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Decoupling strikes again: Karen “decapitated”

October 4th, 2013 - 11:31 pm

As expected, the weather story of the day was not Tropical Storm Karen, but a damaging tornado outbreak in Nebraska, Iowa and extreme southeastern South Dakota, and an epic early-season blizzard in Wyoming and western South Dakota (up to 3 feet of snow already in Lead, SD, and maybe 5 feet total before it’s over!). Both of these events were caused by the same storm over the Plains, which absolutely dwarfs puny Karen in the Gulf right now:

That said, this blog’s focus is on tropical weather (notwithstanding my #SNOWFIREICANENADO references), so I’ll defer to Mike Smith, Jeff Masters and others to wrap up those events.

As for Karen, well, it looks like she’s just about finished. I tweeted earlier (shortly after my last blog update) that, according to the National Hurricane Center’s 5pm EDT discussion, Karen had a chance to restrengthen after 24 hours if she could hold together that long, but first there was a significant chance the storm could completely “decouple,” with its thunderstorms totally separating from its low-level center — much like what happened with Flossie and Gabrielle earlier this season. Flash forward six hours, and it appears that is indeed happening:

The NHC’s 11pm EDT discussion explains:

KAREN HAS BEEN DECAPITATED BY STRONG SOUTHWESTERLY SHEAR AND NOW CONSISTS OF A VERY TIGHT SWIRL OF LOW CLOUDS WITH SOME LINEAR CONVECTION TO THE EAST OF THE CENTER. DATA FROM RECONNAISSANCE PLANES INDICATE THAT THERE IS A WELL-DEFINED CIRCULATION…BUT THE
WINDS ARE GRADUALLY DECREASING. ON THIS BASIS…THE INITIAL INTENSITY HAS BEEN LOWERED TO 40 KNOTS IN THIS ADVISORY. TROPICAL STORMS RARELY RECOVER AFTER BEING STRONGLY DAMAGED BY SHEAR…ESPECIALLY IF THE ENVIRONMENT IS AS DRY AS IT IS OVER THE WESTERN GULF OF MEXICO.

Officially, the NHC is keeping Karen as a tropical storm for now, and even forecasting marginal strengthening after 24 hours, given its “vigorous circulation” at low levels. However: “AS PREVIOUSLY INDICATED…AN ALTERNATIVE SCENARIO IS THAT KAREN REMAINS DECOUPLED FROM THE DEEP CONVECTION AND WEAKENS AT A FASTER PACE. THE LATTER SCENARIO IS BECOMING MORE REALISTIC GIVEN THE CURRENT ORGANIZATION OF THE CYCLONE.” Reading between the lines, and in light of respected meteorologists’ thoughts on Twitter, it’s pretty clear that rapid weakening and dissipation is what the forecasters truly expect, but the NHC is going to wait a few more hours to be sure before making that official.

Here at Weather Nerd, though, we don’t stand on ceremony, so I am hereby treating this storm as basically “dead until proven alive.” Accordingly, this will be my last blog update on Karen unless I wake up in the morning and learn to my surprise that she’s making a comeback. Even if that does happen, my updates will be limited because of weekend family plans — but I’ll at least keep tweeting about Karen if she survives, which you will be able see below. Once Karen officially dies, however, I’ll go back on Twitter #hiatus, and you won’t hear from me again until another U.S. tropical threat arises.

As always, Amy Sweezey’s Twitter list of “Wx Tweeps” is a good resource in my absence.

Anyway, without further ado, here are my most recent tweets, updating live:


Nearly-Naked Karen Twerks Toward Gulf Coast

October 4th, 2013 - 4:10 pm

About an hour ago, Ed Piotrowski called Tropical Storm Karen “nearly a naked swirl.” Around the same time, Rob Perillo tweeted, “Karen’s low-level circulation exposed! (sounds like a New York Post headline).” After I retweeted them, @RINOPundit asked me, “Is this the weather form of twerking? If so, #DOOM.” I couldn’t resist responding with the headline you see above.

But seriously — Karen continues to look disorganized as she moves slowly toward a late-Saturday/early-Sunday date with the Gulf coast. But there are increasing indications in the computer models — most notably a sudden shift by the Euro model toward the GFS-favored easterly track that was once considered an “outlier” — that the storm may remain over water longer, stay further east, and target the Florida panhandle instead of Louisiana — all of which means the “sleeper potential” of strengthening before landfall could become a reality. It will be interesting to see whether the National Hurricane Center adjusts the official forecast at 5pm EDT.

