IV. Discussion
Section 4.1 discusses the results of the case studies individually presented in
sections 3.2 and 3.3. Section 4.2 provides a synthesis of this work and how it
complements prior work completed on precipitation distributions over the Northeast
accompanying landfalling and transitioning tropical cyclones.
4.1 Case Study Discussion
4.1.1 Hurricane Bob (1991)
Precipitation analyses obtained from various sources (i.e., UPD and NWS plots)
during the northward propagation of Hurricane Bob along the east coast of the US
exhibit relative agreement in the spatial distribution of precipitation. However,
discrepancies arise when considering the maximum precipitation accumulations, with
total amounts significantly underestimated by the UPD (Fig. 3.1) in comparison to the
actual amounts constructed from NCDC surface archives (Fig. 3.3). The gridded nature
of the UPD and the smoothing processes that are employed to obtain acceptable results
lead to gross underestimation of mesoscale precipitation signatures related to coastal
fronts and orography. In general these analyses indicate that the axis of heaviest
precipitation stretches northeastward from eastern Long Island, New York, into central
Massachusetts and southern New Hampshire, but a critical absence of precipitation in
the UPD is observed in southeastern coastal areas of Maine where NCDC archives
indicate almost twice the amount of total precipitation.
The examination of daily precipitation accumulations combined with synoptic
analyses provides insight into the cause of the extremely heavy amounts of precipitation
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observed over the southern and eastern New England region well in advance of the
passage of the center of circulation (Fig. 3a). The middle- and upper-tropospheric maps
indicate a persistent and significantly amplified ridge over the western Atlantic Ocean
that contains an area of upper-level divergence associated with the vertical branch of an
implied thermally direct ageostrophic circulation about the equatorward entrance region
of the jet maximum. Ridge building produced by inferred diabatically induced upperlevel outflow associated with convection accompanying the hurricane has modified the
upper-level flow pattern, such that northward amplification of the geopotential height
pattern has caused an enhanced gradient between the western Atlantic ridge and the
trough to the north and west, producing more pronounced confluence and upper-level jet
winds. The jet maximum becomes less mobile as a result of the diabatically induced
enhancement, thereby anchoring the thermally direct circulation over areas of the
Northeast experiencing heavy precipitation (Figs. 3.4b,c). The interaction of the tropical
cyclone with a middle-tropospheric trough has resulted in the preferential left-of-track
shift in the precipitation distribution (Fig. 3.1). The ridging experienced in the upper
levels also extends to the lower levels, and has resulted in an enhanced southerly lowlevel jet occurring in response to a strengthening geopotential height gradient between
the ridge and tropical cyclone. The enhanced low-level flow is providing a supply of
warm, moist tropical air well in advance of the hurricane, extending into the New
England region. The juxtaposition of the vertical branch of the implied thermally direct
circulation and the transport of high θe air over New England have provided conditions
conducive for extremely heavy precipitation production by way of large-scale dynamic
processes.
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Daily precipitation totals near landfall exhibit a broad precipitation shield that
encompasses much of the northeast region and possesses less pronounced accumulations
than the previous 24 h (Fig. 3b). Propagation of the tropical cyclone to coastal locations
results in a gradual transition to forcing functions more commonly observed in
midlatitude systems, including, an upward increase in CVA and an increasing Laplacian
of WAA resulting from the interaction of the tropical cyclone with a remnant baroclinic
zone. The juxtaposition of CVA, WAA, and stationary jet dynamics over northern New
England leads to a broad area of upward motion by way of large-scale forcing for ascent.
A reorientation of the low-level jet has resulted from the interaction of the tropical
cyclone and the building ridge, manifested as an eastward bulge in the geopotential
height field along the western periphery of the ridge. This interaction produces a
pronounced southwest–northeast oriented low-level jet, which acts to advect the main
axis of tropical air east of the most vigorous areas of dynamical forcing located over
New England. Even though the maximum θe air has been displaced northeastward, a
component of the low-level jet wrapping around the north side of the system maintains
easterly flow, thereby supplying air of tropical origin to areas most conducive to vertical
motion and heavy precipitation.
