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Synoptic and Mesoscale Overview –
Precursor:
Saturday through Sunday, January 8th-9th, 2011…
On Saturday January 8th, 2011 a cold front crossed the
Carolina’s with Canadian High Pressure beginning to build in from the
NW (Figure 3). On the evening of January 9th, 2011 the cold High
Pressure over the Great Lakes region built south across much of the
Mid-Atlantic and southeast with cyclone formation off the Texas coast
(Figure 4). Although the surface low was weakening as it migrated eastward
along the Gulf Coast region, the mid and upper level pattern (with a
negatively tilted trough across the central US) was able to provide
sufficient lift north of the system (Figure 5 and Figure 6). The mid and
upper level divergence observed across AL, MS and GA late Sunday night and
early Monday morning coincided well with the observed wintery precipitation
(Figure 7). In addition, with cold high pressure in place across the south
and moist southerly flow moving north ahead of the surface low, a winter
storm started to take shape for much of the southeast. While snow was
falling in many locations, parts of Mississippi and Alabama were
experiencing icy conditions late Sunday evening. Near the surface, before the storm
had approached the Carolina’s, the low levels had a sufficient
temperature gradient between the southern states (AL, MS, LS and GA) and
the Carolina’s with a fairly deep layer of CAA keeping temperatures
at or below freezing (Figure 8). This layer can also be seen in the 00Z
RAOB at MHX (Figure 9). Also, based on this profile precipitable water was
less than a quarter of an inch.
Apart from this limitation, the temperature and upper atmospheric
dynamics were suitable for winter weather development. However, the main questions plaguing
forecasters at this time were the onset of precipitation (low level
saturation) and low level cold pool stabilization (blocking high
pressure).
Figure 3. A cold front crossed the East Coast on Saturday leaving a
residual frontal boundary draped across the Gulf of Mexico Saturday evening
(00Z Sunday). This stationary
boundary combined with mid/upper level forcing helped to promote
cyclogenesis along the Gulf Coast region Sunday.
Figure 4. A strong 1030mb High Pressure across the Great Lakes region
builds down across the southeast Sunday afternoon with a cyclone developing
off the Texas coast.
Figure 5. Pressure continues to build across the eastern CONUS by Sunday
evening with a 1032mb High dropping south into the Carolina’s. The weakening low pressure (now
1008mb) along the Gulf Coast continues to migrate eastward. Note the large dew point depressions
across the Carolina’s.
Figure 6. The deep southern stream jet with a negatively tilted trough
draped from the NW to the Gulf Coast region continued to pivot to the east
with the left entrance of the jet collocated with the surface cyclone. The
majority of the upper level divergence was well correlated with the
precipitation shield.
Figure 7. National Radar Mosaic Sunday overnight period (07Z Monday)
with northern portions of AL, MS and Georgia receiving several inches of
snow and ice. Much of the
moisture observed across the Carolina’s was not reaching the ground
due to lack of upper level forcing and low level dry air.
Figure 8. At the low levels, there is an impressive temperature gradient
between the southern states and the Carolina’s thanks in part to the
deep layer of northerly winds still in place across much of the
Mid-Atlantic region. Surface
high pressure also influenced the CAA in place across eastern NC.
Figure 9. Based on the atmospheric sounding taken on Sunday night (00Z
Monday), the mid and lower levels were dry with plenty of cold air in
place. While the dry air was a
limitation (precipitable waters around 0.16”), evaporative cooling
helped lower temperatures as the atmosphere moistened with time.
Snow Event:
Monday through Early Tuesday, January 10th-11th,
2011…
The unusual nature of this storm resided in both the
northerly advection of precipitation ahead of the surface low and the zonal
nature of the mid level flow (Figure 10). This made forecasting even more
challenging as past event analogs were not able to be utilized.
The first reflectivities moved into the region early
Monday morning (Figure 11).
