Hurricane and storm surge warnings have been issued for most of coastal Louisiana as Tropical Storm Francine, currently over the Bay of Campeche in the southwestern Gulf of Mexico, is expected to make landfall late Wednesday. Francine’s appearance on Sept. 9 ends an exceptionally long period without a named storm in the Atlantic (see tweet below) and comes 11 days after the usual appearance of the sixth named storm of the season on Aug. 29.

Francine is becoming more and more organized

As of 11 a.m. EDT Monday, Francine was located 480 miles south-southwest of Cameron, Louisiana, and moving north-northwest at 5 mph (8 km/h), with peak winds of 50 mph (80 km/h) and a central pressure of 1002 mb. Satellite imagery indicated that Francine had a moderate-sized area of ​​severe thunderstorms that were steadily organizing and with significant lightning activity. Conditions were very favorable for development, with record-high ocean temperatures of nearly 31 degrees Celsius (88°F), light wind shear of five to ten knots, and a moist atmosphere.

Record warm water in the Gulf of Mexico

On Monday and Tuesday, Francine will benefit from record-high ocean temperatures of nearly 31 degrees Celsius (88°F). That’s about 1 degree Celsius (1.8°F) above the 1981-2010 average and the highest ever recorded for this time of year (Fig. 1). Human-caused climate change has made this ocean warming up to 100 times more likely, according to the Climate Central Ocean Shift Index (Fig. 2). The warm waters reach great depths along Francine’s path, giving the storm plenty of oceanic heat energy to favor rapid intensification.

Route forecast for Francine

Because Francine is still developing, there is some uncertainty about the forecast track, both cross-track (which part of the coast the storm will hit) and along-track (when it will arrive). Louisiana is definitely in the storm’s sights (Figure 3) and has the greatest potential impacts from wind and storm surge. However, heavy rainfall of over four inches is expected along a broad swath of the Gulf Coast, from northeast Texas to the Alabama coast.

Intensity forecast for Francine

Near-ideal conditions for Francine’s intensification will continue through Tuesday, although the storm will likely be slow to develop on Monday due to its initial disorganized state. Rapid intensification (defined as a 35 mph increase in wind speed in 24 hours) is quite possible: Monday’s 12Z run of the SHIPS model gave Francine a 36 percent chance of rapidly intensifying by 35 mph in the 24 hours ending 8 a.m. EDT Tuesday, which would bring it to Category 1 strength with winds of 85 mph. The model gave an 18 percent chance that Francine would become a Category 3 major hurricane with winds of 115 mph by Thursday morning.

On Wednesday, as Francine approaches the Louisiana coast, wind shear is expected to increase to 20-30 knots, and dry air on the western side of the storm could attack the storm’s core and weaken it. In addition, less heat energy will be available from the ocean. These factors should cause Francine to stop intensifying six or more hours before landfall, and intensity models predict Francine will make landfall with sustained winds between 65-90 mph (105-145 km/h), or as a strong tropical storm to a powerful Category 1 hurricane. Ensemble models indicate that Francine could become a stronger storm as it moves farther east (which would create a greater threat to New Orleans).

A destructive storm surge for Louisiana

According to NOAA’s Tides & Currents website, a storm surge of about 2 feet (60 cm) was already observed along the Texas coast Monday morning, causing some minor flooding. A much larger storm surge of 5 to 10 feet (1.5 to 3 m) is expected along the Louisiana coast near and to the right of where the center of the storm surge makes landfall, causing severe flooding. The Louisiana coast is one of the most vulnerable places in the world to high storm surges due to the large areas of shallow water offshore.

High tide at Amerada Pass in central Louisiana is early Thursday morning at 3:30 a.m. EDT (7:30Z); low tide is Wednesday afternoon at 5:30 p.m. EDT (21:30Z). The difference in water level between high tide and low tide is about 1.5 feet (0.5 m), so the timing of Francine’s landfall will be a major factor in determining how severe coastal flooding will be. The National Hurricane Center’s 11 a.m. EDT forecast Monday predicted a landfall around 8 p.m. EDT Wednesday, just after low tide. However, we should expect the timing of landfall to change by four or more hours in future forecasts, as the system is still in the organization phase and models do not yet have a good handle on it. The timing of landfall from the 06Z runs of our six major hurricane models on Monday had a span of nine hours. The earliest predicted landfall occurred on Wednesday at 4 p.m. EDT (HAFS-B model) and the latest on Thursday at 1 a.m. EDT (European model).

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