Surface observations before and after landfall show a short-lived subtle coastal
front that has developed in situ across eastern Connecticut and southern Massachusetts
as a result of the intersection of cyclone-induced inland northerly flow and easterly
onshore flow provided by the low-level jet, the presence of which is further substantiated
by inspection of θ and θe gradients. Northeastward propagation and dissipation of the
coastal front occurs east and south of Boston, Massachusetts, as the northward
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progression of the tropical cyclone continues. The positioning of the low-level jet in
relation to the northeastward propagating tropical cyclone is such that the maintenance
of enhanced easterly flow in the northern quadrant of the cyclone is not sufficient to
produce a coastal front of extended duration. As a result, small-scale areas of
precipitation enhancement near the location of the coastal front are evident, but are not
dominant in the overall precipitation distribution.
Overall, the heaviest amounts of precipitation occurred in advance of the landfall
of Bob in association with the juxtaposition of moisture transport by the enhanced lowlevel flow and vigorous vertical motion induced by an implied thermally direct
ageostrophic circulation. Forcing for ascent by large-scale midlatitude processes
becomes more dominant close to the period of initial landfall, with coastal frontogenesis
producing a minor impact on the overall precipitation distribution. Due to incomplete
surface observations over northern New England, it is difficult to diagnose the exact
cause of the precipitation signatures in the mountains of northern New Hampshire and
Maine during the latter half of the event; however, cyclone-induced easterly flow
directed against the windward slopes of the higher terrain could sufficiently explain the
observed accumulations.
4.1.2 Hurricane Gloria (1985)
Maximum accumulations and spatial distributions of precipitation during the
northward propagation of Hurricane Gloria along the east coast of the US exhibit relative
agreement along a broad axis that stretches from eastern Virginia into central New York,
with embedded heavier areas extending from Virginia into eastern Pennsylvania. The
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middle- and upper-level structures investigated during the event show substantial
amplification of a large trough encompassing the entire northern tier of the US, and a
significant ridge over the western Atlantic Ocean. A rather potent and exceptionally
strong jet maximum located over the Great Lakes region strengthens as inferred upperlevel diabatic outflow associated with convection from Gloria acts to produce western
Atlantic ridging, thereby increasing the geopotential height gradient between the ridge
and the trough to the west. These large-scale flow adjustments have caused significant
intensification of the jet winds, while simultaneously slowing the propagation of the jet
streak. The positioning of the intensified jet streak, coupled with its lethargic
movement, have produced an area conducive to heavy precipitation over the MidAtlantic region by way of upward motion through the vertical branch of an implied
thermally direct ageostrophic circulation in the equatorward entrance region of this jet
streak. Interaction of the tropical cyclone with the approaching midlatitude trough to the
west has resulted in increased forcing for ascent due to an upward increase in CVA
along the Mid-Atlantic coast, and a pronounced left-of-track shift in the precipitation
distribution (Fig. 3.14). The interaction with the approaching trough has also led to an
increase in southeasterly flow over the baroclinic zone, thereby developing forcing for
ascent due to the Laplacian of WAA over northern locations of New England. This
WAA occurs late in the evolution of the event, with the vast majority of forcing
occurring north of the region and out of the domain of this research.
Ridging observed in the upper troposphere also extends to the lower troposphere,
causing a significantly enhanced geopotential height gradient between the ridge and the
tropical cyclone, thereby producing intensification of the low-level jet. The strength of
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the western Atlantic ridge is sufficient to maintain the orientation of the low-level jet so
that a strong easterly onshore flow around the north side of the tropical cyclone can be
maintained along coastal locations of the Mid-Atlantic for an extended period of time.
The strong onshore flow created by the enhanced geopotential height gradient has also
created a strong flux of high θe air into the region of vigorous upward motion created by
jet dynamics. The jet dynamics and the flux of warm, moist tropical air over the MidAtlantic region are collocated with the north–south oriented shield of heaviest
precipitation. Surface observations within this dynamically favored region indicate that
the intersection of enhanced onshore flow and cyclone-induced inland northerly flow has
led to the formation of a coastal front in situ that stretches from eastern Virginia to
northern Pennsylvania. This coastal front is manifested as a rather pronounced feature in
the standard surface plots and also is apparent in θ and θe gradients. The strength and
orientation of the flow around the northern side of the tropical cyclone is maintained by
the reinforced western Atlantic ridge, and provides conditions favorable for a coastal
front of extended duration (~12 h). The position of the long-lived coastal front
corresponds closely to the axis of heaviest precipitation, and appears to be the dominant
feature contributing to the focusing of the most extreme accumulations.
