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Flossie to hit Hawaii; Dorian undead?

July 29th, 2013 - 1:42 am

Tropical Storm Flossie has — somewhat surprisingly — maintained its strength over the weekend, and appears set to make a direct hit on the Big Island of Hawaii as a moderate strength tropical storm Monday morning. Tropical Storm Warnings are up for most of the state, and residents are anticipating heavy rains, strong winds, dangerous surf and flash flooding, starting as early as 6am local time (noon Eastern Time) on the Big Island and Maui, from what would be the first active tropical storm to actually make landfall in Hawaii in more than two decades.

pacific_vis_current

The Central Pacific Hurricane Center (CPHC) expects Flossie’s top sustained winds to diminish from 60 mph to around 50 mph by the time she makes landfall on the Big Island shortly before or around noon Hawaii time (6pm Eastern Time) tomorrow. The latest discussion explains:

THE SYNOPTIC SCALE ENVIRONMENT THAT FLOSSIE IS MOVING INTO WILL GRADUALLY CAUSE SYSTEM WEAKENING. [SEA SURFACE TEMPERATURES] WARM SLIGHTLY ALONG THE PROJECTED PATH OF FLOSSIE…ESPECIALLY WEST OF THE MAIN HAWAIIAN ISLANDS. HOWEVER…WATER VAPOR IMAGERY SHOWS THIS SYSTEM EMBEDDED IN AN AREA OF INCREASINGLY DRY AIR ALOFT DUE TO SUBSIDENCE ASSOCIATED WITH THE UPPER HIGH NEAR KAUAI NOTED EARLIER. AS FLOSSIE MOVES WEST…THIS HIGH IS EXPECTED TO IMPOSE INCREASING NORTH TO NORTHEASTERLY SHEAR ACROSS THE SYSTEM. MODELS CONTINUE TO INDICATE A GRADUAL WEAKENING…

However, there is considerable uncertainty about both the track and intensity of Flossie because of something I’d never heard of until tonight: the “split flow effects of the Big Island.” (Hey, I’m an Atlantic basin guy, cut me some slack!) More here, here and here. (Thanks, Google!) The CPHC writes:

TRACK GUIDANCE REMAINS TIGHTLY CLUSTERED IN THE SHORT TERM…SHOWING FLOSSIE NEAR THE BIG ISLAND AND MAUI MONDAY MORNING…THEN PASSING SOUTH OF OAHU MONDAY NIGHT AND SOUTH OF KAUAI THROUGH TUESDAY. … [HOWEVER,] IT IS IMPORTANT TO NOTE THAT FLOSSIE IS VERY LIKELY TO TRACK ACROSS AT LEAST A PORTION OF THE BIG ISLAND OVER THE NEXT 36 HOURS…INTRODUCING THE DIFFICULTY OF ACCOUNTING FOR TERRAIN EFFECTS ON SYSTEM TRACK AND INTENSITY. POST-BIG ISLAND TRACK AND INTENSITY FORECASTS MAY BE QUITE DIFFERENT ONCE THESE TERRAIN EFFECTS ARE KNOWN.

Fascinating! I’ll try to post more on Flossie tomorrow. For now, here’s the NWS Honolulu radar page.

On another note, remember our “survivor” storm in the Atlantic, Dorian, which…um…didn’t survive after all? Well, from survivor to zombie — or, if you prefer, resurrected storm — she’s trying her best to come back to us now, at the turn of the tide, as Dorian the White. Quoth the NHC:

A TROUGH OF LOW PRESSURE…ASSOCIATED WITH THE REMNANTS OF DORIAN…IS PRODUCING A SMALL BUT CONCENTRATED AREA OF SHOWERS AND THUNDERSTORMS…AS WELL AS GALE-FORCE WINDS…A FEW HUNDRED MILES NORTH-NORTHEAST OF THE LEEWARD ISLANDS. ALTHOUGH THERE ARE CURRENTLY NO INDICATIONS OF A CLOSED LOW-LEVEL CIRCULATION AND A WELL-DEFINED CENTER…ONLY A SMALL INCREASE IN ORGANIZED THUNDERSTORM ACTIVITY COULD LEAD TO THE REFORMATION OF A TROPICAL CYCLONE. ENVIRONMENTAL CONDITIONS ARE MARGINALLY CONDUCIVE FOR REGENERATION…AND THIS SYSTEM HAS A MEDIUM CHANCE…50 PERCENT…OF BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS AS IT MOVES WESTWARD TO WEST-NORTHWESTWARD AT 15 TO 20 MPH. THIS DISTURBANCE IS EXPECTED TO PASS WELL TO THE NORTH OF THE LEEWARD ISLANDS AND PUERTO RICO TONIGHT AND MONDAY…AND MOVE OVER THE TURKS AND CAICOS ISLANDS AND THE SOUTHEASTERN BAHAMAS ON TUESDAY.

