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| Supercell Spot Chase |
May 15, 2006 |
BACKGROUND
During the late afternoon on May 15, 2006, a supercell formed SW of Philadelphia in deep moist, cyclonic flow and propagated NE. This storm moved in the wake of another cell, and generated three hail reports (1 in Delaware and 2 in eastern Pennsylvania) before crossing the river into NJ. Based on its trajectory (more or less parallel to US Route 130), it looked like the main updraft would pass very close to Robbinsville, NJ.
Although no mesocyclone was detected by radar, persistent, weak midlevel rotation is seen in the updraft, and the updraft tower itself showed some helical structure. A prominent flanking line and rear flank downdraft combined to make this an interesting chance to observe a marginal NJ supercell. |
AniS animation of thumbnail reflectivity images for the 0.5˚ elevation from KDIX, from 2300Z on 5/15 to 0100Z on 5/16/2006. The two white dots show the stop locations discussed in this log. Full-size animation is linked here. |
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1831 EDT (2231Z)
GOES-EAST image over NE US. Two cells are visible along the NJ-PA border: one just east of Trenton and the other just about cross from DE into PA/NJ. The southernmost cell has a prominent flanking line. A satellite imagery loop is linked here.
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AniS animation of thumbnail radial velocity images for the 0.5˚ elevation from KDIX, from 2300Z on 5/15 to 0100Z on 5/16/2006. Note the strengthening of the RFD gust front as it enters Mercer County, NJ. The two white dots show the stop locations discussed in this log. Full-size animation is linked here. [Note: occasionally the doppler scan appears to cock backwards to the SW, but this appears to be an artifact of the Nexrad Viewer's handling of missing data.] |
| STOP 1 2017 EDT (0017Z 5/16/2006) |
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| After packing a camera, Spencer and I head north on US Route 130 to Gordon Road and head E about 100 yards to a wide area of shoulder. This spot had previously been scouted and is known to provide a decent view to the south and southwest. It is slightly higher than the surrounding terrain, and affords a view over the tops of some wetlands to the south. Sunset was at 0008Z, so it is getting dark. Winds are brisk, and from the ESE when we get out of the car, and remain so until we leave this location. |
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| Above: Looking SW from Gordon Road in Robbinsville, NJ. The twilight silhouettes the main updraft tower and flanking line. The updraft base is just visible over the line of trees that are less than a mile away. Below: tilting up a bit to catch the top of the tower. |
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0017Z KDIX 0.5˚ data (reflectivity and rollover radial velocity image. The labeled white dots show the locations of Stops 1 & 2. The RFD gust front is visible in the velocity data, trailing away SW from the cell.
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| 0017Z Velocity Scans in an animation which varies the Scan Elevation. A persistent rotation at mid-to-upper levels is visible. Click here for a full-size animation. |
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| ~2022 EDT (0022Z 5/16/2006) |
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| Above: By 0022Z, the cell has closed to the point where the front flank is visible, with light from the setting sun just revealing the northern edge of the updraft base. The leaning updraft tower shows signs of weak rotation. To the west and northwest, occasional lightning (intracloud, cloud-cloud, cloud-air, cloud-ground) is noted. No lightning is observed within the updraft base. |
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0022Z KDIX 0.5˚ data (reflectivity and rollover radial velocity image.) The labeled white dots show the locations of Stops 1 & 2. The RFD gust front has strengthened and is very prominent in the velocity data but does not manifest itself in the reflectivity data. A weak mesocyclone is visible in the lowest reflectivity scan data at 0022Z (see below.)
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| 0022Z KDIX 0.5˚ radial velocity image. We are at Stop 1. Click on thumbnail for full-size image. |
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| Above: Rotate camera 90˚ to try and capture the total vertical extent of the tower, and in this shot we are close. From this point on, however, only the fisheye lens will capture the entire storm and for some reason I did not break it out. Below: By 0024Z the tiered base structure is becoming more evident. (This photo is a bit redundant but the color is good and shows the flanking line development.) |
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| ~2026 EDT (0026Z 5/16/2006) |
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| Above: We are able to see beyond the cell's updraft base in the RH side of this image to the fading twilight, and gives the suggestion of a bell-shaped lower base (which will turn out to be a cowcatcher-shaped wall cloud in a few minutes.) With the end of civil twilight fast appraoching (at 0039Z), longer exposure times are needed and the apparent light levels are misleading. Much of the detail in this picture was not apparent to the eye. Below: The development of a new tower on the flanking line suggests that the cell is building to the east as it approaches and about this time I get the feeling we are out of position. There is no way (that I know) to avoid color shifts with the Nikon at these light levels, and correcting for them is more trouble than it's worth, so some of these shots are left as blue-shifted. |
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Above: composite taken at about 0030Z giving an acceptable (not great) view of the cell. Note we can see below the base of the storm and there appears to be little, if any, precipitation falling in this sector of the storm. Below: the component images.
