system's center of circulation, the diurnal increase in convection during the late morning and afternoon hours are more suggestive of a mid-latitude hybrid system.

  • A few related web sites -

    NOAA News Report on Allison
    NCDC June Climate Watch Report on Allison
    FEMA Allison Report

  • Precipitation Totals and Imagery -

    24 Hour Rainfall Totals Ending 12Z 06/14/2001

    24 Hour Rainfall Totals Ending 12Z 06/15/2001

    24 Hour Rainfall Totals Ending 12Z 06/16/2001

    24 Hour Rainfall Totals Ending 12Z 06/17/2001

    Daily Rainfall Graphs for Selected North Carolina Cities

    Image from the
    Southeast Region Climate Center.

    Allison Total Regional Precipitation
    (click to enlarge)

    Image from the Southeast Regional Climate Center.

  • Tropical Depression/Tropical Storm History -

    NHC Graphics
    Archive of NHC Graphics

    Text Products
    Discussions Strike

    Tuesday June 5, 2001
    1: 1900Z 
    2: 2100Z 
    1:   2PM CDT
    2:   4PM CDT
    2a: 7PM CDT
    3:   10PM CDT
    1: 3PM EDT
    2: 5PM EDT
    3: 11PM EDT
    1: 2PM CDT
    2: 4PM CDT

    Wednesday June 6, 2001
    3: 0300Z 
    4: 0900Z 
    3a: 1AM CDT
    4:   4AM CDT
    4: 5AM EDT
    None issued

  • Satellite and Radar Imagery-

    Satellite Image from 22Z on 06/12/2001

    Regional Radar Reflectivity Loop of images every 3 hours from 12Z 06/12/2001 through 12Z 06/17/2001

  • Case study team -
    Jonathan Blaes
    Kermit Keeter
    Phil Badgett
    Brandon Locklear
    Shaun Baines

    For questions regarding the web site, please contact Jonathan Blaes.

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