The first tropical storm warnings of 2021 for the U.S. coast were up Friday afternoon for the expected arrival of Potential Tropical Cyclone Three (PTC 3). The system was growing more organized over the Gulf of Mexico and spreading heavy rains along the Gulf Coast from southeast Louisiana to the Florida Panhandle. PTC 3 is expected to develop into Tropical Storm Claudette before making landfall Friday night or Saturday morning in southeast Louisiana, but regardless of its exact track and landfall intensity, PTC 3 will bring heavy rains and dangerous flash flooding to portions of Louisiana, Mississippi, Alabama, and the Florida Panhandle through Sunday.

The Air Force Hurricane Hunters flew their first mission of the hurricane season on Thursday afternoon into PTC 3: They found the system did not have a well-defined surface circulation, and thus could not be classified as a tropical depression. Nonetheless, since the system was expected to become a tropical storm before making landfall, the National Hurricane Center opted to classify it as a “potential tropical cyclone” so that advisories could be initiated on it.

PTC 3 retained that classification at 11 a.m. EDT Friday, when it had top winds of 35 mph – just below tropical storm strength. A follow-up hurricane hunter mission on Friday morning was also unable to find a well-defined surface circulation center.

Figure 1. Five-day rainfall ending at 8 a.m. EDT June 16, 2021 in Mexico. PTC 3 dumped over 12 inches of rain over portions of Mexico. (Image credit: Conagua)

PTC 3 brings flooding to Mexico

PTC 3 lingered for nearly a week over Mexico’s Bay of Campeche, bringing over a foot of rain to some areas in southern Mexico (Figure 1). reported that 17.32” (440 mm) of rain fell in Jacatepec, Oaxaca, in a 48-hour period ending June 16. River flooding, landslides, and flash floods isolated 30 communities, and lightning caused two deaths in the Ocotlán de Morelos municipality.

Figure 2. Radar image of PTC 3 at 10:55 a.m. EDT Friday, showing heavy rains along the central U.S. Gulf Coast. (Image credit: Mark Nissenbaum)

PTC 3 still disorganized

At 11 a.m. EDT Friday, PTC 3 was headed north-northeast at 14 mph over the central Gulf of Mexico. The system had marginal conditions for development, with warm waters of 28.5 degrees Celsius (83°F), but high wind shear of 20-25 knots was driving dry air to the west of the system into its core, disrupting it. Satellite loops showed PTC 3 with a broad circulation, with a building area of heavy thunderstorms to the east of the center. An upper-level trough of low pressure over the western Gulf was acting to increase PTC 3’s heavy thunderstorms, thereby also giving the developing storm a less-than-tropical appearance.

Figure 3. Predicted seven-day rainfall amounts ending at 12Z Friday, June 25. The central Gulf Coast is predicted to receive 5-10 inches of rain from PTC 3. (Image credit: NOAA)

National Hurricane Center forecast for PTC 3

Steering currents are expected to push PTC 3 roughly north-northeastward at a forward speed of 10-15 mph on Friday, which is expected to result in a landfall over southeast Louisiana Friday night or Saturday morning.

The exact landfall location of PTC 3’s center is relatively unimportant, as heavy rain will be the storm’s main threat, and a wide swath of heavy rain will affect the central Gulf Coast regardless of PTC 3’s exact track. Because of dry air over the western Gulf as a result of the presence of an upper-level trough, the heaviest rains and strongest winds of PTC 3 will lie almost entirely to the east of the center. The storm is likely to dump rainfall amounts of 5-10 inches along the coast June 18 through June 20, causing damaging flash flooding. Already, soil moisture as of Wednesday, June 17, ranked in the top 1-5% of climatology for the date across much of the region likely to experience PTC 3’s heaviest rains.

Figure 4. Predicted wind speed (colors) and sea level pressure (black lines) for PTC 3 at 2 a.m. EDT (6Z) Saturday, June 19, from the 6Z Friday, June 18, run of the HWRF model. The model predicted PTC 3 would be making landfall in Louisiana as a tropical storm with 45 mph winds and a central pressure of 1004 mb. (Image credit: Tropical Tidbits)

Strong model support remains for the continued development of PTC 3 up until landfall, but this development is expected to be slow. As PTC 3 moves northward over the Gulf, it will encounter waters with less heat content. Wind shear will remain high, and dry air to the west of the system will continue to inhibit development. These conditions suggest that PTC 3 will be no stronger than a 50-mph tropical storm prior to making landfall. In an 8 a.m. EDT Friday tropical weather outlook, the National Hurricane Center gave PTC 3 two-day and five-day odds of development of 90%.

Watch for intensification of PTC 3 over the Carolinas

Interestingly, the European model predicts that after PTC 3 makes landfall in Louisiana, it will weaken, and then re-intensify on Monday as it turns east-northeast and traverses North and South Carolina before it emerges out over the Atlantic Ocean. The GFS model is not making this same prediction, but according to retired NHC branch chief Hugh Cobb, a number of systems have re-intensified in this manner, including Cleo (1964), Agnes (1972), and Danny (1997). Technically, this occurs because of vorticity column stretching as the system moves over the lower terrain east of the southern Appalachians.

Bob Henson contributed to this post.

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Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...