The continued northward propagation of Gloria into interior sections of New
England results in the dissipation of the coastal front near the Pocono Mountains. A
slightly weaker enhanced low-level easterly flow around the north side of the cyclone
continues throughout the event without the maintenance of the inland convergent
boundary. The inland track of the cyclone produces prolonged low-level easterly flow
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perpendicular to topographic barriers and enhanced precipitation in southern New York,
Massachusetts, Vermont and New Hampshire (Fig. 3.15c).
Overall, the heavy precipitation resulting from Hurricane Gloria occurred on
rather short time scales. The juxtaposition of jet dynamics, CVA, and a flux of warm,
moist tropical air in the Mid-Atlantic region provided conditions favorable for synopticscale forcing of ascent. The enhanced low-level jet around the north side of the
landfalling tropical cyclone, teamed with inland cyclone-induced northerly flow,
produced a coastal front of extended duration that played a major role in the modulation
of the most extreme rainfall amounts. During the latter stages of the event, prolonged
easterly flow producing orographic precipitation enhancement along the windward
slopes of north–south oriented topographic barriers was the dominant focusing
mechanism for the heaviest precipitation.
4.1.3 Hurricane Belle (1976)
Precipitation analyses during the passage of Hurricane Belle through the
northeast US are in relative agreement when examining both the spatial distribution and
maximum accumulations. The observed precipitation fell in a linear swath extending
from eastern Delaware northward through central Vermont (Figs. 3.29 and 3.31). Two
areas of precipitation over eastern Massachusetts and northern portions of Maine are
separated from the main swath of precipitation, and are represented well by both
analyses.
Examination of upper-level plots shows a ridge over the western Atlantic Ocean,
and a trough propagating through southern Canada and the northern US. The confluence
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formed between the approaching trough and the amplifying western Atlantic ridge has
produced a significant increase in the upper-level geopotential height gradient (Figs.
3.32a,b). The intensification of the geopotential height gradient has produced an
increase in the upper-level jet winds, and therefore has resulted in a more vigorous and
focused area of upper-level divergence over the corridor of maximum precipitation. The
amplification of the western Atlantic ridge has slowed the propagation of the jet
maximum to the northeast, resulting in the stagnation of the upper-level divergence
maximum over areas experiencing the heaviest precipitation. This divergence maximum
represents implied upward motion through the vertical branch of a thermally direct
ageostrophic circulation, and can establish conditions conducive to heavy precipitation.
Observed ridging over the western Atlantic is also evident at lower levels, and
produces an enhanced geopotential height gradient between the ridge and the tropical
cyclone. An intensified southerly low-level jet and high θe air extend well in advance of
the northern periphery of the cyclone, impinging upon areas of the Northeast under the
influence of the implied vertical motion induced by upper-level jet dynamics. This
scenario exhibits upward motion by way of jet dynamics, with heavy precipitation
resulting from the advection and injection of warm, moist tropical air into the implied
vertical circulation. A daily precipitation analysis early in the event (Fig. 3.30a) reveals
an area of heavy precipitation that has occurred well in advance of the tropical cyclone,
and therefore in all probability is a direct result of a process of this nature.
An increase in the interaction between the tropical cyclone and an approaching
short-wave trough has resulted in the introduction of CVA to the Mid-Atlantic region,
with this process shifting northward into New England as time progresses. This
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interaction with the weak midlatitude trough has produced a pronounced left-of-track
shift in the linear swath of heavy accumulations. The juxtaposition of the large-scale
middle-tropospheric dynamics and the stationary jet-induced upper-level dynamics over
the northeast US has resulted in large-scale forcing for ascent over areas of heavy
rainfall. This region of upward motion is sufficiently fed by a low-level flux of high θe
air wrapping around the north side of Belle, establishing conditions favorable for a
heavy precipitation event.