If Dorian re-develops, the computer models seem to genreally take it toward South Florida:

To be very clear, the bottom map is just one “run” of just one computer model (and not one of the best models either — it’s the NAM, not the GFS or Euro), about a hypothetical storm that doesn’t exist right now (which makes the models even less reliable). So it’s by no means a reliable forecast of what will happen; it’s merely representative of the general sort of scenario that could potentially, perhaps, be on the table, along with various other possibilities (including the distinct possibility that nothing happens at all). Seriously folks, there’s no need for the Andrew talk (even though, yes, 1992′s Andrew did lose its circulation for a while before regenerating and, of course, ultimately becoming a Category 5 monster). Again to be clear, there is no reason to believe anything remotely like that will happen here. But this is an interesting situation. There’s absolutely no need for hype or #PANIC — but we may not be done with Dorian yet. So, stay tuned.

As per usual: you can find more frequent updates by me on Twitter at @brendanloy, and by others on Twitter on Amy Sweezey’s “Wx Tweeps” list.

Dorian is a TSINO; Flossie nears Hawaii

July 27th, 2013 - 12:37 am

The National Hurricane Center has officially maintained Dorian’s status as a 40 mph tropical storm as of 11pm EDT. But, my fellow Americans, let me be clear: Dorian is a TSINO (Tropical Storm In Name Only).

Here’s another view of the ASCAT pass. Dorian has also looked horrible all day on satellite, and NOAA’s Satellite Services Division deemed it “too weak” to run the “Dvorak” analysis that’s used to estimate tropical cyclone strength. “Not sure I’ve seen that before with an active ‘tropical storm’,” writes Charles Fenwick.

The NHC’s 11pm advisory makes Dorian’s “TSINO” status pretty clear. The headline on the public advisory is “DORIAN FORECAST TO BECOME A REMNANT LOW IN A DAY OR SO.” From the discussion:

DORIAN…HAS BEEN DEVOID OF DEEP CONVECTION FOR SEVERAL HOURS. I WAS TEMPTED TO DECLARE DORIAN A REMNANT LOW IN THIS ADVISORY…BUT GIVEN THE FACT THAT THE CIRCULATION IS MOVING OVER WARMER WATERS AND SHEAR IS FORECAST TO LESSEN…NEW CONVECTION COULD REDEVELOP. THE BEST OPTION IS TO KEEP THE CYCLONE FOR A COUPLE OF MORE ADVISORY CYCLES AND MONITOR THE EVOLUTION OF THE CONVECTION.

Translating from NHC-ology into English: “It’s not actually a tropical cyclone right now, but we’ll maintain it as a minimal T.S. for now, just in case it redevelops overnight.”

That said, it’s possible that a flare-up of thunderstorm convection in the last few hours could extend Dorian’s official life by a few more advisories, notwithstanding the lack of a closed circulation (which is ostensibly the dividing line between “tropical cyclone” and “tropical wave”). It’s the Monty Python storm!

As Fenwick explains, there’s precedent for ignoring the lack of a closed circulation and keeping alive a seemingly-dead storm when a potential for future reorganization beckons:

It is memories of…[2005's] Irene…that caused me to express a bias towards Dorian persisting. Irene was quite the survivor of shear & dry air, going down to a tropical depression and having “little to no evidence” of a closed circulation at one point, before finally finding more hospitable conditions and eventually becoming a hurricane. The discussions from the 7th through the 11th make for fun reading. Note how it was never forecast to die despite (apparently) utterly failing to meet the criteria for a tropical cyclone at one point.

Now, my recalling of 2005 Irene isn’t to suggest that we are certainly in line for a repeat, it’s just to offer a reminder that a circulation can manage to survive in arduous conditions. A spark of convection here and there and it can survive near death. As the 11 PM discussion noted, should a circulation survive and should it avoid land, there is a promised land ~ 3 days out. That’s the rationale for the thinking that makes this aspect of NHC-ology, a storm being kept alive past its apparent due date, exist. It’s not 100% nuts, just, you know, kind of nuts, at times.

Another 2005 storm which went a step further, actually dying out, and then still managed to come back once it reached a “promised land”: Tropical Depression Ten, which died out near the northern Lesser Antilles on August 14, only to combine with another tropical wave nine days later to form a cyclone you may have heard of: Katrina. Like Fenwick, I’m not suggesting in any way that we’ll see a repeat of that — it’s just a memorable proof of concept.

