When a confirmed EF-0 tornado touched down in Chickasaw on a Friday afternoon, some residents wondered why the National Weather Service office in Mobile never issued a tornado warning for the storm.
Lead forecaster John Purdy said forecasters scrutinize the pre-storm environment before severe weather arrives, and that day the key parameters — instability and wind shear — did not favor tornadic development. The primary threat appeared to be straight-line winds, with instability running high but wind shear low.
The Spin-Up Problem
Purdy said quick spin-up tornadoes are one of the Gulf Coast’s toughest forecasting challenges. These weak, short-lived twisters often occur between radar scans, which run every five minutes, leaving forecasters no time to warn before they dissipate. By the time a warning could be issued, he said, the tornado is often already gone.
There is also a deliberate effort not to over-warn. Purdy noted that warning on every spin-up could mean ten tornado warnings on a single severe weather day — and a public that stops taking them seriously. Balancing alerts for minor spin-ups against making sure people respond to strong, life-threatening tornadoes is a delicate task, he said.