Gust-Front Related Waterspouts
On June 23rd, 2012, a strong thunderstorm produced a spectacular gust front over Boston Harbor. This gust front was associated with intense outflow that helped spawn a waterspout that roared ashore in Scituate as an EF-0 tornado. This waterspout, however, developed ahead of the gust front, yet merged with the cloud structure of the outflow, hinting at a type of interaction between the thunderstorm downdraft and the waterspout. This tornado caused minor damage, and was never documented by the National Weather Service.
A similar situation occurred in Plymouth on July 24th, when three waterspouts formed ahead of the gust front of a severe thunderstorm, with one of these tempests roaring ashore on White Horse Beach as an EF-0 storm. Photos taken of these spouts reveal their formation ahead of the gust front, with a downdraft/waterspout interaction similar to the situation of June 23rd. Time-Lapse photography of the gust front taken moments after the dissipation of the spouts reveals a horizontally oriented vortex a few hundred meters ahead of the storm's outflow boundary.
The spinning of this vortex is likely caused by friction between the cooler air subducting behind the gust front against the warmer air ahead of the front. This warmer air, displaced by the gust front, is forced to rise into the storm above it. The conflict between this cooler, sinking air and the warmer, moisture-laden air is responsible for creating this horizontally-oriented vortex.
The vortex, on occasion, may be tilted onto a vertical axis by the warmer air ahead of the front; this same warm air now infiltrates the vertically-oriented vortex, thus being stretched, and forming a small waterspout. Because the waterspouts, however, are under the influence of the cool air from the thunderstorm downdraft, the spouts may move erratically before being upended. This erratic movement may at times cause them to move ashore.
In his book Nature's Ultimate Windstorm, Thomas P. Grazulis coins the term ‘gustspout', which I believe would be an appropriate manner in which to refer to these gust front related waterspouts. Many people may assume, however, that because of phonetic similarities, ‘gustspouts' and ‘gustnadoes' may be similar. Conversely, both exhibit very different behavioral patterns, with gustspouts a force to be reckoned with particularly in ocean-side communities. The gustnado, on the other hand, is far more innocuous. Gustnadoes also feature descending air currents, whereas gustspouts are associated with rising air.
Because of the inability for NWS radar domes to detect the microscale circulation associated with such gust-front related waterspouts, forecasters at NWS offices in Massachusetts will now focus on the predecessors of waterspout formation. Among these tell-tale parameters is a well-pronounced gust-front moving over warm ocean waters, as well as a sharp, highly localized temperature contrast between the air ahead of and behind the gust front. When the NWS believes that conditions will favor the development of gust-front related waterspouts that may move onshore, a special statement will now be broadcast through the EAS system in Severe Thunderstorm Warnings. Mariners will benefit as well, with Special Marine Warnings advising caution on ‘short-lived waterspouts'.
Despite these alterations in NWS offices in Massachusetts, much has yet to be done to warn the public of the dangers associated with gust-front related waterspouts. It is the responsibility of broadcasters to help convey when these erratic, short-lived waterspouts may move ashore. Hopefully, the public will be made aware of this phenomenon in the not-so-distant future; additionally, other coastal NWS offices may consider using these parameters in their warning systems.