April 16, 2011 North Carolina Tornado Outbreak|
Event Headlines -
...Thirty confirmed tornadoes occurred in North Carolina on 16 April 2011, the greatest one-day total for North Carolina on record...
...A total of 24 individuals lost their lives in North Carolina with thirteen tornadoes classified as strong, some hitting highly-populated areas...
...Nine tornadoes occurred in the National Weather Service (NWS) Raleigh (RAH) County Warning Area (CWA). Among the nine, there were two
EF-3 tornadoes, four EF-2 tornadoes and three EF-1 tornadoes...
...There were 8 fatalities in the RAH CWA, the most in April on record in central North Carolina (official tornado database begins in 1950)
and second only to the 28 March 1984 outbreak...
...There were a total of 304 injuries reported in central North Carolina but the actual number is likely much higher. Two tornadoes, the Sanford-Raleigh Tornado and
the Fayetteville-Smithfield Tornado had more than 100 injuries each...
...Total structural damage in central North Carolina was estimated at $328,610,000. The Sanford-Raleigh Tornado produced $172,075,000 in damage alone and
the Fayetteville-Smithfield Tornado produced $116,100,000 in damage...
Case Study Highlights -
- Several detailed interactive maps that contain a total of 466 separate pieces of
information are available including tornado tracks, tornado summaries, damage locations, fatality information,
photos, and videos across central North Carolina.
- A discussion of several aspects of the outbreak that made it such a historic and unique event
including the rarity of the number, strength, and longevity of the tornadoes as well as the
multiple long track tornadoes, the issuance of a "High Risk" severe weather outlook, the first
use of the “Tornado Emergency” wording by the NWS Raleigh in a Tornado Warning, and the track of the Sanford-Raleigh tornado
that came within 1.75 miles of the NWS Raleigh office.
- A summary of how meteorologist at the NWS Raleigh forecasted the event.
- Several interesting science topics including the atypical evolution of a squall line that
breaks up into multiple discrete supercells and the use of high resolution convection allowing models.
- Multiple radar images and loops as well as discussions of NSSL's Rotational Track Product and
MESH Hail Swath Product.
- An audio recording of the Central Carolina SKYWARN amateur radio transmissions on 16 April.
Event Overview -
A well forecast, recording breaking tornado outbreak occurred across central North Carolina (NC) during
the afternoon of 16 April, 2011. Thirty confirmed tornadoes occurred in North Carolina on 16 April 2011,
the greatest one-day total for North Carolina on record. A total of 24 individuals lost their lives
in North Carolina with more than 400 injuries.
The North Carolina NC Department of Crime Control & Public Safety reported that over 900 homes and
businesses were destroyed and more than 6,400 were damaged across the state. Across central North Carolina
in the National Weather Service (NWS) Raleigh (RAH) County Warning Area (CWA), total structural damage
was estimated at nearly a third of a billion dollars, $328,610,000. The Sanford-Raleigh Tornado produced
$172,075,000 in damage alone and the Fayetteville-Smithfield Tornado produced $116,100,000 in damage.
Nine tornadoes occurred in the Raleigh CWA. Among the nine, there were two EF-3 tornadoes, four EF-2
tornadoes and three EF-1 tornadoes. The nine tornadoes in the RAH CWA were produced by four supercell
thunderstorms, with each supercell producing at least two tornadoes. The Sanford-Raleigh tornado and the
Fayetteville-Smithfield tornado were long tracked and were on the ground for more than 55 miles each.
The 9 tornadoes in central North Carolina were on the ground for a total of 196.4 miles.
The tornadoes in central North Carolina touched down between 200 and 500 PM local time
which is a little earlier than the climatological peak hours of tornado touchdown of 500 to 700 PM as noted in
a Severe Weather Climatology for the Raleigh, NC County Warning Area (Locklear 2008).
There were 8 fatalities in the RAH CWA, the most in April on record in central North Carolina
(official tornado database begins in 1950) and second only to the 28 March 1984 outbreak. Seven
fatalities occurred in mobile homes and the other in a vehicle. Tornado fatalities occurred in the communities of Sanford,
Lemon Springs, Raleigh, Linden and Dunn. All of the tornado fatalities occurred within the boundaries
of tornado watches and were preceded by tornado warnings. The mean lead time for tornado warnings
covering fatalities was 28 minutes and the mean lead time for all tornado warnings issued was 26 minutes.
There were a total of 304 injuries reported in central North Carolina but the actual number is
likely much higher. Two tornadoes, the Sanford-Raleigh Tornado and the Fayetteville-Smithfield
Tornado had more than 100 injuries each.
One tornado came within 1.75 miles (2.8 km) of the NWS Weather Forecast Office Raleigh and the staff
executed a phased evacuation from the operations area which is on the third floor of a three story building.
WFO Blacksburg, VA backed up the Raleigh office for around 6 or 7 minutes. As the storm drew closer, the power went
out but there was no damage to the building or in the immediate area. The staff noted a strong odor of pine
in the air after returning to the operations area.