However, the more significant threat to the U.S. — albeit still a fairly minor one — comes from the Pacific’s Tropical Storm Flossie (yes, “Flossie”), currently with 60 mph winds, which is expected to reach the Big Island of Hawaii late Monday or early Tuesday as either a weak tropical storm or a tropical depression:

Flossie has cold waters and wind shear ahead, which will cause inevitable weakening, as nearly always happens with storms approaching Hawaii from due east (rather than recurving up from the south, Iniki-style). But for now, Flossie certainly looks much more fearsome than Dorian:

Wind impacts should be mild, but flooding is possible in Hawaii from Flossie’s rains.

I may or may not update Weather Nerd over the weekend, depending on what happens with Dorian and Flossie, but regardless, for more frequent updates, follow me on Twitter (@brendanloy). Also, as I always say, Amy Sweezey’s “Wx Tweeps” Twitter list is good for getting the latest info.

Dorian on death’s door

July 26th, 2013 - 4:56 pm

There is an emerging consensus among weather-tweeters I follow that Tropical Storm Dorian is now well & truly falling apart, and is likely on the verge of downgrade and/or dissipation. A few hours ago, it was perhaps “50/50 it makes it through the weekend intact.” Now it seems like 50/50 the storm makes it through today intact. Here’s Brad Panovich, chief meteorologist at WCNC in Charlotte, within the last hour:

Panovich added: “Dorian no longer appears to be a Tropical Storm, may barely be a Depression.” Around the same time, Dr. Ryan Maue of WeatherBell tweeted: “Last few visible images show TS Dorian torn apart, perhaps 2 centers, which means advisories would be discontinued at 5 pm or 11 pm.” Ed Piotrowski of WPDE in Myrtle Beach agreed, saying he sees “two centers oriented SW to NE.” Panovich, responding to Piotrowski, noted, “yeah the mid-level center is way off to the NE now, with dry mid level air can’t hold on much longer.”

As I was typing this up, the NHC’s 5pm EDT advisory came out, roughly a half-hour early, and it maintains Dorian as a tropical storm, albeit weakening its officially estimated top wind speed from 50 mph to 45 mph. However, reading between the lines, and engaging in some “NHC-ology,” it’s pretty clear NHC forecaster Richard Pasch thinks Dorian may be in its final hours:

UNDER THE INFLUENCE OF WESTERLY SHEAR AND DRIER AIR…DORIAN HAS BECOME EVEN LESS ORGANIZED THAN IT WAS EARLIER TODAY. VISIBLE SATELLITE IMAGES INDICATE THAT THE LOW-LEVEL CENTER HAS BECOME DISPLACED TO THE WEST OF AN AREA OF WEAKENING DEEP CONVECTION. ALSO…ANIMATION OF THE IMAGERY SHOWS THAT CIRCULATION IS BARELY CLOSED AT THIS TIME. USING A BLEND OF DVORAK ESTIMATES FROM SAB AND TAFB…THE ADVISORY INTENSITY IS REDUCED TO 40 KT. THE OFFICIAL WIND SPEED FORECAST NOW SHOWS GRADUAL WEAKENING WITH DISSIPATION BY DAY 5…IN GENERAL AGREEMENT WITH THE GFDL AND HWRF GUIDANCE. GIVEN THE CURRENT TRENDS…HOWEVER…THE SYSTEM COULD DEGENERATE INTO AN OPEN WAVE MUCH SOONER THAN THAT.

“Much sooner” like 11:00 PM EDT tonight? I think maybe so. Earlier today, a “pass” by NOAA’s Advanced Scatterometer, or ASCAT, appeared to show a not-entirely-closed circulation. (Here’s the full image.) According to Charles Fenwick, the next pass, assuming the satellite doesn’t “miss,” will be at roughly 8:00 PM Eastern Time. Data from the ASCAT pass would be available 1-2 hours later — in time for the 11pm advisory. If that pass shows no closed circulation, I think it’s lights out for Dorian.

So, barring a comeback by our “survivor” in the next 3 to 6 hours, I’m guessing 11pm will be the final advisory, and the system will be declared an open tropical wave at that time. Alternatively, perhaps NHC will downgrade Dorian to a tropical depression at 11pm, pending visual confirmation in the morning that it continues to look horrible, and then discontinue advisories at 5am tomorrow. Either way, by the time the sun rises tomorrow here at Weather Nerd World Headquarters in Denver, Colorado, I doubt we’ll have Dorian to kick around anymore. We might have to monitor the Artist Formerly Known As Dorian for possible future development once it reaches more favorable climes next week — but in all likelihood, this storm is just about done.

