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Thursday, 31 May 2007 |
Hurricanes have been the cause
of many maritime disasters and
unfortunately, there is no single
rule of thumb that can be used by
mariners to ensure safe separation
from a hurricane at sea. Instead,
constant monitoring of hurricane
potential & continual risk analysis
when used with some fundamental
guidelines become the basic tools to
minimize a hurricane's impact to
vessels at sea or in port.
 Today,
even as our understanding of hurricanes
increases, there is still much
error inherent in forecasting the
movement & intensity of these systems.
Through the use of a recurring
risk analysis, mariners can
minimize potential impacts of a
hurricane encounter. Coincidental
with the fact that NHC issues 4
Tropical Cyclone Forecast/Advisory
Messages (TCM) per day when a
system is active, the risk analysis
needs to be done in conjunction
with each new TCM to ensure that
the sailor is evaluating the latest
information to make navigation
decisions. This risk analysis
includes a number of extremely
important factors needed to make
sound decisions & ultimately
remain clear of hurricanes either at
sea or in-port.
History of Regional Hurricane Tracks
There are climatologically
favored regions/tracks for hurricane
development/movement in the
North Atlantic. Both are important
to vessels at sea or in port in order
to begin assessing risks involved
during the hurricane season.
Knowledge of hurricane climatology
is the first significant aids in helping
mariners avoid these systems in
the North Atlantic.
Impact of Ocean Currents &
Warm Water
Certain areas in the basin support
rapid intensification of hurricanes.
Understanding the contribution
that warm water plays in the
growth of a hurricane, it is easy to
appreciate that ocean regions with
high sea-surface temperatures
(greater than 79° F or 26° C) are
often dangerous locations for
mariners to be found as a hurricane
threatens. Knowing local sea-surface
temperatures (SST) & location
of ocean currents are also important
factors for the mariner. The
two most prominent areas to possess
this danger are the Gulf of
Mexico & the Gulf Stream.
A second impact that the Gulf
Stream places on vessels is
enhanced sea states resulting from
the interaction of ocean current
with hurricane wind field. Winds of
tropical storm or hurricane force
opposing any ocean current can
quickly create very steep, short
period waves making navigation
through these areas extremely dangerous
and difficult.
Knowledge of Inherent
Hurricane Track/ Intensity
Errors
Generally speaking, the smallest
errors associated with hurricane
track forecasts occur while a system
is moving in a general west to westnorthwest
track, south of the
Atlantic subtropical ridge.
Conversely, the largest errors
involved in hurricane forecast
tracks occur during recurvature &
beyond as systems first slow when
starting to recurve, then typically
accelerate northeast after recurvature.
Similarly, increased uncertainty
in track forecasts often
occurs when a system is in an area
of little to no environmental steering,
a situation tending to occur
most often in the Western
Caribbean Sea & Gulf of Mexico.
By themselves, intensity errors
can be quite large through the 72
hour forecast of the TCM. These
errors are accentuated when a poor
intensity forecasts is combined with
the average track forecast errors
occasionally resulting in even worse
forecasts of the radius of tropical
storm force winds associated with
hurricanes, particularly at the 2 to
3 day range.
Guidelines For Avoiding Hurricanes At Sea
In order to help account for the
inherent errors in hurricane forecasting,
a few guidelines should be
used by the mariner in order to
limit the potential of a close
encounter between ship & storm.
34 KT Rule
For vessels at sea, avoiding the
34 KT wind field of a hurricane is
paramount. 34 KT is chosen as the
critical value because as wind speed
increases to this speed, sea state
development approaches critical
levels resulting in rapidly decreasing
limits to ship maneuverability.
It also deserves mention that the
state of the sea outside of the
radius of 34 KT winds can also be
significant enough as to limit
course & speed options available to
the mariner and must also be considered
when avoiding hurricanes.
1-2-3 Rule
This is the single most important
aid in accounting for hurricane forecast
track errors (FTE).
Understanding & use of this technique
should be mandatory for any
vessel operating near a hurricane.
The rule is derived from the latest
10-year average FTE associated
with hurricanes in the North
Atlantic. Application of the rule
requires information from the TCM
and is extremely important to
remaining clear of a hurricane at
sea.
The 1-2-3 rule establishes a minimum
recommended distance to
maintain from a hurricane in the
Atlantic. Larger buffer zones should
be established in situations with
higher forecast uncertainty, limited
crew experience, decreased vessel
handling, or other factors set by the
vessel master. The rule does not
account for sudden & rapid intensification
of hurricanes that could
result in an outward expansion of
the 34 KT wind field. Also, the rule
does not account for the typical
expansion of the wind field as a system
transitions from hurricane to
extratropical gale/storm.
Ship Versus Hurricane Track
Analysis
In the dynamic state of moving
ships & hurricanes, recurring comparison
of hurricane forecast track
versus planned ship movement is
mandatory. The continual monitoring
of the latest official NHC forecasts
compared to current or
planned evasion options can greatly
increase a mariner's confidence
regarding vessel safety.
