Event Summary
     National Weather Service, Raleigh NC

August 21, 2007 Severe Weather Event
Updated 2007/09/06





Event Headlines -
...The remnants of Tropical Storm Erin enhanced thunderstorms that produced widespread wind damage across central North Carolina...
...The National Weather Service in Raleigh issued 35 Severe Thunderstorm Warnings on Tuesday, August 21, 2007, including warnings for 30 of the 31 counties in the NWS Raleigh County Warning Area...


Event Overview -
The Mesoscale Convective Vortex (MCV) remnant of Tropical Storm Erin moved across the central Appalachians late in the day on Tuesday afternoon, August 21. A Mesoscale Convective System (MCS) associated with the remnants of Tropical Storm Erin weakened as it moved into the Appalachians during the late morning hours. Scattered thunderstorms which developed across the mountains of Virginia and North Carolina just after midday were enhanced by the large scale vertical ascent associated with the approaching MCV. The developing thunderstorms organized into a line of severe thunderstorms as they pushed east into the Foothills and Piedmont regions of North Carolina where the atmosphere was primed to support severe thunderstorms with damaging winds.


Surface Conditions -
Surface analysis late in the morning of August 21 showed a thermal boundary near and along the NC/VA state line. Temperatures south of the boundary in central NC were in the lower 90s and temperatures north of the boundary in central VA were in the lower 70s. This thermal boundary was the result of differential heating due to extensive low cloud cover in central and northern VA during the morning hours. Additionally, a surface trough axis was located across portions of NC/VA, stretching from Norfolk VA west southwest to Hickory NC. Surface dew points across central NC were in the upper 60s to lower 70s during the morning hours, although these would fall into the lower 60s during the afternoon in response to strong diurnal mixing and dry air in the lower levels.


Upper Level Conditions -
12Z RAOB analysis, water vapor satellite imagery and short term model (RUC) data showed a shortwave and associated vorticity maximum over central and western KY during the morning of August 21. This mid/upper level feature can be traced back via water vapor imagery and 500 MB height/vorticity fields as the remnants of Tropical Cyclone Erin. Erin made landfall on the Texas coast near Corpus Christi as a minimal tropical storm during the early morning of August 16, 2007. After quickly weakening to a tropical depression, the remnant circulation meandered through Texas into central Oklahoma by early morning on August 19. The mid/upper level remnants of Erin continued east into Missouri on August 20, reaching western Kentucky during the morning hours of August 21. During the late afternoon and evening hours on August 21, the vorticity maximum associated with Erinís remnants served as a catalyst for organized thunderstorm development as it approached and eventually crossed North Carolina and southern Virginia.


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





Mesoscale Data

500 MB heights, temperatures (red), isotachs (blue), and wind barbs (black) from SPC at 21Z on Tuesday, August 21, 2007 (500 PM EDT).
The shortwave trough associated with the remnants of Tropical Storm Erin can be seen across eastern Ohio and eastern Kentucky. Note the enhanced westerly flow just ahead and south of the trough with winds up to 40 kts.

SPC Analysis at 21Z on Tuesday, August 21, 2007.



Analyzed surface temperatures (red/purple), dew points (brown/green), and wind barbs from SPC 21Z on Tuesday, August 21, 2007 (500 PM EDT).
Note the large area with temperatures in excess of 90 degrees shown in the purple contours with much of central and south-central North Carolina experiencing temperatures around 100 degrees. At the same time surface dew points are in the lower 60s.

SPC Analysis at 21Z on Tuesday, August 21, 2007.



Analyzed low level lapse rates in the 0-3 km layer (blue, green, orange, and red) from SPC at 21Z on Tuesday, August 21, 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 8 deg C/km across include much of North Carolina with values exceeding 9 deg C/km across central North Carolina indicating absolute instability. These extreme values are connected to the very hot temperatures around or just above 100 degrees.

SPC Analysis at 21Z on Tuesday, August 21, 2007.



Analyzed surface based CAPE (red) and surface based CIN (shaded in blue) from SPC at 21Z on Tuesday, August 21, 2007 (500 PM EDT).
CAPE values between 1000 and 1500 J/kg are evident across much of central North Carolina.

SPC Analysis at 21Z on Friday, August 10, 2007.



Analyzed precipitable water (green) and wind barbs from SPC at 21Z on Tuesday, August 21, 2007 (500 PM EDT).
Precipitable water values range from 1.6 to 1.8 inches across much of central North Carolina.

SPC Analysis at 21Z on Tuesday, August 21, 2007.



NWS Composite Reflectivity Imagery from 2032Z on 21Z on Tuesday, August 21, 2007 (500 PM EDT).
The composite reflectivity imagery is from the approximate time in which the analysis imagery above is valid.

Composite Reflectivity Imagery from 2032Z on Tuesday, August 21, 2007 (500 PM EDT).




