Weather Nerd

Gabrielle downgraded, flood threat remains; multiple candidates for "Humberto" emerge

Just twelve hours (and two full NHC advisories) after earning the name “Gabrielle,” the seventh tropical storm of the 2013 hurricane season … is a tropical depression again, expected to soon dissipate altogether. As George W. Bush might say, “Gabrielle follows in the path of Chantal, Dorian and Erin. And she will follow that path all the way to where it ends: in meteorology’s unmarked grave of discarded storms.”

Gabrielle just couldn’t survive a quartet of factors ripping it apart, Dr. Jeff Masters notes: “Wind shear, dry air, and interaction with the rough terrain of Puerto Rico and a strong tropical disturbance to its northeast have significantly disrupted Gabrielle.”

More specifically, what has happened to Gabrielle is a rather severe case of “decoupling.” You might remember that term from Pacific Tropical Storm Flossie, which decoupled as it approached the Big Island of Hawaii. Well, it happened again, this time in the Atlantic. As of yesterday, Gabrielle’s mid-level and low-level centers of circulation were close together, but not quite “stacked” — i.e., not directly on top of each other, which is how they should be in a healthy tropical cyclone. I remember reading some discussion yesterday (I forget where) about the possibility that “stacking” might soon occur, which would have opened the door for intensification. But instead, the exact opposite happened. The centers moved in totally divergent directions overnight, and now they’re far apart and continuing to diverge:

As you can see, Gabrielle’s low-level center of circulation, a naked cloud swirl almost completely devoid of precipitation, is southwest of Puerto Rico and approaching Hispaniola. Meanwhile, the mid-level center — which is under the bulk of the rain and thunderstorm convection — is east of Puerto Rico and moving away to the northeast. (The mid-level circulation is the spin you can see on San Juan radar.) This sort of split is fatal to Gabrielle’s vitality as a storm.

As a sign of how unhealthy Gabrielle is, you can see outflow boundaries flowing out of the storm — like you’d expect to see in a big thunderstorm over the Great Plains. Tropical cyclones aren’t supposed to produce those!

Indeed, arguably Gabrielle is not technically a tropical cyclone at all right now, in which case she ought to be declared “dissipated,” rather than merely downgraded to a tropical depression with dissipation forecast in 24 to 36 hours. But the NHC is hedging its bets, for the moment, against the possibility of a new center forming:


It’s also possible, albeit unlikely, that the “old” low-level center, even after degenerating and dissipating, could “pull a Dorian.” I can imagine Gabrielle’s remnants hanging around in the Bahamas region for several days — and getting bypassed by the upper-level trough that was expected to pull a stronger Gabrielle out to sea, which won’t have the same influence on a naked low-level swirl — and then eventually being resurrected next week. But that’s highly speculative, and certainly not in the forecast. By no means am I predicting it.

What would be really interesting would be if Gabrielle does re-form under its mid-level center later today or tomorrow, and then, days later, its long departed low-level center “pulls a Dorian” and re-forms as well. If that scenario were to occur, Gabrielle would essentially have split into two storms. And really, the one that “deserves” the continuation of the name “Gabrielle” would arguably be the resurrected low-level center. But that system would instead be named “Humberto” (or “Ingrid,” if Humberto has already formed elsewhere), because the mid-level center would already have claimed the name “Gabrielle.” … Okay, so maybe this name-related nerdery is only “interesting” to me, but whatever, it’s my blog. And “nerd” is in its title. So sue me. Heh.

Anyway, more on possible candidates for “Humberto” in a moment, but first, let’s get back to the present reality of Tropical Depression Gabrielle for a second.

There’s still a threat of flooding and mudslides in Puerto Rico — even though, on some parts of the island, they’re complaining and wondering where the rain is. Rainfall rates of 4 inches per hour are just offshore, sloooooowly moving toward land. A Flash Flood Watch remains in effect, although the official forecast is now for “2 to 4 inches [of rain] with locally higher amounts,” as opposed to the “6 to 10 inches” number that was being bandied about yesterday.

Here’s the local NWS discussion for Puerto Rico, with more detail on what’s expected. And here are two versions of the San Juan radar loop, the first showing standard base reflexivity (i.e., rainfall right now), the second showing one-hour rainfall rates:

Oh, and here’s a cool 24-hour radar loop, courtesy of Brian McNoldy, RSMAS/Univ of Miami, and discovered via McNoldy’s article on the Washington Post‘s Capital Weather Gang. The loop shows Gabrielle’s approach to Puerto Rico, as well as some of its structural changes over the past day:

Now then… I promised a discussion of candidates for the name “Humberto.” Have a look:

Invest 99L, the Gulf system (#2 on the map), presently has the best chance of development among the three systems designated by the NHC as areas of interest. It has decent odds (30%) of becoming a tropical depression, and perhaps another short-lived, Fernand-type tropical storm. It’d be an underwhelming “Humberto,” if it earned the name. But it’s running out of time before reaching land.

Invest 98L, the east-central Atlantic system (#3 on the map), is “showing some signs of organization” after being left for dead several days ago, but conditions are about to become very hostile, so it seems unlikely to ever become Humberto.

Then there’s the tropical wave near Gabrielle (#1 on the map). I find it hard to believe that this will develop into a new named storm, independent of Gabrielle, especially because Gabrielle’s mid-level center is now moving toward it. If anything, in the scenario where Gabrielle re-forms around its mid-level center, the likely result would effectively be a “merger” between Gabrielle and this unnamed wave, which would almost certainly maintain the name “Gabrielle.” I can imagine such a system developing into a respectable storm — but, given its location and the approaching troughs, it would almost certainly head out to sea, a harmless “fish.”

Another candidate for “Humberto,” as I mentioned, is Gabrielle’s low-level center, if it survives Hispaniola in any form, and if Gabrielle presently re-forms around its mid-level center (thus stealing the low-level center’s chance of keeping the “Gabrielle” name), and if the low-level center subsequently “pulls a Dorian” and regenerates next week. Needless to say, that’s a lot of “ifs.”

But perhaps the most significant candidate for the name “Humberto” isn’t even labeled on the map yet. It’s the next wave expected to emerge off Africa. There’s a lot of buzz, fueled by various computer-model projections in recent days, about the season’s first hurricane finally forming next week — and maybe becoming the season’s first major hurricane, too — from that wave, which will probably eventually be designated “Invest 91L.”

Ingestion of several grains of salt is called for, though. Computer model forecasts are subject to massive errors at long time ranges, especially when they’re making predictions about a hypothetical system. And this is hardly the first time we’ve heard this season about how the “next wave off Africa” is going to be the real deal. So, we shall see.

Anyway… this will probably be my last blog update on Tropical Depression Gabrielle unless something unexpected happens, like more-severe-than-expected flooding in Puerto Rico, or a meteorological development that increases the mainland U.S. threat (which right now asymptotically approaches zero, with the unlikely “Dorian-esque reformation in the Bahamas” scenario being really the only chance).

Also, I’m unlikely to devote much time to 99L (the Gulf system) because even if it does become T.D. 8, or even T.S. Humberto, it probably won’t pose a threat to the United States. I suspect my next blog update will be about that African wave, proto-91L, assuming it ever amounts to anything. But who knows? I don’t control the weather. I just blog about it. :)

In the mean time, as always, follow me on Twitter at @brendanloy for the latest. And, like I said, I’ll blog here again when conditions warrant.