III. Results
Section 3.1 of this chapter will present the results of the work conducted to
obtain the final dataset that led to the selection of individual cases for further study.
Section 3.2 will give in-depth, detailed case analyses of the landfall and transition of
Hurricanes Bob, Gloria, and Belle, with an historical perspective resulting from
available data for Hurricanes Connie and Diane in section 3.3.
3.1 Focusing of Tropical Cyclone Dataset from Initial to Final Size
The results of the initial 41-storm dataset containing landfalling and transitioning
tropical cyclones producing > 100 mm (4 in) of rainfall over the Northeast US during the
time period 1950–2001 are presented in Table I. A modified version of this dataset
(Table II) was produced to take into account the lack of UPD data past 1998. This
dataset weathered the loss of three more recent storms (Floyd, Gordon, and Allison) that
will likely receive increased attention in future investigations of this nature. The final
subset of nine storms containing precipitation distributions that were potentially
influenced by coastal frontogenesis is presented in Table III. The most scrutiny will be
reserved for a small selection of these cases (presented shortly) in an attempt to quantify
the role of synoptic and mesoscale processes on observed precipitation distributions.
3.2 Case Studies
3.2.1 Hurricane Bob
3.2.1.1 Overview
53
Hurricane Bob was the only tropical cyclone to hit the US in 1991 (16–29
August 1991), and the first to strike the northeastern US since Hurricane Gloria in 1985.
The impacts of Bob were quite severe, with 17 reported deaths and two billion dollars of
damage occurring as a result of the storm.
Bob originated from a large disturbance that slowly intensified to tropical
depression status by 0000 UTC 16 August, while centered approximately 330 km east of
the Bahamas. Over the next couple of days, Bob began to turn toward the north and
northeast, propagating parallel to the mid-Atlantic coast (Fig. 3.1). When the system
was located approximately 165 km east-southeast of Norfolk, Virginia, maximum
sustained intensity as a category 3 hurricane on the Saffir–Simpson scale was achieved.
Bob slowly weakened as it accelerated to the northeast, eventually moving over Block
Island, Rhode Island, and making landfall in Newport, Rhode Island, as a category 2
hurricane. After initial landfall, Bob continued on its northeast path, passing over Rhode
Island and Massachusetts, before eventually making a second landfall as a tropical storm
near Rockland, Maine. Bob then became extratropical as it crossed over the Gulf of St.
Lawrence around 1800 UTC on 20 August and continued recurving over the North
Atlantic (Pasch and Avila 1992).
3.2.1.2 Precipitation Distributions
Examination of the tropical cyclone track versus total precipitation (Fig. 3.1)
shows the progression of the system parallel to the east coast of the US, with a
pronounced left-of-track shift in the distribution of precipitation as Bob propagates
northeastward through the New England region. Maximum storm total accumulations
54
are located in a southwest–northeast oriented band extending from western Long Island,
New York, through central Connecticut and Massachusetts, and northeastward into
southern New Hampshire and extreme western portions of Maine.
The daily UPD precipitation plots and topography (Fig. 3.2a) for the 24-h period
ending at 1200 UTC 19 August display a broad and rather intense precipitation shield
well in advance of the passage of the center of circulation. Heavy precipitation values
extend northward from Long Island, New York, to southeastern areas of New
Hampshire. Precipitation totals for the next 24-h period (Fig. 3.2b) show the axis of
heaviest precipitation (although noticeably weaker than the previous 24 h) oriented
through central New England, with an extension northward into Maine, closely
following the track of the system. Smaller areas of enhanced precipitation are
distributed throughout the region, including an area southwest of Boston, Massachusetts,
and signatures tied to elevated terrain in northern New Hampshire and western Maine.
A finer-resolution NWS precipitation analysis (Fig. 3.3) identifies a southwest–
northeast oriented maximum across central New England, which extends to coastal
locations of Maine. Precipitation accumulations approaching 250 mm (10 in) are clearly
defined, and orographic signatures can be identified in northern New Hampshire, near
the vicinity of Mount Washington.
3.2.1.3 Synoptic-Scale Analysis
Investigation of analyses containing 200 hPa geopotential height, wind speed and
divergence, show that approximately 24 h before initial landfall on the southern New
England coast (1800 UTC 18 August), the upper-tropospheric flow pattern was
55
dominated by a positively tilted large–amplitude trough over the upper Great Lakes
region, and a ridge over the western Atlantic ocean (Fig. 3.4a). Coupled upper-level jet
maxima located in southwesterly flow over the Northeast and southern Canada has
produced a broad area of divergence associated with an implied thermally direct
ageostrophic circulation about the equatorward entrance region of the broad jet
maximum. The area containing significant upper-level divergence, which is conducive
to the most vigorous upward motion, is located in the equatorward entrance region of the
southernmost jet streak, immediately north of the center of the tropical cyclone and
impinging upon portions of southern New England.
