An Explanation of Thursday’s DC Tornadoes

Washington DC’s weather gang put together a report explaining the types of tornadoes that hit DC on Thursday. The Weather Gang was seeking to explain that these tornadoes are not the ones that we are used to seeing, or hearing about. The tornadoes which touched down in and around Washington were not tornadoes caused by a super cell  thunderstorm that are created by a spiral updraft also called a mesocyclone, but instead were “squall line tornadoes” which form along a bow-shaped series of thunderstorms. These squall lines, called quasi-linear convective systems (QLCS) are reported as being responsible for 20% of all tornadoes in the U.S., and are smaller, weaker and shorter-lived than their super cell counterparts.

This specific squall line approached the DC area from the southwest. However the Weather Gang report that this squall line was unusual in nature. Firstly there was no lightening, the line itself was very wavy in nature- arching and bowing in several areas, and lastly it formed in area of only relative instability. An extremely supportive upper atmosphere compensated for the low instability ultimately allowing for the squall line to be formed. In this case the supportive upper-atmosphere was a vorticity maximum, which is characterized by an extremely vigorous ascent of air beneath the jet stream. The squall line is a direct reflection of the violently rising air along the outer edge of the vorticity maximum.

This particular squall line had the appearance of a regular QLCS, which is why multiple tornado warnings were issued for areas around DC. However meteorologists are still learning and researching about the way in which these systems form, and develop. What is known about these QLCS systems pose real problems for detection because their parent storms, misocyclones are shallower and shorter-lived than the mesocyclones which cause super cell tornadoes. This means that they can easily fool Doppler radar, and they are also hard to see visually because they are often embedded within storms and are hidden by the heavy rain that falls along the squall line. They are even hard to determine post-mortem because the strong rotary winds become embedded within the powerful straight-line winds of down bursts.

The squall line that formed along Washington, showed the counter-clockwise rotating couplets which are products of a misocyclone system. These rotating couplets were able to be observed because of the highly sensitive radar located at Andrews Air Force Base, south of DC. The rotating couplets of air were seen 10,000 feet over the ground, and while it is likely that these vortical circulations likely extended to touch the ground and became weak tornadoes which caused spotty wind damage across the district. While the spinning column of air characteristic of tornadoes were not present in many of the videos that the Weather Gang studied, but in the case of the likely tornado which hit the Tidal Basin and H. St. Corridor showed a spinning column of water spray on the river, and also rotation in the in the cloud base as well. The reason that the characteristic funnel cloud was not present in this case is most likely due to the fact that the pressure drop at the center of these rotating air columns was not sufficient enough to cause enough the right amount of condensation for a funnel cloud to form.

https://www.washingtonpost.com/news/capital-weather-gang/wp/2017/04/07/how-thursdays-tornadoes-in-the-washington-area-formed/