The
Significant Severe Weather Outbreak on 2007 August 16th Across the North County
On 2007 August 16th a significant severe weather
outbreak occurred across the Weather Forecast Office, Burlington, Vermont
county warning area. The Weather Forecast Office in Burlington
issued 26 severe weather warnings and 24 were verified with nearly 4 dozen
reports of severe weather across the North Country.
This particular event featured two large and long lived supercell
thunderstorms, which produced large hail and widespread significant wind
damage. A supercell thunderstorm is a long-lived violent thunderstorm, which
contains large hail, high winds, and potential tornadoes, and is most common
across the Central Plains. These violent thunderstorms produced a 91mph wind gust
at Cumberland Bay
near Plattsburgh, along with reports of golf
ball size hail near Beekmantown,
New York. The following Storm Prediction
Center graphic shows the
two distinct lines of severe weather reports across our region.
Figure 1 Storm Prediction Centers
reports of severe wind (blue) and hail (green), significant wind (black)
Click the link below for a complete listing of all the storm
reports
http://www.erh.noaa.gov/btv/events/16Aug07/LSRBTV.txt
The following link will provide you with a complete result
of the storm surveys performed by the National Weather Service in Burlington, Vermont
on August 16th.
http://www.erh.noaa.gov/btv/events/16Aug07/PNSBTV.txt
Pre-Storm Environment
Several meteorological ingredients came together to produce
these supercells. First, we had a potent disturbance in a very strong middle to
upper level wind flow across southern Canada
into northern New York and Vermont. This disturbance in the winds aloft
combined with a surface cold front interacting with low level instability,
helped to fuel these two distinct complex areas of severe thunderstorms. The
image below shows the water vapor along with lightning data, note the strong
drying across the Saint Lawrence Valley and the tight gradient of height lines
across the region, which indicates very strong winds aloft.
Figure 2 Water Vapor at 425 PM on Aug
16th
The pre-storm environment for this supercell showed some
clearing developed across the Champlain
Valley as well as parts of central and
southern Vermont.
This helped to warm surface temperatures well into the upper 70s to lower 80s.
These temperatures combined with surface dewpoints (a measure of the amount of
low level moisture) in the mid to upper 50s helped to produce CAPE
values between 1500 and 2000 J/kg. CAPE stands
for Convective Available Potential Energy and is a good indicator for how
unstable the atmosphere. The larger the CAPE,
the more unstable the atmosphere is. In addition, to surface instability we had
very favorable turning of the winds in the atmosphere. Surface winds across the
Champlain Valley were southerly at 10 to 20 mph
with higher gusts, while 6000 to 7000 feet above the surface winds were 40 to
50 mph. This change of wind speed and direction in the atmosphere created
conditions favorable for rotating supercell thunderstorms. Below is a 4 PM
August 16th LAPS sounding from the Central Champlain
Valley. Note the
southerly winds near the surface and the 110 mph jet at 35,000 feet.
Cape=1600 J/kg
Lift Index= -4.0C
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Figure 3 Laps 400 PM Sounding August
16, 2007
The 4 PM surface analysis (pictured
below) showed a cold front approaching northern New York
and Vermont
with scattered thunderstorms developing along and ahead of the front. The
visible satellite picture below shows the over shooting thunderstorm cloud
tops, along with 5 minute lightning strikes and surface observations. You can
see the observation at Burlington,
Vermont was 84 degrees with a
southerly wind at 15 mph with gusts to 28 mph. Furthermore, note all the
clearing ahead of the developing storms across central and southern Vermont, which helped to
destabilize the atmosphere by aiding in daytime heating.
Over-shooting
thunderstorm cloud tops and forward sheared anvil
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Approaching
Surface Cold Front
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Figure
4 Satellite 400 PM with
lightning (green), surface obs (yellow), frontal position (blue)
Storm Analysis (Supercell #1 Plattsburgh to Saint Johnsbury)
On 2007 August 16th two distinct supercells
tracked across our county warning area. The first severe thunderstorm cell
developed across southern Canada/Saint Lawrence River Valley, and tracked
southeast into central Clinton County, then across southern Grand
Isle County
in the Champlain Valley,
then followed the Lamoille River Valley
from Georgia Center
to Cambridge, through the Green
Mountains to The Upper Connecticut River Valley near Saint
Johnsbury. Numerous reports of widespread trees down and power lines were
common along this track, along with several reports of large hail the size of
golf balls. Storm surveys conducted by The National Weather Service showed the
damage was a result of straight line thunderstorm winds of 70 to 90 mph from
the Plattsburgh/Cumberland Head area in New York
thru Grand Isle
County into the Georgia /Fairfax areas
of Vermont. The survey results showed most of the tree
and structural damage to be in the same directions with the center of the
maximum winds appearing to be about 4 miles wide across Grand Isle County.
The radar reflectivity picture below was taken at 3:47 PM on
August 16th, when the storm was very close to the Cumberland
Head/Plattsburgh area. The highlighted area shows a well defined hook-like
reflectivity structure, which is very common to tornadic producing supercells.
