Event Summary National Weather Service, Raleigh NC
July 8, 2008 Excessive Lightning and Severe Weather Event Updated 2008/10/08
Event Headlines -
...58 severe weather events were documented by the NWS Raleigh...
...33 Severe Thunderstorm, Tornado and Flash Flood Warnings were issued...
...Significant Flash Flooding was reported in Anson County ...
...Forecasters anticipated the potential for thunderstorms with excessive amounts of lightning. The
1120 AM Area Forecast Discussion noted that "several parameters including the high CAPE (both MUCAPE and -10 to -30C CAPE)... K-index of 33-36... and normalized CAPE of 0.25 all
support very vigorous updrafts and rapid charge separation... thus the mention of frequent lightning has been added to the forecast through tonight"...
...There were 11,734 cloud to ground lightning strikes in the NWS Raleigh County Warning
Area during the 24 hour period ending at 12 UTC on Wednesday, July 9, 2008...
...In Anson County there were 1,001 lighting strikes in the hour ending at 2300 UTC, or an average of around 16.7 strikes per minute...
Combined, the three rounds of storms were responsible for widespread wind damage and a few severe hail
reports from northern Georgia to southern Virginia. In addition, the number of cloud-to-ground lightning strikes
observed with these storms was remarkably high. At one point, there were
over 2,000 strikes in a one hour period across the Piedmont of southern Virginia, North Carolina, and northern South Carolina.
Forecasters at the National Weather Service Forecast Office in Raleigh, NC, have undertaken a research project to anticipate
the level of lightning activity, in hopes of providing the public with an improved awareness of events where the number
of lightning strikes will be exceptionally high. Forecasters look for certain conditions that are needed for the production
of lightning by completing a checklist each morning. More information on the lightning forecast process is discussed below.
A Java Loop of regional reflectivity imagery from 1538 UTC on July 8 through 0758 UTC on July 9, 2008
is available here. Note - this loop includes 68 frames.
A Java Loop overview of the entire event with images from every hour
between 1602 UTC July 8 through 0700 UTC July 9, 2008 is available
here.
Note - this loop includes 16 frames
A Java Loop overview of the entire event with images from every 15 minutes
between 1602 UTC July 8 through 0700 UTC July 9, 2008 is available
here.
Note - this loop includes 60 frames
A Java Loop overview of the entire event with images from every volume scan
between 1602 UTC July 8 through 0700 UTC July 9, 2008 is available
here.
Note - this loop includes 206 frames
In just the NWS Raleigh CWA
there were 11,734 strikes detected.
The lightning strikes were concentrated in a few locations primarily across the Southern
Piedmont in Anson County; across eastern portions of the Sandhills in Harnett, Cumberland, and
Lee Counties; along the VA-NC border in northern portions of Person, Granville, and Vance Counties; and along the Coastal Plain from Johnston County northward into Halifax County.
The looping image to the right (click here to control, stop, or start the loop)
is from the AWIPS D2D application showing
the cloud to ground lighting strikes in 15 minute
intervals across central North Carolina from 1500 UTC on July 8 through 0900 UTC on
July 9, 2008. Negative lightning strikes are shown with a "-" symbol and positive
lightning strikes are shown with a "+" symbol. (Click here for more information on lightning from the NWS's Jetstream Weather School.)
The greatest cloud to ground lightning activity during this event was located
across Anson
County where 2,212 strikes were detected during the
24 hour period ending at 1200 UTC on July 9. More impressively, there
were 1,001 lighting strikes
in the hour ending at 2300 UTC, or an average of around 16.7 strikes per minute during the
hour across Anson county. The sampling area that approximately covers Anson County is across 41 5x5 km grid boxes, or 1,025 km².
This corresponds to an average of 2.16 cloud to ground lightning strikes per km² during the 24 hour period
ending at 1200 UTC on July 9, with 0.98 cloud to ground lightning strikes per km² in the hour ending at 2300 UTC.
