April 12, 2008 Severe Weather Event

Event Summary
National Weather Service, Raleigh NC

April 12, 2008 Severe Weather and Tornado Event
Updated 2009/06/01

Collage of images related to this severe weather event.

Event Headlines -
...A weak EF-0 tornado damaged several homes near the community of Raynor Town in Wayne County...
...A new tool that maps the track of rotation in thunderstorms allowed meteorologists to easily identify the path the tornado traveled across Wayne County...
...The new rotational tracks tool allows meteorologists to quickly locate target areas for storm verification...

Event Overview -
The upper-level flow pattern on April 12, 2008 was characterized by a positively tilted upper-level trough over the eastern half of the U.S., with a closed low over the Great Lakes region at 700mb and 850mb. A strong 990mb surface low had developed the day before over the Central Plains, where heavy rains contributed to flooding conditions along the Mississippi River and adjacent river basins. The low pressure occluded and tracked slowly to northeast through the Great Lakes region overnight, and was located over western New York by 12 UTC on the 12th. A cold front, mainly in the form of a dew point gradient, trailed the low center, and extended down the western side of the Appalachian Mountains.

A line of showers and thunderstorms extended from Central Georgia to Southern Alabama, ahead of the main front early on the 12th. As temperatures and instability increased with daytime heating, the most intense thunderstorm activity subsequently developed along the leading line. Through the morning and early afternoon, most of the thunderstorm activity over North Carolina was relative benign, as conditions were not overly favorable for intense convection. Instability was not very high, but by 17 UTC, surface based CAPE ahead of the front had creped up to 500 J/kg. With the front well ahead of the upper-level cold pool, mid-level lapse rates were meager in the 5 to 6 C/km range.

Conditions continued to become slightly more favorable for convection in the Coastal Plain of North Carolina as the line moved east during the next couple of hours. Temperatures rose into the upper 70's to lower 80's, and a narrow prefrontal moisture surge raised dew points into the low to mid 60's. Differential heating due to reduced cloudiness over the Coastal Plain helped to create a thermal moisture boundary ahead of the approaching storms. This boundary likely increased low-level confluence and increase the low-level Enhanced Helicity Index. In addition to increasing instability, an area of enhanced upper-level divergence was present over the area with Central NC in the right entrance region of an passing upper-level jet. The upper-level divergence intensified over southeastern NC from 16 UTC to 19 UTC, although this may appear in part due to the increased storm top divergence of the convective line.

At around 17 UTC, a strong thunderstorm formed just north of Cheraw, South Carolina. The storm moved northeast and strengthen over the next hour, and a severe thunderstorm warning was initially issued by the NWS in Raleigh at 1830 UTC (2 PM EDT) as the storm moved just north of Fayetteville, N.C. The storm was located at the southern end of a developing line of storms that extended northeast toward the NC/VA border.

At 1908 UTC (308 PM EDT), the NWS in Raleigh issued a tornado warning for Northern Wayne, East Central Johnston, and Southern Wilson Counties. Radar imagery from 1904 UTC (304 PM EDT) showed the storm entering Western Wayne County with a noticeable hook-like appendage. Velocity data from the lowest radar elevation angle (0.5 degrees) indicated strong rotation in the hook region in Eastern Johnston County Moments later, a weak tornado touched down near the community of Raynor Town (shown by the damage are box). Several homes were damaged near Raynor Town (see pictures below), which is 9 miles west of Goldsboro. A fast northeast track took the storm just north of Goldsboro, where law enforcement also reported seeing a tornado on the ground. The weak EF-0 tornado was only briefly on the ground, and no other damage from the tornado was reported in Wayne County or subsequent counties to the east.

Severe Weather Reports -
Text of severe weather reports across central North Carolina

Severe weather reports via SPC from April 12, 2008

Severe weather reports via SPC from April 12, 2008

Raynor Town Tornado

At 312 PM EDT, a brief EF-0 tornado touched down 2 miles north of Raynor Town, which is located to the west of Goldsboro in Wayne County. The tornado first touched down west of Princeton Rd. and blew a garage door off of a brick house. The roof of a nearby mobile home was then ripped off before the tornado lifted off the ground as it crossed Princeton Rd. The tornado touched back down on the east side of Princeton Rd., where it rolled a mobile home approximately 20 yards into another mobile home. Finally, a fourth mobile home was blown partially off its foundation.

A storm survey conducted by the NWS in Raleigh concluded that based on the isolated nature of the damage, and the fact that the damage path was very narrow, the tornado that occurred was an EF-0 on the Enhanced Fujita Scale. Winds were estimated to have been 75 mph, and the length of the path of damage was around 200 yards.

A Google Earth satellite image of the area along with locations of damage in Raynor Town is shown below (click to enlarge). The storm motion was toward the northeast or toward the upper right of the image. Google Earth map imagery used under license.

NSSL's Rotational Track Tool

One of the most important aspects of the severe weather warning process is the collection of ground truth observations. This is true during the event when ground truth information is used in warning decision making, and also after the event when verification statistics are analyzed.

For over a decade, NOAA's National Severe Storms Laboratory (NSSL) has developed products and tools for severe weather operations in the Warning Decision Support System (WDSS). The tools are developed to assist forecasters in providing the most accurate and timely warnings of severe weather possible.

One such product is the "Rotational Tracks" product which is a gridded dataset that contains rotational shear from single and multiple radars that is accumulated over time providing tracks of radar detected rotation. The basic process for creating these products is initiated when velocity data from each radar is run through a Linear Least Squares Derivative (LLSD) filter creating an azimuthal shear field. The azimuthal shear fields in a 0-3 km layer from each radar across the CONUS are then combined and the maximum value at each 250 m2 grid point is plotted over the time period providing the graphic.

