T.S. Gabrielle forms; Puerto Rico flooding is primary threat; mainland U.S. hit "doubtful"

“Invest 97L” became Tropical Depression Seven this afternoon, and then Tropical Storm Gabrielle tonight:

TD#7 upgraded to Tropical Storm Gabrielle. Winds now 40 mph. Heavy rain moving into Puerto Rico tonight. #tropics pic.twitter.com/oZTlEilfym

— Tim Heller (@HellerWeather) September 5, 2013

Gabrielle’s heaviest rains are approaching Puerto Rico as we speak. Here’s a live radar loop. Because the storm is quite slow-moving, 6 to 10 inches, or more, are expected in spots. This is causing significant concerns about flooding. As Miami meteorologist John Morales explained earlier: “It should come as no surprise to my followers in Puerto Rico that I’m concerned about flash flooding. You’ll EASILY see 10 inches of rain. On flash-floody Puerto Rico, a storm moving less than 10mph skirting the SW tip of the island spells big rain trouble.” More from the National Weather Service’s San Juan office here and here.

What about potential later U.S. impacts, after Puerto Rico? The conventional wisdom is that Gabrielle has essentially no chance of affecting the mainland. Indeed, phrases like “no threat” and “no risk” are being thrown around, based largely on the near-consensus among the computer models that Gabrielle will stay out at sea.

Levi Cowan, however, paints a scenario in his “Tropical Tidbit” video for how Gabrielle could still potentially become an issue for the U.S. East Coast. It begins with recognizing that the interaction between Gabrielle, whose core circulation has become rather compact, and the large sprawling tropical wave to its northeast, creates significant uncertainty about Gabrielle’s exact track in the next few days:

As usual, @TropicalTidbits video has good info http://t.co/7qITQ0I9Ky especially on interaction of #Gabrielle & wave: pic.twitter.com/uNMTWneS7c

— Brendan Loy (@brendanloy) September 5, 2013

“This wave here is rotating around and will be trying to merge with [Gabrielle],” Cowan says, predicting that Gabrielle and the wave — which the NHC now designates as a separate area of interest, with a 10% of development all its own — will “probably eventually combine into a smaller system, and get more energy bundled into one place, and this could try strengthening into a moderate to strong tropical storm once it clears the islands.” (Dr. Jeff Masters has more on this interaction.) Cowan adds that “the interplay between these two features will be interesting, and exactly where the center of this storm ends up in a couple of days could be a little bit more uncertain than it usually is.”

Meanwhile, another source of uncertainty, according to Cowan, is the possibility of land interaction with the islands — particularly mountainous Hispaniola — altering Gabrielle’s track in unexpected ways, including potentially by pulling it west and roughly paralleling Hispaniola’s north coast for a while. If this happens, and Gabrielle (or the eventual Gabrielle/wave hybrid) finds itself centered further west and south than anticipated in a day or two, it would become at least conceivable that a mainland U.S. impact could be on the table, as Cowan explains:

“Can it affect the [mainland] United States?” Cowan asks rhetorically. “I would use the word ‘doubtful’ in this situation, but it’s not completely off the table. … Keep an eye on it, just in case.”

Stay tuned, here and on Twitter at @brendanloy, for the very latest.