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TD 9 Forms in Central Atlantic

By: Bob Henson 7:05 PM GMT on September 16, 2015

The last few days of rare September quietude in the global tropics are coming to an end, with a new tropical depression in the Atlantic and a named tropical storm in the Pacific. Tropical Depression 9, located about 1200 miles east of the Lesser Antilles, is moving slowly toward the north-northwest. Although TD 9 has a closed circulation and a reasonable amount of shower and thunderstorm activity, it remains a weak system, with top sustained winds of only 30 mph. TD 9’s north-northwest motion reflects the increasing southwesterly upper-level flow that will combine with easterly trade winds to put the depression under strengthening wind shear. This shear will dent TD 9’s chances at growth, even as it approaches an area of sea-surface temperatures 2-3°C above average (around 29°C or 84°F). The 0600 GMT runs of the HWRF and GFDL models bring TD 9 up to tropical storm strength for a brief time during the next 24 to 48 hours, but the official NHC forecast keeps TD 9 short of becoming a named storm throughout the next 120 hours. There could be another window for TD 9 to strengthen in the open Atlantic early next week if its circulation survives the trek.

Figure 1. Visible-wavelength image of Invest 95L, collected by the GOES-East floater satellite at 1445 GMT (11:45 am EDT) on Wednesday, September 16. Image credit: NOAA/NESDIS.

Behind TD 9 is Invest 95L, located well southwest of the Cape Verde islands. With its origins further east than TD 9, 95L has a bit more time to potentially develop into a tropical storm before it encounters the wind shear and relatively dry air that’s been dominating the western and central North Atlantic. NHC gives 95L a 70% chance of becoming a tropical cyclone by Friday. Most statistical and dynamical models gradually intensify 95L to tropical-storm strength while bringing it west-northwest to higher latitudes, reducing the odds it will make it anywhere near North America or the Caribbean.

There are no systems of immediate concern for tropical development near the Gulf or Atlantic coast, although a broad area of surface low pressure will combine with some upper-level support and a weak surface boundary to bring heavy rains to much of the Florida peninsula over the next several days. No flash flood watches were in effect on Wednesday morning, but the NOAA Weather Prediction Center is calling for widespread 2-3” amounts over the southern half of Florida. This could be good news for the Everglades region west of Miami, an area in moderate to severe drought that was largely missed by the extensive rains across Florida this summer.

In the Northwest Pacific, Tropical Storm Krovanh is still on track to become a major typhoon over the next several days. Krovanh could affect the very sparsely populated Iwo Jima and nearby islands as it strengthens rapidly; the Joint Typhoon Warning center brings Krovanh to Category 4 strength in the next 48 hours as it begins recurving over warm waters with relatively light wind shear.

Figure 2.. Enhanced infrared MTSAT image of rapidly organizing Tropical Storm Krovanh from 1714 GMT (1:14 pm EDT) on Wednesday, September 16. Image credit: NOAA/NESDIS.

Flash-flood death toll rises in Utah; Henri’s remnants sock Europe
Some of the worst impacts associated with tropical cyclones this year are happening long after the storms dissipate. Moisture from ex-Hurricane Linda was entrained into rich monsoonal flow that swept across the U.S. Southwest on Monday and Tuesday. As an upper-level trough interacted with the moisture, heavy thunderstorms erupted on Monday over far southern Utah, causing two deadly flash floods that killed at least 16 people. Two vehicles were swept away near the town of Hildale (see our blog post from Tuesday), with 12 people killed and one child still missing. The disturbing photo below (Figure 3) shows the ability of floodwaters to wreak destruction. Many people killed in floods die from trauma rather than from drowning, as pointed out in this overview of flood impacts published in the open-access journal PLOS. During flash floods in mountainous areas, the percentage of deaths caused by trauma is especially high.

Figure 3. The twisted wreckage of two vans that were washed away in a flash flood with women and children inside on Monday, September 14, 2015, rests on the bank of Short Creek near Hildale, Utah, on Tuesday, September 15. Image credit: George Frey/Getty Images.

Just to the northwest of Hildale, in Zion National Park, four canyoneers were killed and three others were missing on Wednesday morning after floodwaters coursed through the narrow Keyhole Canyon on Monday. Zion’s slot canyons are notorious for extremely rapid flash flooding, with little or no ability to escape the water. Less than a mile long, Keyhole Canyon is considered a relatively easy trek, although a permit is required. The website canyonyeeringusa.com rates the generalized flash flood risk of Keyhole as “low”, adding “The collection zone is small, and the canyon short. But it does flash big at times, so don't get caught in there!” Rangers warned visitors on Monday morning of probable flash-flood danger, according to a park spokesperson quoted in the Los Angeles Times. After the second of two thunderstorm complexes passed through around 4:30 pm, the flow through the nearby North Fork of the Virgin River rose from 55 to 2630 cubic feet per second in 15 minutes, a level recorded about once every three years.

Monday’s death toll in Utah appears to have been the largest in a local/regional U.S. flash flood episode since June 11, 2010, when at least 20 people were killed by a flash flood that swept through a remote campsite in southwest Arkansas.

Figure 4. WunderMap radar imagery shows intense thunderstorms stretching from western Germany to southern France at 1445 GMT (4:45 pm CEST) on Wednesday, September 16.

The remnants of former Tropical Storm Henri--having traveled thousands of miles across the Atlantic over the past five days--got swept into a strong midlatitude trough that pushed a broken line of powerful thunderstorms into Germany and France on Wednesday evening. The French newspaper Le Monde reported that at least two people in France were killed by high winds in today’s storms, with several other injured. Winds gusted to 75 mph at Lyon’s Bron airport, the highest wind speed on record for that site in September, according to Nick Wiltgen at weather.com.

Bob Henson

Hurricane Flood

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.