Yale Climate Connections - Jeff Masters Weather Blog

Hurricane Watches go up for Louisiana as Tropical Storm Francine forms » Yale Climate Connections

  • Posted by
09 Sep

Published Date and Time: 2024-09-09 12:26:40


A Hurricane Watch and a Storm Surge Watch are up for most of the coast of Louisiana as Tropical Storm Francine, currently located over the southwestern Gulf of Mexico’s Bay of Campeche, heads toward an expected landfall late Wednesday. Francine’s appearance on September 9 ends an extraordinarily long period without an Atlantic named storm (see Tweet below) and comes 11 days after the usual August 29 appearance of the season’s sixth named storm.

Francine steadily growing more organized

At 11 a.m. EDT Monday, Francine was located 480 miles south-southwest of Cameron, Louisiana, moving north-northwest at 5 mph (8 kph), with top sustained winds of 50 mph (80 kph) and a central pressure of 1002 mb. Satellite imagery showed that Francine had a modest-sized area of heavy thunderstorms which were steadily growing more organized, with considerable lightning activity. Conditions were very favorable for development, with record-warm ocean temperatures near 31 degrees Celsius (88°F), light wind shear of five to 10 knots, and a moist atmosphere.

Location of record sea surface temperatures at the end of AugustLocation of record sea surface temperatures at the end of August
Figure 1. Sea surface temperatures at the end of August were record-warm over the portions of the Gulf of Mexico where Francine will be located on Monday and Tuesday. (Image credit: Michael Lowry)
Climate Shift Index for the Gulf of MexicoClimate Shift Index for the Gulf of Mexico
Figure 2. Ocean Shift Index for Sep. 7, 2024, shows that the record-warm ocean temperatures in the western Gulf of Mexico along Francine’s path were made about 100 times more likely because of human-caused climate change. (Image credit: Climate Central)

Record-warm waters in the Gulf of Mexico

On Monday and Tuesday, Francine will benefit from record-warm ocean temperatures near 31 degrees Celsius (88°F). This is about 1 degree Celsius (1.8°F) above the 1981-2010 mean and is the warmest on record for this time of year (Fig. 1). Human-caused climate change made this ocean warmth up to 100 times more likely, according to the Climate Central Ocean Shift Index (Fig. 2). Warm waters extend to great depth along Francine’s path, giving the storm plenty of oceanic heat energy to fuel rapid intensification.

Track forecast for Francine

With Francine still in the formative stages, there is some modest uncertainty in the track forecast, both cross-track (which part of the coast the storm will hit) and along-track (when it will hit). Louisiana is definitely in the bull’s-eye for a hit (Fig. 3) and has the greatest potential impacts from wind and storm surge. However, heavy rains in excess of four inches are expected along a large swath of the Gulf Coast, from northeastern Texas to coastal Alabama.

Track forecasts for FrancineTrack forecasts for Francine
Figure 3. Track forecasts out to five days for Francine from the 6Z Monday, September 9, run of the European ensemble model (left) and GFS ensemble model (right). Individual forecasts of the ensemble members are the lines color-coded by the wind speed in knots they predict; red colors correspond to a Category 1 hurricane. The time in hours from the model initialization time is in gray text. The more easterly ensemble members, with more of a threat to New Orleans, tended to predict a stronger storm. (Image credit: weathernerds.org)

Intensity forecast for Francine

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

On Wednesday, as Francine approaches the coast of Louisiana, wind shear is expected to rise to the high range, 20-30 knots, and dry air on the west side of the storm may be able to attack the core of the storm, causing weakening. There will also be less ocean heat energy available. These factors should cause Francine to stop intensifying six or more hours before landfall, and the top intensity models predict Francine will make landfall with sustained winds between 65-90 mph (105-145 kph), or a strong tropical storm to a strong Category 1 hurricane. Ensemble models suggest that if Francine tracks more to the east (resulting in an increased threat to New Orleans), it may be a stronger storm.

A damaging storm surge for Louisiana

A storm surge of about two feet (0.6 m) was already being observed along much of the lower Texas coast on Monday morning, causing some minor coastal flooding, according to NOAA’s Tides & Currents website. A much larger storm surge of five to 10 feet (1.5-3 m) is expected along the Louisiana coast near and to the right of where the center makes landfall, causing major flooding. The coast of Louisiana is one of the most vulnerable locations in the world for high storm surges because of the large expanse of shallow water offshore.

Forecast from the tide station at Amerada PassForecast from the tide station at Amerada Pass
Figure 4. Observed and predicted water levels from the 6:40 a.m. EDT Monday forecast from NOAA’s National Water Prediction Service for the Amerada tide gauge on the coast of central Louisiana.

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 and low tide is about 1.5 feet (0.5 m), so the timing of Francine’s landfall will be a significant contributing factor in determining how much coastal flooding occurs. The 11 a.m. EDT Monday National Hurricane Center forecast called for a landfall near 8 p.m. EDT Wednesday, not long after low tide. However, we should expect the timing of landfall to change by four or more hours in future forecasts, since the system is still in the organizing stage and the models do not yet have a good handle on it. The timing of landfall from the 06Z Monday runs of our six top hurricane models had a nine-hour range. The earliest predicted landfall occurred at 4 p.m. EDT Wednesday (HAFS-B model), and the latest occurred at 1 a.m. EDT Thursday (European model).

We help millions of people understand climate change and what to do about it. Help us reach even more people like you.





Source link