Progression of time by 12 h (Fig. 3.4b) reveals marked changes in the overall
structural characteristics of the environment at this level. The positively tilted trough
located over the upper Great Lakes has sharpened and propagated slightly
southeastward, while the western Atlantic ridge has amplified due to implied diabatically
induced upper-level outflow from convection associated with the tropical cyclone. The
resultant effect of these two changes has caused an increase in the magnitude of the
geopotential height gradient over the northeast, and produced a single, focused, jet streak
with maximum wind speeds on the order of 60 m s-1. The increase in the maximum
speed of the jet streak is accompanied by a more vigorous implied thermally direct
ageostrophic circulation, identified as an increased divergence maximum in the
equatorward entrance region of the jet, near the New Jersey shoreline. The next two
consecutive time periods (Figs. 3.4c,d) show a rotation of the trough to a more neutral
tilt and detachment of the southern portion of the trough over the Ohio Valley, while
Atlantic ridging continues to occur. A split flow structure between the main flow
56
located to the north and the detaching trough to the south, teamed with Atlantic ridging,
has resulted in a strong confluence region and geopotential height gradient over southern
Canada. Jet streak intensity has increased to 65 m s-1 on 1200 UTC 19 August and
undergoes slight weakening to 60 m s-1 by 1800 UTC. Propagation of the jet to the
northeast is a slow and gradual process, resulting in the stagnation of a vigorous implied
thermally direct ageostrophic circulation over areas experiencing maximum precipitation
in southern New England.
An analysis of geopotential height and absolute vorticity at the 500 hPa level for
0600 UTC 19 August (Fig. 3.5a) shows a rather potent trough digging into the Great
Lakes region in association with the fracturing of the southern branch of a northward–
displaced trough. The mid-tropospheric reflection of the tropical cyclone is evident
along the Mid-Atlantic coast, manifested as a negatively tilted orientation of the
geopotential height field along the periphery of a western Atlantic ridge. A rather
pronounced area of confluence is evident over the Northeast in association with the
intersection of flows created by the Great Lakes trough, the tropical cyclone and the
Atlantic ridge.
Progression through the next two time periods (1200 and 1800 UTC 19 August;
Figs. 3.5b,c) shows the tropical cyclone propagating northward into the northeastern
region, pivoting around the eastern periphery of the southern portion of a trough that has
been cutoff from the main flow to the north. Major interaction does not occur with the
trough located over the Ohio Valley; instead, the tropical cyclone becomes attached to
the eastward propagating positively tilted trough in Canada (identified by interaction
between vorticity centers). The western Atlantic ridge experiences continued
57
reinforcement, acting to efficiently increase the magnitude of the geopotential height
gradient between itself and the tropical cyclone, thereby inducing an enhanced southerly
flow. The confluence region created by the intersection of flows continually shifts with
the movement of the tropical cyclone, and can be identified as it propagates into
southern Canada. Cyclonic vorticity advection over the Northeast increases with the
approach of Bob to the coast, and becomes increasingly important to induced vertical
motions as interaction with the mid-latitude trough occurs in northern New England and
southern Canada (Fig. 3.5c).
Examination of 850 hPa geopotential height, potential temperature, and wind,
shows a remnant baroclinic zone located in southern Canada (Fig. 3.6a) associated with
a recently departed low-pressure system (not shown) off the east coast of Newfoundland.
The juxtaposition of the building Atlantic ridge and the tropical cyclone along the
eastern seaboard has increased the geopotential height gradient, producing
intensification in the southerly flow (over southern New England) at low levels to ~15
m s-1. The increased southerly flow has not produced significant WAA due to a lack of
interaction with the baroclinic zone thus far.
Evolution through the next 18 h (Figs. 3.6b–d) shows a continued increase in the
geopotential height gradient between the Atlantic ridge and the mobile tropical cyclone,
producing further intensification of the low-level flow to ~ 30 m s-1 over southern New
England. The enhanced flow has acted to rotate the baroclinic zone cyclonically, while
intensification of the frontal baroclinic zone has occurred across northern New England
as a result of the flux of warm air on the east side of the tropical cyclone. Warm-air
58
advection has also increased slightly, and can be observed in the northeastern quadrant
of the cyclone as the low-level jet supplies warm air to eastern areas of New England.
Examination of 925 hPa geopotential heights, equivalent potential temperature,
and wind for 0600 UTC 19 August (Fig. 3.7a) reveals an enhanced geopotential height
gradient between the ridge and the tropical cyclone, resulting in the intensification of the
low-level southeasterly jet to ~20 m s-1. The advection of high θe (warm and moist) air
by the southeasterly wind has resulted in the introduction of values in excess of 355 K
along the Mid-Atlantic coast, with somewhat lower (but still relatively high) values
encroaching on the New England region well in advance of the tropical cyclone. This
southeasterly flow provides a continuous supply of warm, moist air to the vertical branch
of the implied thermally direct circulation, thereby establishing conditions conducive for
heavy precipitation. A gradual strengthening and reorientation of the core winds
associated with the low-level jet occurs over the next 18 h (Figs. 3.7 b–d). The
positioning of the jet deviates from the east/northeast quadrant to the southeast quadrant
of the cyclone based upon the fact that the strength of the ridge is not sufficient to
maintain the orientation of the geopotential height gradient (Figs. 3.7b,c). Enhancement
of easterly wind speeds to ~20 m s-1 around the north side of the storm are encroaching
on the southern New England region, but lack the intensity observed further offshore.
The modified offshore low-level jet possesses speeds in excess of 30 m s-1 from a
southerly/southwesterly direction, resulting in the advection of highest θe values (350+
K) across the southeastern areas of New England. Even though maximum θe values
have been transported southeast of New England, an appreciable amount of southerly
flow has resulted in the penetration of warm, moist tropical air in excess of 345 K
59
through central portions of the region. Figure 3.7d illustrates the reconfiguration of the
strongest geopotential height gradient to a southwest–northeast orientation, and the
resultant effect of the low-level flow in efficiently advecting the main axis of warm,
moist air out to sea during the latter stages of the evolution.
