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Synoptic
and Mesoscale Overview –
A cold front approached from the west slowly during the late afternoon
and evening hours of July 17, 2009,
as a well defined mid to upper level short wave passed north of the region.
Ahead of the front a trough developed over central North
Carolina with very unstable air across eastern North
Carolina as temperatures climbed into the upper
80s and lower 90s with dewpoints well into the 70s. MSAS and SPC’s
meso-analysis page showed surface based CAPE values
greater than 3000 j/kg over the region and lifted indices between -5 to -7 C (Figure 2). Instability was
also enhanced over the MHX CWA due to a
cloud free region over the area during peak heating hours.
Figure 2. Visible satellite imagery with
MSAS mean sea level pressures (mb in yellow) and lifted indices (C in
pink).
The mid level short wave passing north of the region helped increase
mid level winds resulting in 0-6km shear values in the 30 to 35 knot range
(Figure 3), in addition to providing lift through positive vorticity
advection. The combination of strong instability and moderate shear was
favorable for storm organization and SPC’s
Super-cell Composite Index showed values greater than 8 over the region.
Additionally, low level 0-1km shear increased to 20-25kts (Figure 4) while
lifting condensation level (LCL) heights were at or below 1000m. These
ingredients were favorable for the tornadogenesis that occurred over the
area. Eastern North Carolina was also near the right
front entrance region of a 250mb jet which enhanced upper level divergence
over the area (Figure 5). This also supported the deep convection over the
area.
Figure 3. NAM12 0-6km bulk shear
vectors.
Figure 4. NAM12 0-1km shear vectors.
Figure 5.
NAM12 250mb winds and divergence (shaded). Green and yellow colors are
divergent and blue values are convergent. Note the upper level divergence
maxima near little Washington.
RADAR and
Warning Overview –
Scattered thunderstorms began to develop inland from the
coast during the mid afternoon hours as along the sea breeze/resultant
boundary near Highway 17. One small storm with a decent 55-65dbz core
developed over central Onslow county and blew the top off a pine tree NE of
Jacksonville. This was the only missed event of the day. Other
thunderstorms began to intensify in Beaufort
County and one developed
supercell characteristics with a fat hook or kidney bean shape with some
decent low level rotation. A tornado warning was issued for this storm
given the above mentioned environment and due to its radar characteristics.
This storm produced a weak tornado east of Washington
where some minor damage was reported.
A more organized broken line of storms along a
prefrontal trough then began to move ESE
into the region early in the evening. As this line of storms moved into the
northern coastal plain they merged with a few weaker cells from the
previous convection. The merger helped increase the intensity of the storms
and the one in Pitt County
began to show some indications of strong winds with good low level velocity
and a strong leading edge reflectivity gradient. A severe thunderstorm
warning was issued for this cell and eventually a report was received of
windows blown out of a home east of Greenville.
As the above mentioned cell moved across eastern Pitt
and into extreme western Beaufort
County (Figure 6) some low
level rotation began to develop along the leading edge of the low level
gust front. Initially these rotational couplets were weak and we held off
on issuing a tornado warning. Eventually the couplets strengthened a bit
(Figure 7) and were also moving east into lower LCL heights. Based on this,
a tornado warning was immediately issued.
Figure 6. 0.5 degree reflectivity from 8:03pm EDT as seen
from KMHX WSR-88D.
Figure 7. 0.5 degree storm relative
velocity from 8:03pm EDT as seen from KMHX WSR-88D.
Shaded circle represents rotational couplet responsible for EF-0 tornado 5
miles SE of Washington.
Damage Survey and Reports
The July 17,
2009 severe
weather outbreak led to 2 confirmed tornadoes and 5 reports of damaging
winds in the WFO MHX CWA (Figure 8). National Weather
Service officials in conjunction with local emergency management performed
a detailed damage survey for the tornado that impacted the far eastern end
of River
Road
in Beaufort County. Damage began 282 feet down Swan Point Road from the intersection of River Road. The tornado tracked east for
about a mile before hitting the river. Several witnesses reported seeing a
waterspout there briefly before it lifted into the clouds. The worst damage
from the tornado was to trees, which took down long spans of electrical
wires and several poles. There was also minor damage to approximately 25
homes from falling trees and limbs. Damage estimates are about $58,000 in
total to the houses.
Figure 8. SPC storm report for July 17, 2009.
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