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Synoptic
and Mesoscale Overview –
A. Synoptic Discussion
The
upper level pattern was representative of meridional
flow with a highly amplified trough centered across the Eastern CONUS
(Figure 4). The upper level trough deepened as it crossed the Eastern U.S.
At the surface, low pressure developed across the northern Gulf of Mexico
on the 24th then pushed northeast across Florida and into the
western Atlantic Ocean on the 25th (Figure 5). The coastal low
then strengthened down to 993 mb and made its closest approach to Eastern
North Carolina around 10 AM EST on the 26th (Figure 6). The
synoptic setup was favorable for frozen precipitation given the cold airmass aloft combined with ample moisture support from
an intensifying coastal low pressure system.
Figure
5: Hydrometeorological Prediction Center’s
significant surface low tracks issued at 0835Z Friday morning, December 24th.
The actual track was a little closer to the coast which is within the
circle of uncertainty shown at 12Z Sunday.
Figure
6: MSAS mean sea level pressure (green lines) and surface temperature
(orange lines) overlaid with METAR and buoy observations at 15Z (10 AM EST)
on December 26th. Notice the changeover to all snow occurred
across most of eastern North Carolina by this time except for the Outer
Banks.
B. Mesoscale Discussion
As is
often the case with complex coastal low pressure systems, the amount of
storm total snowfall and timing of changing precipitation type depend on
the storm’s track. If a storm passes very close to the coast then the
transition line from rain to snow pushes farther inland. Conversely, if the
storm tracks slightly offshore then the rain/snow line would push closer to
the coast. This storm passed about 100 miles off the coast which was far
enough for every location to eventually change over to all snow as the low
pushed northeast.
For this
event the models performed fairly well several days out suggesting light
precipitation would first begin as rain or a rain/snow mix across the
Coastal Plain counties Christmas night. The GFS model low level thickness
values were forecast to change rain over to snow by 7 AM for western
portions with a rain/snow mix across central and eastern portions (Figure
7). Note that the 1290 m and 1540 m isohypse
contours are oriented southwest to northeast across Duplin, Lenoir, Pitt,
and Martin counties where the greatest snowfall was observed. Frontogenesis
fields are also important forecast tools and often highlight areas of
greater precipitation when low level frontogenesis vectors converge (Figure
8).
Figure
7: 12 hour GFS forecast of low level thicknesses, mean sea level pressure,
and 500 mb height. The top two frames show low level thicknesses of
1000-850 mb (left) and 850-700 mb (right) with critical thicknesses of 1290
and 1540 meters respectively. The bottom two frames illustrate the location
and strength of the surface low (left) and upper level trough (right).
Figure
8: 18 hour GFS 850-500 mb layer frontogenesis and convergence vectors. Note
the vectors converge across the Coastal Plain on a line from Duplin County
northeast to the NC/VA border where locally higher 7 to 10 inch snowfall
was observed.
Observations –
Figure 6
above illustrated the surface observations with most locations having
transitioned to all snow by 10 AM EST on the 26th. A vertical
profile or sounding of temperature and dewpoint
also illustrates the changeover process from west to east (Figure 9). By 10
AM EST the Coastal Plain was below zero from the surface aloft while Cape
Hatteras reported 42 °F. As the low pushed northeast of the region
during the early afternoon the freezing line shifted east (Figure 10). The
snowfall map in Figure 2 illustrates the spatial variation of snow from the
Coastal Plain to the Outer Banks. Please view the Public Information
Statement (Figure 11) for specific snowfall amounts for each county.
Figure 9:
LAPS sounding at 10 Z (5 AM EST) Sunday December 26th. The light
blue line marks 0 °C. Notice central and eastern portions of the region
were above freezing at the surface (green and orange lines) while the
entire column was below freezing in across the Coastal Plain.
Figure
10: LAPS sounding at 20 Z (3 PM EST) Sunday December 26th. The
light blue line marks 0 °C. All locations were at or below freezing by
this time.
Figure
11: At 9:10 PM EST the NWS Newport/Morehead City issued a Public
Information Statement for storm total snowfall amounts across the region.
Notice the variability across the more populated counties of Beaufort,
Carteret, Craven, Duplin, and Pitt counties.
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