I’ll be at a movie when the 11pm advisory comes out, but I may be able to tweet anyway, as it’s an outdoor movie. :) Anyway, follow me on Twitter (@brendanloy) for the latest. Also, again, Amy Sweezey’s “Wx Tweeps” Twitter list is good.

This morning’s biggest “storm” might be the Chris Christie-Rand Paul civil liberties feud — and the most exciting “weather forecast” might be the announcement that Sharknado is coming to theaters next Friday at midnight — but there’s also still some noteworthy action in the tropical Atlantic, barely. Tropical Storm Dorian is still chugging west-northwest at 21 mph with estimated top winds of 50 mph. Yesterday I called Dorian a “survivor,” but today, it’s clear that it’ll take all the strength he has not to fall apart, as he tries hard to mend the pieces of his broken circulation’s heart. (Sorry.)

The storm is continuing to struggle mightily against dry air:

precip-water

Image credit: CIMSS and NOAA, via the Capital Weather Gang. As CGW explains, warm colors on the top map indicate moisture, while cool colors indicate it’s too dry for a tropical cyclone (primarily in the lower atmosphere); brown colors on the bottom map represent dry air (primarily in the upper atmosphere).

The NHC explains:

VISIBLE SATELLITE IMAGES SHOW THAT THE CLOUD PATTERN OF DORIAN IS RATHER DISORGANIZED…WITH LITTLE OR NO EVIDENCE OF BANDING FEATURES. WATER VAPOR IMAGERY AND TOTAL PRECIPITABLE WATER DATA [i.e., the maps above] INDICATE THAT DORIAN WILL BE ENCOUNTERING DRY AIR OVER THE NEXT DAY OR SO AND THIS…ALONG WITH MODERATE WEST-SOUTHWESTERLY SHEAR…SHOULD CAUSE SOME WEAKENING OF THE TROPICAL CYCLONE. … AN ALTERNATIVE SCENARIO…WHICH IS CERTAINLY POSSIBLE…IS THAT DORIAN COULD DEGENERATE INTO A TROPICAL WAVE WITHIN THE NEXT COUPLE OF DAYS.

The Capital Weather Gang characterizes the situation thusly: “The increased vertical wind shear and persistent dry air have broken down tropical storm Dorian’s defenses.” Those “defenses” were initially quite strong — creating the impression of Dorian as a “survivor” — in part because, as Levi Cowan notes in an exceptionally informative video update on his “Tropical Tidbits” blog, “Dorian is very small, and small systems like this are good at fighting dry air.” But there’s a lot of dry air ahead of Dorian, cutting off its moisture supply. That’s pretty typical for this still early stage of the hurricane season, as Hurricane Track’s Mark Sudduth noted yesterday:

Anyway, dry air “is encroaching on the storm through a significant depth of the atmosphere” — at both upper and lower levels, as the map shows — which is largely why Dorian’s “has lost a lot of organization in the past day,” Capital Weather Gang writes. “And it’s probably not going to escape this hostile environment for a few more days.”

There are other factors, also worth mentioning, which are contributing to Dorian’s struggles as well. One is wind shear. As Ed Piotrowski explains, “because Dorian is moving so quickly, it’s creating its own wind shear that’s pushing storms away from the center.” That’s somewhat reminiscent of the even-faster-moving Chantal, leading my jokey comment yesterday that Dorian might be turning into “Chantal 2: Cape Verde Boogaloo.” But Levi Cowan says that’s not quite right: “This is not a Chantal situation. When Chantal was trying to keep a closed cirulation, she had strong trade wind flow everywhere around here. Right now, Dorian has a very strong trade wind flow just to his north, but the winds to his south…are much slower in speed.” As a result, even despite Dorian’s fast pace, the shear is only moderate. The trouble, as Cowan also notes, is that while small storms like Dorian are good at fighting dry air, “they’re terrible at fighting wind shear.”

Another factor contributing to Dorian’s weakening over the last 18-24 hours is the simple fact that the storm has moved further west, away from the moist “monsoonal flow” that’s blowing from southwest to northeast over the waters closer to Africa. As Cowan explains, that flow “shuts off” west of roughly 40 degrees west longitude, and trade winds resume, “so Dorian is now completely isolated from Intertropical Convergence Zone, and he is on his own.” Previously, the monsoonal flow and ITCZ were lending Dorian a helping hand, atmospherically speaking. Now, “it will be up to him, and him alone, to maintain himself off of the ocean water beneath him.”

