Funnel, Scud, ??June 10th, 2013 at 11:47 am by Bill Steffen under Bill's Blog, Weather
Click on the picture to enlarge. Jenni Vorenkamp Westra put this picture on my facebook page. This is Sunday evening in Hudsonville, by the RR crossing looking southeast. This was a shower (not a thunderstorm), pretty much a single cell by itself. It did produce brief heavy rain. I was tracking this cell on my phone while I was down at Festival and giving the Festival committee updates on where this shower and others were headed. From a still shot, you can’t tell if the lower cloud in the middle is rotating. This is a relatively small shower in size, not a big Oklahoma thunderstorm, so we are fairly sure that if this shower would have produced a tornado (and you don’t need lightning and thunder to get a small tornado), it would have been small and weak. This may very well be what we call a “scud cloud“. These are low and often scary looking clouds that hang down underneath showers or thunderstorms and can move fairly fast, but they by themselves do not produce tornadoes. The rule to remember…if it’s not rotating, it’s not dangerous. Tornadoes have to rotate. They also have to be strong enough to produce damage. A “tornado” with winds spinning around at 40 mph may not do any damage at all and in fact, the EF scale starts at 40 mph, so technically, a circulation with 39 mph winds isn’t yet a tornado.
Now, what if you’re the meteorologist on duty. You got a picture like this. The first thing you would do is look closely at Hudsonville on the radar. You wouldn’t rush to issue a tornado warning. You’d look for other clues. You might also call law enforcement to get more eyes on this. Next, if you did see or hear something that might lead you to issue a warning…you might think about the area and type of impact. If, by chance, you were to get enough confirmation to issue a tornado warning, you could issue it for a very small area. The shower was small and moving north. Do you really need to issue the warning for western Kent County and have the sirens sound throughout much of Kent County? Probably not. A warning could be issued for a relatively small portion of Ottawa Co. This is no danger from this shower in Holland or Zeeland, no need to sound sirens in Tulip City and disrupt activity there. One of the questions we are looking at now when we issue warnings is impact. Every time a tornado warning is issued, there’s an impact. People take action. If we issue too many tornado warnings when there is no tornado, people become complacent and not take action when there is a tornado that is a significant threat. That’s why trained spotters (and reports from the public like this picture from Jenni) are still a very important part of the severe weather warning process. A very small area of SE Ottawa County did see a quick half-inch of rain from this shower, which weakened some as it got past M-45.