[UPDATE: Welcome, InstaPundit readers! Karen is now basically dead. Here's my final Friday-night update.]

Anyway, you can see my latest updates below via Twitter after the jump is an archive for posterity of this evening’s tweets in between this post and the one after it:

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I woke up this morning to snow in Denver (here’s a quick video) from the back end of a storm that’s producing an early-season blizzard in Wyoming and South Dakota. That might not seem related to Tropical Storm Karen — nor to extreme wildfire danger in California, nor a significant threat of severe weather in the Midwest later today, possibly including violent tornadoes in Iowa — but actually, they’re all connected.

Eric Holthaus explains:

These events comprise a crowded weather map that is actually the manifestation of a single continent-scale choreography of weather: high pressure out west is helping to steer and strengthen an intense low pressure system over the upper midwest that in turn is pulling the tropical storm northward towards the coast. It’s a perfect picture of the physics of the atmosphere, working seamlessly together.

The western high-pressure creates the intense offshore Santa Ana winds that have the forecasters in the National Weather Service’s Los Angeles office shouting from the rooftops about the worst fire danger in five years. The midwestern low pressure fuels both the blizzard and the severe weather and tornado threat, and is also responsible for shearing apart Tropical Storm Karen and, as Holthaus said, pulling it toward the coast.

According to Dr. Jeff Masters, “Wunderground weather historian Christopher C. Burt has done some research to see the last time a blizzard, major severe weather outbreak, tropical storm, and extreme fire danger all threatened the U.S. at the same time, and has not been able to find such an event in past history.”

The tornado and wildfire potential might well be today’s biggest threat, moreso than Karen, to be perfectly honest:

But this blog’s charge is to cover hurricanes and tropical storms that are threatening the United States, so let’s focus on Karen. (For ongoing coverage of the other severe weather events, I’d recommend Mike Smith’s blog and Amy Sweezey’s “Wx Tweeps” Twitter list. I’ll also continue to tweet and RT relevant information as I see it about all of these weather events on Twitter at @brendanloy.)

Karen has been relatively resilient in the wake of significant wind shear, but it nevertheless weakened further overnight, and is now down to 50 mph winds. The storm’s circulation center is exposed, with the thunderstorms sheared off to the northeast — leading to a dramatic visible satellite image this morning at sunrise of the thunderstorms casting a shadow over the center:

As I wrote last night, Karen continues to look unlikely to ever become a hurricane. The “outlier” intensity models, which last night were still insisting on intensification to hurricane strength, have seemingly relented. That said, the 11am EDT National Hurricane Center discussion implies that there’s still a chance:

THE ENVIRONMENT DOES NOT LOOK FAVORABLE FOR SIGNIFICANT INTENSIFICATION…WITH MODERATE SHEAR EXPECTED TO CONTINUE FOR THE NEXT DAY OR TWO. HOWEVER…IF THE SHEAR DOES LESSEN…EVEN FOR A SHORT PERIOD OF TIME…DEEP CONVECTION COULD RE-DEVELOP CLOSER TO THE CENTER AND ALLOW FOR SOME INTENSIFICATION. IN ADDITION…BY 48 HOURS UPPER-LEVEL DIVERGENCE AHEAD OF AN APPROACHING MID/UPPER-LEVEL TROUGH COULD ALLOW FOR SOME STRENGTHENING. THE NEW NHC INTENSITY FORECAST HAS BEEN ADJUSTED DOWNWARD AND SHOWS LITTLE CHANGE IN THE NEXT 24 HOURS AND A STRENGTHENING TO 55 KT BY 48 HOURS.

Mark Sudduth elaborates:

The strong upper level winds have been taking a toll on Karen but despite the relentless shear, deep convection keeps trying to form right over the well defined center of circulation. Water temps are plenty warm and all it will take is a relaxation of the shear for Karen to intensify and in quick fashion. This is discussed very well in the NHC forecast which notes that even the ECMWF model which has been the least “enthused” about Karen shows a 10 millibar drop in pressure before landfall. …

If Karen is strengthening right up until landfall, this will also mean that the wind will be likely be more dramatic. A convectively active storm or hurricane tends to have more downburst winds than one that is weakening. This is something we will need to watch closely over the weekend as Karen approaches the coast.