Cowan further explains that yet another issue hampering Dorian right now is the rise in nearby barometric pressures. “Dorian is becoming isolated in an area of sinking and stabilizing air around him,” Cowan says. He illustrates this with a helpful map, but rather than re-posting it, let me just embed Cowan’s video, which is seriously worth the 8+ minutes it takes to watch. It really explains, succinctly and scientifically but with minimal jargon, what’s going on:

So, what’s next for Dorian? The first question is whether it will even survive the next day or two. As the NHC said, degeneration to a tropical wave — an “open wave” without a “closed” counterclockwise circulation — is distinctly possible. A “decoupling episode” last night, disrupting the synchronicity between the storm’s upper and lower level circulations, coupled with the ongoing dry-air entrainment, wind shear, and the other factors mentioned above, make this a perilous moment for Dorian. Says Cowan: “If he opens up today…and loses the convection, it could be good night to Dorian, and he may actually dissipate here.”

Cowan thinks Dorian will maintain a closed circulation, in part because (contra Chantal) the aforementioned trade-wind pattern — strong winds north of the storm, weaker winds south of it — is creating “an area of natural vorticity in front of” the storm. But it’s a very difficult question to confidently forecast. Charles Fenwick‘s take:

The question for Dorian is whether its environs are absolutely inhospitable for tropical cyclones or merely unfavorable for strengthening. The storm is being pushed along at a brisk pace, but not as fast as Chantal. The shear is there, but not in a debilitating quantity. The air is dry and that’s a negative too. Maybe it adds up to enough that the Hurricane Hunters fly on Sunday and don’t find a circulation. But for now, I am slightly biased towards the Dorian persisting for the duration. Still a bit of time for everyone to wait and see.

I guess we’ll find out in the next couple of days whether this storm really is a “survivor.”

If Dorian does survive the short-term threats to its survival as a tropical cyclone, the next major question will be that occurs when it reaches the area around Puerto Rico, Hispaniola and the Bahamas. Cowan points out a peculiar fact: the atmospheric environment in 4-5 days near those islands looks like it will “not the most unfavorable situation in the world” — indeed, it will have some favorable features, counterbalancing some potentially unfavorable ones (watch the video above for more) — yet although “the environment doesn’t look particularly unfavorable in the southwestern Atlantic, the models are now all pessimistic” about Dorian’s potential for development in that area. You can see that clearly by comparing the GFS model forecast for Dorian’s approach to Hispaniola with the GFS forecast from 2 days ago for Dorian at roughly the same longitude:

gfs_mslp_wind_watl_31

gfs_mslp_wind_watl_65-twodaysago

Cowan, for his part, thinks that something like the bottom forecast (from two days ago) is still possible, if Dorian’s survives intact as a closed circulation over the next few days. “Dorian could either become a hurricane, if he survives and strengthens like the GFS was showing a few days ago, or he could dissipate,” Cowan says. “It’s a little bit hard for me to believe that, if Dorian retains a closed circulation,” he will weaken as much as forecast. Part of the reason is that sea-surface temperatures will get progressively warmer as Dorian moves west, eventually as much as 1.5 to 2 degrees Celsius above what Dorian is traversing now. Thus, “if all else remains equal in his environment,” Dorian ought to strengthen. Cowan predicts: “If he retains a closed circulation, he should be able to be a little bit stronger [north of the Lesser Antilles] than what the GFS shows.”

He adds, though, that “the confidence, at least in my mind, for the intensity forecast [for] days 3-5 is lower than normal…because of all the balancing [of] negative and positive factors…and given Dorian’s small size. It’s always very hard to predict the intensity of tiny storms because they are very fragile, but they can also ramp up in a hurry due to warm water feedback. So this is going to be an interesting storm to track.” The storm could still be a “long-term threat” to become a hurricane, and eventually menace the U.S. East Coast sometime late next week, “but there is still a large, large degree of uncertainty.”