Within the dynamically favorable region, surface plots for the event show a
subtle and short-lived north–south oriented coastal front extending through western
Connecticut, and observed near the intersection of easterly onshore flow and northerly
inland flow. Eastward positioning and weaker strengthening (when compared to
previous cases) of the geopotential height gradient between the western Atlantic ridge
and the tropical cyclone has dictated the evolution of the low-level jet so that a southerly
orientation is obtained earlier. This type of orientation of the low-level flow has limited
the strength of onshore easterly flow on the north side of Belle, thereby providing
unfavorable conditions for a coastal front of extended duration. The weak boundary
formation occurred in situ slightly south of the southern extent of the Berkshire
Mountains; therefore, the development of this feature may have also been inhibited by
the presence of the topographic barrier. Even though the magnitude of the onshore
easterly flow was less pronounced than in previous cases, the significant inland
penetration and maintenance of the flow as the tropical cyclone propagated through the
Northeast produced prolonged upslope flow along the Berkshire and Green Mountains.
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As a result, orographically modulated precipitation was the dominant mechanism for
observed heavy precipitation by way of forcing for ascent through upslope flow.
Overall, a significant amount of precipitation fell well in advance of the passage
of the tropical cyclone, resulting from the interaction of jet-induced dynamics and a
plume of tropical air. As the tropical cyclone approached landfall, upward motion
became increasingly dominated by large-scale dynamical processes (i.e., CVA and jet
dynamics), which efficiently produced heavy precipitation in the presence of warm,
moist tropical air. Within this swath of rainfall, the formation of a short-lived coastal
front did not have a significant effect on the overall precipitation distribution; however,
prolonged enhancement of easterly flow that impinged on the windward slopes of
topographic barriers throughout the Northeast produced significant intensification of
rainfall and was the dominant mechanism for the observed distributions.
4.1.4 Hurricanes Connie and Diane (1955)
Hurricanes Connie and Diane were investigated in an attempt to elucidate both
the synoptic and mesoscale processes that were responsible for the heavy precipitation
distributions. Results show that a trough interaction, while present in Diane and absent
in Connie, played a major role in the spatial distribution of precipitation. Diane
exhibited a precipitation shift from right- to left-of-track, with very heavy precipitation
lying in a narrow band to the left of track. Connie, on the other hand, exhibited
precipitation along both sides of the observed track. Significant dynamical influence
responsible for the maximum accumulations observed during Connie occurred as a result
of inferred upper-level jet-induced vertical motions.
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Atlantic ridging inferred to have been produced by diabatically induced outflow
associated with convection from Connie resulted in the intensification and
reconfiguration of the upper-level jet. Ridging, however, produced modest results in the
case of Diane, in that a true amplifying ridge did not occur; rather a reinforcement of the
ridge acted to intensify the jet dynamics. In both cases, ridge enhancements at upper
levels extended to lower levels, producing an intensification of the low-level jet and θe
advection over the region. As a result, areas conducive to coastal frontogenesis were
common. Coastal frontogenesis showed signs of influence from the position and
strength of the low-level jet. Areas affected by the jet occurred farther north in Diane
than in Connie, resulting in coastal frontogenesis farther to the north in the case of
Diane.
The inland track of Connie ensured that southeasterly flow east of the storm track
would interact with the Berkshire Mountains, producing locally heavy precipitation in
eastern Massachusetts and Connecticut. The antecedent conditions were in place for a
disastrous flood. The development of a coastal front in Diane was dictated by the track
of the tropical cyclone and the position and strength of the low-level jet, with the lack of
orographic processes during the event resulting from the relatively insignificant inland
penetration of enhanced flow perpendicular to orographic barriers. The formation of the
coastal front in association with Diane acted as a flood catalyst for the already saturated
soils over the southern Berkshire Mountains and areas of northwestern Connecticut.
Total precipitation values for the two storms were in excess of 630 mm and produced
widespread flooding throughout the region.
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4.2 Synthesis and Literature Comparison
The main focus of this research has been to diagnose the synoptic and mesoscale
mechanisms observed during the landfall and transitioning of tropical cyclones in the
northeast US, to elucidate their effects on precipitation distributions. The detailed
examination of five cases for this research effort (i.e., Bob, Gloria, Belle, Connie and
Diane) has reinforced and further quantified observations and hypotheses put forth by
previous investigators.
