Event Summary
     National Weather Service, Raleigh NC

June 4, 2004 Severe Weather Outbreak
Updated 2004/06/10


Synoptic Overview - A stationary front extended southwest to northeast across central North Carolina during the late morning and afternoon hours of June 4. A weak wave of low pressure was stalled along the front. In the upper levels, trough was slowly moving eastward across the Ohio Valley at 500 MB with a closed circulation over western North Carolina at 850 MB. Thunderstorms developed south of the frontal zone in a warm and unstable airmass and then moved north and northeast. Several thunderstorms became severe with a few thunderstorms producing tornadoes.

Severe Weather Reports - The severe weather reports were clustered near the frontal zone as convection that developed in the warm air south of the front became enhanced near and along the font. There were approximately six reports of tornado touchdowns with dozens of reports of funnel clouds. Flooding became an increasing problem as thunderstorms redeveloped and repeatedly moved over the same areas dropping several inches of rain and producing flash flooding.

There were several factors limiting wind damage and tornado touchdowns. While directional wind shear along the boundary was impressive, the magnitude of the winds in the low and mid levels was lacking. This limited the amount of wind shear available to the storms. The direction of storm motion had a significant northerly component which caused the storms to quickly move across the frontal boundary.

Had the surface front been oriented more north to south or had the storm’s steering winds moved the thunderstorms parallel to the front, the individual storms would have been exposed to frontal boundary wind shear for longer periods of time. This would have likely extended the life time of the rotating mesocyclones that only lasted 1–2 radar scans (about 5 to 10 minutes). Had the mesocyclones persisted for longer durations, there may have been more funnel clouds reaching the ground as tornadoes.

The mesocyclone that did produce a tornado across Johnston, Wayne, and Wilson Counties was persistent, causing damage all the way into Martin County. It was also very deep, with the radar MESO algorithm showing a mesocyclone depth from 1,500 feet to 12,000 ft at one point. This storm also display good rotation on the radar's 0.5 Storm relative velocity imagery, while most other storms had weak to no rotation in the lowest slice.








Upper Air Analysis

500 MB Upper Air Analysis at 18Z on Friday, June 4, 2004 (2 PM EDT).
The

SPC Upper Air Analysis from 18Z on Friday, June 4, 2004.

850 MB Upper Air Analysis at 18Z on Friday, June 4, 2004 (2 PM EDT).


SPC Upper Air Analysis from 18Z on Friday, June 4, 2004.



Surface Analysis

NCEP Surface Analysis at NCEP Surface Analysis from 18Z on Friday, June 4, 2004 (2 PM EDT Friday June 4, 2004).

NCEP Surface Analysis from 18Z on Friday, June 4, 2004.

Java Loop of Surface Analysis from Surface Analysis from 00Z Friday June 4, 2004 through 06Z Saturday June 5, 2004.



Mesoscale Data

Analyzed surface temperatures (red), dewpoints (green) and wind barbs from SPC at from 18Z on Friday, June 4, 2004 (2 PM EDT).
The frontal zone is clearly evident over the coastal plain. Note the convergent winds and the tight gradient in surface temperatures and dewpoints.

SPC Analysis from 18Z on Friday, June 4, 2004.



Analyzed BRN shear (green) and surface CAPE (red) from SPC at from 18Z on Friday, June 4, 2004 (2 PM EDT).
Note the intersection of the BRN values between 40 and 60 with CAPE values of around a 1000 units (along the surface front/boundary). This is the general location in which the tornadic storms developed. This area was where the low-level shear and updraft strength were maximized to support supercells. As the afternoon progressed, the BRN values remained steady while the instability or CAPE decreased. The near storm environment became less unstable with time. This likely contributed to the diminishing thunderstorm intensity and the diminishing strength of the mesocyclones as the afternoon progressed.

The "BRN shear" is the Bulk Richardson Number shear term (the denominator of the BRN). Values of 35-40 m2s-2 or greater have been associated with supercells. The "CAPE" (Convective Available Potential Energy) is a measure of instability through the depth of the atmosphere, and is related to updraft strength in thunderstorms.

SPC Analysis from 18Z on Friday, June 4, 2004.



Satellite

Visible satellite imagery at 1515Z on Friday, June 4, 2004 (1115 AM EDT).
Visible satellite imagery and radar showed overcast skies with numerous showers and a few embedded thunderstorms on the cool side of the surface front. Scattered thunderstorms developed in the warm sector where the air mass was the most unstable.






Infrared Satellite image depicting an "Enhance-V Signature" associated with a Tornadic Thunderstorm at 1545Z on Friday, June 4, 2004 (1145 AM EDT).
A tornado was reported on the ground approximately 4 minutes earlier in eastern Johnston County, just southeast of Princeton. The enhanced-V signature is an area of colder cloud tops (green colors in the image below) in infrared satellite imagery in the shape of a "V". This indicates that the environmental wind flow is diverging around a very strong updraft. It is a very reliable signature of severe storms, though in this case it was apparent until after the tornado warnings were issued.





Radar Imagery

Regional reflectivity mosaic across the Carolinas and Virginia at 1600Z on Friday, June 4, 2004.
Click on the image to enlarge.

Click to enlarge

Java Loop of base reflectivity imagery from krax WSR-88D from 1317Z Friday June 4, 2004 through 2142Z Friday June 4, 2004.
This imagery highlights the development of the numerous thunderstorm clusters across central North Carolina during the afternoon and evening of June 4, 2004.




Click to enlarge

4-Panel KRAX "8-bit" Storm Relative Velocity Imagery from 1647Z Friday, June 4, 2004.
Click on the image to enlarge.

Click to enlarge

4-Panel KRAX "8-bit" Storm Relative Velocity Imagery from 1702Z Friday, June 4, 2004.
Click on the image to enlarge.



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Jonathan Blaes

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