Several days of soaking ahead
Today and Tonight: Showers and thunderstorms will begin to develop as early as this afternoon. Most of the action should remain west of an Amite Count to St. Mary Parish line—not to rule out a shower elsewhere. The main point is that today’s activity will be more scattered in nature, and some locations could stay dry until tomorrow. High temperatures won’t exceed the mid to upper 70s on account of thicker clouds. Showers and thunderstorms may wane a bit during the overnight hours with a loss of instability. Lows will be in the mid 60s.
Up Next: The remainder of the week will be very unsettled. Periods of rain and thunderstorms are in the forecast through Friday. A few severe thunderstorms will be possible Wednesday to Thursday Morning with heavy rain, gusty wind and isolated tornadoes being the main threats. Several inches of rain will be possible on Thursday. At this time, it looks as though the final full day of rain chances will be on Friday. A general 5-8” is expected area wide with locally higher amounts possible. Temperatures won’t move much overall the wet stretch with highs in the mid 70s and lows in the mid 60s.
Forecast Discussion: Several ingredients are coming together to support a long duration rain event across the region. A strong upper level low is cutting off over the Rio Grande. Deep return flow of moisture has been loading the atmosphere for several days. By Tuesday Night, perceptible water values will increase to around 1.5”. As mid to upper level heights begin to fall, some instability will arrive allowing showers and thunderstorms to spark off by the afternoon. Any severe weather should stay over the Atchafalaya Basin and points west. On Wednesday, the better dynamics for severe storms percolate eastward bringing helicity values of over 300 into the Baton Rouge area. Moisture will continue to increase, with some instability possible dependent on the upper air temperatures. The Storm Prediction Center is highlighting areas west of I-55 in a slight risk for severe weather and this seems appropriate. Gusty wind, heavy rain and even a few tornadoes may be the result. By Thursday, the upper low will move toward East Texas with a surface low in Northeast Texas. Perceptible water will have increased to over 2”. In an environment of steady positive vorticity advection and elevated omega (uplift), an axis of heavy rain is expected to set up over our forecast area. 2-4”+ could fall on Thursday alone—especially where training bands of rain become established. Both river rises and flash flooding will be a concern and we can expect the National Weather Service to issue a flash flood watch in the near future. Additionally, minor coastal flooding may be a concern as southerly winds slow drainage from the Amite, Tickfaw and Tangipahoa Rivers. By Friday, forecast models show some discrepancy. The GFS model brings precipitation to a close as the upper low tracks northeast and away from the area. The slower ECMWF and GEM models hold the trough axis west of the Baton Rouge area until Saturday Afternoon which would keep showers in the forecast. Given that these trends have persisted for a few days, we’re blending the model forecasts and calling for a rain shutoff in the Friday Night timeframe—that part of the forecast is certainly subject to scrutiny with updates needed.