The lowest common denominator for the five cases examined was the presence
of an amplifying western Atlantic ridge downstream of the tropical cyclone, which acted
to intensify North Atlantic geopotential height gradients and subsequently produced
intensification of upper-level jet winds. This phenomenon has been documented in
previous studies of extratropical transition (e.g., Sinclair 1993a,b; Bosart and Lackmann
1995; Sinclair 2002; Atallah and Bosart 2003; Jones et al. 2003), and in wintertime snow
events such as the March 1993 Superstorm (Dickinson et al. 1997; Bosart 1999). This
intensification process is diabatically induced, resulting from enhanced convection and
manifested as upper-level outflow. In all cases examined for this study, the increasingly
vigorous jet maximum coupled with the northward-propagating tropical cyclone as
amplification of the ridge slowed/stopped the progression of the jet maximum over the
building ridge. Implied jet streak-induced vertical motions (upper-level divergence
maxima) forming the vertical branch of a thermally direct ageostrophic circulation
within the equatorward-entranced region of the jet maximum were situated over the
areas of heaviest precipitation, and have previously been shown to play a critical role in
the intensification and distribution of precipitation during landfalling and transitioning
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events (e.g., Klein et al. 2002; Sinclair 2002; Atallah and Bosart 2003; Colle 2003; Jones
et al. 2003; McTaggart-Cowan et al. 2003; Ritchie and Elsberry 2003).
Western Atlantic ridging in the upper levels also resulted in a more potent ridge
at lower levels for all events examined. The placement of an intense cyclonic circulation
in close proximity to the amplifying low-level ridge results in a significantly enhanced
geopotential height gradient between the two features, and subsequently causes an
intensified low-level jet. This low-level jet acts to advect copious amounts of tropical
moisture into regions of the Northeast, priming the area for heavy precipitation. When
the vertical branch of the thermally direct circulation associated with jet dynamics
becomes collocated with a flux of tropical air, heavy precipitation can be produced well
in advance of the tropical cyclone, as was the case during Hurricanes Bob and Belle, and
during the passage of Tropical Storm Agnes (1972) through the Northeast documented
previously by Bosart and Carr (1978). Identification and analysis of heavy precipitation
produced by the collocation of jet-induced vertical motions and moisture advection were
also quantified in depth during the examination of heavy snowfall events along the East
Coast of the US by Uccellini et al. (1984) and Uccellini and Kocin (1987). Collectively
these two studies provide direct evidence as to the existence of a jet-induced thermally
direct circulation and its impact on precipitation production when combined with
significant moisture transport and injection into the area of vertical motion.
The propagation and interaction of the tropical cyclone with an approaching
middle-tropospheric trough (observed in all cases except Connie) increases forcing for
ascent by midlatitude processes (e.g., differential CVA). The juxtaposition of
differential CVA, the coupled jet dynamics, and the flux of tropical air produces a
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dynamically favorable region for the production of heavy precipitation closer to initial
landfall and inland penetration. The presence of a remnant baroclinic zone is not
required for the evolution of heavy precipitation (significant baroclinicity was observed
northwest of Gloria only); however, in the case of Hurricane Floyd (1999) the presence
and interaction of a tropospheric-deep baroclinic zone aided in the focusing and
intensification of the observed precipitation (Atallah and Bosart 2003). The
investigation of precipitation distributions relative to the track of the tropical cyclone
verifies the previous findings of Atallah (2003), showing that systems under the
influence of an upper-level trough exhibit a left-of-track shift in precipitation
distributions (Bob, Gloria, Belle and Diane), while systems governed primarily by jet
dynamics (Connie) exhibit a right-of-track shift in precipitation distributions.
Within this dynamically favorable environment, the development and
maintenance of the low-level jet is also critical to the evolution of mesoscale processes
as the tropical cyclone progresses closer to landfall and inland penetration. The strength
and positioning of the western Atlantic ridge governs the orientation of the low-level jet.