However, near surface conditions were fairly dry due to the strong
1030mb high pressure across the region (Figure 12). By 12Z, the upper level jet was
across the Carolina’s with the left entrance region collocated with
the 1010mb surface low offshore of the Florida Panhandle (Figure 13 and
Figure 14). With heavier bands developing inland, low level saturation
started to become realized across much of South Carolina and southern
NC. A heavier band of
reflectivity developed from West to East in the southern portion of the
County Warning Area with heavier precipitation rates occurring at or just
above the surface allowing low level saturation to increase more rapidly
(Figure 15). This banding was
predominately fueled by mid level vorticity (Figure 16). Snow inland and a
wintery mix along the immediate coast quickly developed Monday
morning. Surprisingly, no
frontgenesis was noted, at least based on model output. The orientation of
the snow band helped to promote higher precipitation rates while ultimately
impacting the snowfall accumulations across Duplin, Onslow, Jones and
Carteret Counties. Locations
north of HWY-70 did not receive enough low level forcing and moisture
initially to drown out the dry air in place near the surface (Figure 17). Therefore, it wasn’t until the
late afternoon and evening that those locations received mixed
precipitation. Higher
reflectivities started propagating north of HWY-70 by Monday late afternoon
helping to saturate the lower levels (Figure 18). However, by the time the
northern CWA became saturated enough to generate precipitation to the
ground, the majority dynamic forcing needed to generate precipitation had
diminished and moved offshore.
Monday evening, the surface low pressure formed off Georgia coast
(Friday 19). Based on the 00Z RAOB sounding on Monday evening, the dendrite
zone became unsaturated and the mid to lower levels were beginning to warm above
freezing (Figure 20). This
atmospheric profile is indicative of wintery precipitation transitioning to
freezing rain. Any
precipitation that did fall late Monday evening through early Tuesday was a
wintery mix across much of the area (especially inland), with mainly
freezing rain along the coast.
Through the overnight hours much of the dynamic forcing needed to
continue precipitation began to diminish in addition to upper level dry air
intrusion. The surface low continued to move NE along the eastern seaboard
eventually phasing with a potent upper level system which wreaked havoc in
New England (Figure 21)
Figure 10. The mid level flow became more zonal across the
Carolina’s by early Monday morning (12Z). The once amplified trough across
central US became deamplified allowing the surface low to continue to
weaken.
Figure 11. Composite reflectivity early Monday morning (09Z) with no
reports of snow across most of NC and SC. However note the higher
reflectivities moving across southeastern NC.
Figure 12. Surface high pressure building in across eastern NC Monday
morning (12Z) keeps temperature low with dry conditions observed near the
surface. Snow is falling across SC where a weaker high pressure gradient is
in place.
Figure 13. Upper level jet stream moving NE has limited directional
divergence. However, speed
divergence is observed across Georgia and the Carolina’s with the
left entrance region collocated with the surface low.
Figure 14. A surface low continues to move eastward south of the Florida
Panhandle.
Figure 15. Composite reflectivity Monday morning (13Z) illustrates
banding across SE NC. With
heavier precipitation, low levels became saturated and snow was falling
across most of Wilmington’s CWA.
Figure 16. The 00Z GFS Sunday evening illustrates precipitation banding
across southeastern NC with vorticity maxima (850mb-700mb layer) collocated
with the heavier precipitation.
Figure 17. Surface analysis illustrates strong high pressure in control
over much of eastern NC with dew very low dew points across central and
northeast NC.
Figure 18. Composite reflectivity Monday afternoon (22Z) illustrates
heavier precipitation moving north helping to saturate the lower levels
across northeastern NC.
Figure 19. Surface analysis of the surface low moving off the SE coast
with high pressure eroding along the Eastern CONUS.
Figure 20. 00Z Monday RAOB sounding at MHX suggests mid to upper level
drying of the dendrite zone (most commonly referred to as the -10- -20
degree C layer). With warming
in the mid levels as illustrated with warming above freezing between
925mb-800mb and below freezing conditions near the surface, sleet and
freezing rain will develop.
Figure 21. Surface analysis shows the surface low off the Carolina coast
early Tuesday morning with precipitation inland and to the north.
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