The process was further improved when the WDSS-II (Warning Decision Support System - Integrated Information) group at NSSL made the "Rotational Tracks" data available for display in Google Earth. Using Google Earth with an overlay of near real-time rotational tracks allows forecasters to estimate where a storm’s low-altitude circulation was most intense and to locate locations of possible damage. The satellite images and high density maps in Google Earth often make it possible to determine the location down to the street. This simplifies the verification process by reducing the amount of time that is spent searching for reports.

The image above is an example of the rotational track guidance from the Wayne County tornado. The shaded areas represent areas where radar detected rotation, and the yellows and reds are indicative of the strongest rotation. Overlaid are NWS storm reports where tornadic damage was reported (Red icons with "T"). From this image, it is evident that the track of the tornado was just north of the town of Goldsboro, across highway 70. The two storm reports fall directly in the path of the radar detected rotation.

Satellite

A Java Loop of infrared satellite imagery from 1645Z (1245 PM EDT) through 2231Z (631 PM EDT) on Saturday, April 12, 2008 shows the line of convection developing in the Piedmont and moving northeast across North Carolina. The storm that produced the Raynor Town Tornado can be seen as the bright white clouds tops in the 1901, 1915, and 1931 UTC images.

KRAX Radar Loops

KRAX base reflectivity image from 1923Z on Saturday, April 12, 2008 (323 PM EDT)

The radar image below with the tornado warning area outlined in red, shows the tornadic thunderstorm as it moved across Wayne County, just to the north of Goldsboro.

Java Loop of KRAX Base Reflectivity from 1601Z (1201 PM EDT) through 2055Z (455 PM EDT) Saturday, April 12, 2008.

Mesoscale Data

Analyzed surface temperatures (red/purple), dew points (brown/green), and wind barbs from SPC at 18 UTC on Saturday, April 12, 2008
Southwesterly surface winds transported moist air with dew points in the lower to mid 60s across the Coastal Plain. Sunshine earlier in the day allowed surface temperatures to climb into the lower 80s.

SPC Analysis at 18 UTC on Saturday, April 12, 2008

SPC Analysis at 18 UTC on Saturday, April 12, 2008

850 MB heights, temperatures (red/blue), dew points (green), and wind barbs (black) from SPC at 18 UTC on Saturday, April 12, 2008
A southwesterly jet with wind speeds of 50 to 55 knots and an enhanced area of moisture was present across the Coastal Plain. A short wave trough is located just east of the Appalachians.

SPC Analysis at 18 UTC on Saturday, April 12, 2008

Analyzed low level lapse rates in the 0-3 km layer (blue, green, and orange) from SPC at 18 UTC on Saturday, April 12, 2008
A lapse rate is the rate of temperature change with height and the image below is for the layer from the surface to around 10,000 feet. Note the surface based, low level lapse rates shown below range in the 6.5 to 7.0 deg C/km across include much of central North Carolina. Values less than 6 degrees C/km represent "stable" conditions, while values near 9 degrees C/km are considered "absolutely unstable." The greatest lapse rates and greatest instability is located across the Coastal Plain and Coast.

SPC Analysis at 18 UTC on Saturday, April 12, 2008

Analyzed surface based convective available potential energy (SBCAPE) (red) and surface based convective inhibition (blue lines - shaded) from SPC at 18 UTC on Saturday, April 12, 2008
SBCAPE values ranged between 500 and around 1000 J/kg across the Coastal Plain.

SPC Analysis at 18 UTC on Saturday, April 12, 2008

Analyzed mixed layer convective available potential energy (MLCAPE) (red) and mixed layer convective inhibition (blue lines - shaded) from SPC at 18 UTC on Saturday, April 12, 2008
MLCAPE values ranged between 250 and around 500 J/kg across the Coastal Plain.

SPC Analysis at 18 UTC on Saturday, April 12, 2008

0-3 Km Storm Relative Helicity (blue) and storm motion (brown) from SPC at 18 UTC on Saturday, April 12, 2008
Note the 0-3 Km SRH values ranged around 150 units in the Coastal Plain.

SPC Analysis at 18 UTC on Saturday, April 12, 2008

Analyzed Lifting Condensation Level (red, blue, and green) from SPC at 18 UTC on Saturday, April 12, 2008
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 the development of significant tornadoes. The LCL values during this event were between 1,000 and 1,250 meters in the Coastal Plain.

SPC Analysis at 18 UTC on Saturday, April 12, 2008

NWS Composite Reflectivity Imagery from 1830 UTC on Saturday, April 12, 2008
The composite reflectivity imagery is from the approximate time in which the analysis imagery above is valid.

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

from

Selected Photographs of the Severe Weather Event

Photos courtesy of Jeff Orrock.
(Click the image to enlarge)

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. Radar imagery was obtained from the National Weather Service web site. Damage photos are courtesy of Jeff Orrock. Base maps for the tornado tracks were provided by Google Maps and Google Earth - Google Earth map imagery used under license.

More information on the use of Rotational Track Tool from NSSL can be found at:
Smith, T.M. and K. L. Elmore, 2004: The use of radial velocity derivatives to diagnose rotation and divergence. Preprints, 11th Conf. on Aviation, Range, and Aerospace, Hyannis, MA, Amer. Meteor. Soc., P5.6 - CD preprints.

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