The light blue circle in the upper left image represents where the KCXX radar
was indicating a mesocyclone/rotation in the storm. In addition, note the
strong low level reflectivity gradient in the southwest part of the storm, also
suggesting a well organized storm, capable of producing a tornado.
Figure 5 The Lowest 4 Base
Reflectivity Elevation Scans of the KCXX Radar at 3:47 PM
As you can see from figure 6 pictures below, the radar
structure of the storm near Plattsburgh
exhibited classic supercell features, like the schematic diagram on the right.
These characteristics included a sharp hook-like reflectivity gradient on the
southwestern portion of the storm, which is the most likely place for a
tornado. The storm also had a core of stronger radar returns, which is
indicated in the brighter purples and pinks on the left hand image. Reflectivities
of this nature are associated with strong winds, large hail and very heavy
rain, which occurred with this storm.
Figure 6 Supercell Near Plattsburgh at 3:47 PM Compared to Supercell
Schematic From OCS
A reflectivity cross section (seen below in Figure 7) is a vertical look into the storms
reflectivity structure. This radar data is used to determine the height of the
strongest radar returns, along with the reflectivity gradient. Figure 7 shows a
very strong storm near Plattsburgh,
New York with a 50 dbZ return to
30,000 feet. The reflectivity tilt is caused by strong west to east winds
displacing the radar returns downstream. Furthermore, the strong radar returns
aloft are being held above the ground, by the storms strong updraft. The air
associated with the storms updraft is warm and moist air and positively
buoyant, which is being carried into the storm by southerly inflow winds. This
strong reflectivity core aloft is called a Bounded Weak Echo Region (BEWR). The
white returns are indicating dbZ of 70 or more, which indicates the presence of large hail. The dark red shows dbZ of 50 or
higher shown below.
Figure 7 Cross Section Reflectivity Near
Plattsburgh at
3:42, 3:47, and 3:52 PM on August 16th
The image below is the lowest elevation angle of velocity
data off the KCXX radar taken at 3:47 PM. The light green shows velocities of
40 to 50 knots, the dark blue indicates wind speeds of 50 to 60 knots coming
toward the radar, and the light blue color shows incoming winds of greater than
70 knots moving toward the radar, just above the ground. This image was taken
very close to the time when all the damaging thunderstorm winds were occurring
near Cumberland Head, New York.
The warm colors or red/orange colors indicate air flowing away from the radar,
which suggests southerly winds feeding into the storm.
Figure 8 Low Level Velocity at 3:47 PM
Figure 9 below is a velocity cross section of the Champlain
Valley Storm near Plattsburgh
at 3:47 PM. The green colors are winds coming toward the radar, while the red
colors are winds moving away from the radar or outbound winds. This cross
section shows some very interesting features associated with the potential
rotation in the low to middle part of the storm. Note the inbound (green color
showing winds coming toward the radar) and outbound (red color indicating winds
moving away from the radar) couplet circled in yellow. This suggests a cyclonic
storm rotation between 2000 and 4000 feet above the surface. In addition, look
how the strong updraft has tilted the low to mid level velocity signature from
horizontal to vertical, which is outlined in the white circle. The storm had
very good cyclonic or counter-clockwise rotation between 2000 and 4000 feet
above the surface, but a cool outflow boundary cutoff the circulation and
prevented the rotation from reaching the ground. The lightest color green in
the picture below indicates inbound winds of greater than 50 knots or 58 mph.
The darker red color is winds between 40 and 50 knots moving away from the
radar.
Strong updraft,
causing horizontal rotation to become vertical
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Low level storm
outflow under cutting the cyclonic storm rotation
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Figure 9 Vertical Velocity Cross
Section Near Plattsburgh
at 3:47 PM
Figure 10 below shows low level velocity data on the right
and low level reflectivity data from the KCXX radar at 4 PM. The green and blue
colors in the velocity data on the left display shows inbound winds of 40 to 70
knots. Note the very small outflow boundary, which cutoff the storms updraft
and prevent the cyclonic rotation from reaching the ground. The image at the
right shows the hook like reflectivity gradient, along with a core of very
strong radar returns in the red/pink colors associated with the storms core.
- Hook-like signature
- Weak reflectivity outflow boundary
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Figure 10 Velocity Data (left) and
Reflectivity Data (right) at 4 PM
Supercell #2 (Rutland County
from near Benson to just south of Killington)
A second supercell thunderstorm
developed on August 16th, which tracked across northern Rutland County
into west-central Windsor
County before weakening.
The damage path started near Benson in northwest Rutland
County and tracked east through
Chittenden and toward the Hubbardton/Pittsford areas of northern Rutland County. A team of meteorologists from
the National Weather Service in Burlington,
Vermont surveyed the damage and
determined the damage was caused by straight-line thunderstorm winds. The
survey team estimated the winds ranged from 70 to 90 mph based on damage indicators
and concluded the path was about 1/2mile wide and 2 to 3 miles long. They noted
several hundred trees uprooted or snapped...but all blown down in the same
direction, roughly northwest to southeast near Chittenden, Vermont.
