Hotz, D, L. Baker Perry, Steve Keighton, 2013: Northwest Flow Snowfall (NWFS) Aspects of Sandy, Part III: Moisture and Trajectory Analysis
in the Southern Appalachians. Extended Abstract, 38th Natl. Wea. Assoc. Annual Meeting, Charleston, SC, 6.3.
6.3
Northwest Flow Snow Aspects of Sandy, Part III:
Moisture and Trajectory Analysis in the Southern Appalachians
DAVID HOTZ
NOAA/National Weather Service, Morristown, Tennessee
L. BAKER PERRY
Appalachian State University, Boone, North Carolina
STEVE KEIGHTON
NOAA/National Weather Service, Blacksburg, Virginia
ABSTRACT
At the end of October 2012, as Hurricane Sandy moved north, the system interacted with a
strong mid-latitude shortwave over the eastern United States, causing the storm to become posttropical and move rapidly to the west on 29 October. The cold air associated with the shortwave
trough combined with the abundant moisture from Sandy to produce historic snowfall in the
Appalachian Mountains from eastern Tennessee into southwestern Pennsylvania. Sandy broke
records for October snowfall. The tight pressure gradients around the storm produced near
blizzard conditions over the higher elevations.
To gain greater insight into how unusual the northwest flow snow (NWFS) associated with
Sandy was compared to more typical NWFS events studied closely in this region, details of the
moisture distribution and atmospheric flow in the lower and mid-troposphere during the storm
were reviewed. The very high moisture content and strong northwest winds observed during
Sandy were found to be well above the climatological norms for a typical NWFS event. The mean
precipitable water during a NWFS event is around 0.762 cm (0.3 inches). However, during Sandy,
precipitable water values ranged from 1.27 to 1.65 cm (0.50 to 0.65 inches).
The trajectory of the air around the circulation of Sandy was also compared to more typical
northwest flow snow events. Initial results suggest that a Great Lakes connection was evident in
the low-level (e.g., 850 hPa) backward trajectories, which has been commonly observed during
NWFS events. However, mid-level (e.g., 700-500 hPa) trajectories had a connection to the North
Atlantic, which is atypical for NWFS events. The implications of these similarities and differences
will be discussed.
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1. Introduction
This study is part of a 4-part series on
discussing the climatology of Northwest Flow
Snowfall (NWFS), and comparing the Sandy
snowstorm to more typical NWFS events.
The objectives of the research are to gain a
greater insight into how unusual the NWFS
associated with Sandy was compared to more
typical NWFS events. A detailed study of the
__________
Corresponding author address: David Hotz, National Weather Service, 5974 Commerce Blvd.,
Morristown, TN 37814
E-mail: david.hotz@noaa.gov
Point B – Poga Mountain, North Carolina with
an elevation of 1150 meters (3773 feet)
Point C – High Knob, Virginia with an
elevation of 1287 meters (4222 feet)
Point D – Snowshoe, West Virginia with an
elevation of 1436 meters (4711 feet)
Point E – Roan Mountain, Tennessee with an
elevation of 1916 meters (6285 feet)
moisture distribution and atmospheric flow in
the lower and mid-troposphere during the
event was compared to more typical NWFS
events. Another focus was to compare the
trajectory and moisture sources with Sandy
and compare to more typical NWFS events.
2. Data collection and methods
Perry (2006) produced a synoptic
climatology from 859 NWFS events. The
study evaluated different synoptic fields
derived from the NCEP reanalysis dataset, and
from Huntington, WV (HTS) rawinsonde data.
We compared the Sandy snowstorm to this
synoptic climatology of NWFS, primarily
focusing on the High Peaks (> 1219 meters)
(Fig. 1), which is area 14 on the map.
Figure 2. The map shows the topography of the central
and southern Appalachian Mountains. The points A
through E show the locations used for this study.
3. Analysis
The deep cyclogenesis associated with
Sandy pulled abundant moisture into the
central and southern Appalachians (Fig. 3).