3.2.1.4 Surface Analysis
Approximately 2 h before the landfall of Bob, the center of circulation was
located just south of the western tip of Long Island, New York (Fig. 3.1). Surface plots
for this time (Fig. 3.8) show a northerly flow of ~10–13 m s-1 along the eastern slopes of
the Berkshire Mountains in Massachusetts, continuing south through central Connecticut
along a streamline bound for the center of circulation. The intersection of enhanced
easterly flow on the order of 15 m s-1 and the cyclone–induced northerly flow has
produced a weak convergence zone (i.e., a coastal front) in eastern Connecticut, western
portions of Rhode Island, and southern Massachusetts. Temperature and dewpoint
gradients across the frontal zone are rather subtle, and on the order of 3–4oC/100 km and
2oC/100 km, respectively. Inferences pertaining to the mesoscale frontal zone are
subjective in nature due to the limitations imposed by a critical hole of surface data in
the eastern Connecticut region. Inspection of an interpolated analysis containing
contours of θ and shaded θ gradients, as well as contours of θe and shaded θe gradients
for this time (Figs. 3.9a,b) shows that weakly enhanced θ and θe gradients (albeit weaker
than less reliable adjacent oceanic calculations) have developed in the aforementioned
region while in the presence of a convergent wind.
60
Progression of time by 1 h (Fig. 3.10) shows that the cyclone induced northerly
flow has remained unchanged, while the enhanced easterly flow has intensified slightly
to 18–20 m s-1. The position of the coastal front has shifted slightly eastward following
the propagation of the system to a location near Block Island, Rhode Island, and is
weakly apparent in analyses of θ and θe gradients in eastern Connecticut and Rhode
Island due to the small-scale and subtle nature of the feature (Figs. 3.11a,b). Weakening
of the feature continues into the next hourly time period (1800 UTC 19 August), and is
identified as progressively weaker θ and θe gradients (Figs. 3.12a,b). The analysis
procedure used to resolve the thermal gradient is objective and should be interpreted
with caution due to inaccuracies arising from smoothing processes. Surface
observations for 1800 UTC show a northward movement of the coastal front from
central Rhode Island, to north of Boston, Massachusetts, as Bob makes landfall in
southern Rhode Island (Fig. 3.13). The thermal boundary possesses temperature and
dewpoint gradients on the order of 2–3oC/50 km and is collocated with an area of
enhanced precipitation southwest of Boston, Massachusetts. Northeastward propagation
of the cyclone during subsequent hours produces a brief cyclonic rotation and rapid
dissipation of the very weak coastal front (not shown).
Previously mentioned in the presentation of precipitation analyses was the
identification of what appeared to be significant orographic modification in northern
New Hampshire and western Maine. Unfortunately, a lack of surface data in northern
portions of New England inhibits the analysis of conditions important to possible
orographic enhancement observed in the area.
61
3.2.2 Hurricane Gloria
3.2.2.1 Overview
Hurricane Gloria (16 September–2 October 1985) developed from an African
easterly wave, reaching tropical depression status near the Cape Verde Islands on 16
September. Hurricane status was achieved on 22 September approximately 400 km east
of the Lesser Antilles. Gloria then made a slow northward turn and passed northeast of
the Lesser Antilles and eventually north of the Bahamas, in route to the mid-Atlantic
region of the United States. A minimum central pressure of 919 hPa was reached
approximately 1500 km southeast of Cape Hatteras, North Carolina, at 0120 UTC on 25
September. The storm then passed over the Outer Banks of North Carolina and
weakened while rapidly accelerating parallel to the eastern seaboard (Fig. 3.14),
eventually making landfall on Long Island, New York. Eight deaths and total damage of
$900 million along the east coast of the US were the end results when Gloria finally lost
tropical characteristics as it propagated into the Maine region (Case 1986).
3.2.2.2 Precipitation Distributions
A plot of storm track and total precipitation (Fig. 3.14) shows the progression of
Gloria parallel to the east coast of the US, passing over the Outer Banks of North
Carolina in transit to landfall on Long Island, New York. Maximum precipitation
accumulations are highlighted in a relatively narrow band that extends from northeastern
North Carolina to the Catskill region of New York State. The distribution of
precipitation possesses a preferential left-of-track shift when compared to the track of
the system.
62
Examination of a plot containing 24-h precipitation accumulations and
topography for 26 September (1200–1200 UTC) (Fig. 3.15a) shows the northern fringes
of the precipitation shield working into coastal locations of the Mid-Atlantic US. The
vast majority of the rainfall during the event occurs over the next 24-h period (ending
1200 UTC 27 September) when significant amounts fall in areas of eastern Virginia,
stretching northward into eastern Pennsylvania and south central portions of New York
(Fig. 3.15b). The most extreme rainfall amounts are focused in an area from
southeastern Virginia north into Maryland, Delaware, eastern Pennsylvania, and western
New Jersey. Lighter precipitation amounts exhibiting embedded areas of possible
orographic enhancement were observed as Gloria quickly accelerated and raced through
the northeast, exiting in northern Maine (Fig. 3.15c). Precipitation signatures indicative
of orographic modulation are evident along the eastern slopes of the Catskill and
Berkshire Mountains, as well as the mountainous regions of northern New Hampshire.
A finer-resolution NWS precipitation analysis for storm total accumulations (Fig.
3.16) shows the heaviest values occurring in a band that extends from eastern Virginia,
northward through eastern areas of Pennsylvania into the southern tier of New York.
Maximum rainfall on the order of 150–250 mm is embedded within the larger-scale
band, in areas west of the New Jersey border, stretching south through Maryland and
Virginia. Signatures of precipitation enhancement due to orographic processes are
consistent with UPD analyses discussed previously.
