3. Climatological Aspects of Freezing Rain and Ice Storms
3.1 Freezing Rain
Results of the freezing rain climatology are shown in Figs. 3.1–3.8. Based on Fig.
3.1, the climatological frequency of freezing rain is relatively high along the spine of the
Appalachians and throughout southeastern Canada. Regional maxima in freezing rain
hours (> 15 h per season) exist across portions of the Blue Ridge Mountains and
Piedmont, the Appalachian Plateau, the New England province (northern Appalachians),
and the Ottawa and St. Lawrence River Valleys. The number of freezing rain hours
decreases notably toward the western slopes of the southern and central Appalachians,
the Great Lakes, and the Atlantic coast. These results are consistent with those of
previous studies, including Bernstein (2000), Robbins and Cortinas (2002), and Cortinas
et al. (2004). As Cortinas (2000) and Rauber et al. (2001b) note, the spatial variability of
freezing rain is strongly influenced by local and regional topography, synoptic weather
patterns, and access to primary moisture sources such as the Gulf of Mexico and the
Atlantic Ocean.
The temporal distribution of freezing rain hours exhibits both seasonal and
interannual variability. Collectively, Figs. 3.2a and 3.2b suggest that freezing rain occurs
more frequently in late fall/early winter (Dec–Jan) than in late winter/early spring (Feb–
Mar). Seasonal differences are especially large across the eastern slopes of the southern
Appalachians, the interior Northeast, and the St. Lawrence River Valley, where nearly
twice as many freezing rain hours are observed during the Dec–Jan period. This disparity
likely results from the fact that the land–ocean temperature contrast is greatest in
December and January. Increased baroclinicity enhances coastal frontogenesis (e.g.,
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Bosart 1975) and establishes a favorable environment for cold surface anticyclones over
eastern North America, which in turn support cold-air reinforcement via processes such
as cold-air damming (e.g., Bell and Bosart 1988) and pressure-driven channeling (e.g.,
Carrera et al. 2009). Time series of seasonal freezing rain hours at individual stations are
also characterized by considerable interannual variability (Fig. 3.3). For example, the
mean and standard deviation (assuming a normal distribution) of seasonal freezing rain
hours at Albany, NY (KALB), are 20.00 h and 12.55 h, respectively. However, the
frequency distribution is negatively skewed such that only a few seasons account for
most of the observed variance (Fig. 3.4).
Figure 3.5 illustrates the spatial distribution of the average number of freezing
rain events per season. Relatively high frequencies of freezing rain events (> 4 events per
season) occur north of a line from eastern Indiana through southern Pennsylvania and
southern New England, with regional maxima (> 6 events per season) in west-central
Pennsylvania, east-central New York, northern New England, and the Ottawa and St.
Lawrence River Valleys. In a previous study, Changnon and Karl (2003) analyzed the
spatial variability of freezing rain days throughout the contiguous U.S. and found similar
results. The geographical distribution in Fig. 3.5 is generally consistent with that in Fig.
3.1, except along the eastern slopes of the southern Appalachians. Despite the
comparatively high number of freezing rain hours, this region experiences relatively few
freezing rain events. This difference not only suggests that the frequency of freezing rain
events depends on latitude (gradual increase from south to north), but also implies that
freezing rain events last longer across the interior southeastern U.S.
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On average, significant freezing rain events occur at least once every two seasons
in the vicinity of the Appalachians (excluding portions of the Allegheny Plateau,
Cumberland Plateau, and Tennessee River Valley), the Ottawa River Valley, and the St.
Lawrence River Valley (Fig. 3.6). Areas particularly vulnerable to significant freezing
rain events include interior North Carolina and southern Virginia, west-central
Pennsylvania, east-central New York, and the Ottawa–St. Lawrence River Valley
intersection. Locally higher frequencies of significant freezing rain events (as well as
total freezing rain events and freezing rain hours) are also observed within the Lehigh
Valley and the eastern New England Upland. The distribution of significant freezing rain
events more closely resembles Fig. 3.1 (freezing rain hours) than Fig. 3.5 (total freezing
rain events), except across northern New England, which experiences comparatively few
significant events. Although the number of total freezing rain events generally increases
with latitude, the proportion of significant events primarily decreases with latitude (Fig.
3.7). Consequently, while freezing rain events are more (less) common in New England
(the Southeast), they are typically less (more) severe in New England (the Southeast).
This inference is further supported by Fig. 3.8, which indicates that freezing rain events
have longer mean durations in the Southeast than in New England.
3.2 Ice Storms
3.2.1 Temporal and Spatial Variability
Results of the ice storm climatology are shown in Figs. 3.9–3.14. For the 1993–
2010 period (17 cool seasons), we identified 137 individual ice storms, which
corresponds to a mean of roughly 8 ice storms per season. The seasonal frequency of ice
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storms exhibits significant interannual variability, with a standard deviation of 3.25, and a
minimum (maximum) of 3 (15) ice storms during the 2001–02 (2002–03) season (Fig
3.9). Ice storms predominantly occur between December and March, and ice storms are
more common during the Dec–Jan period than during the Feb–Mar period (Fig. 3.10). As
one may expect, the temporal distribution of ice storms closely parallels the temporal
distribution of freezing rain hours in Figs. 3.2–3.4.