The NOAA/GOES-13 satellite picture at 1300
UTC 30 October 2012 clearly showed the
warm conveyor belt of moisture from the
Atlantic Ocean wrapping around Sandy. The
circulation around Sandy pulled abundant
moisture into the central and southern
Appalachians for excessive snowfall.
Figure 1. Snow regions used in Perry (2006) NWFS
climatology. The High Peaks (Region 14) climatology,
which is shaded in yellow, will be used for this study.
Since observed soundings do not exist across
the higher elevations, the Rapid Refresh
(RAP) model at 13km was used to construct
the 0-hour (or analysis) for this study. We
gathered moisture data for the following
points across the central and southern
Appalachians (Fig. 2):
Point A – Mount LeConte, Tennessee with an
elevation of 2010 meters (6594 feet)
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relative humidity value was 80 percent or
higher. The graph (Fig. 5) shows that the
thickness of the moist layer during Sandy was
much deeper than typical northwest flow
snowfall events.
Figure 3. NOAA's GOES-13 satellite captured the
image of the storm moving inland at 1300 UTC 30
October 2012.
a. Moisture Anomalies of Sandy Snowstorm
Figure 5. Graph shows typical thickness of the
moisture layer for High Peaks NWFS climatology of
light and heavy events, and Sandy snowstorm range
values, which is shaded in blue.
First looked at the precipitable water or
PWs during Sandy at the 5 observation sites
and compared to climatology. You can
quickly notice that the PWs during Sandy at
all sites were quite a bit higher than the high
peaks climatology for both light and heavy
snows. Typical climatology PW values are
near 1.14 cm (0.45 inches), but during Sandy
PWs were between 1.27 to 1.65 cm (0.5 and
0.65 inches) (Fig. 4).
Since orographic lift is generally the strongest
at 850 hPa, a high moisture value at this level
is important for the development of heavy
snowfall. During the Sandy snowstorm, the
850 hPa mixing ratio was abnormally high
(Fig. 6) contributing to the heavy snows and
low snow-to-liquid ratio content.
Figure 6. Graph showing typical 850 hPa mixing ratio
values for High Peaks NWFS climatology of light and
heavy events, and Sandy snowstorm range values,
which is shaded in blue.
Figure 4. Graph showing typical PW values for High
Peaks NWFS climatology of light and heavy events,
and Sandy snowstorm range values, which is shaded in
blue.
The 850 hPa winds were between 40 and 50
knots across most of the central and southern
Appalachians during the Sandy snowstorm
(Fig. 7). The strong northwest flow continued
from late evening on 29 October through early
31 October. The strong northwest flow
Another way to depict how abnormally deep
the moisture was for the Sandy snowstorm is
to look at the thickness of the moist layer. A
layer was determined to be moist if the
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produced persistent and strong orographic lift
over the west to northwest facing slopes of the
central and southern Appalachians.
Figure 9. The image is a cross-section from the map
(blue line) on Figure 8. The cross-section depicts the
topography from southeast Kentucky to northern South
Carolina.
Even though the wind direction during the
Sandy snowstorm was more westerly than
typical northwest flow events, the much
stronger 850 hPa jet produced strong
orographic forcing over the windward slopes
(Fig. 10).
Figure 7. The 850 hPa height, temperature, dewpoint
and wind at 1100 UTC 30 October 2012.
The map of the southern Appalachians shows
the importance of northwest flow into the
Appalachians (Fig. 8). The northwest flow is
lifted by the terrain producing orographic lift
across the west to northwest facing slopes (Fig.
9).
Figure 10. Graph of typical 850 hPa wind speed values
for High Peaks NWFS climatology of light and heavy
events, and Sandy snowstorm range values, which is
shaded in blue.
During the Sandy snowstorm, temperatures at
850 hPa were slightly warmer than typical
northwest flow events (Fig. 11) which
contributed to the low snow-to-liquid ratio.
Figure 8. The map above shows the topography of the
central and southern Appalachians with the yellow
arrows perpendicular to the terrain.