The threat of severe weather on Saturday, 16 April 2011, was first mentioned during the morning of the Tuesday, 12 April
Discussion (AFD) and the Hazardous
Weather Outlook (HWO). On the morning of 13 April,
forecasters became even more concerned as noted in the AFD with confidence
increasing on 14 April. By the afternoon of 15 April,
forecasters expressed a growing confidence of a significant tornado outbreak in the AFD. On Saturday morning,
forecasters at WFO Raleigh and the Storm Prediction Center (SPC) issued statements and alerts that emphasized the unique and serious dangers presented
by this particular weather pattern. WFO RAH issued 31 Tornado Warnings during the event with a probability of detection
of 97%, false alarm rate of 48%, and an event average lead time of 19.5 minutes.
Synoptic and Mesoscale Summary-
The upper air pattern on the morning of 16 April 2011 featured an impressive eastward
advancing upper level trough extending from the western Great Lakes into the lower
Mississippi Valley. The 500 hPa trough at 00 UTC
had moved eastward and became negatively tilted at 12 UTC with
significant height falls of more than 120m noted across the Ohio and Tennessee
Valleys. A 90 kt mid-level jet was rounding the
base of the 500 hPa trough at 12 UTC across northeast Alabama, northwest
Georgia, and far southeast Tennessee with a diffluent
pattern noted across Georgia and the Carolinas. At 300 hPa, a strong jet core
of 110 kts was located across northern Mississippi, northern Alabama,
and southeastern Tennessee.
At 850 hPa, a closed low was analyzed over the western Great Lakes with a trough axis
extending south along and near the Mississippi River. Another
trough axis was located near and just west of the Southern Appalachians in eastern
Kentucky, eastern Tennessee, and northwestern Georgia. Ahead of this trough, a
region of enhanced south-southwesterly to southerly winds of 50 kts or more was
analyzed across the western Carolinas, West Virginia, and western Virginia.
A thermal ridge at 850 hPa extended into the Carolinas with dew points ranging
into the 9 to 11 degree C range. A strong southeasterly flow at 925 hPa was noted
with winds of 35 to 45 kts across central North Carolina.
At the surface, a cold front
was analyzed near or just west of the Appalachians, extending
from eastern Kentucky south across far western North Carolina into Georgia and the
Florida panhandle. A warm front
noting the leading edge of a warmer, more moist and unstable surface
air mass extended west to east across southern North Carolina to the northeastern North
Carolina Coast. North of the warm front, surface dew points were generally in the mid
to upper 50s with temperatures in the upper 50s to lower 60s.
South of the front, dew points climbed into the lower to mid 60s. The setup of
synoptic scale features with this event was somewhat similar to
the composite map of major synoptic scale
features typically associated with severe weather outbreaks provided by Barnes and Newton (1983).
composite radar at 1158 UTC showed some scattered light to moderate rain showers
across the Piedmont and Foothills of North Carolina with some convection developing
south of the warm front across South Carolina and Georgia.
The air mass across central North Carolina at 12 UTC was stable with
little or no surface based or mixed layer
CAPE. The 60
degree isodrosotherm marking the leading edge of 60+ dew points was
located near the North Carolina-South Carolina border. The strong
low level flow resulted in strong, deep layer shear with bulk shear
values around 50 to 60 kts. SPC analyzed 0-3km storm relative
helicity values were extreme, ranging between 500-800 m2/s2.
Showers and thunderstorms developed along and just ahead of the cold
front in western North Carolina between 1300 UTC and
1500 UTC. By 1500 UTC, the
had advanced into central and northern North Carolina. The 60 degree isodrosotherm
extended into the northern Piedmont, very close to the North Carolina-Virginia border with
dew points approaching the mid 60s in far southern North Carolina.
The convection across western North Carolina intensified and
grew into a squall line between 1500 UTC
and 1700 UTC. The
squall line moved quickly east, advancing off of, and out ahead of the slow moving cold front.
There was very little convection ahead of the squall line and the limited amount of convection
that developed ahead of the squall line was weak and dissipated
fairly quickly. One thought is that the strong vertical wind shear
had a detrimental effect on the convection that tried to develop ahead
of the squall line. The developing updrafts essentially tilted
and ripped the immature convection apart. Further west, the large scale forcing at
the mid and upper levels and the surface cold front were sufficient
to initiate and sustain the convection in the highly sheared environment.
By 1800 UTC, the cold
front was located across the Foothills of North Carolina. The warm front
reached the Virginia border and dew points were well into the lower to middle 60s across
central North Carolina. During the previous 6 hours, the
surface based CAPE had
increased dramatically during the late morning and afternoon hours. It is worth
noting that the increase in the surface instability was dramatic and
only preceded the squall line by just a few hours. The surface flow
increased and backed more, resulting in incredible hodographs.
The convection had intensified into a well-developed line of
thunderstorms, and was able to race ahead
of the cold front and survive in the strongly sheared environment as it reached
the western Piedmont. The outflow behind the squall line was not especially cold, and
there was also very little trailing stratiform precipitation behind the squall line, both suggesting that the
environment was not supportive of long lived linear convection.
During the next few hours, the well-developed pre-frontal squall line fractured into
multiple, discrete, long lived tornadic supercells. The
large vertical wind shear,
strongly curved clockwise hodographs, and
minimal low-level line-normal shear appeared to strongly
favor the development and maintenance of discrete supercells.