In cases where an extended coastal front plays a dominant role in the overall maximum
precipitation accumulations (i.e., Gloria, Connie and Diane), an orientation of the lowlevel jet is obtained so that a significant portion of the strong jet winds are wrapped
around the north side of the tropical cyclone. When the tropical cyclone is collocated
with a continental landmass and the proper configuration of the low-level jet is obtained,
cyclone-induced northerly inland flow intersects the enhanced easterly onshore flow to
produce a mesoscale convergence zone (i.e., a coastal front). The boundary differs from
the conceptual model of a traditional cool-season coastal front in that it is a much more
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subtle feature than initially described by Bosart et al. (1972) and Bosart (1975), with
thermal gradients on the order of 3–4oC/80 km instead of the 5–10oC/5–10 km observed
during New England coastal front events. The properties of the boundaries analyzed in
this research effort are consistent with those of the coastal front analyzed during the
northward propagation of Tropical Storm Agnes (1972) by Bosart and Dean (1991).
Formation of the coastal front appears to be a more dynamically driven process induced
primarily by the effects of the tropical cyclone, similar to orographically induced coastal
fronts during wintertime storms described by Forbes et al. (1987) and Nielsen (1989),
where trapping of low-level cold air against the windward slopes of a mountain barrier
induces a coastal front when warm maritime air is forced up and over the dense lowlevel continental air mass. In the case of a landfalling tropical system, contrasting air
masses result from the introduction of continental air from inland cross-contour
northerly flow and tropical air from enhanced easterly onshore flow. This contrast
produces a situation similar to CAD events described by Bell and Bosart (1988) and
Bailey et al. (2003), albeit much less pronounced and on much smaller-scales. In cases
where the coastal front formed and was unable to persist for a significant amount of time
(i.e., Bob and Belle), the orientation of the low-level jet was not maintained long enough
to provide strong jet winds on the north side of the cyclone, thereby failing to establish
dynamical conditions necessary for sustained and intense intersection of distinct air
flows.
Figure 4.1 represents a schematic diagram of the significant features associated
with heavy precipitation in the Northeast US during landfalling and transitioning tropical
cyclones. This diagram is an idealization of the placement of each individual feature,
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with precise locations varying on a case-by-case basis and dependent upon the track of
the cyclone, the strength and position of the western Atlantic ridge, and the configuration
of the middle-tropospheric flow. A broad area of precipitation resides in the most
dynamically favorable area resulting from the juxtaposition of an implied thermally
direct ageostrophic circulation associated with the enhanced jet maximum over northern
New England and southern Canada, and the enhanced low-level jet wrapping around the
north side of the cyclone. The heaviest precipitation resides near the area where the
intersection of inland northerly flow and easterly onshore flow has formed a coastal front
that extends northward from the center of the circulation, with the coastal front position
varying from case-to-case due to different configurations obtained by the low-level jet.
Proper configuration during translation of the tropical cyclone through the
Northeast, teamed with coastal front dissipation, can result in significant inland
penetration of enhanced flow. Depending upon the direction of the flow and the
orientation of the topographic barrier, warm and moist flow perpendicular to the
windward slopes of the terrain can be forced upward, resulting in enhancement of
maximum accumulations. This scenario occurred during Tropical Cyclone Bola (1988)
in the Gisborne Ranges of New Zealand (Sinclair 1993b). With the exception of Bob
and Diane, where the track of system did not allow for the development of significant
inland flow, this scenario was observed during all of the other events examined in this
study (i.e., Gloria, Belle and Connie) for both large and small spatial scales.
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Features Contributing to Heavy Precipitation over the Northeast US
Accompanying Landfalling and Transitioning Tropical Cyclones
Fig. 4.1. Features critical to the occurrence of heavy precipitation over the northeast US
accompanying landfalling and transitioning tropical cyclones. Enhanced upper-level jet
winds are represented by the blue-hatched area, maximum upper-level divergence is
represented by the magenta circle, enhanced low-level jet winds are represented by the
orange-hatched area, the broad precipitation shield is represented by the light green
dashed line, the heaviest precipitation is represented by the dark green dashed line, the
western Atlantic ridge is represented by the blue “H”, and the approximate coastal front
location is represented by the pink line extending northward from the center of
circulation, which is denoted by the red hurricane symbol.
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