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Synoptic
and Mesoscale Overview –
A. The Main Players
A cold front moved
south into eastern North Carolina during the morning hours of March 27,
2009. The boundary then stalled across the area generally between the
Central Outer Banks and Pitt County during the afternoon and evening hours
(Figure 2). Aloft, the area was initially influenced by short wave ridging
within mean SW flow. However, a well defined mid level short wave trough
approached the area late in the afternoon from the SW (Figure 3). Lift
ahead of this feature associated with positive vorticity advection
undoubtedly enhanced vertical motion over eastern North Carolina.
Figure 2. NCEP Unified Surface Analysis
from 00Z March 28 2009 showing slow moving front over eastern North
Carolina.
Figure 3. Water Vapor imagery at 20:45Z
Friday 27 March 2009. NAM 12 (18z run 3/27/09) 500mb heights overlaid in
red and wind vectors in yellow. Dashed black line is mean location of short
wave trough axis.
B. Instability and Shear
Minimal instability
affected the WFO MHX CWA most of the day given widespread cloudiness over
the area. The atmosphere was marginally unstable just offshore near the
Gulf Stream most of the morning and afternoon, but no thunderstorms
developed in this region. An area of light to moderate precipitation
affected much of the northern half of the WFO MHX CWA before 3pm, then
pushed NE over the Northern Outer Banks, areas near the Albemarle sound,
and eventually offshore into the Atlantic. The dense cloudiness thinned in
wake of this precipitation and led to some filtered sunshine over much of
Duplin, Greene, Onslow, Jones, Carteret, and Craven counties late in the
afternoon. This same area was in the warm sector south of the stalled
frontal boundary noted in Figure 2. As a result, surface based lifted
indices diminished from near zero to between -2 to -4 after 20Z (Figure 4).
Surface based CAPE meanwhile increased to 400-600 J/kg (Figure 5). The
available computer model guidance was much more stable at this time frame.
Even though instability
increased, it was still marginal and significant convection was not
anticipated. However, shear parameters were more than sufficient to promote
rotating updrafts. Low level shear (0-1km) increased steadily between
18z-00Z from 15-20 kt to 25-35 kt (Figure 6). LAPS surface helicity values
also increased into the 150-200 m2/s2 range (Figure
5). In addition, LCL values
were below 1500 ft. This environment was favorable for the low topped super
cells and corresponding tornadoes that developed.
Figure 4. Visible Satellite Imagery and
MSAS Lifted Index (pink lines) and mean sea level pressure (yellow lines)
from 2040Z.
Figure 5. LAPS surface CAPE (shaded) and
Helicity (orange countours) from 21Z 27 March 2009.
Figure 6. NAM12 lifted indices and 0-1km
shear vectors from 00Z 28 March 2009.
C. Radar Data
Convective showers
increased in coverage and intensity across eastern South Carolina and SE
North Carolina after 3pm (19Z) due to the features discussed in sections A
and B above. Very little lightning was observed given the marginal
instability across the area, and echo tops generally did not exceed 20-25
kft. However, individual cells occasionally exhibited supercell
characteristics. Pronounced low level couplets associated with the Greene
and Pitt county tornadoes were observed from both the Raleigh and Morehead
City/Newport radars between approximately 2140-2240Z. The strongest
rotational couplets and gate to gate shear was noted at 22Z (Figure 7).
Tornado warnings were issued for both of these storms prior to any damage
reports or tornado touchdown visuals received. It should be noted that
upstream reports of confirmed tornadoes in adjacent CWA’s just prior
to this event were very helpful in the warning process.
Figure 7. 0.5 degree Storm Relative
Velocity from KMHX at 22Z 27 March 2009 showing strong rotational couplets
just west of Hookerton in Greene County and just S of Greenville in Pitt
County.
Damage Survey
The NWS
WFO MHX damage survey team determined that both tornadoes during the
evening hours of March 27, 2009 were EF1's. The tornado that affected
Greene County touched down approximately 2 miles west of Hookerton along
the intersection of Shady Grove Church Road and SR 1436. It moved northeast
for 1.5 miles to near the intersection of Dixon Farm Road and SR 1437. This
was a high end EF1 with winds estimated from damage of 105 to 110 mph.
Several homes received minor to moderate roof damage, pines trees layed
down and snapped, one large hardwood tree down, windows were blown out of
residences, and a light truck was spun around. There were no deaths or
injuries. An interview conducted with a resident of a manufactured home
took refuge in an interior closet during the tornado. His blue truck was
spun around and home was damaged.
The Pitt County Tornado that affected communities near Greenville was
determined to be a low end EF1 with winds estimated from damage of 85 to 90
mph. This tornado struck a more populated area causing minor damage to
approximately 40 homes. Shingles were off roofs, debris blown into roofs,
siding taken off homes, and some small trees were down. This tornado
started out at Dudleys Grant Road, crossed Firetower Road. moving
northeast, and affected a subdivision along Ashcroft Drive. The tornado
damage path was approximately 100 yards wide, and was on the ground for 1/4
to 1/2 mile. Damages were estimated at $200,000. There were no deaths or
injuries.
In
addition to archived radar data, the Google Earth based rotational track
product produced by the National Severe Storms Laboratory (NSSL) in Norman,
OK proved invaluable in the damage review (Figure 8). This and damage
pictures are shown below.
Figure 8. NSSL rotational track product
from March 27, 2009.
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Case Study Team: