91L Slow to Develop, But a Heavy Rain Threat for Mexico, Texas, and Louisiana

A potential early-season tropical storm is brewing in the southern Gulf of Mexico’s Bay of Campeche, where Invest 91L is slowly organizing. The system has the potential to be a heavy rainmaker for the northeast coast of Mexico and the coasts of Texas and Louisiana this week.

Satellite images on Monday showed that 91L was headed northwest at about 5 mph and had a moderate-sized area of heavy thunderstorms that were slowly growing more organized. The system was under low wind shear of 5 - 10 knots, was over warm waters of 29°C (84°F) and was embedded in a moist atmosphere with a mid-level relative humidity of 70 - 75%. These conditions favor development. Due to 91L's continuing disorganization, the Air Force hurricane hunter aircraft scheduled to investigate 91L on Monday afternoon was cancelled, and has been rescheduled for Tuesday.

Invest #91L remains disorganized today, but still has at least 36-48 hr over water to work with.

Best chance to organize will come when major axis of the elliptical circulation rotates perpendicular to mean flow, which will maximize curvature vorticity. Right now it's parallel. pic.twitter.com/gJoQKFwJCl

— Levi Cowan (@TropicalTidbits) June 3, 2019

Forecast for 91L

A trough of low pressure that was over Southern California on Monday morning will move west this week, and the southerly flow of wind in advance of this trough will force 91L to take on a more northerly track at a faster forward speed of 5 – 10 mph on Tuesday and Wednesday. This motion will bring the center of the storm very close to or just inland along the coast of northeast Mexico a few hundred miles south of the Texas border on Tuesday night or Wednesday morning. The SHIPS model predicted that the approaching trough would increase wind shear to a moderate 10 – 20 knots on Tuesday, and a high 20 – 30 knots on Wednesday, which would likely slow or halt development.

Figure 1. Predicted five-day rainfall amounts ending on Saturday, June 8 for 91L made by the 0Z Monday, June 3, 2019 run of the HWRF model. The model predicted that 91L would intensify into a strong tropical storm with winds of 70 mph and bring a large area of 4 – 8” of rain (dark yellow colors) to the coasts of northeast Mexico, Texas, and Louisiana. This rainfall forecast is at the upper end of what 91L will likely be able to achieve, and lower rainfall amounts are more likely. Image credit: NOAA.

By Thursday, the approaching trough will likely bring steering currents that will take 91L more to the northeast, allowing heavy rains to affect the coasts of Texas and Louisiana. It remains to be seen how far inland moisture from 91L will penetrate to affect the flood-swamped areas of the central U.S. The 0Z Monday run of the HWRF model, which predicted that 91L would develop into a strong tropical storm with 70 mph winds, kept the heaviest rains of 4 – 8” from 91L within 200 miles of the coast, with rains of 2 – 4” penetrating well inland to the Tennessee Valley late in the week. This rainfall forecast is at the upper end of what 91L will likely be able to achieve, and lower rainfall amounts are more likely.

91L has good model support for development, with over 80% of the ensemble members of the 0Z Monday runs of the GFS and European models calling for development. The 8 am EDT Monday  5-Day Tropical Weather Outlook from NHC gave 91L 2-day and 5-day odds of development of 60%. The next name on the Atlantic list of storms for 2019 is Barry.