Event Summary      National Weather Service, Raleigh NC

July 14, 2004 Severe Weather Event Event Headlines - ...Multicells and supercells in the northeast Piedmont and northern and central Coastal Plain... ...Limited moisture but good dynamics and steep low level lapse rates leads to intense convection... ...Severe weather generally limited to hail with some isolated straight line wind damage south of Raleigh... Event Overview - Isolated strong storms developed in an unstable airmass over southern and central Virginia late during the afternoon of Wednesday, July 14, 2004. The thunderstorm cells began to split, with right-movers heading across the border during the early evening into Person and Granville counties, where hail of 3/4 to 1 inch diameter was reported. Storms then increased in coverage with redevelopment to the southwest, and one cell produced wind damage from downburst straight line winds and quarter sized hail over southern Wake County. The storms move to the east and southeast across much of north central and northeast North Carolina through the rest of the evening to just past midnight. Synoptic Overview - A prefrontal trough was located over central North Carolina, as a cold front approached from the northwest during the afternoon hours on Wednesday, July 14, 2004. The prior passage of a vorticity maximum earlier in the day brought in quite a bit of dry air (the precipitable water on the GSO upper air sounding went from 1.51” at 12Z to 1.27” at 00z; the dry air was also evident on water vapor imagery). The relatively dry airmass along with a diffluent surface flow made it hard for convection to develop or maintain itself in the early afternoon, despite strong surface heating (temperatures peaked in the upper 80s to near 90 with plenty of sun). The arrival of second vorticity maximum diving down into the incoming mid level trough base, a 80 knot upper jet (North Carolina was in the left exit region) and increasing surface convergence, facilitated the development of more widespread and sustained storms. Some of these thunderstorms became severe with a few exhibiting some weak supercell characteristics. Severe Weather Reports -

Surface Data NCEP Surface Analysis at 00Z on Thursday, July 14, 2004 (8 PM EDT Wednesday July 14, 2004). Java Loop of Surface Analysis from 12Z Wednesday July 14, 2004 through 12Z Thursday, July 15, 2004.

Mesoscale Data Analyzed mean sea level pressure (black) and surface wind barbs from SPC at from 23Z on Wednesday, July 14, 2004 (7 PM EDT). The surface trough is clearly evident over central North Carolina. Analyzed surface temperatures (red), dewpoints (green), and wind barbs from SPC at from 23Z on Wednesday, July 14, 2004 (7 PM EDT). Note the warm temperatures (upper 80s to lower 90s) and high dewpoints (mid 60s to lower 70s). Analyzed precipitable water (green) and wind barbs from SPC at from 23Z on Wednesday, July 14, 2004 (7 PM EDT). Note the relatively dry airmass with precipitable water values less than 1.5 inches over most of the state with an axis of slightly higher values associated with the front stretching from near Richmond VA to just west of Charlotte NC. Analyzed surface moisture convergence (dark blue), mixing ration (green), and wind barbs from SPC at from 23Z on Wednesday, July 14, 2004 (7 PM EDT). Note the area of surface moisture convergence near the front and along the South Carolina Coast. NWS Composite Reflectivity Imagery from 2325Z on Wednesday, July 14, 2004 (7 PM EDT). The composite reflectivity imagery is from the approximate time in which the analysis imagery above is valid.

Satellite Water vapor satellite imagery at 0045Z on Thursday, July 15, 2004 (845 PM EDT). The passage of short wave earlier in the day allowed quite a bit of dry air to move into the region. The relatively dry airmass was one factor that made it difficult for convection to develop or maintain itself during the early afternoon, despite strong surface heating.