Satellite

A Java Loop of visible satellite imagery from 1145Z (745 AM EDT) through 2331Z (731 PM EDT) Tuesday, August 21, 2007 is available here. This loop highlights the evolution of the event from the initial thunderstorms across the mountains to their organization into a line of severe thunderstorms across central North Carolina. Note - this loop includes 39 frames.

The visible satellite image below is from 2045Z (445 PM EDT) just as the storms were entering the Triad area. The line of enhanced cloudiness associated with the convective line can be seen moving into the Piedmont region of North Carolina and Virginia. There were scattered cumulus clouds across central and eastern North Carolina but these clouds displayed very little vertical growth. There was a relative absence of cumulus clouds across northeastern North Carolina and southern Virginia as this area had a fair amount of low clouds earlier in the day which kept temperatures a little cooler.

Visible satellite imagery



Discussion

As noted previously, the shortwave trough that enhanced the convection across the North Carolina and Virginia on August 21 can be traced back as the remnants of Tropical Storm Erin. Erin moved onshore along the Texas coast near Corpus Christi as a minimal tropical storm during the early morning hours on August 16, 2007. After quickly weakening, the remnants moved through Texas on the 17th and 18th of August and then into central Oklahoma by the morning hours of August 19. The remnants of Erin moved east into Missouri on August 20, and then reaching western Kentucky during the morning hours of August 21.

The system was responsible for significant rainfall across Texas and Oklahoma. Numerous flash flood warnings and severe thunderstorm warnings were issued in Oklahoma on August 19 when the remnants of Erin briefly intensified producing significant flooding and wind gusts in excess of 60 MPH.

National Hurricane Center advisory archive on Tropical Storm Erin
Hydrometeorological Prediction Center advisory archive on Tropical Depression Erin


Map of Erin's track and remnants along with precipitation amounts from the Hydrometeorological Prediction Center

Map of Erin's track and remnants along with precipitation amounts from the Hydrometeorological Prediction Center - click to enlarge

During the morning of August 21, the shortwave trough associated with Erinís remnants was located across western Kentucky and southern Indiana. At this early morning hour convection was ongoing across Kentucky and southern Ohio. A significant boundary was located across southern Virginia on the southern edge of a much cooler and drier air mass. Low stratus clouds across central and northern Virginia as seen on the visible satellite image from 1445Z highlighted the cooler air mass. Skies were clear across central North Carolina allowing maximum solar insolation which resulted high temperatures in the 95 to 103 range.

The atmosphere across central North Carolina contained moderate amounts of moisture and modest instability with high temperatures expected to reach the 95 to 103 range with dew points in the 60s to near 70. Just after midday, differential heating lead to the development of enhanced cumulus clouds across the mountains of North Carolina with the first radar returns appearing at around 1700Z. The convection was rapidly developing across the mountains at 1800Z. As the shortwave trough associated with Erinís remnants approached the mountains, upward motion and vertical speed shear were increasing which allowed the convection to intensify further and organize into a single line of thunderstorms. Despite the westerly low level flow and the drying of low levels associated with diurnal convective mixing and some down slope flow, the convection persisted.

The atmosphere across central North Carolina was supportive of severe thunderstorms with damaging winds on August 21st. Some factors that contributed to this environment include:
  • The approaching shortwave trough associated with the remnants of Erin coincided with the time of day in which the maximum heating occurred.
  • Modest instability with mixed layer CAPE values between 1000 and 1500 J/kg but with little inhibition.
  • Relatively high precipitable water values (between 1.60" to 1.90")
  • Steep low level lapse rates and resultant surface dew point depressions between 30 and 40 degrees, typical of an inverted-V sounding profile, were favorable for strong sub cloud base evaporative cooling and strong, organized surface cold pools.
  • Precipitation loading from the stronger updraft cores also enhanced downdraft strength.
  • Generally westerly, unidirectional wind shear with 30 knot mid-level flow (shown with a VWP plot from the krax radar at 22Z) provided optimal conditions for a balanced mesoscale circulation along the eastward moving surface cold pool, according to RKW theory.
  • 4mb/hr pressure rises occurred in the wake of the gust front/leading edge of the surface cold pool.


    The line of convection weakened somewhat as it moved into a relative theta-e minimum across the Piedmont of North Carolina in the vicinity of US Route 1. This was only a temporary change as the convection reintensified quickly as it approached richer moisture and a much more unstable atmosphere across the Coastal Plain and Coastal region of North Carolina.