I agree. But most likely, this will be primarily a rainmaker for the areas it affects, rather than a major destructivewind and storm surge event. That said, as the recent devastating floods here in Colorado reminded me, flooding rains are nothing to take lightly. Sudduth again:

One of the bigger issues here is going to be heavy rain for a fairly prolonged amount of time. Since Karen is not moving very fast, once the rain shield makes its way onshore, we could be looking at nearly a foot in some areas. Even half of that amount is enough to cause serious flooding concerns and dangerous driving conditions for the region affected.

As for the exact track, that remains uncertain.

And it may remain uncertain for a while, as the NHC discussion explains:

THE FUTURE TRACK WILL BE QUITE SENSITIVE TO THE STRUCTURE OF KAREN OVER THE NEXT COUPLE OF DAYS. IN THE SHORT TERM A WEAKER SHALLOWER SYSTEM WILL BE STEERED MORE TOWARD THE LEFT BY THE LOW-LEVEL FLOW…WHILE A DEEPER MORE VERTICALLY COHERENT CYCLONE WOULD TURN NORTHWARD MORE QUICKLY DUE TO A MID-LEVEL RIDGE TO THE EAST.

In other words, as I wrote in my initial update, further west = weaker storm; further east = stronger storm. Also…

ALL OF THE GUIDANCE SHOWS A NORTHEASTWARD TURN IN 36 TO 48 HOURS…BUT WITH LARGE DIFFERENCES IN THE LATITUDE AT WHICH THE TURN OCCURS AND SIGNIFICANT SPREAD IN WHERE THE CENTER CROSSES THE COAST.

So, although the computer models are fairly confident about the general concept of the forecast, the timing and angle of approach are suc that relatively minor variations within that conceptual consensus will have large ramifications from the perspective of coastal residents who (quite understandably) want to know whether or not they will get hit.

Having said that, don’t focus too much on the exact track “line,” because this storm is so asymmetrical right now that the “center” isn’t nearly as meaningful as in, say, a major hurricane. Indeed, barring significant last-minute strengthening and storms “wrapping around” the center, the worst weather will likely be somewhat east of the eventual track “line” — which could mean that the Florida panhandle may be the most likely area for significant rain and wind, even if the center stays a bit further west.

Anyway, as always, stay tuned to this blog, my Twitter feed @brendanloy, and the aforementioned “Wx Tweeps” list for the latest.

P.S. Capital Weather Gang has an excellent overview on the situation with Karen.

Tropical Storm Karen continues to move north-northwest toward a weekend date with the northern Gulf coast of the United States, somewhere between Louisiana and Florida (exact location TBD). But, as expected, Karen is struggling with upper-level wind shear, and it looks increasingly unlikely that she will ever become a hurricane.

Here’s the current water-vapor satellite loop:

A couple of things to note about the above map. First, the brown stuff over the western Gulf, Texas, Louisiana, and Mississippi is dry air. That will likely become an issue for Karen as it gets closer to shore, and its circulation potentially sucks some of that in. Second, the wind shear — i.e., those strong winds from southwest to northeast across the system, pushing the high clouds and thunderstorms away from the center of circulation, which is exposed off to the southwest of the main area of overcast and rain. That’s not a sign of a healthy, strengthening tropical cyclone — and it’s not likely to improve. In fact, conditions are only expected to get less favorable, which is why this 65 mph tropical storm seems unlikely to gain the last 9 mph needed to become a hurricane.

Here’s what the official 11pm EDT discussion had to say about Karen’s intensity prospects:

KAREN CONTINUES TO BE A SHEARED CYCLONE WITH A VIGOROUS CIRCULATION…AND WITH MOST OF THE THUNDERSTORM ACTIVITY TO THE NORTH AND EAST OF THE CENTER. … THE SHEAR AFFECTING KAREN IS FORECAST TO INCREASE AND THIS WOULD SUGGEST WEAKENING. IT IS BECOMING LESS REALISTIC THAT KAREN WILL BECOME A HURRICANE IN A DAY OR TWO…BUT GIVING CREDIT TO SOME DYNAMICAL MODELS…LIKE THE HWRF…WHICH DO [FORECAST INTENSIFICATION TO HURRICANE STRENGTH]…THE NHC FORECAST CALLS FOR A SLIGHT STRENGTHENING. IT IS ANTICIPATED THAT KAREN WILL BE ON A WEAKENING TREND BEYOND 48 HOURS…AND BECOME EXTRATROPICAL BY DAY 4.

