On May 20th, 2019, NOAA's Storm Prediction Center (SPC) released their severe weather outlook. This outlook called for a high risk of a severe weather outbreak. The last time that the SPC went high-risk was on May 18th, 2017. The SPC only chooses to forecast for a high-risk day when they are confident in the chances of numerous long tracked tornadoes or a long-lived thunderstorm system such as a derecho. The high-risk forecast occurs when they see the four main ingredients, shear, lift, instability, and moisture in excess and if they could cause these storms to last.
Looking back on forecasts for May 20th, they showed a low-pressure system that would be position in the upper part of the Great Plains. This low-pressure system had a central pressure less than 1000 millibars. Systems with pressure less than 1000 millibars can help to initiate storm development. Often with a low-pressure system, fronts start to form, where they are connected to the low-pressure system. Fronts can also provide a way for the storms start, after all, they are a boundary where two different types of air masses meet.
Looking at another ingredient for severe weather on this day, moisture, the lower portion of the Great Plains has a special type of boundary called a dryline. The dryline is a boundary that divides the warm, moist air from the warm, dry air. As a result, the dew points on either side can not only provide a measure of the moisture in the air, but the dryline can also provide another lifting mechanism for parcels of air to rise. The dew point values in the area highlighted by the Storm Prediction Center were into the upper 60’s to lower 70’s, showing that there was plenty of moisture for convection to occur. These high dew points also added more instability into the atmosphere as well. So in order for severe weather to happen, one more ingredient was needed: shear.
Analyzing the soundings from the NWS offices located in this area, the winds were moving from a slight southeast direction the surface to turning more towards the southwest as the weather balloon moved higher up into the atmosphere. Not only that, the speed of the wind increased as the parcel of air rose.
So all four ingredients were present, now why was May 20th not as devastating when looking at the tornadic potential? These storms, produced baseball-sized hail, numerous flooding incidents, and a couple of tornadoes, kept merging together. As a supercell started to show signs of becoming tornadic, it would merge with another cell. After this pattern occurs, instead of discrete supercells, there would be a quasi-linear convective system. These systems can produce rapid spin-up tornadoes and hail, but these types of severe weather are often limited due to the fact that they are in a line and connected to one another. The linear nature of the storms on May 20th caused heavy downpours, leading to multiple flash flood warnings throughout the high-risk area.
While any type of severe weather is damaging, the high-risk area did not see long-lasting tornadoes, instead, the high-risk area got hail and rain, leading to a major flooding event. These areas did experience high wind gusts along with a few brief spin-up tornadoes. These areas did not have to experience the results of a long track tornado as the SPC predicted.
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Sources: NOAA SPC, https://blog.nssl.noaa.gov/ewp/2019/05/21/nucaps-data-from-yesterdays-may-20th-event/
©2019 Weather Forecaster Shannon Sullivan