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
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
upstate of South Carolina of large hail and damaging thunderstorm winds with some of the
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
Case study team - |
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