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
How 2 Tornadoes Helped Initiate Severe Weather Forecasting in the U.S. (Credit: The Weather Channel)
DISCUSSION: In the early 20th century, official tornado warnings were not allowed in the U.S. because the Weather Bureau (precursor to today’s National Weather Service) didn’t want to cause panic based on low-confidence forecasts (given our limited understanding of tornadoes). However, two tornadoes struck Tinker Air Force Base (AFB) in Oklahoma in March 1948 while two prominent Air Force meteorologists (Major Ernest Fawbush and Captain Robert Miller; pictured above) were stationed there that would begin to change that.
Initially, there was no mention of thunderstorms in the forecast for the evening of 20 March 1948. Later that evening, reports came in of a tornado that had damaged an airport to the southwest of Tinker AFB and was moving to the northeast toward Tinker. Unfortunately, those reports came in too late to warn and prepare the AFB. The result was $10 million in damages including the destruction of 52 aircraft.
After this event, a study was put together to determine if better severe storm forecasts could be issued to prevent such damage in the future. Fawbush and Miller identified several large-scale conditions that tended to precede the formation of tornadoes and observed these conditions form again five days later on 25 March. The two base meteorologists decided to issue a forecast similar to today’s tornado watch. This forecast was a success when a second tornado in five days struck Tinker. This tornado still caused $6 million in damage, but there were no injuries likely due to the base preparations that were enacted after the forecast was issued.
According to information from Tinker AFB, 89% of the tornado forecasts issued by Fawbush and Miller verified, which is pretty incredible given the rudimentary observing system and infancy of severe weather science at that time relative to today’s tools and understanding. This success led to the development of the Severe Weather Unit of the Weather Bureau in 1952 which is the precursor to today’s Storm Prediction Center.
Essentially, two tornadoes that occurred in 1948 prompted two Air Force meteorologists to study the conditions under which tornadoes form and issue the first official tornado forecast. This successful forecast and subsequent successful ones helped spawn the severe weather forecasting enterprise that currently exists in the U.S. and which has undoubtedly helped save countless lives.
To learn more about other past historic weather and science events from around the world, be sure to click here!
©2019 Meteorologist Dr. Ken Leppert II