Tornadic Activity During the Month of October (photo credit: NWS Storm Prediction Center, PSU Meteorology)
The season of Fall in meteorological terms is also known as the "second" severe weather season. Just like in the spring, the severity of the fall season varies from year to year. However, the important thing to remember is that severe weather, including tornadoes, doesn't just happen in the spring. As this October comes to a close, there has been an uptick in tornadic activity. In fact, multiple states across the U.S have had a tornado outbreak of some magnitude since October 1st.
First, we begin with Pennsylvania's historic outbreak. On October 2nd, 2018, multiple supercells and bow echos marched across the commonwealth producing 14 tornadoes. These 14 tornadoes completely shattered the state's previous October record dating back to October 5th, 1979, of four tornadoes, according to the National Weather Service (NWS) office in State College, PA. And what's more stunning is there were only 13 October tornadoes reported in the Keystone State since records began in 1950. Although this was suppose to be a low-end severe event, there were several atmospheric factors that played a huge role in the development of the severe thunderstorms that unfolded in Pennsylvania.
A disturbance in the upper-levels provided a combination of very strong winds aloft and a very unstable airmass. For severe thunderstorms to develop, there first needs to be decreasing clouds throughout the region of interest so that the suns rays can heat the surface. On the morning of the event, clouds stuck around for the most part. But ahead of the upper-level disturbance, temperatures at the surface surged well into the 70's while temperatures aloft became rather cold.
Despite the cloud cover throughout the region, the air mass became very unstable and severe thunderstorms began to develop. Many of these severe thunderstorms began rotating due to the very strong winds aloft as well as the differentiation of wind direction and speed in different levels of the atmosphere or what is known as wind shear. Surveys conducted on all 14 tornadoes concluded that these storms produced 4 EF-2's, 6 EF-1's and 4 EF-0's. This was one of the most active tornado days for Pennsylvania in over 20 years. Below is a graphic created by Weather World at Penn State University showing the locations of the 14 confirmed tornadoes.
Next is the state of Oklahoma. Sitting right in the "heart" of tornado alley, Oklahoma had a very slow start to the year tornado wise. To recap, zero tornadoes formed in Oklahoma from January through the end of April as cooler temperatures stuck around into the early weeks of spring. The first tornado didn't occur in Oklahoma until May 1st where a tornado touched down near Harper, Oklahoma. This now stands as the state’s longest wait on record for the first tornado of the year breaking the previous record set back in 1962, when the first tornado of the year hit on April 26th.
On the morning of October 9th, a line of pre-frontal thunderstorms associated with a cold front posed a threat for a tornado or two and damaging wind gusts as these storms moved slowly across central Oklahoma. These storms became strong enough to where they were able to form embedded low-level circulations which is the of the first of many steps to the formation of a tornado.
Right before the NWS Storm Prediction Center issued a tornado watch, a few of these circulations tightened up and were able to produce 5 brief and weak tornadoes just east of the Norman-Oklahoma City area. After multiple tornado reports came pouring in, the SPC followed up with a tornado watch that would include a majority of eastern Oklahoma. Most if not all of these weak tornadoes were rated an EF-0 by the NWS. The image below has the storm reports from that morning plotted with red T's indicating a tornado report.
In total, there were 27 tornado reports on October 9th stretching from Oklahoma to Iowa. It stands as of right now one of the top ten most active tornado days in 2018. For Oklahoma, there were 5 total tornado reports. Now 5 may not seem like that much, but when you compare it to how slow 2018 has been tornado wise, it was definitely a big day for the Sooner State. One last thing to point out is that Oklahoma is currently sitting at 22 tornadoes on the year, 34 shy of its annual average of 56.
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©2018 Meteorologist Joseph Marino