Cloud to ground lightning strikes across Anson County and adjacent areas during a
15 minute period ending at 2300 UTC (700 PM EDT) on July 8, 2008 are shown below. Note that 603 lightning strikes were
observed over this area during the 15 minute period with an average of 40 lightning strikes
each minute.
Cloud to ground lightning strikes across Harnett and Cumberland Counties and adjacent areas during a
15 minute period ending at 0015 UTC (815 PM EDT) on July 8, 2008 are shown below. Note that 425 lightning strikes were
observed over this area during the 15 minute period with an average of 28 lightning strikes
each minute.
Lightning is produced when there is a buildup of electrical
charges within a cloud. In a typical thunderstorm structure,
as the updraft increases, positive charges are vaulted into
the upper portion of the storm, negative charges pool in the
mid levels of the storm, and another pool of positive charges
collects near the cloud base. Negative charges are sent down
the storm in what is called a stepped leader, which then draws
a stream of positive charge upward called a "positive streamer."
As these two charges come together, the electric current forms.
This slow motion movie clip below from the BBC documentary The Power
of the Planet – Atmosphere shows the negative charges "snaking" their way down from the cloud bases.
These negative charges are called "stepped leaders" and when one of them is close enough to the surface, it
draws a stream of positive charge upward. As these two charges come together, the electric current forms and
the lightning strike takes place.
Within a storm,
two primary ingredients are critical for lightning formation:
(1) the presence of graupel, or small balls of water-coated snow
(collisional charging between graupel pellets and lighter ice crystals
facilitates lightning production) in the layer between approximately -10 and -30 degrees Celsius, referred to as the "mixed
phase" region of the storm, where ice coexists with supercooled water;
and (2) strong instability in this same layer of the storm, to
facilitate strong updraft velocities and rapid charge separation
within the storm. While we cannot measure precisely the presence
of graupel inside the storm and the velocity of the storm's updraft,
several near-storm parameters can indicate a favorable environment for
extreme amounts of lightning.
One such parameter that
forecasters monitor to anticipate extreme amounts of lightning
is the layer CAPE within the -10 to -30
degree Celsius layer (the "mixed phase" region
of the storm). Strong instability within this layer has been shown to be favorable for rapid hail growth.
In local studies of extreme lightning events, values over 200 J/kg have been associated with
high-frequency lightning strike events.
Another parameter that helps forecasters measure the potential for
vigorous updrafts which favor lightning is the normalized CAPE,
or N-CAPE. N-CAPE is the CAPE measured from the LCL (lifted condensation
level) to the EL (equilibrium level), divided by the depth of that layer, and it gives a measure of the
"shape" of the CAPE. A wide or "fat" CAPE will
equate to high N-CAPE and indicates the potential for strong
updrafts, whereas a narrow
or "skinny" CAPE will have a low N-CAPE and indicates
weak updrafts. N-CAPE values above 0.1, and especially those above
0.2, indicate a "fat" CAPE and better chance for very
strong updrafts.
Also important to consider is the potential for sufficient graupel in the "mixed
phase" region of an updraft. The amount of moisture present in the hail growth layer (-10 to -30C) is difficult
to quantify. This quantity is not readily available from numerical models for forecasters, so forecasters
are forced to subjectively identify whether or not there is sufficient moisture
available. However, past studies have shown that column total precipitable water may be a good
indicator, even though it does not isolate the "mixed phase" layer. Upcoming refinements to
the lightning forecast process may include evaluation of precipitable water.
Finally, forecasters also consult experimental output from two specialized
model-based algorithms run by the Storm Prediction Center (SPC). One algorithm is based on the NAM model,
while the other is based on the Short Range Ensemble Forecast (SREF). Both sets of output show the
probability of 100 or more lightning strikes. These products, along with the other parameters mentioned above,
are considered together by forecasters by means of the lightning checklist.
