A review of the 0.5 degree regional radar mosaic indicates that an outflow boundary
originated from a large area of thunderstorm activity over south central Virginia
between 03-04Z. While hard to discern in composite reflective due to a high amount
of ground clutter and anomalous propagation (AP), the boundary is observed traveling
toward the southwest during this time with isolated to scattered shower activity
developing along this boundary as it moved toward the southwest. As the outflow
boundary entered into Rockingham and Guilford Counties, it interacted with an area
of scattered showers, likely initiated by the encroaching upper level jet dynamics.
This shower activity quickly organized itself into deep convection with the passage
of the outflow, first occurring over Rockingham County around 0530Z, developing
downstream in NW Guilford County shortly thereafter. Little movement was exhibited
by any of the convection; therefore these storms were very heavy rain producers.
By 0630Z, approximately an hour after convective enhancement began, Doppler radar
estimates from both KRAX and KFCX showed around 2 inches in NW Guilford County.
Just after 07Z, the large thunderstorm that had been observed in Rockingham County
collapsed. It appears the cold pool generated by the storm collapse helped to
further enhance the convection over extreme NW Guilford County. With little to no
movement noted with this activity, Doppler radar estimates quickly climbed to 10-12
inches by 0830Z, just under 3 hours from the beginning of the rainfall accumulation
across the area.
The excessive rainfall and the resultant flash flooding appeared to be caused by a
combination of factors. Some of the primary factors include...
1. A strong upper level jet (for this time of the year) crossed the area with the
Northwest Piedmont being in the favored area for upper level divergence (left exit
region of the jet). A mid level cold pool was also noted along with the strong upper
level jet. At 00Z, the Greensboro, NC (GSO) upper air sounding showed a mid level
warm nose (cap) around 600mb. A look at the upstream sounding from Roanoke, VA
at 00Z does not show such a cap. By 12Z, the cap at the GSO had been eroded as the mid level cold
pool moved through the area. This further enhanced the convective potential.
2. An outflow boundary from earlier convection in south central Virginia, which was
seen on the looping 0.5 degree mosaic reflectivity, raced southwestward and interacted
with the aforementioned upper level jet and mid level cold pool.
3. Weak steering flow in the
850 to 500 MB layer of the atmosphere was present from
00Z sounding data and model sounding data (averaging 5 kts). The weak steering flow
allowed the thunderstorms in the area to pulse up and down and continue to fuel each
other through a series of outflow boundaries each storm produced as they collapsed.
4. Precipitable water (PW) values based from the GSO soundings showed a moist
atmosphere. The PW at 00Z was
1.57” (117% of normal) while the 12Z PWAT value
climbed to 1.72” (129%) of normal. This suggested that any convective activity would
be capable of producing heavy rainfall.
5. 0-6 km Bulk Shear was 20 kts. This
indicates that there was not
sufficient wind shear to sustain organized convection. Instead, the atmosphere was
more favorable for pulse thunderstorm development, which greatly enhances the
potential for storm collapses, resultant outflow boundaries, and cold pool interactions.
Sometimes isolated summertime flash flood events can be caused by low echo centroid
storms. These storms exhibit most if not at all their reflectivity below the freezing
level. In such a case, warm rain processes dominate and there is little to no
lightning/hail production of the storm. These types of storms can appear very
innocuous on infrared satellite. The storms that occurred in the early morning of
July 30th do not appear to follow the description of a low echo centroid storm.
Cross sections of reflectivity data show that the updrafts of the storms reaching well
above the freezing layer with echo tops exceeding 40,000 feet. Over 50 cloud to
ground lightning strikes were produced by these storms.
In summary, the reason why only this small area was affected was essentially timing
and placement. The favored left exit region of the strong upper level jet was moving
into the affected area at the same time as the outflow boundary was moving into the
area. These two features coming together, coupled with a weak steering flow layer, a
moist atmosphere, and further cold pool interactions, resulted in flash flooding for
Northwest Guilford County.