3.2.2.3 Synoptic-Scale Analysis
63
Inspection of 200 hPa geopotential height, wind speed and divergence for 1800
UTC 26 September (Fig. 3.17a) shows a significantly amplified geopotential height
pattern, possessing a large, broad trough that encompasses the entire northern tier of the
US, and a strong ridge over the western Atlantic Ocean. A positively tilted short-wave
feature propagating through the base of the trough has acted to enhance the geopotential
height gradient between itself and the ridge to the east, thus resulting in a significant and
expansive jet streak centered near Lakes Erie and Ontario that is on the order of 60 m s-1.
The combination of upper-level effects from an implied thermally direct ageostrophic
circulation in the equatorward entrance region of the jet and the tropical cyclone has
produced a broad area of upper-level divergence along the eastern seaboard, stretching
from the Carolina region to southern areas of New England.
Progression through the next 18 h (Figs. 3.17b–d) shows ridging over the western
Atlantic downstream of a rather substantial trough to the northwest, resulting in a
significant increase of the geopotential height gradient, and an intensification and
reconfiguration of the jet maximum to 75 m s-1 over eastern Canada. The ridging
occurring over the western Atlantic most likely can be attributed to diabatically induced
upper-level outflow associated with convection from Gloria. The rapid increase in jet
intensity and the highly amplified nature of the flow have produced a vigorous
divergence maximum that slowly propagates northward along the eastern seaboard and
through northern New England, resulting in conditions conducive to upward motion over
the region of heaviest precipitation for an extended period of time.
An analysis of 500 hPa geopotential height and absolute vorticity for 0000 UTC
27 September (Fig. 3.18a) shows a positively tilted double vorticity short-wave center
64
propagating through the large-scale flow near the western Great Lakes, stretching south
to eastern portions of the Midwest, while a strong ridge is evident over the Atlantic
Ocean. Nestled between these two mid-latitude features is the mid-tropospheric
reflection of Gloria (located near coastal areas of the Carolinas), which is causing minor
distortion of the geopotential height field (manifested as an eastward bulge) along the
western periphery of the ridge. Six hours later (Fig. 3.18b), increased north Atlantic
ridge building is observed, manifested as a northwestward bulge of the high over coastal
locations of Maine. The northward vorticity center associated with the Great Lakes
short wave is showing signs of being sheared by the increased southwesterly flow over
the ridge. Cyclonic vorticity advection associated with the northward movement of the
tropical cyclone is impinging upon regions of the mid-Atlantic, providing support for
ascent and precipitation.
Interaction of the tropical cyclone with the approaching trough is evident as the
short wave over the Ohio Valley has become negatively tilted, and the vorticity fields
have merged (Fig. 3.18c). Cyclonic vorticity advection has continued to press
northward, promoted by the enhanced flow generated between the cyclone and the
building ridge. Six hours later (1800 UTC), the northern portion of the Ohio Valley
trough has been sheared off to the northeast over the continually building ridge (Fig.
3.18d). A minor eastward bulge on the western side of the high, a northward spreading
of CVA, and a less recognizable vorticity center associated with Gloria are evident as the
system quickly propagates northward into northern New England.
A signature of the intense ridge over the western Atlantic Ocean is the most
noteworthy feature in the analysis of geopotential height, potential temperature, and
65
wind at 850 hPa for 0000 UTC 27 September (Fig. 3.19a). An enhanced geopotential
height gradient between the ridge and the tropical cyclone has produced flow on the
order of 25 m s-1 on the eastern side of the cyclone, with 20 m s-1 easterly flow wrapping
around the northern side of the system and encroaching on coastal locations of the MidAtlantic. A southwest–northeast oriented baroclinic zone associated with the trough
over the Great Lakes and eastern regions of the Midwest can be identified from the
southeastern US into southeastern sections of Canada. This baroclinic zone is a distinct
entity that exhibits very little interaction with the approaching tropical cyclone.
A continued increase in the geopotential height gradient between the Atlantic
ridge and the cyclone has produced an enhanced southeasterly low-level jet on the order
of 30 m s-1 (Figs. 3.19b–d). The nose of this jet is wrapped around the northern side of
the cyclone during passage along the Carolinas (Fig. 3.19b), producing a significant
increase in easterly flow along the Mid-Atlantic coast. Interaction of the tropical
cyclone with the approaching baroclinic zone does not fully occur until 1200 UTC 27
September (Fig. 3.19c) when the enhanced southeasterly flow intersects the temperature
gradient, causing WAA in extreme northern portions of New England. Gloria continues
its northward propagation and shifts the main axis of enhanced southerly flow to the east
side of the cyclone, penetrating into southern Canada (Fig. 3.19d). The baroclinic zone
has been deformed into an “S” shape due to the advective effects of the cyclonic
circulation; however, any WAA is well displaced into Canada and out of the domain
considered for this case study.
Examination of the 925 hPa geopotential height, equivalent potential
temperature, and wind reveal similar flow characteristics previously described at the 850
66
hPa level. Figure 3.20a shows a readily identifiable ridge and enhanced geopotential
height gradient between the ridge and the cyclone along the east coast of the US. The
rather pronounced low-level jet possesses wind speeds on the order of 28 m s-1, with 20
m s-1 flow on the north side of the cyclone impinging upon Mid-Atlantic coastal
locations. The core of highest θe air (~355 K) is for the most part symmetrically
distributed in accordance with the geopotential height contours of the cyclone; however,
advection of warm, moist air of tropical origin (~350 K) is beginning to occur on the
north side of the cyclone near coastal locations experiencing a maximum in easterly
onshore flow.