I haven’t said much yet about Dorian’s track yet. Here’s the official NHC forecast as of 11am EDT:

Ed Piotrowski explains:

Let’s assume Dorian survives. The track for the next 3-4 days is pretty straightforward. A huge ridge [of upper-level high pressure] to the north of the storm will steer it westward to a point near or north of Puerto Rico and Hispaniola. By early next week, it’s not as clear, since the long range models have been inconsistent with their tracks. For example, on Thursday morning, forecast tracks for Dorian ranged from a landfall near Houston, TX to curving out to sea near Bermuda. On Thursday night, many of the reliable forecast models shifted significantly farther south and are in much better agreement with no turn toward the southeast U.S. or Bermuda. Most of these tracks take Dorian over the Greater Antilles (Puerto Rico, Hispaniola and Cuba) and eventually into the Gulf of Mexico. The big difference? The trough [of upper-level low pressure] that could turn Dorian northward is weaker [than previously forecast], which allows the ridge of high pressure to build westward into Florida. This extension of the ridge protects the Carolinas by blocking a turn to the north. If this pans out, land interaction [with the Greater Antilles] could kill Dorian. While this is certainly encouraging, its not etched in stone. Let’s see if the models continue this trend over the coming days.

It’s worth adding that the possibility of Dorian going into the Gulf of Mexico is a good news/bad news thing. It’s bad news for folks on the Gulf coast, who thought they didn’t need to worry about this storm. But it’s good news in that it implies a weaker storm, as Capital Weather Gang explains: “[T]he longer it stays weak, the more westward it will move, since the low-level easterly trade winds will govern its motion as opposed to a stronger storm being steered by winds higher in the atmosphere.” Also, because the mountains of the Greater Antilles lie between Dorian and the Gulf of Mexico, a track that aims at the Gulf increases the likelihood of the storm’s tiny circulation being destroyed by interaction with mountainous land masses, as opposed a track through the Bahamas.

Anyway, as always, stay tuned. Check in here for periodic updates, and follow me on Twitter (@brendanloy) for more frequent tweets and RTs. Also check out Amy Sweezey’s “Wx Tweeps” Twitter list for frequent updates from various meteorologists and fellow weather nerds.

T.S. Dorian appears to be in trouble again

July 26th, 2013 - 3:26 am

Did I jinx Tropical Storm Dorian? No sooner did I declare the storm a “survivor” and an “overachiever” (and got Instalanched for it) than it began to struggle mightily. First, it started to look more ragged. Then, the computer models began to adjust their intensity expectations downwards, many calling for weakening or even outright dissipation:

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(Graphic via Tropical Atlantic, which is an awesome site, BTW.)

Then, the National Hurricane Center, in its 11pm EDT discussion, explained why the models were souring (again) on Dorian:

THE CONVECTIVE STRUCTURE OF DORIAN HAS NOT BEEN PARTICULARLY ORGANIZED THIS EVENING…AND RECENT MICROWAVE IMAGERY INDICATES THAT THE LOW-LEVEL CENTER LIES TO THE SOUTHWEST OF THE DEEP CONVECTION. … DORIAN HAS WEAKENED A BIT…AND THE INITIAL INTENSITY IS NOW [50 MPH, DOWN FROM 60 MPH]. …

DORIAN IS BEING DRIVEN QUICKLY WEST-NORTHWESTWARD BY A LOW- TO MID-LEVEL ANTICYCLONE OVER THE CENTRAL ATLANTIC…AND ITS ESTIMATED MOTION IS 285/17 KT. … DORIAN IS EMBEDDED IN AN ENVIRONMENT OF FAST EASTERLY LOW-LEVEL FLOW AND RELATIVELY LIGHT UPPER-LEVEL WINDS…WHICH APPEARS TO BE INDUCING SOME WESTERLY TO SOUTHWESTERLY SHEAR IN THE LAYER BELOW THE CIRRUS OUTFLOW. SINCE THE FORWARD SPEED OF THE CYCLONE IS NOT EXPECTED TO SLOW DOWN ANY TIME SOON…DORIAN COULD EXPERIENCE PERSISTENT SHEAR FOR THE NEXT FEW DAYS. ON TOP OF THAT…MID-LEVEL RELATIVE HUMIDITIES ARE EXPECTED TO DECREASE…AND THE DRY AIR COULD INHIBIT DEEP CONVECTION. NONE OF THE GUIDANCE SHOWS MUCH CHANGE IN INTENSITY DURING THE NEXT FEW DAYS…SO THE OFFICIAL FORECAST WILL KEEP THE WINDS FLATLINED AT 45 KT THROUGH DAY 3 WITH NOT MUCH STRENGTHENING THEREAFTER.

In other words, Dorian’s rapid forward speed may be starting to turn it into a scaled-back version of Chantal 2: Cape Verde Boogaloo. “A weaker Dorian’s low level center [is] outracing the thunderstorms…he will struggle to survive,” wrote Miami meteorologist John Morales.

