Introduction
On a Saturday morning in late July 2012, nearly four-hundred junior sailors were assembled at the
Sandusky Sailing Club (SSC) in Sandusky, Ohio to participate in the Optimist Dinghy National
Championship Regatta. Saturday was the third day in the Open Championship which was held on the
protected waters of Sandusky Bay, east of downtown Sandusky and west of the Cedar Point causeway.
Optimist dinghies, affectionately called “Optis” by their owners, are competitively sailed by children who
are 15 years old or less. The boats are small – less than 8 feet long and 4 feet wide – and carry a single
sail measuring approximately 35 square feet. The sailors are typically organized into fleets based upon
their age and sailing experience. But don’t allow the size of the boats or the age of the sailors deceive
you -- Opti regattas are extremely competitive events.
Forecast
The Forecast Summary for July 28, 2012 (link to pdf) suggested that winds on the race course would
range from eight to fourteen knots from the northeast, and wave heights would remain below one foot.
The Summary also suggested a slight risk of showers and thunderstorms until early afternoon associated
with a low pressure system located over northeast Ohio. This system had a proven track record of
producing thunderstorms, with several clusters occurring over Lake Erie just before dawn. In response to
the combination of very warm Lake Erie water temperatures and cold air several thousand feet above
the surface, the Summary also raised the potential for waterspouts.
The Skipper’s Meeting
As with most regattas, the day’s activities opened with a skipper’s meeting at 9:00 am, an opportunity
for race officials to make important announcements, remind competitors about the format of the racing
and provide a short weather briefing. Several minutes into the meeting and just before the weather
briefing was to begin (between 9:10 and 9:15), “there’s a waterspout” was heard above the din and the
competitors, parents and coaches rushed from the tent for a closer look. The waterspout, and its parent
storm, was located approximately four miles east of the race course and was moving south.
Although I have sailed on Lake Erie for nearly twenty years and have “chased” waterspouts on several
occasions, this was my first sighting. It was an extraordinary experience, and although I lost track of the
time, I estimate the waterspout persisted for thirty or forty-five seconds. At no time was it a threat to
the Opti sailors, and much to my surprise, they quickly returned to the skipper’s meeting after it
dissipated.
Waterspouts: Two Varieties
Although both are intense columnar vortices and are considered “tornadoes over water”, waterspouts
come in two varieties – tornadic and fair weather. The fundamental difference being the type of storm
they are associated with and the manner in which they form. Tornadic waterspouts are produced by
supercell thunderstorms and form when the relatively long-lived mid-level rotation within the storm
extends down to the surface of the water. The self-sustaining nature of supercell thunderstorms may
promote long-lived and cyclic waterspouts. Fortunately, only 5% of all thunderstorms are supercells,
therefore the tornadic waterspouts occur very infrequently.
Far more common are the awkwardly-named fair weather waterspouts. Rather than descending from
the cloud to the surface, fair weather waterspouts form upwards from the surface of the water in
association with the updraft of a rapidly growing cumuliform cloud. Although fall less powerful than
their tornadic cousins, winds associated with fair weather waterspouts are capable of reaching sixty
knots or more and have a long history of capsizing small boats, shredding sails and damaging equipment.
They should always be avoided.
Fair Weather Waterspout Environments And Formation
Fair weather waterspouts harness the energy associated with the temperature difference between very
warm water and relatively cold air several thousand feet above the surface. When analyzing the
potential for waterspout development, meteorologists compare the lake temperature to the air
temperature at approximately five-thousand feet to determine if the difference will be 13°C (23.4°F) or
greater. Satisfying this condition determines that the atmosphere is supportive of rapidly developing
thunderstorm updrafts.
Large temperature differences between the surface of the lake and five thousand feet are easier to
achieve during late summer. As the chart of average monthly water and air temperatures shows, Lake
Erie typically reaches its maximum temperature in August, approximately one month behind the peak
air temperature over the Lake. As summer gives way to fall, transient weather systems promote the
invasion of much cooler air from the North which sets the stage for a dramatic increase in waterspouts
from July to October (link to image).
However, temperature difference isn’t the only ingredient needed to produce an environment that is
favorable for waterspouts. Since the vortex forms from the surface upward to the cloud base, a preexisting source of low-level rotation (vorticity) must be available. This rotation is predominantly counterclockwise and can be found along surface boundaries, such as a cold, warm or stationary fronts. Across
the Great Lakes, waterspouts frequently tap the rotation associated with land-breeze boundaries (link to
graphic) which typically form during early evening and persist until early or mid-morning.