Maintenance of the enhanced easterly flow is achieved as northward propagation
into Maryland and New Jersey occurs (Fig. 3.20b). A tongue of high θe air has
propagated northward over the Northeast in advance of the system, and possesses 340 K
air, while even higher values (approaching 355 K) are being advected into eastern
Virginia and Maryland. The reanalysis wind field produces a signature of possible
coastal frontogenesis near the Chesapeake Bay area associated with the intersection of
southeasterly onshore flow and inland northeasterly flow (investigated in more detail
later in this chapter).
The enhanced southeasterly flow approaching 20 m s-1 continues to advect the
tongue of warm, moist air farther northward throughout the Northeast in advance of the
passage of the tropical cyclone (Fig. 3.20c). Immediately north of the center of
circulation, in eastern Pennsylvania, the wind field continues to indicate a possible
coastal front, manifested as the intersection of 10 m s-1 northerly inland flow and 10
m s-1 easterly onshore flow. The wind shift is positioned in an area possessing a
67
significant θe gradient, resulting from the intersection of westward–advected warm,
moist air, and a midlatitude air mass associated with the upper-tropospheric trough. The
progression of Gloria into interior sections of the Northeast results in the continued
northward penetration of high θe air by the southeasterly flow, and a more circular inland
flow that does not indicate the presence of a coastal front signature (Fig. 3.20d).
3.2.2.4 Surface Analysis
A surface plot for 0600 UTC 27 September roughly places the center of
circulation near the North Carolina coast (Fig. 3.21). A cyclone-induced northerly
inland flow exists along eastern areas of Pennsylvania, central Maryland and eastern
Virginia. A broad area of easterly onshore flow around the north side of Gloria extends
along coastal and immediate inland locations, stretching from Virginia to New Jersey. A
convergence zone formed in situ by the intersection of these two flows extends from the
Chesapeake Bay area northward to the border of New Jersey and Pennsylvania.
Temperature and dewpoint gradients across the coastal front are approximately 4oC/80
km and 6oC/80 km, respectively, within the most intense section located near
Chesapeake Bay. A slightly weaker gradient exists across northern regions of the
boundary, where gradients of temperature and dewpoint across western New Jersey and
eastern Pennsylvania are on the order of 3–4oC/100 km and 3–4oC/100 km, respectively.
An analysis of θ and θe gradients for the same time (Fig. 3.22) further verifies the
location of the coastal front, identifying enhanced θ gradients of 2–3 x 10-4 K (100 km)-1
and θe gradients of 10–12 x 10-4 K (100 km)-1 under the influence of convergent wind
68
flow in the regions previously discussed, and collocated with the band of heaviest
precipitation.
Three hours later (0900 UTC), a boundary extending northward through the
center of Maryland and into eastern Pennsylvania is sustained by the intersection of a 15
m s-1 northerly inland flow and a 10–15 m s-1 easterly onshore flow (Fig. 3.23).
Temperature and dewpoint gradients possess similar magnitudes as those discussed
earlier, with θ and θe gradients (Fig. 3.24) identifying the position of the north–south
oriented coastal front.
The convergent coastal front boundary slowly encompasses less areal coverage
and steadily intensifies as Gloria moves north along the east coast (Figs. 3.25 and 3.26).
Overall, the baroclinicity intensifies due to the interaction of a midlatitude baroclinic
zone with the baroclinicity produced by the tropical cyclone, amounting to θ and θe
gradients of 3–4 x 10-4 K (100 km)-1 and 12–14 x 10-4 K (100 km)-1 respectively (Figs.
3.27 and 3.28). The enhanced baroclinic zone is occurring in the presence of a relatively
convergent wind flow that can be initially identified from northern Maryland through
eastern Pennsylvania (Fig. 3.25), and is experiencing a cyclonic rotation to a position in
northern New Jersey and northeastern Pennsylvania (Fig. 3.26). Directional wind shear
slowly ceases over the next 2 h, resulting in significant weakening and dissipation of the
coastal front feature near the Pocono Mountains (not shown).
Investigation of surface observations during the passage of the cyclone through
the northeast US shows that easterly flow was maintained along the north side of the
tropical cyclone during much of the translational period. This easterly onshore flow
intersected the windward slopes of topographic barriers in southern New York (Fig.
69
3.25), Massachusetts, Vermont and New Hampshire (not shown), producing areas of
orographic precipitation modification.
3.2.3 Hurricane Belle
3.2.3.1 Overview
Hurricane Belle (6–10 August 1976) developed from an easterly wave
propagating off of the African continent on 28 July. Belle attained tropical depression
status on 6 August approximately 450 km east-northeast of the Bahamas, and proceeded
to strengthen and quickly move northward parallel to the east coast of the United States
(Fig. 3.29). Belle reached hurricane status during the late afternoon of 7 August,
achieving a minimum central pressure 24 h later of approximately 958 hPa. Landfall
occurred in the vicinity of Jones Beach, New York, at 0500 UTC 10 August, as a much
weaker system. Belle then continued propagating to the north-northeast through central
New England before succumbing to dissipative forces and losing its tropical
characteristics over the New Hampshire and Maine region. Belle was the deadliest and
costliest Atlantic hurricane to strike the mainland US in 1976 (Lawrence 1977).
3.2.3.2 Precipitation Distributions
The examination of an analysis displaying tropical cyclone track versus storm
total precipitation (Fig. 3.29) shows the axis of heaviest precipitation located in a narrow
band exhibiting a preferential left-of-track shift as Belle progresses northward. The
linear band of precipitation stretches from coastal locations of Maryland and Delaware
70
north into central locations of New England, with two localized precipitation maxima
observed in southeastern Massachusetts and northern Maine.
