12
Jun
13

Heavy weather tomorrow?

Well, as you may have read or heard about today, storms forming in the Midwest may produce a “derecho”, a Spanish term (much like tornado), and what it is is an area of very strong straight-line winds in advance of and onset of a band of thunderstorms that are racing across a region. T-storms act a lot like a vacuum cleaner, in terms of air movement. Upon initial development, they suck up air into their core, and that air has to go somewhere, so it exits at the top of the storm cell. That air then descends down the front and the rear of the cell, and if you have a situation where cells are forming a line (ie a line from north to south), they act in concert, where the sucking part is concentrated, and the exit part becomes concentrated, and the exiting air to the front of the cell line is a lot akin to a wave moving ashore. As the wave moves up the shore, it grows in height (bc the floor is coming up in this case), and it “rolls” over itself, crashing ashore. The wind is not much different in the physics of movement than the water crashing ashore. It rolls down the front of the cells, and forms a “wind” line. Frequently, the t-storm cells, and thus the air rushing ahead, form the shape of a backwards “C”, and that air is referred to as a “derecho”, and can exceed speeds of 70mph. It will be preceeded by a weird low-hanging dark cloud (that’s the evidence it exists, it forms this cloud, which is just moisture and dirt), and the wind rush will last from a few minutes to maybe 45 minutes. In the worst of cases, you can get a debris wall of ground objects that have been lifted into the wind line, which can cause as much or more damage than the wind itself. (Flying debris at 70 mph can do some damage! In fact, much of a tornado’s damage is precisely from previously lifted debris that is then part of the tornado.) Anyway, the short story is this: First, the Media and the Weather Service (NWS) are in a state of heightened alert in general, because of Oklahoma in recent weeks. So, they are not going to err on the side of conservatism. The NWS is going to sound the alarm, and the media is going to act in concert because it is looking for stories. Second, the “event” tomorrow, would be concentrated farther south than here, over much of PA and extreme southern NYState. That’s not to say we won’t get some gusty wind and some very heavy rain moving as far north as approximately the Thruway, but this is not as prime an area for widespread damage (wind or flooding) as 150-500 miles south. So, bring in loose patio furniture, be prepared for some minor to moderate river/stream/street flooding, and pay attention to my forecasts. You’ll be fine.

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