Key Factors that Came Together to Enhance the Precipitation across Central North Carolina
Based on research conducted by Croke/NCSU
(Tropical Cyclone Landfall and Precipitation), several key factors came
together to enhance the precipitation across central North Carolina
on June 14th.
- A Very Moist Airmass - Precipitable water
values across the region were near to slightly above 2 inches
(
GSO RAOB |
MHX RAOB).
Model moisture transport vectors indicated strong moisture advection
into the region from coastal sections of Georgia and South Carolina.
- Low Level Focus - A weak surface cold front crossed the region
Monday night then stalled across South Carolina on Tuesday, the 13th.
Northeast flow behind the boundary advected cooler drier air into the
region as dewpoints midday Tuesday were in the mid and upper 50s.
As the remnants of Alberto moved north northeast across southern
Georgia late Tuesday afternoon and Tuesday night, the winds just
above surface veered to a more south southeast direction, pulling
moisture over the surface cool dome. This caused patches of light
rain to break out across central North Carolina Tuesday evening.
This patchy precipitation helped to establish a limited area of
cold air damming across most of the Piedmont. The development of
the cold air damming aided in the formation of an inverted surface
trough across the coastal plain of central North Carolina by 0600
UTC/14th with northeast winds across the Piedmont and east to southeast winds across the
coastal region. The heavy rain axis coincided along and just north of
this boundary. Finally, there was significant speed convergence
over central North Carolina at 850 and 700 MB. Winds of 35-45 knots
were across South Carolina and southeast North Carolina while winds
across central North Carolina were 10-15 knots.
- Upper Level Support - In the upper levels, a distinct mid
level trough
was crossing the Ohio River valley, headed toward the central and
northern Appalachians. This feature likely increased the potential
vorticity over central North Carolina. At 250 MB, a jet maxima of
100-105 knots was positioned across New England, placing central
North Carolina in the favorable right entrance region. The
approaching mid level trough and the jet max produced strong upper
level divergence over the area.
Another feature, not well depicted by the models, but detected in
the 500 and 300 MB upper air subjective analyses, 12Z on June 13, was a
weak short wave diving south along the southern Mississippi River
valley. It was hypothesized, though not documented in external
discussions, that such a feature could possibly pull the tropical
low slightly westward of the forecast track. It does appear that
the surface low center did indeed move left of the 24 hour forecast
track position for 12-21Z on June 14 across interior NC.
- Tropical Cyclone Precipitation Distribution -
In the preliminary forecast mode using numerical model output and
latest NHC/TPC forecast track positions for Alberto, product bundles
from the Tropical Procedure on AWIPS were used to estimate the
likely area of heaviest rainfall associated with the storm track.
Deep layer shear, 850-300 MB, showed the dominant shear vector
pattern along or in the direction of the path of the storm suggesting
the heaviest rainfall would likely occur along the track of the
low pressure center. Divergence aloft, 250 MB, showed a maximum
of divergence to the north of the storm track. Given a predicted
storm track running from near Dillon, SC at 12z/14 to just east
of Virginia Beach, VA at 00z/15, it appeared the heaviest rain
based on these parameters would occur just to the east of US 1
or from Laurinburg to Raleigh to Roanoke Rapids. It was also
noted that heaviest rainfall within this zone would likely
occur near the stalled front or boundary between cooler, drier
air to the northwest and subtropical warm, humid air to the
southeast. This was thought to most likely occur in the northeast
Sandhills and central Coastal Plain, roughly from Fayetteville to
Wilson.
As shown in the HPC analysis below, the heaviest rain fell along and left of the storm track across North Carolina.
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