Translation: “The human forecasters ain’t buying it, but some of the computers still think Karen will become a hurricane, so we’ll split the difference and forecast mild strengthening to 70 mph, followed by weakening. But we don’t really believe even that.”

As for Karen’s track, there isn’t much more clarity tonight than there was 11 hours ago. The most reliable American global model (GFS) continues to be a far-right outlier [INSERT GOVERNMENT SHUTDOWN JOKE HERE], forecasting a landfall near the Big Bend of Florida, followed by re-emergence off the Carolinas!

But that seems pretty unlikely to most observers. The model consensus calls instead for a landfall point somewhere between southeastern Louisiana and the western Florida panhandle, in the vicinity of Destin, where my family vacationed earlier this year. (Very glad we picked May over October, which we also considered!) That’s still a wide range, though. Hopefully the models will get a better handle on the track tomorrow.

In the mean time, folks in Hurricane or Tropical Storm Watch areas should make prudent precautions now. Friday is really the last full day to prepare; the storm will hit on Saturday or early Sunday. Heaviest impacts will be to the east of wherever the center makes landfall. This isn’t going to be any sort of catastrophe — as I wrote earlier, there’s no reason whatsoever to hype it as some sort of Hellstorm — but if you’re in it’s path, please take it seriously, prepare prudently, and heed the advice of trusted local authorities.

UPDATE: A helpful summary from 28storms.com:

996623_679285955417248_648415340_n

Karen’s Low-level circulation is continuing to get further displaced from the Mid-level circulation due to strong wind shear. IF this trend continues a much more westerly/weaker track would verify (similar to the EURO).

If convection can re-develop near the center, however, in the next 12 hours, Karen could still track towards the north followed by a turn towards the NE.

Bottomline: The next 12 hours will be key.

I’ll be tweeting and RT-ing updates as feasible tomorrow on Twitter at @brendanloy, and will also update this blog as conditions warrant.

In addition, here’s a reading list of helpful sources. A good one-stop shop on Twitter is Amy Sweezey’s “Wx Tweeps” Twitter list. Another good blog source is Mike Smith’s site.

Worth noting: Friday may also be a big day for severe thunderstorms and tornadoes in the Plains and Midwest, a blizzard in Wyoming and South Dakota, and wildfires in Southern California — so there will be a lot to watch! (And all of it being monitored by NOAA folks working without pay, and without the help of furloughed “non-essential” support staff.)

Tropical Storm Karen has formed in the Gulf of Mexico, and is expected to hit somewhere along the northern Gulf coast this weekend. Hurricane Watches are up from Louisiana to Florida.

Photo Oct 03, 11 48 33 AM

The computer models are split on Karen’s likely landfall location, as you can see above. Shown are the Canadian model (left), the European model (center) and the American GFS model (right), via @RyanMaue. Here’s the official forecast from the not-shut-down-because-it’s-essential National Hurricane Center. But the bottom line is, forecasters aren’t sure yet where this thing is headed. One key point: the further east Karen goes, the stronger she’s likely to be. Conversely, further west = weaker.

The models agree, however, that Karen won’t get stronger than a Category 1 hurricane, and many models doubt she’ll ever graduate from tropical storm status. So this isn’t looking like a HELLSTORM OF DEATH worth hyping (not that that’ll stop Drudge & the cable newsies). But it certainly bears watching, and should be taken seriously by folks in the potentially affected areas. The storm is only ~2 days away, so the time to begin prudent preparations is now.

In order to avoid the massive time sink and brain damage of arguing with People Who Are Wrong On The Internet about the government shutdown and debt ceiling debate, and for other personal reasons, I’ve been on my own personal “shutdown” — an indefinite Twitter “hiatus” — this week, posting only the occasional Instagram and Tumblr link (totaling 2.5 tweets per day, vs. my normal pace of 100+ tweets per day), and avoiding Twitter interactions altogether. However, because of Karen, I will temporarily return to Twitter later tonight, and remain through the weekend before resuming my “hiatus.” You can follow me @brendanloy. I will also post a more complete update on Karen here on the blog later tonight.

