Dense Fog — St. John the Baptist, Louisiana

2023-10-23 · St. John the Baptist, Louisiana

Event narrative

A chain reaction event of car accidents occurred on I-55 near mile marker 14 as dense fog reduced visibility to near zero. The pile up that took place included 158 vehicles and resulted in dozens of injuries and several fatalities. Many vehicles caught fire and one toppled over the interstate guard rail and ended up in Lake Maurepas.

Wider weather episode

Typical radiation fog forms when the surface quickly cools at night to become nearly saturated and can become dense when conditions trap more moisture near the surface, usually in the presence of high pressure as what was observed on 10/23. Super fog forms when a mixture of smoke and moisture released from damp smoldering organic material such as brush, leaves and trees, mixes with cooler, nearly saturated air or fog. Ongoing drought conditions set the stage for marsh fires caused by an unknown reason to develop and persist. Smoke from fires located near New Orleans East and Lafitte was trapped near the surface due to a strong temperature inversion that is typically present in fog setups. Smoke gradually spread westward during the morning hours due to the light easterly winds. This combo of smoke and fog conditions led to superfog over I-55 during the morning commute. A chain reaction event of car accidents occurred on I-55 near mile marker 14 due to the superfog. There were dozens of injuries and 7 fatalities.

Source: NOAA Storm Events Database, event_id 1148310. Narrative written by the NWS forecast office that issued the report.