Back to hurricane background
and preparedness information
courtesy
of National
Hurricane Center
MARINE
SAFETY
RECEIVING
TROPICAL CYCLONE FORECASTS AT SEA
MARINE
BUOY OBSERVATIONS
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.
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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 west-northwest 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. See Marine Safety Rules of Thumb at right
for details on applying this most important technique.
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.
Calculating
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.
1
- 2 - 3
Rule of Thumb
1
- 100 mile error radius
for 24hr forecast
2
- 200 mile error radius
for 48hr forecast
3
- 300
mile error radius for 72hr forecast
Steps
to Determine the Hurricane Danger Area
1. Plot
the initial and forecast hurricane positions on
a navigation chart.
2. Find the maximum radius of 34 KT winds
at the initial, 24, 48, and 72 hour forecast times
of the TCM.
3. Apply the 1-2-3 rule to each of the radii
at the 24, 48, and 72 hour forecast positions.
4. Draw a circle around the hurricane initial
position with radius equal to the maximum radius
of 34 KT winds given in the TCM.
5. Draw circles around the 24, 48, and 72
hour forecast positions of the hurricane using the
respective radii found in step 3.
6. Connect tangent lines to each circle constructed
in steps 3 and 4 along both sides of the hurricane
track.
7. The area enclosed by these tangent lines
is known as the danger area of the hurricane and
must be avoided as a vessel attempts to navigate
in the vicinity of the hurricane.