In the mean time, here’s a reading list of weather-bloggers and tweeters who you should consider checking out.

Remember what I said on Thursday, near the end of my last post, about a then-hypothetical tropical wave “91L” near Africa, which could earn the name “Humberto” this week? Well, it has happened. 91L formed, was designated as Tropical Depression 9 yesterday, and then, this morning, became Tropical Storm Humberto. Now, it is expected to become the season’s first hurricane — and perhaps, if some of the computer models are to believed, a major hurricane soon afterward:

Humberto is expected to be a “fish” storm, posing no threat to land, but it’s a notable meteorological curiosity because of its timing. The all-time record for the latest date of Atlantic basin hurricane formation in the satellite era was set in 2002, when Gustav became a hurricane on September 11. More specifically, Gustav was declared a hurricane at 11:00 AM Eastern Time on 9/11/2002 (though the official NHC postmortem says it actually “became [a] hurricane just before 1200 UTC,” which is 8:00 AM Eastern). Wednesday of this week is September 11, 2013, and Humberto is forecast to become a hurricane sometime right around Wednesday morning. So it’s a race against the clock. Will Humberto 2013 “beat” Gustav 2002 to hurricane status, or will 2013 break the record for the season with the latest hurricane formation? Weather nerds will be watching closely!

Because it’s no threat to land, I won’t be posting regular updates on Humberto here. But I’ll be tweeting about it at @brendanloy, and also following the rest of the Atlantic tropics, including the potential for development in the southwestern Gulf of Mexico (proto-”Ingrid”?) this weekend or thereabouts. You can also keep abreast of the latest in the tropics by following Amy Sweezey’s “Wx Tweeps” Twitter list.

P.S. You probably remember the name “Gustav” from the 2008 hurricane that killed 153 people, mostly in the Caribbean; caused $4.3 billion of damage in Louisiana & environs; triggered a massive evacuation and a silly statement by Ray Nagin about the “Mother of All Storms”; and cancelled Day 1 of the 2008 Republican National Convention. The name “Gustav” was retired after 2008, but previously, it had been in the sextennial storm name rotation, so 2002′s Gustav, which brushed North Carolina and hit Nova Scotia and Newfoundland, was the penultimate “Gustav” in the Atlantic. There were also “Gustavs” in 1996, 1990 and 1984. “Gustav” will be replaced by “Gonzalo” in 2014.

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:

CONVECTION CONTINUES NEAR THE MID-LEVEL CENTER…SO THERE IS STILL A CHANCE OF THE LOW-LEVEL CENTER REFORMING IN THAT AREA. IF THAT DOES NOT OCCUR…GABRIELLE IS EXPECTED TO WEAKEN AS IT INTERACTS WITH THE TERRAIN OF THE DOMINICAN REPUBLIC AND ENCOUNTERS INCREASING WESTERLY SHEAR. THE REVISED INTENSITY FORECAST NOW CALLS FOR THE CYCLONE TO DEGENERATE TO A REMNANT LOW PRESSURE AREA IN 24 HOURS AND TO DISSIPATE COMPLETELY THEREAFTER.

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.

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

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:

“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.

The tropical wave that I blogged about late Sunday night, “Invest 97L,” is continuing to remain fairly disorganized as it moves through the eastern Caribbean Sea. Things have gotten interesting this morning, as the wave has been looking a bit better — Dr. Jeff Masters writes that 97L “has a moderate amount of spin and heavy thunderstorms, with a respectable upper-level outflow channel to the north,” albeit “no sign of a well-organized surface circulation” — but the system also finds itself competing for moisture with a new area of thunderstorm convection off to its east (not designated as an “invest” yet):

Hurricane Tracker App posted a video this morning about the interaction issue, and even referenced the possibility that the two systems could “merge,” which would slow any development — but would also, I think, make things even more unpredictable for a while. I haven’t seen others discussing such a “merger,” but certainly, the interaction of these two areas of convection is significant. Dr. Masters calls this issue a “wild card,” writing that the “new tropical wave may compete for moisture, slowing development of 97L, and could also modify the track of 97L.”