In order to evaluate ice storm spatial variability, we calculated the number of ice
storms impacting each county during the 1993–2010 period. Ice storms are quite rare near
the Atlantic coast and eastern Great Lakes, but common throughout the interior
Northeast, particularly over elevated terrain, along prominent mountain ranges, and
within protected valleys (Fig. 3.11). On average, many counties within the Appalachian
Plateau and Valley-and-Ridge zones experience more than one ice storm per season.
Other areas highly susceptible to ice storms include the Blue Ridge Mountains, the
Lehigh Valley, the Berkshires, the Lake George–Saratoga region, and eastern Maine.
From Fig. 3.11, we can infer that ice storm occurrence is strongly modulated by synoptic
and mesoscale topographic features, as well as proximity to large bodies of water. A sideby-side comparison of Figs. 3.6 and 3.11 exposes several major differences between the
climatological frequencies of significant freezing rain events and ice storms. For
example, relatively few ice storms were reported across portions of the Buffalo, NY
(BUF), Gray, ME (GYX), and Boston, MA (BOX) CWAs, despite most areas averaging
nearly one significant freezing rain event per season. On the contrary, the highest ice
storm frequencies were observed in sections of the Pittsburgh, PA (PBZ) and
Philadelphia, PA (PHI) CWAs, where, based on Fig. 3.7, less than one significant
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freezing rain event occurs per season. These differences likely result from population
density biases, manual observation errors, and inconsistencies regarding how WFOs
archive significant weather events, all of which reduce the reliability of NCDC Storm
Data. The absence of stations with long-term surface data in locally favored ice storm
areas also contributes to the observed discrepancies between ice storms and significant
freezing rain events.
3.2.2 Characteristics
Figure 3.12 illustrates the relative frequency distribution of ice storms by spatial
coverage. An overwhelming majority (81.8%) of ice storms were classified as local,
regional, or subsynoptic events, whereas only 18.2% of ice storms qualified as synoptic
events. Moreover, Fig. 3.13 reveals an inverse relationship between the number of ice
storms and the number of CWAs impacted. Approximately 69.3% (95) of all ice storms
impacted three or fewer CWAs, while only 16.8% (23) impacted more than six CWAs.
Although these results imply that ice storms are predominantly mesoscale phenomena,
ice storms typically occur within preferred synoptic-scale environments, and thus we
cannot ignore the importance of synoptic–mesoscale linkages. One intriguing feature of
Fig. 3.13 is the distinct second peak in the frequency distribution for ice storms impacting
seven CWAs, which likely represents a clustering of synoptic events.
As Fig. 3.14 indicates, the synoptic weather pattern most commonly associated
with ice storms in the northeastern U.S. was Type G (65 events), followed by Type BC
(30 events), and Type EF (25 events). Again, Type BC events occur on the cold side of a
warm/stationary front or within the occluded region of a surface cyclone, whereas Type
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EF and Type G events are associated with cold-air damming along and east of the
Appalachians. Note that Type EF events are associated with a surface trough or cyclone
along the Atlantic coast and a dominant anticyclone to the north, but Type G events are
associated with a surface cyclone approaching the Appalachians or Great Lakes from the
west or southwest. Type G events comprised nearly half of all ice storms (47.4%), while
Type BC and Type EF events comprised 21.9% and 18.2% of all ice storms, respectively.
Together, these three event types accounted for roughly 87.6% (120) of all ice storms. Of
the remaining 12.4% (17), several ice storms were associated with Type A and Type D
patterns, but most were unclassifiable due to ambiguous synoptic weather patterns.
Northern portions of the ice storm domain (upstate New York and New England) were
primarily impacted by Type BC and Type G events, while the southernmost areas (West
Virginia and northern Virginia) were almost exclusively affected by Type EF and Type G
events. If we only consider the subset of synoptic ice storms (N=25), the frequency
distribution is clearly dominated by Type G events (16; 64.0%). Furthermore, Type G
events accounted for nine of the 14 ice storms (64.3%) impacting seven CWAs.
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Figure 3.1: Average number of freezing rain hours per season (contoured every 5 h).
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(a)
(b)
Figure 3.2: Average number of freezing rain hours during: (a) the Dec–Jan period and
(b) the Feb–Mar period (contoured every 5 h).
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Figure 3.3: Time series of seasonal freezing rain hours at KALB.
Figure 3.4: Frequency distribution of seasonal freezing rain hours at KALB.
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Figure 3.5: Average number of freezing rain events per season.
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Figure 3.6: Average number of significant freezing rain events per season.
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Figure 3.7: Percentage of total freezing rain events qualifying as significant (contoured
every 10%).
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Figure 3.8: Mean freezing rain event duration (contoured every h).
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N = 137
Figure 3.9: Number of ice storms observed during each season.
N = 137
Figure 3.10: Monthly distribution of ice storms during the 1993–2010 period.
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Figure 3.11: Number of ice storms impacting each county during the 1993–2010 period.
N = 137
Figure 3.12: Frequency distribution of ice storms by spatial coverage. Values in
parentheses indicate the corresponding number of ice storms.
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N = 137
Figure 3.13: Number of ice storms as a function of the number of CWAs impacted.
NTOT = 137
NSYN = 25
Figure 3.14: Distribution of total ice storms (blue) and synoptic ice storms (red) by event
type.
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