Event Summary
     National Weather Service, Blacksburg VA
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June 16, 2004 Flash Flood
Last Updated 2005, 09/11

Synoptic Overview - An upper ridge of high pressure was centered over eastern Georgia, the forecast area (southwestern Virginia) on the northern periphery. An upper low was drifting slowly northward into east Texas. Southerly flow between the 2 systems was advecting tropical air into the forecast area. The PWAT (00 UTCWED roab: RNK 1.86, GSO 2.37) was near 200% of normal. A quasi-stationary boundary in the lower troposphere (showed up as a trof on the 00 UTC 925, 850, and 700 mb analysis) was the focus for deep convection over the northwest piedmont of North Carolina and south side Virginia. Thunderstorms were oriented west-east near the NC/VA border and kept regenerating along this boundary.

Severe Weather Reports - The flash flood resulted in two fatalities. Peak rainfall was around 10 inches in about 4 hours time.  Runoff from this rainfall resulted in flash flooding along Stuart and Matrimony creeks in the extreme southern part of Henry County, Virginia, and the extreme northern part of Rockingham County, North Carolina (F).  It is believed Stuart creek which is normally a few feet wide, covered an area half the width of a football field and rose to a depth that was 6 feet above Meeks Road.  This is where the two fatalities occurred.  

Map of Region

Image of Survey Rod Showing Depth of Flood Waters

Stuart Creek looking downstream:

The car in the picture is approximately where the other car was when it was swept off the road.

 Picture of a car in a location that was flooded

 



Upper Air Analysis

300 MB Upper Air Analysis at 00 UTC on Wednesday, June 16, 2004 (8 PM EDT, Tuesday, June 15)
Streamlines clearly define position of the upper ridge and upper low over the southeast CONUS.  The flash flood location was at the apex of the upper ridge.

300 Millibar Analysis 


500 MB Upper Air Analysis at 00 UTC on Wednesday, June 16, 2004 (8 PM EDT, Tuesday, June 15).
Note wind diffluence between the RNK and GSO raobs. (Data over the pacific northwest is corrupted.)
 500 Millibar Analysis

700 MB Heights (solid black lines) and Equivalent Potential Temperature (dashed yellow lines) from the 00 UTC Upper Air Analysis Wednesday, June 16, 2004 (8 PM EDT, Tuesday, June 15).
The 700 mb trof was oriented west-east along the NC/VA border (blue dashed line). Note ridging to the north of the trof which likely resulted in the trof remaining quasistationary. The boundary was also evindent at both 850 and 925 mb.

700 Millibar Heights and Equivalent Potential Temperature

 

850 MB Heights (solid black lines) and Equivalent Potential Temperature (yellow dashed lines), from the 00 UTC Upper Air Analysis on Wednesday, June 16, 2004 (8 PM EDT, Tuesday, June 15).

850 Millibar Heights and Equivalent Potential Temperature

 

925 MB Heights (solid black lines) and Equivalent Potential Temperature (yellow dashed lines), from the 00 UTC Upper Air Analysis on Wednesday, June 16, 2004 (8 PM EDT, Tuesday, June 15).

925 Millibar Heights and Equivalent Potential Temperature



Surface Analysis

Surface Analysis at 06Z on Wednesday, June 16, 2004 (2 AM EDT).

 Surface Analysis at 06Z

Moisture Transport

06 UTC ETA Forecast Moisture Transport and Advection at 850 MB for Wednesday, June 16, 2004.
In spite of the heavy rainfall, moisture continued to be replentished over the area allowing for the heavy rain to continue for several hours.

850 Millibar Forecast Moisture Transport

 

07 UTC Surface Theta E Analysis for Wednesday, June 16, 2004.
The persistent transport of moisture results in pool of high Theta-E air along the Virginia - North Carolina border.

07Z Surface Theta E analysis



Sounding Data

Analyzed rawinsonde data for Greensboro, NC  (GSO) and Blacksburg, VA (RNK) for 00Z Wednesday, June 16, 2004 (8 PM EDT, Tuesday, June 15).
The PWAT in the GSO and RNK soundings were 2.38 and 1.87, respectively.  This was 200 percent of normal. 