Meanwhile, that scourge of tropical cyclones — dry air — has finally cracked Dorian’s previously solid circulation, and begun to take its toll:

Morales added that “days ago,” he had “mentioned 3 hurdles” to Dorians’ development: “(1) cool water, (2) dry air, (3) future shear. Whatever (1) didn’t do (2) is doing now.”

Meteorologist Michael Watkins of Hurricane Analytics summarized the sudden change this way:

Watkins added that Dorian now looks a bit “wave-like” — in other words, its circulation may be losing some of its core organization.

North Carolina meteorologist Brad Panovich pointed out that both the GFDL and HWRF computer models — two of the most reliable American models, after the GFS — now “show ‪Dorian‬ dissipating as it gets into the Caribbean.”

1078582_518520588220402_685770796_o

A single run of those two models isn’t gospel, of course. (And the GFS, for its part, keeps Dorian intact long enough to slam Hispaniola — in which case the mountains would defeat it, probably.) But the overall trend in the models toward a more southerly track (i.e., closer to due west than WNW or northwest) is also indicative of weakening. If Dorian is a weak and disorganized system, it will be pushed more by lower-level steering currents, and will be unable to respond to shortwave troughs that could otherwise pull it more to the north. Thus, a more southerly track implicitly means a more disorganized storm.

Panovich comments: “The writing on the wall with this one.”

We’ll see. I’m not quite ready to give up on Dorian just yet. But a significant U.S. threat is certainly looking less likely tonight than it did this morning and afternoon.

Stay tuned. Follow me on Twitter (@brendanloy). Also check Amy Sweezey’s “Wx Tweeps” Twitter list for frequent updates from various meteorologists and assorted weather nerds.

For about a week-and-a-half now, meteorologists and weatherbloggers, myself included, have been watching a potential Cape Verde hurricane that some computer models were forecasting way back when it was just a vigorous batch of thunderstorms over the African continent, many days away from the warm ocean waters that fuel tropical cyclones. Countless grains of salt were consumed, especially given that the previous hypothetical “proto-Dorian,” forecasted for mid-July by some models — you might recall me mentioning it in a Chantal update — had proved to be a total dud. But when the Canadian model (CMC) joined the premier American model (GFS) on July 16 in predicting a late-month wave with significant potential for development, things started to get interesting.

Dorian as a wave

By July 20, as the vigorous African wave (represented by the green line above) neared the Atlantic Ocean, the model runs were becoming more and more consistent in predicting storm formation (or “cyclogenesis,” to use the fancy #science term). But then, even as the wave emerged over water late on July 21, and was officially designated as “Invest 98L” on Monday (July 22), the models backed off and stopped predicting development.

Invest 98L now looked like a marginal threat — an unlikely “proto-Dorian,” even after all that computer-model hype the previous week. 98L, however, was apparently committed to living up to the hype. Once it got over the unseasonably warm waters just off Africa, 98L did unexpectedly well. Its odds of development steadily increased, and by late Tuesday, it was a “WINO” (Wave In Name Only) on the verge of Tropical Depression status. Early yesterday (Wednesday) morning, 98L officially became Tropical Depression 4, and six hours later, it was Tropical Storm Dorian. (Not DeLorean!)

At the risk of overly anthropomorphizing tropical systems, it sometimes feels like certain storms are “survivors,” tending to consistently outperform the expectations of computer models and forecasters, while other storms are consistent underperformers, never quite “getting their acts together” even when the meteorological conditions suggest that they “should.” Thus far, Dorian seems to fall into the “survivor” category. Even after it defied the odds by becoming a depression and then a storm yesterday, the naysayers were everywhere, noting Dorian’s seemingly “dim future” thanks to cool sea-surface temperatures ahead. The conventional wisdom was that “significant strengthening [was] unlikely,” and that total dissipation was entirely possible. I was one of the naysayers:

I intended to post a “Weather Nerd” update yesterday, and I had the lede pre-written in my head. Referencing my first Chantal update, I was going to say: “Stop me if you’ve heard this one before. An unseasonably early ‘Cape Verde’ tropical storm has formed in the eastern Atlantic, and could eventually pose a threat to the southeastern U.S. — if it lasts that long.”

Alas, politics took precedence over weather in my blogging endeavors yesterday, and because of the time I spent explaining why I support Justin Amash and oppose lawless NSA/FISA spying, I never got around to publishing that post. Then again, perhaps that’s just as well, because 24 hours later, the story looks quite different.

