Event Summary
     National Weather Service, Raleigh NC

September 14, 2007 Severe Weather and Tornado Event
Updated 2008/07/22





Event Headlines -

...Several thunderstorms developed low level rotation and produced funnel clouds with seven thunderstorms producing tornadoes...
...The tornadoes were all of EF-0 strength with estimated wind speeds of 65 to 85 MPH...
...While the thunderstorms across central North Carolina may have been indirectly influenced by Humberto, the core of the remnants of Humberto remained south of central North Carolina...
...A cursory examination of the radar and lightning rates/polarity data and their relation to tornadogenesis was inconclusive...


Event Overview -

A progressive short wave trough moved across the Ohio Valley and Northeast late on Tuesday, September 11, 2007. The associated cold front moved through central North Carolina on Wednesday morning. An area of high pressure behind the front moved into the Northeast late on Wednesday and extended southward into the mid Atlantic on Thursday. The cold front stalled across South Carolina on Wednesday and Thursday as the upper level flow became westerly and parallel to the front.

Tropical storm Humberto developed off the Texas coast during the morning hours on Wednesday, September 12. A small tropical cyclone, Humberto rapidly intensified and became a Hurricane just after midnight on Thursday, September 13, 2007. The hurricane made landfall near High Island, Texas, at around 200 AM CDT on Thursday.

In response to an impressive short wave trough with an 80 knot jet at 500 mb, the zonal upper air pattern across the Mid Atlantic on Thursday morning became more amplified by Thursday evening. In addition, a short wave trough over far northwestern Texas early Thursday morning began to increasingly interact with Humberto during the afternoon.

As Humberto moved northeast across far eastern Texas and Louisiana it experienced increasing shear and weakened quickly. Humberto was finally downgraded to a Tropical Depression at 500 PM on Thursday while the system was centered over central Louisiana.

The amplifying pattern and increasing southwesterly flow pushed the stalled front that was across South Carolina slowly northward late Thursday and Friday. The increasing southwesterly flow also transported the deep moisture associated with Humberto across the Deep South into Mississippi and Alabama on Friday morning and then into the southeastern portion of the Carolinas by Friday evening.

The low level flow across the Carolinas increased and became more southerly on Friday morning which allowed showers and thunderstorms to break out across far western North Carolina. The lower levels of the atmosphere across central and northeastern North Carolina was considerably more stable at 12Z then across western North Carolina. During the afternoon, the low level stable air mass quickly eroded .

As the upper trough approached central North Carolina on Friday afternoon, the atmosphere evolved into an environment supportive of thunderstorms with isolated Tornadoes. A short wave trough at 500 mb across the Tennessee and lower Mississippi Valleys along with divergence aloft associated with the right entrance region of the 300 mb jet over Pennsylvania provided sufficient forcing for convection. In addition, a wave of low pressure developed on a warm front that was moving slowly northward across North Carolina. The low pressure center and warm front produced an area of low level convergence and enhanced helicity. Scattered convection developed across the Western Piedmont initially and then spread east into the Northeast Piedmont near the Triangle Area. Several thunderstorms developed low level rotation and produced funnel clouds with six thunderstorms producing tornadoes.


Severe Weather Reports -

Text of severe weather reports across central North Carolina

Text summary of tornadoes across central North Carolina





Numerous Funnel Cloud Reports

Mesocyclone and wall cloud near Four Oaks in Johnston County - photograph provided by Lani Johnson - Click to enlarge Over 25 funnel cloud reports were received during the event with 8 confirmed tornadoes (all of them were weak EF-0 tornadoes). The lower levels of the atmosphere contained a significant amount of shear as noted in the 0-1 km storm relative helicity and the 0-3 km storm relative helicity. In addition to the synoptic scale environment, the presence of a northward advancing thermal moisture boundary (front) likely provided additional shear to support rotating updrafts in thunderstorms.

The photograph to the right was provided by Lani Johnson who took the picture looking northwest on Hilltop Drive in Four Oaks, Johnston county near Highway 210. Without animation it cannot be confirmed but the photograph suggests that the thunderstorm was rotating with a mesocyclone and a wall cloud clearly visible. The photograph was taken shortly after 700 PM and it appears consistent with a storm report of a funnel cloud at 710 PM in Coats Crossroads.

Summary of tornadoes across central North Carolina


Satellite

A Java Loop of visible satellite imagery from 1145Z (745 AM EDT) through 2301Z (701 PM EDT) on Friday, September 14, 2007 is available here. This loop highlights the evolution of the event from the early morning hours to just before sunset. Note - this loop includes 45 frames.