 Upper Air Sounding at Greensboro

 Upper Air Sounding at Blacksburg


Integrated Precipitable Water Vapor (IPW) from the Blacksburg ground-based GPS station.  Plot is from June 14 through
June 18 2004.   For comparison, the annual normals are in the figure that follows.
The RNK GPS station reported a maximum IPW of 5.0 centimeters between 00 and 02 UTC on June 16th.  This is equivalent to a PWAT of 1.97 inches.

 Integrated Precipitable Water Vapor (IPW) from the Blacksburg ground-based GPS station

1995 to 2000 Blacksburg Surface to 500 Millibar Precipitable Water 



Satellite

IR Imagery from 0015Z to 0845Z, Wednesday, June 16, 2004.
Click thumbnail to start loop.
(File size: 3.7MB)

 Satellite Image

Water Vapor Imagery from 0015Z to 0845Z, Wednesday, June 16, 2004.
(
File size: 2.0MB)

Satellite Image



Radar Imagery

Radar Loops from 0000Z to 0830Z, Wednesday, June 16, 2004.
Click thumbnail to start loop.
(File size: 3MB).

Radar Loop

Radar Cross Section During the height of the Storm. Note the higher reflectivity cores were concentrated in an environment that was warmer than 0 degrees Celcius.

Radar Cross Section


Iflows Summary - Bargraph of 15 minute rainfall for the Marrowbone reservoir in southern Henry County Virginia.
The The Integrated Flood Observing and Warning System (IFLOWS) at the Marrowbone Reservoir captured two back to back periods of very heavy rainfall (Midnight to 1:00 am EDT and 1:30 to 3:00 am EDT). Note the significant rate increase after 1:30 am EDT with upwards of an inch of rain falling every 15 minutes. This coincided with the increase in cold cloud tops in the satellite imagery. Total rainfall in the gage from 9:00 pm through 3:00 am was 10.38 inches, 8.28 inches of that falling between midnight and 3:00 am EDT, and the heaviest burst coming at the end with 3.52 inches between 2:00 and 3:00 am EDT.

Bar Graph Depicting Rainfall Amounts at a Gaging Station in Henry County


Rainfall Analysis

The Areal Mean Basin Estimated Rainfall (AMBER) for both Stuart and Matrimony Creeks, using a ZR relationship of Z=300R^1.4, was about 2 inches above the 6 hour Flash Flood Guidance (FFG).

Basin map showing the distibution of the 6 hour basin average rainfall using Z=300R^1.4 Basin Map Showing 6 Hour Basin Average Rainfall


Gage vs Radar Comparison - This is a plot of the rainfall recorded at the Marrowbone Reservoir IFLOWS gage (1451-G), and the radar estimated rainfall at the 9 associated radar bins. The graph clearly shows that the radar was underestimating by nearly half. Other IFLOWS gages in the area also showed heavier rain than was estimated by the radar.

Line Graph Showing Plot of Rainfall Versus Radar Estimation

Rain Gage Compared to 1 Kilometer Radar Bins

 

ZR Comparison - The tropical ZR estimate was double the convective estimate, and was closer to the actual amount of rain that fell during the event. Forecasters were aware of this at that time and reacted accordingly by issuing the Flash Flood Warnings based on the tropical estimates. For the tropical ZR, the 6 hour basin averages (not shown) for Stuart and Matrimony Creeks were 5 to 7 inches above FFG.

Tropical Z=250R^1.2 <------------------------------------------> Convective Z=300R^1.4
ZR Comparisons


River Hydrographs

Hydrographs for 3 downstream gages.
Click thumbnail to enlarge.

Smith River Hydrograph at Eden North Carolina   Eden North Carolina on the Smith River.

Dan River Hydrograph at Danville Virginia  Danville Virginia on the Dan River.

Dan River Hydrograph at South Boston Virginia  South Boston on the Dan River.

Minor flooding was reported along the Dan River at South Boston where the river crested at 21.4 feet.  This was 2.4 feet above flood stage, and occurred about 30 hours after peak rainfall.

 


 

Case study team -
Phillip Manuel
Mike Gillen
Jan Jackson
Robert Stonefield

For questions regarding this case study, please contact Phillip Manuel or Jan Jackson