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0026Z KDIX 0.5˚ data (reflectivity and rollover radial velocity image.) The labeled white dots show the locations of Stops 1 & 2. The RFD gust front has strengthened and is very prominent in the velocity data but, again, is not apparent in the reflectivity data. The small, detached reflectivity core on the leading edge of the complex appears to be associated with the tower developing on the flanking line and moving into the main tower but this is a guess based on observations.
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| 0026Z KDIX 0.5˚ radial velocity image. We are at Stop 1. The circulation around the rear flank of the cell, as outlined by the RFD gust front, is very obvious in this scan. Click on thumbnail for full-size image. |
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| ~2030 EDT (0031Z 5/16/2006) |
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| With just a few minutes left before twilight ends (at which point it will be too dark to see), a wall cloud clears the line of trees to our south. At this point there is no doubt we are north of the updraft base and the front flank is approaching. We can see below the base and it looks pretty clear; however, the cloud base is so low we do not have a good view of the wall cloud. |
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0030Z KDIX 0.5˚ data (reflectivity and rollover radial velocity image.) We are at Stop 1, and we really need to be at Stop 2 location or even a bit further to the southwest. Link to Full-Size Reflectivity Scan.
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| The composite image above gives a nicer view than we had on the spot. The last photo is taken at 0033Z and we pack up to head E on Gordon Road to one of two sites which offer good visibility to the west and south. |
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| This image of the new cell building shows just enough detail to be interesting. At the time I was concerned the wall cloud under the updraft base was a shelf cloud and the storm was becoming more linear in nature. Regardless, the wall cloud was south and east of us, so it was time to move closer to the updraft base. |
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0030Z KDIX 0.5˚ radial velocity image. We are at Stop 1, and we really need to be at Stop 2 location or even a bit further to the southwest. A prominent double gust front trails SSW from the cell vertex. Click on thumbnail for full-size image.
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| STOP 2 2039 EDT (0039Z 5/16/2006) |
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The route along Gordon Road is not very open, so we get brief glimpses of the cell as we drive. It looks like we can see a lowering of the wall cloud to our east and south, but we don't get a chance to see anything clearly until we stop.
We get out and the wind is blowing strongly from the west-northwest--apparently we crossed a wind shift line while in transit. (This proved to be the edge of the RFD.)
It is a bit more gusty than at Stop 1. And lot cooler. And a lot darker. |
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0034Z KDIX 0.5˚ data (reflectivity and rollover radial velocity image.) We are enroute to Stop 2, but looking at the velocity data, we missed a chance to see what was going on with the small protrusion of wind shear (for want of a better term) which developed very close to Stop 1. Link to Full-Size Reflectivity Scan.
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| 0034Z KDIX 0.5˚ radial velocity data. The leading edge of the gust front has almost reached our position, and the trailing front has persists. Click on thumbnail for full-size image. |
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2040 EDT (0040Z)
We are out of the car for only a minute--enough time to shoot a few shots using the autoexposure--when we hear the hiss of approaching heavy rain.
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| Looking SW. The lowering cloud base we saw from the van is actually an arc of (apparent) shelf cloud which passed from SW, curved around to our E, and then N; but a nearby farmhouse blocks our view of everything E-N. In retrospect, this cloud arc mirrors the the RFD windshift line seen in the KDIX radial velocity scan at right. |
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| Another view--not much better, but shows the sharp vertical rear boundary of the cloud mass. Not apparent from these pictures is the impression, which the shape of the cloud base gave, of being like the "whale's mouth" clouds sometimes reported. |
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Looking WNW. The cloud arc passes from left-behind us-right, and this view is to its rear, or wake. The sound of rain in the (not so) distance causes me to rush this shot even though I was planning on watching this clearing for a while before deciding it if was interesting--at the time it gave the impression of a clear slot. There was simply no time to do anything but get wet.
Although there appears to be ample light in this last photo, in reality it is dark and gusty at our location,and the cell is moving off into deeper darkness so we call it a day, and Spencer spends the rest of the short drive home drying off my camera. |
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0039Z KDIX 0.5˚ data (reflectivity and rollover radial velocity image.) We are at Stop 2. The gust front is hinted-at in the reflectivity data by the rear flank notch of clear air SW of the cell, which conforms in shape and position to the radial velocity wind shift line.
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| 0039Z KDIX 0.5˚ radial velocity data. The leading edge of the gust front (aloft, at least) appears to have reached our position. The optimal stop would have been halfway between Stops 1 and 2, with a view S-W-N, but a spot offering all those qualities does not exist here. |
NOTES: The sudden shower which cut the trip short was announced by the sound of approaching heavy rain. It was unmistakable, and--accompanied by high winds--hit with remarkable force. The rain came in from the northwest in sheets which drenched us in the few seconds it took to get back into the car. In a minute or two the full force had passed. The shots below detail what little we were able to observe before the weather closed us down.
As one can see from the image at left, there was no sign of rain coming in from the waning sunset.
(Note: The "Stop 2" site was selected specifically for its view W->S, and another, more elevated nearby site is intended for viewing N-SW.) |
