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Waterspouts - are they a
threat to aviation? 
Pilots that frequently fly over large bodies of water or along the
coastlines should be extra-sensitive to the potential of waterspouts
especially during the warm season from April through October. They are
more prevalent in the tropical climates, but can occur any place there
is water such as the Great Lakes. The image above shows
several
waterspouts in various stages over Lake Huron after the passage of a
cold front. Be especially careful flying along the south
Florida
coast. More waterspouts occur in the Florida Keys than
anywhere
else in the world!
Waterspouts that are not associated with deep, moist convection are
rarely dangerous to the public. Occasionally, they will move
inland and can damage small buildings or can be a risk to small craft
on the open water. But are they a threat to
aviation?
Generally speaking, they are not a serious threat. In most
locations in the U.S. they are relatively rare. They usually
occur over large bodies of water where few aircraft fly. And
when
they are mature waterspouts are ordinarily easy to see and
avoid.
But you definitely do not want to cross paths with one; winds within
and close to many waterspouts can exceed hurricane
strength.
The NWS classifies waterspouts into one of two
categories,
namely, fair weather waterspouts and tornadic waterspouts.
Fair
weather waterspouts can develop below fairly innocuous-looking cumulus
clouds usually oriented in a long line over the water. They work upward
from the water’s surface, forming under light wind within a moist and
unstable air mass. While deep, moist convection might be in
the
vicinity, most are not directly associated with
thunderstorms.
Tornadic waterspouts, on the other hand, are the
typical
tornadoes formed over water or tornadoes that move from land to water
(the most common). Therefore, tornadic waterspouts are associated with
thunderstorms. These waterspouts form by moving down from the
thunderstorm base, towards the water below similar to how tornadoes
form over land. Treat these like you would any other
thunderstorm
- keep your distance.
Waterspouts frequently
occur on subsequent days. Often when there's an outbreak on a
particular day, as long as the general weather conditions don't change
drastically, you can expect an increased risk for waterspouts for the
next couple of days.
The Great
Lakes also experience a relatively large number of waterspouts each
year. The highest frequency (shown in the bar graph below)
occurs
in September especially when the water temperature is higher than
normal due to an excessively hot summer. In many cases these
occur after a passage of a cold front where cold air aloft is ushered
in behind the front with very warm temperatures near the surface of the
water creating a very unstable atmosphere, a perfect environment for a
waterspout. Surprisingly, these events
are fairly well forecast, sometimes even a few days in advance.
If your proposed route takes you along the coastal
regions
or over a large body of water during the warm season, take a moment to
read the latest NWS marine forecast and advisories.
You may just discover there is an elevated risk of
waterspouts as
it was on this day (below) in mid-August off the Georgia and South
Carolina coastline.
Click here to read the complete
marine weather statement.
About an hour after this marine weather statement
was
issued, a pilot reported (below) seeing a tornado just off the coast of
Savannah, Georgia. More than likely, this pilot witnessed a
waterspout not a tornado.
HXD UUA /OV HXD180015 /TM
1340 /FL018 /TP PA30 /RM TORNADO MOVG E
GROWING IN SIZE
If a special weather statement has been issued for
an
elevated risk of
waterspouts, here are a few things to look for while
flying below the cloud bases.
1. Look for a dark spot on
the water (as shown to the right). This is usually the
beginning stage of the
waterspout.
This is not always easy to see unless you are reasonably
close looking
down
on the spot. This is your first clue to fly away
from this area.
 2.
Next, look for patterns of light
and dark-colored spiral bands originating around the
dark spot similar
to what is shown in the image on the right. Given that calm
wind conditions are favorable for the development of waterspouts, this
spiral pattern will show up clearly on the surface of the water.
 3.
As the waterspout grows, look for a "sea spray" to form at
the
base of the waterspout (as shown on the right) similar to the rotating
debris field you see
when a tornado first contacts the ground. The sea spray is
often
visible from a distance.
4. A mature waterspout
will have a rope-like appearance (see image at the top) similar to a
small tornado and may
eventually connect with the cumulus cloud above although much of the
waterspout may be nearly invisible. Waterspouts can occur in
small groups of two or more in various stages of
development. If you see one in the distance, keep a close
watch below the clouds for others that may subsequently form.
The best advice is to avoid flying under these long lines of
cumulus clouds or depart the area.
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