Central Alabama Chapter of the
National Weather Association
Wednesday, March 21st, 2012
After a brief summary of past/upcoming chapter activities and other news, Jim Westland, forecaster at NWS Birmingham, took the floor.
Jim Westland spoke about the January 23, 2012, tornado event
He talked about why the storms transitioned from squall line to discrete/supercells. There was a typical negative tilt & other factors pointing to severe weather. There was a squall line ahead of the front, but later on discrete cells began to develop.
What were the main reasons that occurred?
There was a 45 degree angle between the squall line orientation and the 0-6km bulk shear vectors after midnight. He mentioned how this was the key factor in the transition from linear to discrete cells in this event.
He also showed us a flow chart that can be used to help determine whether storms will be severe or non-severe.
After some discussion following Jim’s talk, the floor was passed to Kevin Laws, Science and Operations Officer (SOO) at NWS Birmingham.
Kevin Laws spoke about using Dual Pol data during and after the March 2, 2012, severe weather event.
The clearing of skies and subsequent warm-up was not anticipated....which effectively doubled the LCL heights. There was dry air aloft and moisture surging northward by 00Z with a strengthening low level jet. By 03Z, the best moisture and shear was across the south.
He pointed out that the Autauga/Chilton County storm had what appeared to be a debris signature associated within the inflow region. However, this was not indicative of a tornado.
Why was this feature present if it wasn’t associated with a tornado? The answer…non-uniform beam filling.
The consequence…We are going to have to be VERY careful with debris signatures.
Kevin then talked a little about tornado warnings and his proposed strategies to help lower the false alarm rate/perception. His suggestion is to extend the warnings in time, but not in space. In other words, make sure the warning has enough time, but do not include portions of counties that will not be impacted for quite some time. This will help in turn alleviate the problem of counties being warned numerous times for the same storm.
It seemed like everyone was very interested in this approach and the possible benefits it would provide.
The meeting was adjourned around 1230pm.
John De Block