Franklin County Supercell 3-24-12

My son Ben and I had a golf outing planned with my brother Jack at Smith Mountain Lake on Saturday morning.  After seeing some potential for severe weather (especially east of the Blue Ridge) early in the morning,  I packed a camera and iPad in with the golf clubs.  The golf outing unfolded without a hitch, and afterward I saw an isolated cell on radar move into Patrick County just north of the VA/NC border.  With my son running radar and navigation (11 years old) we targeted the cell hoping to intercept it as it moved into Franklin County.  The cell became tornado-warned and driving toward Ferrum we could clearly see the wall cloud SW of town.  The theme of the day:  weak shear at low-levels would limit tornado potential despite fairly strong rotation picked up by Doppler radar.  A photo of the rotating wall cloud southwest of Ferrum:

As we tracked the storm northward (which at times exhibited a beautiful green/aquamarine coloration due to the filtering effects of a very deep cloud layer), it became less organized over Ferrum.  Undergoing a weak RFD occlusion, a new circulation/wall cloud developed north toward Rocky Mount.  Finally securing a good viewing spot in a heavily forested area, we watched the wall cloud move north over U.S. 220 and Rt. 40 toward Smith Mountain Lake.  The wall cloud quickly began to become rain-wrapped (photo below):

Note the rain/hail curtain wrapping around the wall cloud (coming in from the left).  The storm was north of Rt. 40 at this point, nearing SML.

The picture above was taken about 60 seconds after the previous photo, and notice how the rain/hail curtain begins to enshroud the low-hanging wall cloud.  Sixty seconds later all visuals were lost of the lowering as it moved over SML.  The favored severe threat was certainly hail, as the low-level wind field was not supportive of an environment that favored efficient tornadogenesis in storms.  Over the years, our chasers have come to expect much stronger inflow into storms/wall clouds that become tornadic…sometimes amazing wind speeds are experienced as air funnels into the wall cloud.  Not this day.  But the storms were efficient hail producers ( I have yet to look over data from this day, but would imagine a combination of temp/moisture profile, wet-bulb zero height, and updraft rotation all played a role), sometimes piling up inches of hail:

Hail fog along Rt. 40 in Franklin County.  It was along this stretch of road that we did see a van with windows blown out due to hail.

The largest hailstones my son and I gathered were approx. 1.5″ in diameter(quarter shown for reference).

This day was damaging enough for many people in the area;  if instability was higher, and low-level conditions more favorable for tornadoes, it could have been far, far worse.   -Dave


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