Severe Weather Topics

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href="/ilx/winter_monitor"> Winter Weather Monitor</a> </li> </ul> </div> </li> </ul> </div> </div> <div class="div-full"> <div class="cms-content"><p><strong style="color: rgb(0, 128, 0); font-size: medium; font-family: Arial, Helvetica, Geneva, sans-serif;">Week 2: What is Wind Shear?</strong></p> <p>Wind shear describes how the wind changes speed and/or direction with height.  There is almost always some degree of wind shear present within the atmosphere: however, it tends to be much greater across central Illinois during the winter and spring when strong storm systems frequently impact the region.  For example, when a powerful jet stream shifts overhead, wind speeds from the surface to jet stream level can increase by over 100mph.  The example below shows a diagram of winds from ground-level up to an altitude of 6km (about 3.7 mi).  The direction the wind barb points tells us the direction <strong>from which</strong> the wind is blowing and the number of sticks or flags on the barb tell us the speed of the wind.  A small stick equals 5kt, long stick 10kt, and a flag is 50kt.  As you can see from the diagram, the wind at the surface is from the southeast at 20kt while the wind 6km aloft is from the west-southwest at 55kt.  This represents a wind speed change of 35kt (40mph) and a wind direction change of about 45 degrees.         </p> <p><img alt="" src="/images/ilx/SWOP/directionalshear.jpg" style="width: 156px; height: 345px;" /></p> <p>Wind shear is important to severe thunderstorm forecasting, because if it becomes strong enough, it can tilt a storm's updraft in such a manner that the updraft and downdraft remain separate from one another.  If this occurs, the storm's energy source (it's warm/moist updraft) will not get choked off by the downdraft, and thus the storm will be able to maintain itself for a prolonged period of time.  The image below shows two storms...one in a very weakly sheared environment on the left, and another in a strongly sheared environment on the right.  The storm experiencing little or no wind shear will produce a vertical updraft, which will quickly get killed off by falling rain.  On the other hand, the storm experiencing strong wind shear will develop a tilted updraft with the rain falling away from the updraft.</p> <p><img alt="" height="451" src="/images/ilx/SWOP/updraft.jpg" width="735" /></p> <p> </p> <p>Wind shear can also enhance rotation within the thunderstorm updraft, which in turn can lead to the development of a tornado.  The picture below shows light westerly winds at the surface with very strong westerly winds aloft.  This creates a rolling motion within the atmosphere as shown by the red arrow, which if ingested into a storm's updraft can lead to the formation of a supercell thunderstorm and thus enhance the probability of a tornado.</p> <p><img alt="" height="280" src="/images/ilx/SWOP/tube.jpg" width="308" /></p> <p> </p> <p>For these reasons, strong wind shear is a chief indicator of long-lived and potentially severe thunderstorms.  Meteorologists assess wind shear in a number of ways, but one of the primary tools is 0-6km bulk shear.  This parameter describes the deep-layer wind shear from the surface up to 6km aloft.  Supercell thunderstorms become more likely as values increase to greater than <span style="color:#FF0000;"><strong>35kt</strong></span>.  As always, wind shear is just one of many tools available for severe weather forecasting and should not be used alone in order to determine the potential for strong to severe thunderstorms.</p> <p> </p> <p> </p> <p><a href="https://www.weather.gov/ilx/swop-severetopics-CAPE"><strong style="color: rgb(0, 128, 0); font-size: medium; font-family: Arial, Helvetica, Geneva, sans-serif;">Week 1: What is CAPE?</strong></a></p> </div> </div> <div style="clear:both"></div> </div> </div> </div>   <div class="full-width-bordertop"> <div class="full-width-first communication-links"> <div class ="one-sixth-first divs"> <div 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