The visible satellite image below is from 2215Z (615 PM EDT) which is within a few minutes of tornado touchdowns near Lillington in Harnett County and Clayton in Johnston County (see the text of severe weather reports across central North Carolina for more details). The base reflectivity image from the KRAX WSR-88D just a minute later at 2216Z (616 PM EDT) shows the location of the storms and the locations under Severe Thunderstorm and Tornado Warnings.


Visible satellite imagery



Regional Radar Loop

A Java Loop of regional reflectivity imagery from 1558Z (1158 AM EDT) through 0548Z (148 AM EDT) on Saturday, September 15, 2007 is available here. Note - this loop includes 68 frames.

The regional reflectivity image below is from 2308Z (708 PM EDT) on Friday, September 14, 2007. At around this there were numerous reports of severe weather in the Triangle area. A few minutes later, tornado touchdowns were reported near Lillington in Harnett County and Clayton in Johnston County This is also the time in which the mode of the severe weather evolved from discrete cells into a linear squall line structure.


Regional reflectivity image from 2308Z on Friday September 14



KRAX Radar Loops

Overview of the entire event with images from every 15 minutes
Java Loop of KRAX reflectivity imagery from 1603Z (1203 PM EDT) on Friday September 14 through 0600Z (200 AM EDT) on Saturday, September 15, 2007.
Note - this loop includes 57 frames

Overview of the entire event with images from every volume scan
Java Loop of KRAX base reflectivity from every volume scan between 1814Z (214 PM EDT) through 0458Z (158 AM EDT) on Saturday, September 15, 2007.
Note - this loop includes 144 frames

The KRAX reflectivity image below is from 2216Z (616 PM EDT) on Friday, September 14, 2007. At this time there were numerous reports of funnel clouds across Harnett and Johnston counties. In fact, multiple tornado touchdowns were reported between 610 and 620 PM EDT across Harnett and Johnston counties.

KRAX radar imagery from 2216Z on Friday, September 14, 2007



Warning Strategies

Forecasters used surface and meso-analysis throughout the warning process. The SPC meso-analysis page was invaluable and it was used frequently to find the locations that had the greatest tornado threat with low LCL heights, high 0-1 km SRH, sufficient deep layer shear (30-35 kts) and moderate instability (1000-1500 J/kg). Later in the event a storm with marginal shear and but a respectable reflectivity pattern was correctly not warned for, as it was in an area where the LCL heights were too high (over 1500 m) and instability was lowering quickly.

SPC mesocale discussion graphic After the initial thunderstorms and damage were reported in the Triad area near Clemmons, it was quickly assessed to be that of a weak tornado. This allowed the staff to quickly begin employing radar strategies suggestive of those best practices for remnant tropical systems tracking along a thermal moisture boundary.

Forecasters used sectorized warning strategies during the event. The CWA was divided into three sectors from west to east along the front.

It was critical for radar operators to know the location of the front. Any convection that approached or tracked along the boundary had to be interrogated continuously for tornadic activity. Since the front was located west to east across the CWA at the onset of the event, all 3 radar operators had to deal with this possibility. Forecasters were able to take observations and recognize the evolving environment, such as the rising dew points and possibly lowering LCL’s across central North Carolina.

Once the storms morphed into lines and bow segments, forecasters rapidly transitioned to issuing Severe Thunderstorm Warnings in stead of Tornado Warnings. In addition, the growing threat of flash flooding was recognized when storms moved over urban areas while producing rainfall rates in excess of 2 inches per hour. Flash Flood Warnings were issued when cells began training from west to east along the boundary.

Early during the event, forecasters recognized that the KRAX rainfall estimates were underdone by about 25 percent. This was achieved by quick inspection of Doppler radar estimates versus ground truth reports by cooperative observers and a CoCoRaHs real time report from 10.1 N of Raleigh. The flash flooding occurred when central North Carolina was in an extreme drought and had not had significant widespread rainfall in over a month. Flash flood guidance values ranged from 3 inches per hour to 5 inches in 3 hours to as low as low as 3 inches per hour and 4.4 inches in 3 hours for urban Durham County.





Humberto's Remnants

National radar loop showing the motion of the remnants of Humberto A portion of the remnants of Humberto moved across Georgia and South Carolina during the afternoon and evening of Friday September 14, 2007. While the showers and thunderstorms across central North Carolina may have been indirectly influenced by Humberto, the remnants of Humberto remained south of central North Carolina.

Tropical storm Humberto developed off the Texas coast during the morning hours on Wednesday, September 12. Humberto rapidly intensified and became a hurricane just after midnight on Thursday. The hurricane made landfall near High Island, Texas, at around 200 AM CDT on Thursday.