Radar Imagery KRAX 4-Panel Composite Reflectivity, VIL, Base Reflectivity, and Echo Tops Imagery from 2215Z Wednesday, July 14, 2004. Hail the size of quarters was falling across extreme northern Person County at about the of time the 4 panel image shown below. The four panel image contains Composite Reflectivity (upper left), VIL - Vertically Integrate Liquid (upper right), 8bit 0.5 degree Reflectivity (lower left), and Echo Tops (lower right). Note the purple pixel of VIL (upper right panel) which is indicative of a VIL of 65 Kg/m2 with a corresponding Echo Top height of 44,000 feet at the same location. Click on the image to enlarge. KRAX 4-Panel Composite Reflectivity, VIL, Base Reflectivity, and Echo Tops Imagery from 2255Z Wednesday, July 14, 2004. Hail the size of pennies was falling across northern Granville County at about the time of the 4 panel image shown below. The four panel image contains Composite Reflectivity (upper left), VIL - Vertically Integrate Liquid (upper right), 8bit 0.5 degree Reflectivity (lower left), and Echo Tops (lower right). This is the same thunderstorm that produced even larger hail over northern Person County earlier. The storm was a "right mover" or demonstrating deviant motion since the storm motion should have been more eastward and this storm was traveling southeast at times. Click on the image to enlarge. KRAX 4-Panel Composite Reflectivity, VIL, Base Reflectivity, and Base Velocity from 0055Z Thursday, July 14, 2004. The small thunderstorm over southern Wake county shown in the four panel image below was responsible for some isolated reports of wind damage. The four panel image contains Composite Reflectivity (upper left), VIL - Vertically Integrate Liquid (upper right), 8bit 0.5 degree Reflectivity (lower left), and 8bit 0.5 degree Velocity (lower right). Note the bright green colors on the 8bit 0.5 degree Velocity panel (lower right) between Cary and Holly Springs (near and just east of Apex. The very bright green colors are indicative of winds approaching 50 knots or about 58 miles per hour. This storm was responsible for wind damage resulting from straight line winds across a few locations in southern Wake county at about 8:50 to 9:00 PM EDT. Click on the image to enlarge. KRAX 4-Panel Storm Relative Velocity Imagery from 0055Z Thursday, July 14, 2004. There was very little or no indication of rotation within this thunderstorm as noted by the Storm Relative Velocity imagery below. The four panel image contains 8bit 0.5 degree Storm Relative Velocity (upper left), 8bit 1.5 degree Storm Relative Velocity (upper right), 8bit 2.4 degree Storm Relative Velocity (lower left), and 8bit 3.4 degree Storm Relative Velocity (lower right). When comparing the SRM to the Base Velocity radar products, note the Base Velocity provided a better indicator of the damaging winds in southern Wake County associated with the thunderstorm moving eastward, directly toward the KRAX Doppler Radar. Click on the image to enlarge.

Warning Decision Process Thunderstorms first developed during the mid afternoon hours on Wednesday in the foothills of central and western Virginia. The thunderstorms moved east with a cluster of cells dropping into Pittsylvania, Halifax, and Mecklenburg counties in south central Virginia. This cluster of thunderstorms continued to move east and build southward with the first storms moving into Caswell, Person, and Granville counties after 2100Z Wednesday (5:00 PM EDT). Several cells in this cluster of thunderstorms split or merged as they moved across northern North Carolina. Forecasters monitored the storm tracks for right movers and used VIL/VIL density as the mechanism for most warnings. Rapid increases in lightning activity was also utilized as an intensity gauge. During the evening 25-30 knot westerly low level jet developed and the thicknesses pattern became more diffluent across central North Carolina (a favorable condition for backward propagation of convection). The thunderstorms began to propagate on the southwest side, redeveloping back over Chatham, Wake, and Johnston counties after 0100Z (9:00 PM EDT). The 00z Thursday GSO sounding had a deep near-dry-adiabatic mixed layer up to around 715 mb, which contributed to the threat for a damaging downburst. This threat was realized in southern Wake County, where straight-line winds in an isolated storm knocked down a few trees across southern Wake County shortly before 9:00 PM. Base velocity data showed localized westerly winds in excess of 50 knots with the thunderstorm that moved across southern Wake County. The thunderstorms continued to move east and build slightly southward as they moved across the Coastal Plain. With the loss of daytime heating, the storms diminished in coverage and intensity by around midnight.

Selected Photographs of the Severe Weather Event Photos courtesy of Mike Shumate and Jonathan Blaes (Click the image to enlarge.)

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