Saturday, September 21, 2024

Tropical Storm Gordon embarks on a slow Atlantic crossing » Yale Climate Connections

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Tropical Storm Gordon became the seventh named storm of the 2024 Atlantic season at 11 a.m. EDT Friday, September 13, launching what looks to be a long, slow journey across the eastern and central tropical Atlantic – assuming that Gordon survives the trek. In records spanning the period 1991-2020, the seventh named Atlantic storm of the year arrives on average on September 3, so Gordon developed more than a week later than the typical “G” storm.

As of 11 a.m. EDT Friday, Gordon was located in the remote tropical North Atlantic roughly 1000 miles west-northwest of the Cabo Verde Islands. Gordon’s top sustained winds of 40 mph were at minimal tropical storm strength, and Gordon was moving west-northwest at 12 mph.

A satellite photo of Tropical Storm Gordon
Figure 1. Satellite image of Tropical Storm Gordon at 1740Z (1:40 p.m. EDT) Friday, September 13, 2024. (Image credit: NOAA Satellites)

Showers and thunderstorms (convection) pulsing around Gordon’s center were being pushed to the east by wind shear of around 15 knots. This wind shear should gradually lessen over the next several days, and Gordon will traverse increasingly warm sea surface temperatures, rising from around 27 degrees Celsius (81°F) up to 28-29°C (82-84°F) by early next week. However, Gordon will also be surrounded by an expanse of dry air, bringing mid-level relative humidity down from around 65% on Friday to around 50% by Sunday-Monday, so any initial strengthening could be tamped down over the weekend.

Gordon will pose no threat to any land areas for at least the next few days, and perhaps never will. Weak steering currents will prevail into next week across the central and eastern tropical Atlantic, and this will likely slow Gordon’s general westward motion. Dry air could bring Gordon back down to tropical depression status at almost any point. Should Gordon manage to strengthen along the way, it would also become more likely to recurve far east of the Caribbean or North America.

Francine’s remnants continue to pelt parts of the South

Showers and thunderstorms continued to rotate on Friday around Post-Tropical Cyclone Francine, which was centered in northeast Arkansas and drifting southeast at just 3 mph. Francine stormed inland as a Category 2 hurricane in central Louisiana around 5 p.m. CDT Wednesday, injuring at least three people and bringing torrential rains and wind gusts of 70-100 mph well to the east, including the New Orleans area. More than 550,000 people lost power in Louisiana, Alabama, and Mississippi, and widespread flooding led to numerous water rescues.

The largest preliminary rainfall totals from Francine from late Sunday through late Wednesday included 9.69” at Covington, LA; 8.63” at Waveland, MS; and 8.05” at Nonconnah Creek, TN.

“Francine is likely going to be a low- to mid-single-digit-billion economic loss,” said Steve Bowen, chief science officer at Gallagher Re, one of the world’s largest insurance brokerages, in a direct message. He added: “This could have been a lot worse, but fortunately landfall occurred in a sparsely populated area of Louisiana. While wind-related losses will be perhaps a bit lower than initially feared, the expectation is Francine’s flood-related impacts will be a bit higher – including in areas outside of Louisiana.”

This weekend the heaviest rains from ex-Francine, which could total 6 to 8 inches, are expected across northern Alabama. A few intense downpours could emerge from eastern Arkansas to western Georgia and the Florida Panhandle. Apart from the risk of localized flash flooding, Francine’s rains will be largely beneficial to this swath of the South, most of which was covered by moderate to severe drought in the weekly U.S. Drought Monitor released on September 12.

Elsewhere in the Atlantic, a weak disturbance named Invest 94L moving through the northern Leeward Islands was struggling with dry air and unlikely to develop further. In its Tropical Weather Outlook issued at 2 p.m. EDT Friday, the National Hurricane Center lowered the development odds of this disturbance to near zero. Of more concern is a stalled frontal boundary extending from the Carolinas offshore. A number of European and GFS ensemble model members indicate that an area of low pressure could develop along the front and morph into a tropical or (more likely) subtropical cyclone late this weekend or early next week, most likely moving northwestward toward the coast. This system was given 10 percent odds of development through Sunday and a 40 percent chance through next Friday. The next name on the Atlantic storm list is Helene.

An infrared satellite photo of tropical storm IleanaAn infrared satellite photo of tropical storm Ileana
Figure 2. Infrared satellite image of Tropical Storm Ileana at 2 p.m. EDT Friday, September 13, 2024. (Image credit: NOAA Satellites)

Tropical Storm Ileana heading into the Gulf of California

Though disorganized, Tropical Storm Ileana was hauling an expansive shield of intense convection from the northeast tropical Pacific toward southern Baja California on Friday and will likely push moisture into the southwestern United States over the weekend. As of 11 a.m. EDT Friday, Ileana was located about 55 miles southeast of Cabo San Lucas, Mexico, with top sustained winds of 40 mph.

Ileana is predicted to clip the southeast Baja Peninsula on Friday night and traverse the Gulf of California most of this weekend, probably weakening into a tropical depression by Sunday. Wind damage will likely be minimal, but Ileana could dump pockets of 8-12+ inches of rain across parts of the southern Baja Peninsula and northern Sinaloa state, with localized flash flooding and mudslides possible. Moisture sweeping north from Ileana could help spawn scattered intense thunderstorms over Arizona from late Saturday through Monday, again with localized flash floods possible.

If nothing else, Ileana’s clouds should help bring an end to Phoenix’s record-long streak of 100°F-degree weather – which was at 109 consecutive days as of Thursday, September 12, including every day of meteorological summer (June through August).


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