A short wave trough over far northwestern Texas early Thursday morning began to interact with Humberto during the afternoon. Increasing shear weakened Humberto as the storm moved across far eastern Texas and Louisiana. Humberto was downgraded to a tropical depression late Thursday afternoon. The weakening remnants of Humberto then moved across northeast Louisiana, Mississippi, and Alabama.

Tracking the remnants of Humberto across Tennessee, Georgia, South Carolina, and North Carolina on Friday and Saturday the 14th and 15th of September was difficult. In many cases, weakening tropical cyclones that have made landfall, have their low level and mid/upper level remnant circulations separate. There are several ways forecasters can track the remnants of Humberto and see if the remnants moved across central North Carolina and possibly played a role in the severe convection including radar and satellite imagery, surface pressure analysis, and potential vorticity.

The radar loop above shows a relatively persistent area of convection that appears to be associated with the remnants of Humberto. These remnants appear to track across Mississippi, northern Alabama, northeast Georgia, and northern South Carolina.

Satellite imagery provided another method to track Humberto and its remnants as they move inland. A visible satellite loop from 1245 UTC on 9/13 through 2145 UTC on 9/14 shows a portion of the circulation splitting away from the main circulation center and moving into the Gulf of Mexico. The circulation center was located over north-central Louisiana around midday on Thursday September 13th. By midday Friday, the circulation center had advanced northeastward into central Mississippi and then turned sharply southward and moved into the Gulf of Mexico toward sunset.

The water vapor loop is not as clear, but if you look closely you can see the mid/upper level circulation center advance northeast into northern Georgia and the western Carolinas. The advancing short wave initiates convection across eastern Tennessee and western North Carolina on the afternoon of the 14th making it difficult to track the mid/upper level circulation center.

Surface pressure and wind analysis early Friday morning shows the low level circulation over Mississippi at 09 UTC. At midday on Friday, the surface pressure center weakens and drifts south but a circulation center can be seen in the wind field across northern Alabama. By late afternoon the wind circulation had moved into western North Carolina and merged with a developing surface trough east of the Appalachians.

A low level Potential Vorticity (PV) anomaly was located over northern Louisiana at midnight Friday morning. The PV anomaly moved into Mississippi just after daybreak on Friday and then elongated across Mississippi and northern Alabama. The PV anomaly then moved into northern Georgia and western South Carolina toward sunset. The PV anomaly over northern North Carolina and Virginia at the same time was associated with an area of showers and thunderstorms and not the remnants of Humberto.



Cloud to Ground Lightning Rates and Tornadogenesis

Over the past few decades there have been numerous studies of cloud to ground lightning rates and polarity with tornadogenesis. Many of the studies suggest that examining cloud to ground lightning rates or polarity changes to exclusively anticipate tornado formation is not practical. The amplitude of the flash rate changes are too inconsistent to be used as a forecasting tool but there still appears to be some sort of relationship with tornado formation. It is possible that cloud to ground lightning rates in conjunction with other operational tools could be useful in analyzing severe thunderstorm evolution and the potential for tornado development.

A study of the polarity changes with a severe thunderstorm that produced an F2 tornado with a twelve mile track across Mississippi provides a summary of recent studies relating cloud to ground lightning rates and polarity with tornadogenesis. An excerpt of the summary follows. Kane (1991) analyzed lightning data from several tornadic storms across the northeastern United States. He noted that the tornadoes and large hail followed the peak 5-minute lightning rates by 10 to 15 minutes and were accompanied by a rapid decline in CG strikes. MacGorman and Burgess (1994) studied the lightning characteristics of 15 severe storms. In MacGorman and Burgess’ dataset, the majority of storms dominated by positive CG flashes produced tornadoes. They noted that the storm’s most damaging tornado began after positive CG flash rates decreased from their peak value, before or near the time when negative flashes became dominant. During some storms, the CG flash rates decreased to near zero after positive CG flash rates decreased from their peak value, and before a polarity shift to predominantly negative CG flashes in the storm. Seimon (1993) studied lightning data associated with the F5 tornado in Plainfield, Illinois in August of 1990. He noted an anomalous predominance of positive-polarity CG flashes during the development of the severe thunderstorm that produced the tornado, a 20 minute span of reduced CG activity coinciding with tornado formation and intensification, and a reversal in dominant CG flash polarity from positive to negative at the time of tornado touchdown. Finally, Knapp (1994) conducted a thorough study of 264 tornadic thunderstorms east of the Continental Divide. For Positive Strike Dominated (PSD) tornadic storms, he noted that storms often went through a rapid polarity shift from PSD to Negative Strike Dominated (NSD), about 10 minutes prior to tornado occurrence. He also noted a rapid increase in flash rates, beginning 20 minutes prior to tornado touchdown.

