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.
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. There may be multiple waterspouts developing in the 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 simply depart the area.
Most pilots are weatherwise, but some are otherwise™
Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist
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