Dissection of the total precipitation accumulations into separate daily analyses
shows a plume of precipitation along the Mid-Atlantic coast extending northeastward
into southern New England (Fig. 3.30a), well in advance of the northward–propagating
center of circulation located east of the Outer Banks of North Carolina (Fig. 3.29).
Maximum accumulations are observed during the following 24 h (Fig. 3.30b) as Belle
penetrates inland, producing a relatively linear and intense area of rainfall from eastern
New Jersey north along the eastern slopes of the Berkshire and Green Mountains.
Embedded within the broad precipitation shield that encompasses a significant portion of
the Northeast are both large and small regions of heavier rainfall that are evident in
central and extreme northern areas of Maine, and near Mt. Washington, New Hampshire.
Lighter, residual rainfall accumulations associated with the northward movement of the
system into Canada dominate the last 24-h period across the extreme northern New
England region (Fig. 3.30c).
The NWS precipitation analysis shows a narrow and intense linear band of
precipitation extending from southern Maryland northward into central Vermont (Fig.
3.31). Accumulations throughout this broad area of precipitation are on the order of 75–
100 mm, while smaller regions embedded within the larger precipitation shield possess
amounts that approach 100–180 mm. An area of heavy precipitation detached from the
main axis of precipitation is observed south of Boston, Massachusetts, and in northern
Maine, consistent with UPD analyses discussed previously.
71
3.2.3.3 Synoptic-Scale Analysis
Inspection of 200 hPa geopotential height, wind speed and divergence for 0600
UTC 9 August (Fig. 3.32a) shows a split-flow structure in the geopotential height field
exhibiting an amplified southern branch of the flow across the US, and a more zonal
westerly flow across Canada. A weak trough is evident over the Great Lakes region
stretching south into the southeastern states, with a ridge visible across the western
Atlantic Ocean. A confluent region formed by the intersection of southwesterly flow
over the ridge and westerly flow across Canada has produced a pronounced jet
maximum of approximately 50 m s-1 over the Newfoundland area, stretching
southwestward into central New England. A broad area of divergence and vigorous
upward motion is located in the New England region, and is likely associated with a
thermally direct ageostrophic circulation about the equatorward entrance region of the jet
maximum.
Six hours later (1200 UTC), minor building of the western Atlantic ridge has
resulted in a slight tightening of the geopotential height gradient between the ridge and
the trough located over the Great Lakes region (Fig. 3.32b). The overall structure and
intensity of the jet maximum has been relatively unchanged by the subtle geopotential
height field adjustment; however, a broader and less focused area of upper-level
divergence produced by the introduction of tropical cyclone induced outflow has become
evident along the east coast. Examination of the following 12 h (Figs. 3.32c,d) shows an
increase in western Atlantic ridging, with a slight westward retrogression of the ridge,
and a northeastward lifting of the northern portion of the Great Lakes trough. The
overall intensity of the jet maximum does not change appreciably; however, a
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reconfiguration of the jet core to the southwest in association with the tightening of the
geopotential height gradient between the trough and the ridge has placed the maximum
upper-level divergence in a band oriented from Maryland, north through central
Vermont, near the area of maximum precipitation.
Examination of the 500 hPa geopotential heights and absolute vorticity
approximately 24 h before landfall (0600 UTC 9 August; Fig. 3.33a) confirms the splitflow structure containing a weakly amplified southern branch of flow across the US,
which is detached from the main flow in Canada. A weak trough located over the Great
Lakes is identified by a maximum of vorticity, while the beginning stages of interaction
between the tropical cyclone and the vorticity maximum are evident across the Carolina
region. The western Atlantic ridge is firmly entrenched at this time, exhibiting an
enhanced geopotential height gradient between itself and the trough, thereby intensifying
the southerly flow along the eastern seaboard.
Continued northward movement of the tropical cyclone along the east coast
produces a progressive increase in the degree of interaction with the isolated short-wave
trough slowly lifting to the north across southern Canada (Figs. 3.33b–d). The ridge
located over the western Atlantic Ocean shows signs of amplification and retrogression
westward as time progresses. An eastward bulge in the geopotential height field
produced by the northward propagation of the tropical cyclone along the western
periphery of the ridge is weak due to the magnitude of the ridge; thereby maintaining a
north–south orientation of the region of increasing geopotential height gradient along the
east coast. The intensified geopotential height gradient results in the enhancement of
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southerly flow along the east coast, subsequently producing an increase of CVA into the
southern New England region.
An analysis of 925 hPa geopotential height, equivalent potential temperature, and
wind for 0600 UTC 9 August (Fig. 3.34a) shows the reflection of the tropical cyclone
located east of the Carolina region. An enhanced geopotential height gradient between
the ridge over the western Atlantic and the tropical cyclone is already evident at this
time, accompanied by enhanced southeasterly flow on the order of 15 m s-1 some
distance offshore. A core of warm, moist high θe air (approaching 350 K) is located
within the area of maximum low-level flow in the northeastern quadrant of Belle, while
the orientation of the ridge adjacent to the tropical cyclone has resulted in a persistent
southerly flow that is acting to transport a tongue of higher θe air (~335 K) into eastern
portions of New England.
As the tropical cyclone continues to move northward along the coast (Fig.
