The Goldsboro Severe Thunderstorm
A Closer Review of the Storm -
The severe thunderstorm that moved across Wayne County and the city of Goldsboro
was at least in part, a result of convection that initially developed over
southeastern Virginia and far northeastern North Carolina earlier in the afternoon. Forecasters knew
that there was a high likelihood of
thunderstorms during the afternoon and evening.
The Storm Prediction Center recognized the threat early in the day with
eastern North Carolina and eastern Virginia located in the "Slight Risk" area on the Day 1 Severe Weather Outlook.
As expected, conditions were favorable for convection with much of eastern North Carolina in
an extremely unstable air mass with mixed layer CAPE (MLCAPE) values exceeding 3000 J/kg.
A surface trough oriented from northeast to southwest was located just
east of the Piedmont fall line. This boundary provided a focus for surface moisture
convergence along with a western boundary for storms to intensify along. In addition, a series of impulses in the mid and upper levels of the
atmosphere over Virginia provided a trigger for the storms to develop.
Because of the growing threat, the Storm Prediction Center initially issued a
Mesoscale Discussion at 115 PM and then
issued a Severe Thunderstorm Watch for much of eastern
North Carolina and southeast Virginia from 145 PM EDT to 900 PM EDT.
The intense thunderstorms that developed across northeastern North Carolina
created several outflow boundaries that raced southwestward between 200 and 400 PM. These
outflow boundaries were enhanced by severe thunderstorms that developed over Franklin County before 400 PM
and Edgecombe County between 400 and 500 PM. These
large outflow boundaries moved southwest and they often intensified other storms, especially the thunderstorms that were in
a very unstable air mass whose thunderstorms intersected the outflow boundaries at favorable angles.
The animation to the right (click here for a larger view in a new window),
clearly shows the numerous boundaries that moved across the area.
At 500 PM the Storm Prediction Center issued a
Mesoscale Discussion that highlighted
the growing threat for the central coastal plain, in particular Wayne, Duplin, Lenoir, and Greene Counties.
These locations were just east of the surface trough and a weak low center. In addition, the air mass in
this region was very unstable and significant pressure falls which typically result in the backing of
the surface winds and enhanced convergence. In this environment, the intersection of the outflow
boundaries with one another or with the surface trough/low center
would lead to enhanced convection and the potential for damaging wind gusts.
In the animation shown to the right
(click here for a larger view in a new window),
the 0.5 deg Base Reflectivity product is shown in the upper left, the Grid
based VIL product is shown in the upper right, the Composite Reflectivity
is shown in the lower left, and the Echo Top product is shown in the
lower right. The loop runs from 2139Z through 2229Z.
An outflow boundary from an intense thunderstorm across northern Wayne County that was moving
southwestward, begins to interact with a cell developing in southeastern Johnston
County at 2144Z. Forecasters anticipating the interaction of this outflow boundary and
the developing convection issued a Severe Thunderstorm Warning at 2145Z (545 PM) for
eastern Johnston and Southern Wayne Counties. At 2200Z, the southwestward moving boundary was
now enhancing the developing storm as seen in the developing
elevated reflectivity values.
The thunderstorm updraft over western Wayne County rapidly intensifies between
2204Z and
2213Z.
The storm's explosive intensification appeared to peak between the 2213Z and 2217Z volume scans. The
12 degree
Base Reflectivity product at 2213Z showed an elevated reflectivity core of 67 dBZ up to 37,000 feet with the corresponding
cross section showing a large area of
greater than 60 dBZ aloft. By 2217Z, the 15 degree
Base Reflectivity product showed an elevated core of 61 dBZ exceeding 47,000 feet with the corresponding
cross section showing the area of
60 dBZ extending just a little higher but also reaching a little lower to around 15,000 feet.
Downbursts, defined as an area of strong winds produced by a thunderstorm downdraft, often
times result from the collapse of a large precipitation core from aloft, as described above. There
exists a spectrum of different types of downbursts.
Downbursts are classified by the environment in which the parent thunderstorm develops, and
the resultant physical processes which provide the forcing for the downdraft acceleration. On
one end of the spectrum lie dry microbursts - Those that are characterized by high-based, weak
updrafts and associated precipitation cores. Dry microbursts derive their energy from evaporation
and sublimation of raindrops and snowflakes beneath the thunderstorm, respectively, which latently
cool and accelerate the air within the downdraft earthward. On the other end of the spectrum,
however, are wet microbursts, which
derive their energy from the downward force created by the weight of excessive water content, known as precipitation loading. In addition, the entrainment of dry mid level air and the melting of hail as it falls below the freezing level both latently cool and accelerate the downdraft air toward the earth. The environment in which the thunderstorms that developed over central North Carolina on the 10th could be classified as a hybrid microburst one, skewed toward the wet end given the size and composition of the precipitation cores that developed within the strong updrafts. The massive precipitation core that developed with the Wayne County storm undoubtedly imparted an incredible downward force as it descended. The collapse of this elevated reflectivity core produced one such downburst. The downburst is visible when comparing the radar cross section at
2217Z and
2221Z. The
0.5 degree Base Reflectivity and Storm Relative Velocity imagery at
2221Z shows the downburst with an arc of enhanced reflectivity, along with an area of enhanced outbound velocities (shown in red) up to 45 kts, which can be seen extending across Goldsboro.