Lightning Checklist
In order to quickly assess the potential for high-frequency lightning activity, forecasters complete a daily checklist on each midnight shift, with the option to update the checklist as new model data and
observations arrive during the day. The checklist is broken up into three categories of lightning activity - low, medium, or high.
Corresponding thresholds for the parameters discussed above were chosen for each level based on previous research and experience. The parameters that
are objectively evaluated are K-index, Most Unstable CAPE (MUCAPE), CAPE in the -10C to -30C layer, Normalized CAPE (N-CAPE), SPC-produced model-based probabilities, and
persistence of the weather pattern (what was the activity level the day before?). Other parameters that are more subjectively evaluated include the
moisture in the hail growth layer, the presence of synoptic forcing, and the location and orientation of high equivalent potential temperature air at 850mb. Each of these factors was noted in past studies as being correlated with excessive lightning events.
During the early morning hours of the 8th, forecasters completed the checklist, and noted several indicators that would suggest that storms would produce an unusually high number of lightning strikes. The main parameters that suggested high potential included
K-index values above 30. (K-index measures mid-level lapse rates and moisture.)
CAPE in the -10C TO -30c layer. (Values of greater than 200 J/kg suggest strong updrafts, and the forecast layer CAPE on the 8th was
over 900 J/kg.)
Normalized CAPE of 0.25.
Persistence. Forecasters noted that lightning activity was high the day before, and there would be little change to the synoptic
pattern and air mass.
Regarding the more subjective parameters, forecasters also noted that there would be a strong synoptic feature in the vicinity of central North Carolina to initiate convection.
Specifically, an upper-level low would be over Central Virginia, with a strong vorticity maximum rotating through North Carolina with the
shortwave mentioned in the event overview. The vorticity center and its differential positive vortcity advection (DPVA) would provide
strong lift to help develop widespread thunderstorms. The final evaluation on the checklist is of any ridging of equivalent potential
temperature at 850mb, which in this case was found to be fairly uniform across the entire CWA. Forecasters will often use extra space
on the checklist to describe spatial differences in the values of parameters across the CWA, but given that no comments were made, and
that the 850mb equivalent potential temperature would be essentially uniform, there was no indication that one area would be favored for
high lightning activity versus another.
As noted above, in just the NWS Raleigh CWA
there were 11,734 strikes detected.
The lightning strikes were concentrated in a few locations primarily across the Southern
Piedmont in Anson County; across eastern portions of the Sandhills in Harnett, Cumberland, and
Lee Counties; along the Virginia-North Carolina border in northern portions of Person, Granville, and Vance Counties;
and along the Coastal Plain from Johnston County northward into Halifax County. Although the task of identifying individual storms
that will realize their full lightning potential is difficult, focusing on parameters that lead to high lightning frequency
can assist forecasters in targeting days where the environment is conducive to more frequent lightning strikes.
Analysis Data
Two of the main forecast parameters are readily available via the
SPC mesoanalysis Page,
Most Unstable CAPE and CAPE in the -10 to -30C layer.
These plots can be used to show what the conditions were in and around Anson County at the time of the excessive lightning. At 23 UTC,
Most Unstable CAPE values were highest, from 2000 to 3000 J/kg, from Anson County eastward
into the Sandhills. This is a good indicator that the environment in that area was highly unstable and conducive to strong convection. Even
more representative of the potential for very strong updrafts is the CAPE in the -10 to -30C
layer, which was 250 to 300 J/kg in the Southern Piedmont, with a maximum analyzed very near Anson County. Finally, the K-index is not
analyzed on the SPC mesoanalyis page, but can be computed using various mesoanalysis products and the equation
(T850 - T500) + Td850 - (T700 - Td700), where T is the temperature and Td is
the dewpoint temperature). Each value can be approximated from the upper air analyses
(850mb, 700mb,
500mb). An evaluation of the K-index based on these images gives a value of 33C, which is exactly what was noted by forecasters 18 hours earlier in the lightning checklist.