3.34b), the building ridge remains firmly in place and does not become extremely
deformed in the presence of the circulation. Consequently, a further increase in the
magnitude of the north-south oriented region of geopotential height gradient occurs,
resulting in an increase of the low-level jet to 18 m s-1. The highest θe air is located in
the northeastern quadrant of Belle and is close to impinging upon areas of southern New
England, while weaker transport of tropical air induced by the southerly flow between
the two systems has occurred well in advance of the center of circulation, extending into
northern portions of Maine.
Twelve hours prior to the landfall of Belle, the geopotential height gradient
between the western Atlantic ridge and the tropical cyclone continues to strengthen,
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resulting in a southeasterly low-level jet on the order of 20–23 m s-1 (Fig. 3.34c). The
location of the most pronounced low-level flow is displaced slightly offshore, with the
leading edge of the jet wrapping around the northeastern side of Belle and advecting
warm, moist air into central and eastern regions of the northeast US. Closer to landfall,
the orientation of the low-level jet becomes less southeasterly and proceeds to become
more southerly as a slight weakening of the high over the western Atlantic Ocean is
observed (Figs. 3.34d,e). This subtle reorientation of the low-level jet advects the main
axis of tropical air across eastern portions of New England; however, an appreciable
amount of flow exhibiting an easterly component on the north side of the system is able
to maintain a significant flux (although less pronounced than to the east) into interior
portions of New England.
3.2.3.4 Surface Analysis
Inspection of a manual surface analysis close to the time of landfall on Long
Island, New York, reveals a weak thermal boundary (~3oC/100 km) in the presence of
convergent flow along a north–south line extending through eastern Connecticut (Fig.
3.35), coincident with an area of observed heavy precipitation. The subtle coastal front
is a very short-lived feature that is not well represented in analyses containing θ and θe
gradients due to a critically low number of observations in the area, and a close
proximity to questionable interpolated analyses over the open ocean (not shown).
A surface plot immediately after the initial penetration of Belle into interior
sections of the Northeast (Fig. 3.36) shows that the air mass encompassing the area has
become rather homogeneous, with little evidence remaining of the subtle coastal front
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boundary in northwestern Connecticut. The positioning of the tropical cyclone center is
such that an enhanced easterly flow around the northern periphery of the cyclonic
circulation is beginning to impinge upon the southern extension of the Berkshires
located in extreme northern areas of Connecticut. Intersection of the low-level flow with
the topographic barrier farther north is inferred from multiple reports of due easterly
surface winds in Massachusetts and Vermont. Movement of the cyclonic circulation into
central Massachusetts by 1100 UTC 10 August (Fig. 3.37) simultaneously repositions
the easterly flow on the north side of the system to southern areas of New Hampshire
and Vermont, resulting in flow mechanically forced upward along the eastern slopes of
the Green Mountains. A signature of a weak inland coastal front along the Vermont and
New Hampshire border is manifested as the intersection of northwesterly wind over
Vermont and southeasterly wind over New Hampshire; however, the presence of the
coastal front cannot be further verified due to a lack of data in the region (the coastal
front is not analyzed on Figs. 3.37 and 3.38). The observed easterly wind flow continues
to produce upsloping along the windward facing slopes of the high terrain of the
Northeast as Belle continues to move in a northward direction through northern New
England (Fig. 3.38).
3.3 The Events Surrounding the Passage of Hurricanes Connie and Diane
3.3.1 Overview
Within a nine-day period (12–20 August 1955), Hurricanes Connie and Diane
made landfall in the Mid-Atlantic region of the US. Poleward propagation of the two
tropical systems produced copious amounts of precipitation throughout the region. The
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subsequent collocation of heavy precipitation associated with the two events combined
to produce major flooding in the southern New England region. Nearly five billion
dollars of property damage, 180 deaths and 200 partial or total dam failures were
reported.
3.3.2 Hurricane Connie: 12–14 August 1955
Hurricane Connie made landfall in Cape Lookout, North Carolina, on the
morning of 12 August 1955. Subsequent to landfall, Connie slowly propagated
northwestward. Precipitation fell along both sides of the storm track (Fig. 3.39), with
the heaviest accumulations in excess of 250 mm located in a narrow band extending
from the Mid-Atlantic region northward into western New England (Fig. 3.40).
The 500 hPa geopotential heights and absolute vorticity analyses (Figs. 3.41a,b)
show a northward-displaced flow passing through southern portions of Canada with a
short-wave feature lifting northeastward over a ridge anchored over the western Atlantic
Ocean. Minimal interaction between the tropical cyclone and the short-wave trough is
evident from the geopotential height and vorticity fields, resulting in relative isolation of
Connie from the main westerly flow. Parallel positioning of the geopotential height and
absolute vorticity contours along the Mid-Atlantic States suggests that CVA is very
weak to nonexistent throughout the event.
Upper-level western Atlantic ridging has led to delayed propagation of an
intensifying jet core to the north and east, allowing the maximum upper-level divergence
associated with the equatorward entrance region of the jet to stagnate over the areas of
heaviest precipitation in the Mid-Atlantic and southern New England regions for an
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extended period of time (Figs. 3.42a,b). Examination of the lower levels shows that at
850 hPa (Figs. 3.43a,b) the main baroclinicity is displaced well to the north in Canada.
An increase in the geopotential height gradient between the western Atlantic ridge and
the tropical system has resulted in an increase of the low-level southeasterly jet to ~20
m s-1 over the Mid-Atlantic and southern New England regions. The 925 hPa analysis
(Figs. 3.44a,b) reinforces the observation that an intensification of the geopotential
height gradient has occurred. In response to the increase of the geopotential height
gradient, both the speed of the low-level southeasterly jet and the advection of high θe air
into the Mid-Atlantic and southern New England region have increased, efficiently
feeding the lower branch of the inferred thermally direct ageostrophic circulation
associated with the upper-level jet. Low-level wind analyses (not shown) suggest the
occurrence of coastal frontogenesis along an area stretching from eastern Maryland to
Pennsylvania, which will be investigated in more detail shortly.