For now, though, the big-picture reality is that 97L hasn’t developed much since my last update on Sunday. That’s roughly as expected; most forecasters figured that, if 97L didn’t develop before crossing the Lesser Antilles, it probably wouldn’t get its act together until it reached the more favorable central or western Caribbean later this week. The key change in the forecast, however, and the reason for my “less likely to hit U.S. mainland” headline, is that 97L is now expected by most computer models to “turn right” and head toward the Greater Antilles (i.e., Puerto Rico, Hispaniola and/or Cuba), instead of moving into the open waters of the western Caribbean:

That’s both good news and bad news. It’s bad news for Puerto Rico and Hispaniola, which could see significant flooding from the wave’s heavy rain. Even though 97L doesn’t have a name or an official tropical-cyclone status right now, it has plenty of moisture, and the mountainous terrain of those islands lends itself to flash flooding and mudslides. Here’s the San Juan radar. As Mark Sudduth notes, “Even tropical waves/disturbances can dump excessive rainfall and…the terrain of the Greater Antilles islands is mountainous and is just inviting disaster each time a heavy rain event comes around.” Dr. Masters writes:

Regardless of whether or not 97L becomes a tropical depression today, the major danger from this slow-moving storm will be heavy rains. Three to six inches of rain are predicted to fall over Puerto Rico and the Virgin Islands by Thursday morning, with 3-day rainfall totals of 5-10 inches expected along the south and southeast shores of Puerto Rico. These rains are capable of causing dangerous flash flooding and mudslides, and a Flash Flood Watch has been posted. Similar rainfall amounts will fall in the eastern Dominican Republic, and heavy rains of 3-6″ are also likely to affect the Southeast Bahamas and Turks and Caicos Islands later in the week. Since 97L is relatively small, Haiti may see lower rainfall amounts of 2-4 inches.

Conversely, on the “good news” side, 97L’s turn toward the Greater Antilles will likely delay strengthening, due to land interaction and to the mountains disrupting any nascent low-level circulation that 97L might try to form. Instead of a window for intensification beginning on Thursday or so, as had been predicted previously, 97L now probably won’t have a chance to seriously organize until the weekend. Again quoting Dr. Masters:

The models take 97L to the west-northwest, bringing the center over the eastern tip of the Dominican Republic on Thursday morning. This track will allow the high mountains of Puerto Rico and Hispaniola to disrupt the circulation of 97L, forcing the storm to regroup on Friday over the Southeast Bahamas and Turks and Caicos Islands. The best chance for development of 97L would appear to be on Saturday, after the storm has had time to recover from its encounter with Hispaniola. The UKMET model predicts that 97L will become a tropical depression just north of the Southeast Bahamas and Turks and Caicos Islands on Saturday, but the GFS and European models show little development over the next three days.

Here’s a chart demonstrating this wide spread among the computer models — some calling for significant intensification, others calling for little or none at all:

The smart money, though, seems to be on less pronounced intensification. Here’s Mark Sudduth again:

Once the sprawling area of low pressure gets past the large islands of the Caribbean, it should gradually develop somewhere in the Bahamas. … Fortunately, none of the traditionally accurate intensity models show much coming out of this system as far as strengthening goes. That being said, I would not be surprised to see 97L eventually become a tropical storm before it gets pulled out to the northeast and away from the Southeast U.S. coast and the Bahamas.

That last point — the track — is the other reason why a significant hurricane approaching the United States mainland, like some models were predicting over the weekend, seems less likely now. As shown in the computer-model spaghetti map earlier in this post, the models now generally favor “recurvature” — i.e., turning further right and out to sea — rather than movement toward the mainland. That isn’t the universal prediction, but it’s certainly the trend. Dr. Masters again: “There will be a strong trough of low pressure off the U.S. East Coast this weekend, and the models predict that this trough will be strong enough to turn 97L to the north and northeast by Sunday, keeping 97L well offshore from the U.S. East Coast, but with a possible threat to Bermuda next week.”

One other areas of disturbed weather, in the southern Gulf of Mexico, merits a brief mention.

Sudduth: “[A] tropical wave continues to move across the Yucatan towards the southern Gulf of Mexico with no solid signs of development just yet. As we saw with the evolution of Fernand several days ago, it is possible that we could see development from this system as it moves across the southern Gulf. This area seems to favor quick development so we’ll see what happens. Anything that does manage to get going should be short-lived as was the case with Fernand.”