In addition, the team discovered the most significant and widespread damage to
be near Chittenden Road,
which was where the winds were funneled between two hills and accelerated in
the valley. In the Hubbardton and Pittsford areas had only sporadic tree damage
occurred and the team estimated the winds to be between 45 and 65 mph.
The storms
environment was characterized by modest instability with CAPE
values around 1500 J/kg, but very highly sheared. Shear means, there was plenty
of turning and changing of wind speed and direction with height in the
atmospheric profile. This shear helped to organize the supercell and caused the
storm to exhibit strong rotation.
Figure 11
below shows the lowest level reflectivity display from the KCXX radar, along
with the tornado vortex signature in the white triangle. The radar displayed a
hook-like signature, very similar to the storms that impacted the northern Champlain Valley earlier in the day. The circled
region on the figure below would be the area most favorable for tornadic
development within this cell.
Figure 11 Reflectivity and Tornado
Vortex Signature (White Triangle) on August 16th at 5:47 PM
Figure 12 below shows the storm
relative motion data from the KCXX radar at 5:47 PM on August 16th, along with
the digital mesocyclone detection display. This radar algorithm, which is highlighted
in purple below will radar data to determine if the storm has circulation and
the potential to produce a tornado. In addition, it will provide warning
meteorologist a storm track of where the circulation is moving, which is shown
in the purple line below in figure 12. Once again, the lighter green colors in
the display show inbound winds or wind coming toward the radar of 40 to 50
knots, while the red colors are winds moving away from the radar. Note the
strong gate to gate couplet shown in the storm relative motion display below,
which is circled and indicating show low level rotation just north of West Castleton.
Figure 12 Storm Relative Motion and
Digital Mesocylone Detection (purple) on August 16th at 5:47 PM
The following figure 13 below shows
a 4 panel display of, composite radar reflectivity upper left panel, vertically
integrated liquid upper right panel, one hour storm total precipitation lower
left, and echo tops in the lower right display. The image was captured at 5:51
PM on August 16th, during the height of the damage produced by this
storm across Northern
Rutland County.
The strongest part of this storm, that developed into a line of thunderstorms
had vertical integrated liquid (VIL) values around 50 kg/m^2, along with 65 to
70 dbZ reflectivity returns, and storm echo tops of 50,000 to 55,000 feet tall.
These values are very characteristic of s supercell thunderstorm, which would
be found on the Central Plains of the United States. Furthermore, this
storm produced one hour storm total precipitation amounts of around 1 inch.
Composite
Reflectivity= 65 to 70 dbZ
VIL=50 to 55
Kg/m^2
Echo Top= 50 to
55 kft
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Figure 13 Radar 4 Panel of Composite
Reflectivity (upper left), Vertical Integrated Liquid (upper right), One Hour
Precipitation (lower left), and Echo Top (lower right) on August 16th
at 5:47 PM
The figure below shows two
reflectivity cross sections taking over northern Rutland County
on August 16th at 5:42 PM and 5:47 PM. These cross sections are used
to determine the height of the strongest radar return, along with a vertical
examination of storm structure. The image at the lower left shows a weak
bounded echo region (circled in yellow) between 4,000 and 8,000 feet above the
ground, in the lowest elevation angles. This signature suggests a strong
updraft holding the reflectivity core aloft. Furthermore, the first image shows
a height of the 50 dbZ to 26,000 feet, which suggests the potential for strong
winds and hail. The cross section on the right shows a very strong reflectivity
core aloft (circled in yellow), which is being held aloft by the strong
updraft. The reflectivity cross section on the right shows a 60 dbZ core to
almost 26,000 ft. This is a very strong thunderstorm capable of producing
widespread wind damage and large hail, which was the result, based on the storm
reports we received from spotters and local law enforcement. Warning
meteorologists look at vertical reflectivity cross sections to determine
strength and height of the maximum radar reflectivity return, and the potential
damage the storm can cause. Many times if we see a 50 dbZ return above the -20C
isotherm or a 60 dbZ return above the 0C isotherm, the storm will be capable of
producing severe hail penny size or large or damaging thunderstorm wind gusts.
Figure 14 Reflectivity Cross Sections
at 5:42 PM and 5:47 PM on August 16th
Storm Summary:
The 2007 severe weather season
continues to be very active across northern New York
as well as central and northern Vermont.
The significant outbreak on August 16th featured two distinct
supercells, which produced extensive damage. This on August 16th had
numerous reports of large trees down and many people were left without power
due to the strong winds knocking down trees and power lines. In addition, a tractor
trailer was blown off Interstate 87 between exits 40 and 41 during the height
of the storm, along with a measured wind gust to 91 mph at Cumberland Head near
the Plattsburgh
area. The following are several interesting storm structure pictures taken by
National Weather Service Meteorologists from the South Burlington airport,
along with a few damage survey pictures from Plattsburgh/Grand Isle area as
well as a picture from northern Rutland
County.
weather.gov 