Never Cross The "T"
Never plan to cross the
track (cross the "T") of a
hurricane. Done out of
respect for the negative
effects that heavy weather
places on vessel speed/handling,
sudden accelerations
in hurricane motion can
ultimately place a vessel in
conditions not originally
expected thereby resulting
in disaster. Adjustments to
course & speed in order to
remain clear of the danger
area in a hurricane are the
most prudent navigation
decisions a mariner can
make in these instances.
Forecast Track
Tendencies
Comparison of the most
recent NHC forecast track
with forecast tracks from
the past 24 hours can sometimes
prove useful for determining
a trend in the forecast
motion of a hurricane.
For instance, a comparison
of forecast tracks issued
every 6 hours over the last
24 hours, may show a
noticeable shift right or left
(with respect to storm
motion) in the forecast track
of a hurricane. This information
may provide some
indication as to how the
forecast & actual hurricane
track are tending and provide
more guidance in navigation
planning for avoidance,
particularly in the 2-3
day forecast range &
beyond.
Calcula ing Closest
Point of Approach (CPA)
The last item to complete
in the at-sea risk analysis is
comparison of CPA between
current & possible evasion
options. Over time, increases
in CPA between vessel &
hurricane based on current
navigation decisions should
increase the mariner's confidence
in current avoidance
plans. However, decreases
in CPA should be dealt with
using the utmost urgency.
An immediate review of all
evasion options combined
with a detailed look into the
latest official forecasts/discussions
needs to be accomplished
with a goal of establishing
a new evasion
course/speed option to once
again increase CPA from
the hurricane.
Assessing Options
Mariners must be cautioned
never to leave themselves
with only a single
navigation option when
attempting to avoid a hurricane.
Sea room to maneuver
is not a significant factor
when operating in the open
waters of the North
Atlantic, but becomes
extremely important in the
confined waters of the
Western Caribbean
Sea/Gulf of Mexico. More
often than not, early decisions
to leave restricted
maneuver areas are the
most sensible choice.
Port Specific Risk
Analysis Considerations
Vessels seeking shelter in
port or considering movement
toward or away from
port need to consider all the
factors discussed above
while acknowledging some
other factors in order to
finalize their risk analysis.
Hurricane Approach
To Port
In general, hurricanes
forecast to make a perpendicular
landfall tend to have
the smallest amount of
FTE. Conversely, systems
that are forecast to parallel
the coast, as is often noted
in the Mid-Atlantic region
of the United States, tend to
have larger track errors.
Additionally, hurricanes
that make landfall within
50-100 NM of a particular
port tend to be more
destructive than those that
approach the port from over
land or parallel the coast in
the vicinity of the port. Also,
ports located in the right
front quadrant, based on
direction of movement of
hurricanes during landfall
tend to have higher winds,
seas, and storm surge.
Go & No Go Decisions
To Leave Port
The decision to leave port
for hurricane avoidance
must be made very early.
Consideration to the latest
safe departure time & likely
avoidance routes must be
balanced with a number of
other factors. Most important
of these is time versus
distance. The risk of damage
to a vessel at sea
increases as the motion of
the hurricane increases
towards the maximum safe
speed of the vessel attempting
to leave port in advance
of that system. When
reviewing these time/distance
considerations,
mariners must include the
effects "squally weather"
associated with the rainbands
in a hurricane will
have on underway preparations
& movement from
port. Similarly, building
wind & sea conditions found
at sea, ahead of the hurricane,
can also hamper vessel
speed & maneuverability.
Recognizing these
time/distance problems, it
cannot be emphasized
enough that early decisions
to leave port in attempt to
avoid hurricanes are crucial.
There have been a
number of recorded
instances where vessels
have made the right decision
to leave port in
attempts to avoid hurricanes,
yet were still either
damaged/lost because that
decision came too late.
Berthing & Shelter
Requirements
Considerations to remain
in port during hurricane
passage must include an
evaluation of the amount of
protection afforded by the
port. The direction from
which the strongest winds
are forecast to blow along
with the potential for storm
surge must be factored in
when deciding whether to
seek haven pier side, at
anchorage, or further inland
to more protected anchorage.
For instance, storm
surge can pose significant
problems to vessels tied pier
side. Substantial rises in
water level may place a vessel,
previously in a protected
wind/wave regime, into
an area exposed to significantly
greater winds &
waves. Similarly, many port
& dock facilities, particularly
in the Caribbean region
are fixed. Although sufficient
to support the normally
small tidal range of the
region, they can quickly
become submerged when
exposed to even minimal
hurricane related surge.
Additionally, attention to
the tying of lines is also of
considerable importance.
This is because the force on
a moored vessel will nearly
double for every 15 knots of
wind from tropical storm
force (34 KT) to hurricane
force (64 KT). Therefore, a
vessel tied to the pier under
normal situations can
quickly break from the pier
in periods of higher winds
causing substantial damage
to itself or other vessels.
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