A cursory examination of the lightning and radar data from this event was inconclusive. While there were numerous rotating thunderstorms with corresponding funnel clouds, only a few of these storms had tornado touch downs. A more rigorous review of this and other events will need to be completed in the future before any notable conclusions can be made.



Mesoscale Data

Analyzed mean sea level pressure (black) and surface wind barbs from SPC at 21Z on Friday, September 14, 2007 (500 PM EDT)
An area of low pressure is analyzed across western North Carolina with a warm front extending east across North Carolina. A trailing cold front extends southwest across the western Carolinas and Georgia.

SPC Analysis at 21Z on Friday, September 14, 2007



Analyzed surface temperatures (red), dew point temperatures (green), and wind barbs from SPC at 21Z on Friday, September 14, 2007 21Z on Friday, September 14, 2007
Note the gradient in dew point temperatures across North Carolina associated with the warm front. The shear along the front likely assisted in the development of the rotating thunderstorms and tornadoes.

SPC Analysis at 21Z on Friday, September 14, 2007



Analyzed low level lapse rates in the 0-3 km layer (blue, green, orange, and red) from SPC at 21Z on Friday, September 14, 2007 (500 PM EDT)
A lapse rate is the rate of temperature change with height and the image below is for the layer from the surface to around 9,000 feet. Note the surface based, low level lapse rates shown below exceed 7 deg C/km across southern portions of central North Carolina indicative of moderate low level instability.

SPC Analysis at 21Z on Friday, September 14, 2007>
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<B>Analyzed mixed layer CAPE (red) and surface based CIN (shaded in blue) from SPC at 21Z on Friday, September 14, 2007 (500 PM EDT)</B> 
<br>CAPE values ranged between 500 and 1500 J/kg across central North Carolina 
with the most stable atmosphere across northern portions of the region and 
most unstable airmass near the south Carolina border. 
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Analyzed Lifting Condensation Level (red, blue, and green) from SPC at 21Z on Friday, September 14, 2007 (500 PM EDT)
The LCL (Lifting Condensation Level) is the level at which a parcel becomes saturated. It is a reasonable estimate of cloud base height when parcels experience forced ascent. Note that much of central NC has LCL values less than 1000 meters. Research has shown that LCL heights generally are low (< 1000 meters) for most significant tornado cases.

SPC Analysis at 21Z on Friday, September 14, 2007



NWS Composite Reflectivity Imagery from 2132Z on Friday, September 14, 2007 (532 PM EDT)
The composite reflectivity imagery is from the approximate time in which the analysis imagery above is valid.

Composite Reflectivity Imagery from 2132Z on Friday, September 14, 2007




Archived Text Data from the Severe Weather Event

Select the desired product along with the date and click "Get Archive Data."
Date and time should be selected based on issuance time in GMT (Greenwich Mean Time which equals EDT time + 4 hours).


Product ID information for the most frequently used products...

RDUAFDRAH - Area Forecast Discussion
RDUZFPRAH - Zone Forecast Products
RDUAFMRAH - Area Forecast Matrices
RDUPFMRAH - Point Forecast Matrices
RDUHWORAH - Hazardous Weather Outlook
RDUNOWRAH - Short Term Forecast
RDUSPSRAH - Special Weather Statement
RDULSRRAH - Local Storm Reports (reports of severe weather)
RDUSVRRAH - Severe Thunderstorm Warning
RDUSVSRAH - Severe Weather Statement
RDUTORRAH - Tornado Warning


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References

Kane, Richard J., 1991: Correlating lightning to severe local storms in the northeastern United States. Wea. Forecasting, 6, 3-12.

Knapp, D., 1994: Using cloud-to-ground lightning data to identify tornadic thunderstorm signatures and nowcast severe weather. Nat. Wea. Dig., Vol. 19, No. 2, 35-42.

MacGorman, D.R., and D.W. Burgess, 1994: Positive cloud-to-ground lightning in tornadic storms and hailstorms. Mon. Wea. Rev., 122, 1671-1697.

Seimon, A., 1993: Anomalous cloud-to-ground lightning in an F5-tornado-producing supercell thunderstorm on 28 August 1990. Bull. Amer. Meteor. Soc., 74, 189-203.



Acknowledgements

Many of the images and graphics used in this review were provided by parties outside of WFO RAH. The surface analysis graphic was obtained from the Hydrometeorological Prediction Center. The upper air analysis images were obtained from the College of DuPage and the University of Wyoming. GOES satellite data was obtained from National Environmental Satellite, Data, and Information Service. SPC meso-analysis graphics provided by the Storm Prediction Center. The photograph of the thunderstorm with a wall cloud near Lillington was provided by Lani Johnson.


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