Mesoscale Data
Analyzed surface pressure and wind barbs from SPC at 21 UTC on Tuesday, July 8, 2008 A weak surface trough extended across the Western Piedmont providing some weak convergence.
Analyzed surface Theta-E (green) and Theta-E advection (purple) from SPC at 21 UTC on Tuesday, July 8, 2008 The greatest theta-e values ranged in an axis from northeast South Carolina into
southern North Carolina where theta-E values 352K or greater.
Analyzed low level lapse rates in the 0-3 km layer (blue, green, and orange) from SPC at 21 UTC on Tuesday, July 8, 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 7.0 to 7.5 deg C/km across 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." An axis of steep low level lapse rates extended from
north to south across central North Carolina.
Analyzed surface based convective available potential energy (SBCAPE) (red) and surface based
convective inhibition (blue lines - shaded) from SPC at 21 UTC on Tuesday, July 8, 2008 SBCAPE values ranged between 2500 and around 3000 J/kg across eastern portions of the
Southern Piedmont and the Sandhills with no significant convective inhibition (CIN).
Analyzed mixed layer convective available potential energy (MLCAPE) (red) and mixed layer based
convective inhibition (MLCIN) (blue lines - shaded) from SPC at 21 UTC on Tuesday, July 8, 2008 MLAPE values ranged between 1500 and around 2000 J/kg in a north-south axis across
central North Carolina with no significant convective inhibition (MLCIN).
CAPE in the layer from -10 C to -30 C, 0-6-km shear vector, and the freezing level height from SPC at 21 UTC on Tuesday, July 8, 2008 Large CAPE in the layer from -10 C to -30 C favors rapid hail growth. 0-6-km shear in excess of 30-40 knots
supports supercells with persistent updrafts that contribute to large hail production.
Finally, lower freezing level heights suggest a greater probability of hail reaching the surface prior to melting,
though melting impacts small hail much more than very large hailstones.
Normalized CAPE or (NCAPE) (red) from SPC at 21 UTC on Tuesday, July 8, 2008
The NCAPE (Normalized CAPE) is CAPE that is divided by the depth of the buoyancy layer
(units of m s**-2). Values near or less than 0.1 suggest a "tall, skinny" CAPE profile with
relatively weak parcel accelerations, while values closer to 0.3 to 0.4 suggest a "fat" CAPE
profile with large parcel accelerations possible.
NWS Composite Reflectivity Imagery from 2130 UTC on Tuesday, July 8, 2008 (530 PM EDT). 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
Forecasters anticipated the potential for thunderstorms with excessive amounts of lightning. The
1120 AM Area Forecast Discussion noted that "several parameters including the high CAPE (both MUCAPE and -10 to -30C CAPE)... K-index of 33-36... and normalized CAPE of 0.25 all
support very vigorous updrafts and rapid charge separation... thus the mention of frequent lightning has been added to the forecast through tonight".
The excessive lightning checklist was successfully used preceding this event to highlight the potential for excessive amounts of lightning. This high lightning potential was mentioned in the Zone Forecast Product and the
Hazardous Weather Outlook.
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 University
of Wyoming. Lightning data was collected from AWIPS and the Graphical Forecast Editor.
lightning data made available in GFE via the
Ltg Procedure
created by Timothy Barker. Objective analysis of the Graphical Forecast Editor lightning data was produced via the
Lightning Tools Procedure created by Ryan Knutsvig. Slow motion lightning movie clip
is from the BBC documentary The Power of the Planet – Atmosphere. Photos courtesy of John
Hamilton, Mark Calaway, and Michael Moneypenny.
Case study team -
Gail Hartfield
Barrett Smith
Lara Pagano
Jonathan Blaes
For questions regarding the web site, please contact Jonathan Blaes.