Fair weather waterspouts form when the updraft associated with an existing cumulus congestus cloud
(link to image) – fueled by the temperature difference discussed above -- encounters, lifts and stretches
the pre-existing low-level rotation up to its base. Contrary to popular belief that a column of water is
being lifted up to the cloud, the visibility of the waterspout is due to the condensation of water vapor
that occurs as the air molecules cool as they are lifted skyward.
A fledgling waterspout is relatively fragile and easily disrupted by too much wind, variations in wind
direction or an encounter with cold air. Based on a study conducted by Wade Szilagyi, a meteorologist
with Environment Canada, waterspouts rarely form when winds at approximately 5,000 feet are greater
than 35 knots. It is also beneficial if the direction of the wind in the lowest few thousand feet is uniform,
or in technical terms, when directional wind shear is weak. Relatively fast winds or strong wind shear
inhibit the strength and continuity of the storm’s updraft and suppress the formation of the
waterspout’s small-scale vortex. In addition, because rain-cooled air would quickly undercut the
developing updraft, waterspouts typically form before the storm produces any precipitation.
Examining the Opti Regatta Waterspout
The storm that produced the Opti Regatta waterspout first appeared on radar east of Kelley’s Island at
approximately 8:36 am (Figure ???). Observations at the Marblehead Light several miles southwest of
Kelley’s Island, indicate that surface winds in the vicinity of the storm were northwest (330°) at 12.1
knots gusting to 14.0. During the next 15 minutes, the storm strengthened significantly as it moved
south-southeast at nearly 20 knots (link to image with track).
Typically by the end of July, Lake Erie reaches its seasonal high surface temperature. The analysis of
surface water temperatures issued by NOAA’s Great Lakes Coastal Forecasting System (GLCFS) indicates
the mean temperature for Lake Erie on July 28, 2012 was 76.6° F (24.8° C). Of particular interest
however, is the narrow pool of warmer water (80.0° F / 26.7° C) along the Lake’s southwest corner
including the area east of the Cedar Point peninsula.
The track of the storm suggested it remained over very warm water as it travelled south. An analysis of
air temperature at approximately 5,000 feet (850 mb) indicated that temperatures over southwestern
Lake Erie were approximately 55.4°F (13°C) at 8:00 am, resulting in a 24.6°F (13.7°C) difference between
the surface of the water and 5,000 feet. This temperature difference slightly exceeds the 13°C threshold
used to identify when the atmosphere is supportive of waterspout development. (talk about lakeinduced CAPE?)
Winds in the lowest 5,000 feet were also supportive of waterspout development. At 8:00 am, wind
speeds at 5,000 feet (850 mb) were approximately 16 knots, well below the previously discussed
threshold of 35 knots. In addition, the wind direction from the surface to 5,000 feet as observed by the
Doppler radar station at Cleveland was remarkably consistent (link to vwp image), satisfying the
threshold that directional wind shear not be too strong.
Identifying the nature of the pre-existing low-level rotation is the most challenging aspect of an analysis
of a waterspout. I suspect that a land breeze boundary or an outflow boundary associated with the
earlier thunderstorms may have provided the low-level rotation. However, the sparse nature of the
observational network simply doesn’t capture the data necessary to reach a definitive conclusion. In
some instances, a line of storms on radar or satellite imagery suggests the existence of some type of
boundary, but this was not the case for this incident. However, the analysis of surface vorticity (rotation)
produced by the Storm Prediction Center at 9:00 am on July 28 (add link to cropped mesoanalysis)
indicated an elongated area of surface vorticity (in blue) over the southern portion of Lake Erie
extending west to the vicinity of the Opti Regatta. And, of course, the waterspout itself is proof that a
source of low-level rotation was present.
8:41 am
8:46 am
8:50 am
9:00 am
9:05 am
9:10 am
8:55 am
9:14am
The series of radar images shows the development of the waterspout-producing storm from 8:41 am to
9:14 am. During this same period, the storm underwent significant vertical development, growing from
approximately 15,000 feet to nearly 31,000 in the 25-minute period ending at 9:10 am (link to page of
echo top imagery). This dramatic vertical development is consistent with the conceptual model of a
waterspout in which a rapidly strengthening updraft transitions to a waterspout. The waterspout, with a
lifespan of 30 to 45 seconds, occurred between 9:10 am and 9:14 am as the storm reached its maximum
height. And just 10 to 15 minutes after producing the waterspout, the storm reached the southern
shore of Lake Erie and dissipated.