Tropical Storm Rina has become the newest named storm in the hyperactive Atlantic hurricane season of 2023. A disturbance called Invest 91L in the central tropical Atlantic was upgraded to Rina by the National Hurricane Center at 11 a.m. Thursday. With top sustained winds of 40 mph, Rina was located about 1,200 miles east of the northern Leeward Islands, moving north-northwest at 10 mph.
Rina’s formation brings the total 2023 activity in the Atlantic to 18 named storms (including a unnamed subtropical storm in January that was belatedly recognized), as well as 6 hurricanes, 3 major hurricanes, and an accumulated cyclone energy (ACE) index of 121.5. The 1991-2020 averages for this date are 10.7 named storms, 5 hurricanes, 2.3 major hurricanes, and an accumulated cyclone energy (ACE) index of 91.7. Only 11 other seasons since 1851 have had as many as 18 named storms. If 2023 manages to record just two more named storms, it would be tied with 1933 for fourth-busiest year on record behind 2020, 2005, and 2021, which had 30, 28, and 21 named storms, respectively. On average, three named storms form after September 28.
The Fujiwhara effect will complicate forecasts of both Philippe and Rina
Rina has developed in unusually close proximity to another tropical storm, Philippe, which was located about 560 miles east of the northern Leewards at 11 a.m. EDT Thursday. Philippe’s sustained winds were 50 mph, and it was drifting west-northwest at just 2 mph.
The next several days are likely to see a classic example of the Fujiwhara effect playing out in the Atlantic. This phenomenon is when two tropical cyclones are close enough to influence each other’s motion and sometimes interfere with each other’s development. There is no accepted maximum distance for triggering Fujiwhara interaction, but according to the Hong Kong Observatory, the outer limit is no more than about 1,350 kilometers (850 miles). On Thursday morning, Philippe and Rina were already within 1,000 km (620 miles) of each other, and Philippe’s slower westward component means that the two will be getting even closer. It’s not out of the question that the two storms could eventually merge, with one of the storms essentially swallowing up the other in its circulation.
The hallmark Fujiwhara interaction is for two tropical cyclones to essentially rotate counter-clockwise around a common center between them. In a case like Philippe and Rina, this would tend to push Philippe more southwestward and Rina more northwestward, and this is pretty much what is being predicted by the National Hurricane Center.
Forecast for Philippe
The westerly wind shear that has plagued Philippe for several days has pushed most of its showers and thunderstorms (convection) toward Rina and well east of its amorphous low-level center, and the strongest winds are associated with that convection. However, wind shear has lessened to around 5-10 knots, and although the mid-level air around Philippe is only moderately moist (relative humidity around 50-60 percent), the storm is located over unusually warm waters of around 29 degrees Celsius (84 degrees Fahrenheit).
Philippe’s future will depend in large part on how it interacts with Rina. If the convection well east of Philippe’s center ends up in Rina’s orbit, Philippe’s low-level circulation could become little more than a swirl heading slowly toward the nothern Leeward Islands. If Philippe can maintain itself as a tilted but distinct entity, it has a chance to intensify. In fact, experimental ensemble guidance from the HAFS model—the most recent state-of-the-art intensity model, operationally adopted by NOAA this year—shows the possibility of Philippe strengthening into a major hurricane (see embedded animation below), or weakening into a tropical depression.
Assuming Philippe remains distinct, the Fujiwhara influence would likely keep the storm drifting slowly west or even southwest. With steering currents weak, any potential approach to the Lesser Antilles would be at least several days away. Given the vast uncertainty in Thursday’s forecast, the official National Hurricane Center forecast keeps Philippe as a 50-mph tropical storm into the weekend.
Forecast for Rina
Although Rina was the weaker of the two tropical storms on Monday, it may have a more straightforward future. Rina is enveloped in extensive convection and a moist mid-level atmosphere (relative humidity 70-75 percent), and like Philippe, it will be passing over warmer-than-average waters for this time of year of around 29 degrees Celsius (84°F). Strong wind shear will keep any strengthening gradual, and as noted above, there is a high degree of unpredictability in how Rina and Philippe will interact. The Fujiwhara effect will tend to propel Rina toward the northwest into the weekend and beyond, so there is little chance that Rina will pose any threat to land areas, although Bermuda should keep an eye on it.
King tides and Ophelia’s remains bringing coastal flooding
The upcoming full moon on Friday will occur when the moon is near perigee (its closest approach to the Earth), bringing much of the U.S. East Coast “king tides” for the remainder of the week – some of the highest high tides of the year. The high tides are coming during strong onshore winds caused by a combination of high pressure over Atlantic Canada and low pressure from the remains of Tropical Storm Ophelia, located offshore of the Carolinas.
The high astronomical tides and onshore winds led to “Minor” to “Moderate” coastal flooding during the Wednesday night high tide cycle at many sites along the U.S. East Coast, from Georgia to New Jersey; “Major” flooding was observed along the North Carolina Outer Banks at Duck. More minor to moderate coastal flooding is predicted for the Thursday night high tide cycle, with major coastal flooding predicted at Charleston, South Carolina. Minor to moderate high tide flooding is expected to continue into the weekend at many locations along the East Coast.
In the New Jersey/New York City/Long Island/Connecticut region, coastal flooding will be augmented by very heavy rains as moisture and rainbands from Ophelia’s remnants stream northward into the area. Up to 3-5 inches are predicted by the National Weather Service through Saturday, and ensemble models on Thursday morning suggested that localized totals could push into the 5-10 inch range or even beyond that (see Figure 1). Residents should be on alert for sharp differences in rainfall totals and flooding across small areas, and New York metro residents in particular need to be prepared for flash flooding that could be locally intense and dangerous. Central Park has already received 3″ of rain over the five days ending on Wednesday, and more than 8″ for September thus far.