Waterspout in USVI, 2019 -Chelsea

Happy #WeatherWednesday everyone! Yesterday we had a question about the properties of waterspouts in the “TSC Community” group on Facebook. A waterspout is simply a tornado that forms over a body of water. Tornadoes generally form when there are nasty storms that develop mesocyclones and turn the skies black, blue and green. Waterspouts, however, generally form in partly cloudy conditions and are accompanied by little to no precipitation. In the photo above you can see a skinny funnel coming down from a cloud base producing nothing more than a light rain shower. So how can a waterspout form without a nasty severe storm brewing it up?

Besides a few waterspouts that form from supercells located over bodies of water, most waterspouts are very weak and develop on fair weather days near coastlines, requiring a line or area of cumulus to cumulonimbus clouds. They occur in many places like Italy, Croatia, the Florida Keys, and the Caribbean to name a few hot-spots.  Really, they can be found anywhere there exists relatively warm bodies of water. The warmer the water the higher the spout chances. Waterspouts may occur in the Great Lakes as well when cool Canadian air flows over water that is still very warm, typically in the late summer and early fall. The photo below dates back 21 years when one formed in the Sandusky Bay and hit Cedar Point Amusement Park in Ohio.

Waterspout forming in Ohio, 1998 -Jason

The normal waterspout is weak and lasts only 5-20 minutes. It will usually be stationary, positioned under a line of cumulus clouds near a subtle surface boundary (temperature/velocity gradient). The subtle boundary between the air mass over the water and the air mass over land is a source of horizontal vorticity needed to develop spin in the atmosphere. When you have this, waterspouts can really be quite common on days with fair weather and a light E/NE’ly wind. Too much sustained wind will have a negative impact on circulation generation in this microscale weather scenario. It is also fairly common to have multiple waterspouts happening simultaneously or in quick succession of each other underneath the same field of cumulus clouds. As many as 9 waterspouts have been reported all at the same time once on Lake Michigan.

When looking for waterspouts, the best place to find them is in tropical climates like the Florida Keys where the conditions (warm water, light wind, surface boundary, etc.) come together often. They are spotted when funnel clouds drop from the bases of the clouds offshore. Although a funnel may not appear attached to the water, a waterspout is almost certainly in progress. The water content in the funnel may just not be sufficient enough to fully condense the rotating column and visually represent the tornado. The funnel is normally formed closer to the end of the waterspout’s life cycle as this type of tornado starts at the surface and stretches its way up into vertically developing cloud structures. The spout is only visible in its early stages as a dark spot in the water, followed by spiral bands and even a spray ring as the spout becomes stronger. Winds in the spray ring are typically under 67mph, but still dangerous to mariners. It is illegal to intentionally intercept a waterspout on a boat in some jurisdictions. The NWS may issue Special Marine Warnings to alert mariners of severe weather and/or higher potential for waterspouts. Stay tuned for a future TSC venture exploring waterspouts later in 2019!