Masters: “A tropical wave over Mexico’s Yucatan Peninsula and adjacent waters of the Gulf of Mexico is causing scattered disorganized heavy thunderstorms. This activity will move over the extreme Southern Gulf of Mexico’s Bay of Campeche on Wednesday and Thursday, then moving ashore on the Mexican coast between Veracruz and Tampico on Friday. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC put the 5-day and 2-day odds of development at 20%.”

Meanwhile, far off in the eastern Atlantic, waves continue to emerge off Africa. A notable run last night by the best American computer model, the GFS, developed one of them into an enormous, larger-than-Sandy hurricane in a week’s time — but safely over the open ocean, bothering only the fish:

But that’s extremely speculative, and I’m really only posting it because OMG PRETTY COLORS. :) The bottom line is, there’s nothing to worry about right now, in terms of waves emerging off Africa. If that changes, I’ll let you know, of course. Stay tuned to this blog and my Twitter account for the latest.

The Atlantic tropics have been eerily quiet over the last couple of weeks. Indeed, the big headline has been that we turned the calendar to September without having a hurricane, the first time since 2002 that’s happened. Overall, August was remarkably quiet, despite the fact that conditions broadly favor development:

August 2013 had one of the lowest Accumulated Cyclone Energy (ACE) totals on record for an August in the Atlantic. … This year’s combination of no El Niño, warm [sea surface temperatures], and an exceptionally low August ACE is an event unparalleled in the historical record, going back to 1966. … The main reason for the quiet August has been the large amount of dry, stable air over the Atlantic. This dry air has two sources: the Sahara desert of Africa, and sinking air from aloft, which warms and dries as it sinks. Even so, I find it highly perplexing that activity has been so low when all of the other factors–lack of an El Niño, low wind shear, an active African Monsoon spitting out plenty of tropical waves, and above average ocean temperatures–have favored development.

But we may now finally have a candidate for significant tropical development. “Invest 97L” near the Lesser Antilles, a wave that emerged from Africa a week ago (and was written off by computer models days ago), seized the attention of the weather community Sunday as it took advantage of favorable conditions caused by a split-in-half upper low system (which Jonathan Belles explains further), and started looking unexpectedly robust:

97Lrgb0

As the convection fired up, 97L also started producing some attention-grabbing long-range computer model forecasts:

Now, let’s not get carried away here. “97L” isn’t even a tropical depression yet, let alone the major hurricane that some models are predicting it will eventually become. It may very well fizzle entirely, or become another Chantal/Dorian/Erin — a minor storm that develops, but never becomes a significant threat. Notably: “Several of the models indicate that this tropical wave will, in fact, become a hurricane. However, the advanced global models, such as the GFS, show next to nothing at all happening.”

The smart money is that, at a minimum, those bullish intensity models are developing 97L too quickly, for the reasons Dr. Jeff Masters notes:

The main factor keeping the disturbance from developing over the next two days would appear to be the fact that 97L is quite large, and is stretched out from east to west over a wide expanse. Large, elongated systems like 97L usually take several days to consolidate and spin up. Another factor that will likely retard development is the presence of strong surface trade winds over the Eastern Caribbean ahead of 97L, south of the Dominican Republic. These strong trade winds are a common feature of the Eastern Caribbean, and make the region something of a hurricane graveyard. As the surface wind flow to the west of 97L accelerates into this wind max, air will be sucked from aloft downward towards the surface, creating sinking air, interfering with the formation of thunderstorm updrafts. The best chance for development of 97L would appear to be on Wednesday or Thursday, when the disturbance reaches the Central Caribbean.

Here’s a graphical representation of that concept:

By the way, that video update by Levi Cowan really is very helpful and informative. If you want to understand what’s going on with 97L, I recommend taking 8 minutes to watch it:

Another important caveat: if this system does become “Gabrielle,” we cannot even begin to know where, specifically, it will go, because any continental landfall would be many days away — plus, storms that don’t exist yet are very difficult to accurately forecast! So there’s no cause for #PANIC or hype. It bears watching for anyone in Florida or anywhere on the Gulf Coast, but guidance more specific than that is impossible. As Brian McNoldy writes:

We will clearly have to watch this closely, as model solutions range from near-nothing to a Category 3 hurricane in 5 days near Jamaica. Right now, it’s too soon to get concerned, but not too soon to pay attention.

I’ll certainly be paying attention. Stay tuned to this blog and my Twitter account for the latest.