Severe Thunderstorms in the Southeast U.S; Flash Flooding Threat for the Northern Gulf Coast

Severe thunderstorms are expected across portions of the Southeast and Carolinas today where a Slight Risk (Level 2 of 5) has been issued. A Slight Risk Excessive Rainfall Outlook (Level 2 of 4) has been issued for part of the northern Gulf Coast today due to the threat of flash, urban, and riverine flooding. Read More >

LOADING...

Overview

On March 26, 2018 two undular bores were observed across portions of the St. Louis Metropolitan Area. An undular bore is a wave or waves in the atmosphere that can be seen on radar and sometimes by the clouds they produce. They form when an air mass, such as a storm outflow boundary, collides with another air mass characterized by cool, stable air.

The first undular bore passed through the St. Louis Metropolitan area around 1:00 pm CDT. Two associated waves could clearly be seen in the base velocity radar data from both the WSR-88D (KLSX) and the Terminal Doppler (TSTL) radars. These waves traveled across St. Charles and St. Louis County at 50 to 60 mph, causing fluctuations in wind speed and direction, temperature, dew point, and pressure across the area. A time trace (pictured right) was constructed from 5 minute observations at St. Louis Lambert International Airport (KSTL). As the first wave moves across the airport the wind shifts from southeast to northwest and increases rapidly with a gust of 36 mph reported, while the temperature drops and the pressure rises nearly 4 mb. In the lull behind the first wave the wind shifts back to the northeast. The second wave causes a similar response at the observation site but is not as dramatic. This dampening of the wave is similar to throwing a rock into a pond and seeing the ripples expand from the center, and lessening with time.

The second undular bore developed around 2:30 pm CDT and was also observed by radar and captured on video by local storm chaser and weather spotter Tom Stolze in O"Fallon, MO.