At 2214Z (614 PM) an 85 MPH wind gust was reported at the Finch's Station RAWS observing site,
just a few miles northwest of Goldsboro. The core of the thunderstorm collapsed as it
moved directly over the city of Goldsboro with numerous reports of
damage between 620 PM and 635 PM.
A damage survey conducted the following morning found no indications of a tornado touchdown. The broad damage swath and other
indicators found by the survey team allowed them to come to this conclusion with some confidence.
It should be noted that as the storm moved across Goldsboro there was some rotation noted
in the Storm Relative Velocity (SRM) imagery. The 2225Z 4 Panel SRM image
shows the rotation signature near the intersection of U.S. 13 and U.S. 70
The rotation appears to be strongest in the
3.1 and 6.4 elevation angles. A 4 panel
loop of the Storm Relative Velocity imagery from 2200Z through 2242Z
shows that the circulation has some limited temporal continuity at 6.4 degrees as it first appears at 2217Z and is
most impressive at 2225Z. The 4 panel image at 2225Z also shows that the circulation appears to
extend down to the 1.3 degree elevation angle but not the 0.5 degree angle. It is likely not
coincidence that as the outflow from this storm rapidly moved away, the circulation weakens.
The LCL height has been used to determine the threat of tornadoes and significant tornadoes. The
LCL height is the height at which a parcel becomes saturated when lifted dry adiabatically.
The importance of LCL height is thought to relate
to sub-cloud evaporation and the potential for outflow dominance. Low LCL heights imply less evaporational
cooling from precipitation and less potential for a strong outflow that would likely inhibit
low-level mesocyclone development. Thunderstorms that produce tornadoes generally have a lower LCL height
with LCL heights less than 1,000 meters typically favorable for tornado development. The
SPC mesoanalysis page at 21Z showed LCL heights that were around 1,500 meters which would suggest a reduced tornado
threat across the central Coastal Plain.
Cross Section of the Goldsboro Storm -
Radar reflectivity cross section of a severe thunderstorm as it moved across
Wayne County and the City Of Goldsboro on August 10, 2007. The reflectivity
cross section is from the western (left) end of the white line in the inserted
image in the upper left to the eastern (right) end.
A Java Loop of radar reflectivity cross section from 2156Z (556 PM EDT) through 2229Z (629 PM EDT) on Friday, August 10, 2007
is available here.
Note - this loop includes 15 frames.
Damage Survey Report -
Starting at 614 PM on August 10th, severe winds from a large-scale (diameter of 4 km or greater,
with damaging winds persisting for 2 to 5 minutes) downburst, otherwise known as a macroburst, created widespread damage in Goldsboro and the surrounding area.
The macroburst began near the Finch’s Station site on Claridge Nursery Road just northwest
of downtown Goldsboro where an 85 MPH wind gust was recorded at 614 PM by a RAWS
mesonet station. A severe thunderstorm warning was issued for Wayne County at
545 pm highlighting damaging winds in Goldsboro. The warning was effective
until 630 PM EDT. Emergency calls started coming into the 911 center around
620 PM EDT reporting damage throughout the city. The National Weather Service
contacted the 911 center at least twice alerting them to the damaging storm
developing west of Goldsboro. The severe thunderstorm warning was reissued at
626 PM EDT extending the warning until 715 pm for Wayne County.
Straight line winds raced east through the north side of Goldsboro with peak
wind speeds estimated between 85 and 90 MPH based on the damage survey. The
overall macroburst was very large reaching a width of about two and a half
miles and extending downwind from the point of origin for about 6 miles.
Damaging winds extended from Claridge Nursery road just northwest of downtown
Goldsboro through the city ending at Highway 13. Tree damage was extensive all
along U.S. 70, Business 70, Royal Avenue and Ashe Street. About a dozen homes
in the area experienced damage from falling trees. Numerous roads were blocked
by trees and power lines especially from Wayne Memorial Drive east to Highway 13.
The worst damage occurred to the Days Inn on Highway 70. A straight line wind
of about 85 to 90 MPH caused the roof of the hotel to vault or become airborne
and flip over. The wind was trapped under the eave of the hotel and exerted an upward
force on the roof resulting in uplift failure. The hotel roof was
constructed of wood joists and lacked any wind straps to anchor the roof
based on initial inspection. The hotel was built to code at the time of
construction according to the local building inspector. However, new
construction requires additional roof anchoring.
This partially explains the lack of significant damage to surrounding newer
buildings. The failure of the roof structure was due largely in part to the
combination of uplift forcing and toe nail type construction in the trusses
and joists. The roof became airborne landing in the parking lot and damaging
about a dozen vehicles. One person in the parking lot narrowly escaped serious
injury running from their car into the hotel mere seconds before the roof
crushed the vehicle. At least a dozen vehicles sustained damage from flying
debris in the Days Inn parking lot.
Wayne County emergency management reported a total of four minor injuries
and no fatalities.
Special thanks goes to Wayne County Emergency Management, North Carolina
Emergency Management and the Wayne County building inspector for assisting
with the disaster survey.
Map of Damage and Wind Gusts across the Goldsboro Area -
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