A surface analysis for 0330 UTC 13 August (Fig. 3.45a) shows Connie centered
in the vicinity of extreme southeastern Virginia. An intense baroclinic zone draping
southwest to northeast across central Maryland and southeastern Pennsylvania is
collocated with convergent flow, producing an area of coastal frontogenesis and
enhanced precipitation over the northwestern shores of the Chesapeake Bay (refer back
to Fig. 3.40 as necessary for coastal front locations). By 0630 UTC (Fig. 3.45b) the
baroclinicity and convergent flow have continued to propagate northeastward pushing
into extreme southeastern Pennsylvania. The coastal front continues its northeastward
movement through 0930 UTC (Fig. 3.45c), until it shows signs of weakening across
east-central Pennsylvania by 1230 UTC (Fig. 3.45d). The weakening can be attributed
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to a slight rotation of the baroclinic zone, such that it has become oriented quasi-parallel
to the wind flow. A shift in the wind direction from northeasterly to easterly between
0930 UTC and 1230 UTC occurs in southern New England, resulting in flow
perpendicular to the Berkshire and Green Mountain ranges.
Figure 3.40 contains a plot of fine-resolution precipitation accumulations and
approximate coastal front locations analyzed every three hours. A general swath of
extremely heavy precipitation extends from the Chesapeake Bay area northeastward into
central New England. The movement of the coastal front can be seen to coincide with
the axis of heaviest precipitation. Daily precipitation analyses for the 24 h period (1200–
1200 UTC) ending on 12 and 13 August (not shown) indicate that the heavy total
accumulations located in eastern areas of Virginia were not due entirely to the coastal
front, but were partially induced by the approach of the tropical cyclone to coastal
locations of the Mid-Atlantic.
3.3.3 Hurricane Diane: 17–20 August 1955
Hurricane Diane made landfall on the morning of 17 August 1955 in Carolina
Beach, North Carolina. Initial precipitation distributions (Fig. 3.46) position the greatest
accumulations to the right of track, occurring through the central Virginia area. Slow
northward post-landfall movement is evident, with a preference for a slow right turn and
eventual acceleration to the northeast. The precipitation shield exhibits a shift to a leftof-track orientation as the right turn takes place, leaving southern New England to bear
the brunt of the heaviest precipitation, which accumulated to 400–450 mm (Fig. 3.47).
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A large negatively tilted trough near the center of Hudson Bay at 500 hPa (Figs.
3.48a,b) exhibits a short-wave feature propagating through the extreme southeastern
extent of the trough, while a ridge is positioned over the western Atlantic Ocean. A
marked influence on the propagation of Diane can be inferred from the large-scale flow,
manifested by the increased translational speed and the change from northward to
northeastward movement. Interaction between Diane and the short wave is revealed by
the partial merger of the respective vorticity fields surrounding the two features, with the
short wave acting as the catalyst for the right turn and movement of Diane parallel to the
southeastern New England coast.
Upper-level amplification of the western Atlantic ridge has acted to intensify and
reconfigure a jet over Newfoundland into a more potent and compressed jet core on the
order of 45 m s-1 (Figs. 3.49a,b). Maximum upper-level divergence associated with the
equatorward-entrance region of the developing jet has positioned itself in a broad area
that is collocated with the belt of heavy precipitation over southern New England.
Examination of the lower levels shows that at 850 hPa (not shown) the main axis
of baroclinicity is still contained well to the north in association with the trough over
Hudson Bay. The tropical system maintains a cutoff signature in the geopotential height
field along the Mid-Atlantic, albeit weak, and a brief intensification in the
south/southwesterly low-level jet occurs over southern New England. The 925 hPa
analysis (Figs. 3.50a,b) reveals an increase in the geopotential height gradient between
the reinforced Atlantic ridge and Diane, producing an acceleration of the low-level
southerly/southwesterly jet to ~18–20 m s-1 and a significant increase in the transport of
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very warm and moist high θe air into the southern New England region (approaching 360
K).
A surface analysis for 0630 UTC 19 August (Fig. 3.51a) shows Diane centered in
southeastern Pennsylvania. A slight compression of the isotherms exists over southern
New England and is collocated with convergent flow in extreme southern portions of the
region to form a coastal front. The coastal front is formed and maintained by the
intersection of tropical-cyclone induced northeasterly inland flow and southerly onshore
flow. The coastal front remains relatively stationary over the next 3 h (Fig. 3.51b). A
slight progression of the front to the north and east occurs by 1230 UTC as Diane passes
over Long Island, New York (Fig. 3.51c), while the precipitation at this time is driven
predominantly by the collocation of and interaction between convergent flow and a
small-scale baroclinic zone. At 1530 UTC (Fig. 3.51d) Diane has moved toward the
eastern tip of Long Island, New York, pushing the coastal front even farther to the north
and east, impinging upon the Boston, Massachusetts, area. Toward the end of the
analysis period, the temperature gradient coinciding with the coastal front has weakened
to ~3oC/100 km, resulting in the slow dissipation of the front as Diane moves eastward,
south of Rhode Island. The heaviest precipitation was found to reside in a narrow band
north of the coastal front, as exhibited in a finer-resolution precipitation analysis with
approximate coastal front locations analyzed every 3 h (Fig. 3.47).
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