Posted: Apr 29, 2014 1:40 PM by Meteorologist Josh Eachus
A TORNADO WATCH has been issued by the National Weather Service to include the entire viewing area with the exception of Assumption and St. Mary Parishes until 9 PM.
Locations within the watch area may find strong to severe thunderstorms developing through the afternoon hours before activity lessens after dusk. The primary risks with today's storms include heavy downpours, strong winds and isolated tornadoes.
Remember that a WATCH means conditions are favorable for the development of a particular weather event and that you should plan accordingly. A WARNING means a severe weather event is imminent or occurring and you should act on your plan.
DISCUSSION: An upper-level low pressure system is centered over the Great Plains, while a surface cold front positioned ahead of the upper low and is slowly sagging southeastward. Like Monday, the atmosphere is juiced with instability and any storms that develop could quickly become strong t severe. However, the upper level waves of energy that enhance storm development will remain northeast of the Baton Rouge area. Mid-level winds and forcing will be more favorable in those locations as well. But, a key ingredient that was missing on Monday, a lifting mechanism, is present this afternoon as a surface cold front arrives. Additionally, an outflow boundary from morning thunderstorms south of the Mississippi and Alabama coastlines has also migrated westward into Louisiana and has also become a focal point for storm development. Precipitation chances are in the 50% range meaning that there is about a 1 in 2 chance that any given point in the viewing area experiences measurable precipitation.
A supplemental afternoon weather balloon launch displays a significant amount of instability aloft, and a strong morning cap has greatly eroded. This did occur yesterday but surface thermometers missed the temperature needed to release warm, moist air into the atmosphere by a degree or two and as was noted, no external lifting mechanism was present. Severe weather parameters also remain this afternoon. A morning vertical wind profile showing sufficient speed but lacking directional shear has now shifted with both types present. This will give developing thunderstorms the ability to rotate and potentially spawn a tornado or two.
Simulated radar runs show scattered showers and storms throughout the afternoon before congealing into a weak line near the 6pm hour. This is also a change from yesterday when models correctly projected little to no precipitation. Activity seems to diminish from west to east by midnight.