    Loop of regional reflectivity imagery from 21/1202Z (802 AM EDT 08/21/2007) through 22/0730Z (330 AM EDT 08/22/2007)

    regional reflectivity image



  • Warning Methodology

    KRAX base reflectivity imagery and warnings from 21/2104Z (504 PM 08/21/2007) through 22/0003Z (803 PM 08/21/2007)



    The radar loop rather plainly shows the leading edge of the strong winds, located well ahead of the precipitation, denoted by a thin line racing out in front of the area of showers and thunderstorms. Rather than issuing warnings that coincided with the thunderstorms themselves, forecasters warned for the leading edge of the damaging winds. The average lead time (the time between the warning issuance and the earliest report of storm damage) was about 30 minutes.

    Initially, forecasters anticipated, based on the near storm environment, that the outflow from the isolated convective cells that had developed over the mountains and foothills would merge and produce forcing for line segments capable of damaging wind gusts. Such a scenario quickly evolved, and as the line of thunderstorms propagated east through central North Carolina, a well developed cold pool raced east ahead of the line. The leading edge of the cold pool (the outflow boundary) showed up very well on radar as a long, continuous fine-line in the 0.5 degree reflectivity. Damage reports from storm spotters and the general public, in addition to surface observations showing 40 to 50 MPH wind gusts as the outflow boundary passed, led to the decision to issue warnings based on the progress of the outflow boundary. The warning decision process was further complicated by thunderstorms that were developing along and ahead of the outflow boundary. In this case, given environmental conditions characterized by steep low level lapse rates, inverted V sounding profiles and 30-35 knot mid level flow, it was possible that damaging winds would occur not only in association with the outflow boundary, but with thunderstorms developing along and ahead of the outflow boundary and with the actual line of thunderstorms to the west, as well.

    KRAX base reflectivity imagery from 21/2236Z (636 PM 08/21/2007) along with 22Z surface METAR reports





    KRAX Radar Loops

    Overview of the entire event with images from every 15 minutes between 21/1929Z (329 PM EDT 08/21/2007) through 22/0100Z (900 PM EDT 08/21/2007).
    Java Loop of KRAX reflectivity imagery from 21/1929Z through 22/0100Z.
    Note - this loop includes 23 frames

    Overview of the entire event with images from every volume scan between 21/1929Z (329 PM EDT 08/21/2007) through 22/0100Z (900 PM EDT 08/21/2007).
    Java Loop of KRAX reflectivity imagery from 21/1929Z through 22/0100Z.
    Note - this loop includes 79 frames

    The KRAX reflectivity image below is from 2207Z (607 PM EDT) on Tuesday, August 21, 2007.





    Precipitation

    Much of central North Carolina received measurable rainfall from this event; however it was not nearly enough rainfall to ease the ongoing severe to extreme drought conditions across the area. Note that the most significant rain fell across the coastal region where there was much more instability and greater low level moisture.

    24 hour rainfall estimate ending at 12Z (800 AM EDT) Wednesday, August 22, 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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    Final Thoughts

  • The catalyst for this event was the circulation center associated with the remnants of Tropical Storm Erin. This system was responsible for remarkable weather as it moved across Oklahoma and the Missouri Valley. Forecasters should certainly pay special attention to upstream systems that produce significant or anomalous weather, this obviously includes the remnants of tropical systems.

  • Excellent team work during the event was critical in the success of the event. Multiple coordinators were a great help. In such a fast-paced event, with some warnings being issued simultaneously, keeping track of warning issuance, NAWAS coordination, telephone calls to counties and to RDU, and warning expiration was very difficult. One person would have been extremely taxed to keep up with all of this.

  • There was great communication among the seven office staff. Primary radar operators, with well defined areas of responsibility, communicated very well on radar signatures observed in areas under the responsibility of others, or when storms were ready to transition to other parts of central North Carolina. Storm reports were passed to radar operators quickly. Severe weather statements were issued with frequency, and there were nearly 40 LSR's issued during the event, several after the event.

  • Event recognition of wind took place very quickly. This recognition was aided by numerous severe weather reports from spotters and others, primarily law enforcement. Lead time was very good, on the order of 30 minutes. Some warnings needed to be issued for as much as 90 minutes considering both the progress of the outflow and redevelopment behind the outflow. In an event such as this, long severe thunderstorm warnings are warranted. Due to the frequency of warning issuance, a few counties did not receive a follow-up statement. This was intended, as the hierarchy of information for this event was decided to be warnings, storm reports, and statements.

  • Several forecast updates were made. The movement of the line of storms to the east required higher chances for rain, and forecast updates were complicated by multiple watches in effect and their subsequent cancellation in part or whole. Media and other partners are especially appreciative when portions of severe convective watches can be cleared.



  • Case study team -
    Gail Hartfield
    Michael Strickler
    Brandon Vincent
    Phil Badgett
    Barrett Smith
    Scott Sharp
    Darin Figurskey
    Jonathan Blaes


    Credits -
    Thanks to Dr Gary Lackmann for providing the multiday 500mb radar loop.



    For questions regarding the web site, please contact Jonathan Blaes.


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