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Figure 13. The left map is trajectory class 2, which has
no Great Lakes Connection (GLC). The map on the
right is trajectory classes 3.1 and 3.2, which have a
GLC.
Figure 11. Graph of typical 850 hPa temperature for
High Peaks NWFS climatology of light and heavy
events, and Sandy snowstorm range values, which is
shaded in blue.
HYSPLIT backward trajectories were created
at Mount Leconte, Tennessee and Snowshoe,
West Virginia at 2100 UTC 28 October 2012
(Fig. 14). The backward trajectory at 850 hPa,
which is depicted by the red line, showed a
GLC at both locations. The blue line, which
represent 700 hPa backward trajectory,
showed an Atlantic moisture connection into
Snowshoe, West Virginia.
Besides the meteorological factors, an
important contribution to the heavy Sandy
snows was the duration of the event. The
duration of the Sandy snowstorm was
abnormally long compared to typical
northwest flow events (Fig. 12).
Figure 12. Graph of typical snowfall duration for High
Peaks NWFS climatology of light and heavy events,
and Sandy snowfall duration.
Figure 14. The left map is the HYSPLIT backward
trajectory from Mount Leconte, Tennessee, and the
right is the backward trajectory from Snowshoe, West
Virginia at 2100 UTC 28 October 2012.
b. Trajectory Analysis of Sandy Snowstorm
Perry (2006) NWFS research also looked
at the contribution of the Great Lakes
Connection (GLC) to snowfall. Three
backward trajectories were studied.
Trajectory class 2 has no GLC, while classes
3.1 and 3.2 have GLC (Fig. 13). His research
showed that GLC produces higher snowfall
NWFS amounts than compared to no GLC.
At 1000 UTC 30 October 2012, the HYSPLIT
backward trajectories at 850 hPa (red line)
continued to show a GLC at both locations
(Fig. 15). Also, a mid-level Atlantic moisture
feed was evident by the backward trajectory at
500 hPa for Mount Leconte, Tennessee and
Snowshoe, West Virginia.
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 The backward trajectory analysis of
the Sandy snowstorm showed a
prolonged period of the GLC across
the central and southern Appalachians.
 The GLC contributed to the high
moisture content, which enhanced both
snowfall and low snow-to-liquid ratio
content of the snow.
 The backward trajectory analysis also
shows the warm conveyor belt from
the circulation around Sandy at both
the 700 and 500 hPa levels. The warm
conveyor belt pulled deep moisture
into the Appalachians.
 The 700 and 500 hPa trajectories also
showed at least a brief fetch over the
Atlantic which is likely not typical for
a NWFS event.
 Moisture at all levels was much higher
than typical NWFS events. The deep
moisture likely contributed to the
heavy snowfall amounts.
 Even though the boundary layer
temperature (850 hPa) was warmer
than normal, the deep moisture in the
colder mid-levels did allow for
favorable dendritic snow growth.
Figure 15. The left map is the HYSPLIT backward
trajectory from Mount Leconte, Tennessee, and the
right is the backward trajectory from Snowshoe, West
Virginia at 1000 UTC 30 October 2012.
Near the end of the Sandy snowstorm at 1700
UTC 31 October 2012, the HYSPLIT
backward trajectories at 850 hPa (red line)
indicated that the GLC at Mount Leconte,
Tennessee was diminishing, but continued at
Snowshoe, West Virginia (Fig. 16). Also, the
mid-level Atlantic moisture feed was
remained at Snowshoe, West Virginia, but
ended at Mount Leconte, Tennessee.
Acknowledgements. The authors wish to
thank David Gaffin and George Mathews for
their review of the paper and helpful
suggestions.
Figure 16. The left map is the HYSPLIT backward
trajectory from Mount Leconte, Tennessee, and the
right is the backward trajectory from Snowshoe, West
Virginia at 1700 UTC 31 October 2012.
5. Conclusions
The comparison of the Sandy snowstorm
moisture and trajectory analysis to
climatology revealed several interesting
anomalies during the event. They are the
following:
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