As Levi Cowan (@TropicalTidbits) explained in an excellent video update last night, Dorian held together remarkably well yesterday, and it has continued to do so overnight and into today, even as it traverses the coolest waters it will face in its life as a tropical cyclone. The storm shows no sign of significant weakening; indeed, its maximum sustained winds ramped up to 60 mph yesterday, making it a relatively strong tropical storm, and they’ve remained steady ever since. It’s possible the National Hurricane Center — which is relying primarily on satellite estimates right now, as Dorian is too far away for Hurricane Hunter reconnaisance flights, and will remain so until Sunday — is being a little bit generous with those winds, but what’s clear is that Dorian is by no means falling apart, as might have been expected by now. Here’s how it looked this morning:

And here’s how it looks this afternoon:

Like I said, Dorian feels like a “survivor.” I know that’s unscientific, but I can’t tell you how many times I’ve seen storms persistently “live up” — or “live down” — to their seeming “character” as either overachievers or underachievers. Of course, I suppose there’s some confirmation bias at work: for every Irene (a bit of an underachiever) and Epsilon (epic overachiever and “survivor”), there are probably countless storms that one doesn’t remember as having a “character,” because no particular pattern emerges. But whatever. It may be silly, but I’ll confess I’m skeptical of those computer-model projections — even though one of them is the almighty Euro — which show Dorian significantly weakening or dissipating due to upcoming encounters with wind shear, dry air and the like. For instance:

I don’t buy it. I know there’s wind shear ahead, several days down the line. But Dorian is a geographically small storm with a well-developed, tightly-wound circulation, which may be a plus, as Joe Bastardi says: “This was supposed to fire up and start falling apart…I think its being compact is helping it beat models.” As for the dry air that’s supposed to impede it? Well, that seems to be shrinking. And again, my gut just tells me he won’t go quietly into the night. That’s why I lean, unscientifically, toward the more bullish model runs, like this one from last night:

Please don’t focus on the details of that particular track, though. Not only is it an old model run — and using an old supercomputer to run its projections, no less; the new one, this afternoon, spit out a more southerly track — but there’s just a huge amount of uncertainty about where this storm will ultimately go, a week to 10 days hence:

That’s why I urge people NOT TO #PANIC about individual computer model graphics like this one (today’s 06z GFS):

Again, DO NOT #PANIC!! It’s waaaaaay too early to get exercised about specific landfall scenarios. At this distance, you can expect the tracks to keep shifting around from model to model and from run to run. We really don’t have a remotely clear idea yet of where Dorian will go, and we won’t for a while yet. Everybody on the East Coast should keep a loose eye on it.

At this point, rather than specific landfall scenarios, the focus should be on big-picture trends, like the issue articulated this morning by Charles Fenwick:

Track forecast continues to appear to be simple for the forecast period [i.e., the next five days]; same course [as previously predicted], albeit slowing, puts Dorian N of PR by Monday night. It’s in the days over the horizon of the forecast period that the track forecast for Dorian becomes complicated. Still a split in how models depict Dorian several days out. Those that keep it a weak entity continue to move it westward. The GFS, on the other hand, keeps Dorian as an organized storm and turns it northwest *slowly* & moves it to the Carolinas. Speculative stuff.

Indeed. Dr. Jeff Masters has more.

Anyway, stay tuned. I’ll keep updating Dorian periodically here on Weather Nerd. For even more frequent updates, follow me on Twitter (@brendanloy). Another good resource for the latest information, in tweet form, is Amy Sweezey’s “Wx Tweeps” Twitter list.

Okay, so “nothingburger” isn’t the technical meteorological term. But Tropical Storm Chantal was downgraded, at 5:00 PM Eastern Time Wednesday, to a mere tropical wave — a disorganized mass of thunderstorms with no closed circulation. There will be no further advisories by the National Hurricane Center unless the storm makes a comeback. Over on Tropical Tidbits, weatherblogger Levi Cowan has a helpful video summary of what happened:

Here’s what Chantal’s remnants look like as of the wee hours Thursday morning:

chantal-watervapor-thursday

Could redevelopment occur once Chantal’s remnants get a bit further west, and counter a more favorable environment? Perhaps. Joe Bastardi thinks so:

And so does the Canadian computer model:

But for now, that’s fairly speculative. We’ll just have to watch and wait.

This will be my last Weather Nerd update on Chantal unless conditions dictate otherwise — e.g., if restrengthening occurs or begins to look likely. Please follow me on Twitter (@brendanloy) for any updates in the meantime.

Chantal lives! (Barely!)

July 10th, 2013 - 12:17 pm

National Hurricane Center meteorological discussion, 11:00 AM Eastern:

ALTHOUGH SATELLITE IMAGERY SUGGESTED THAT CHANTAL NO LONGER HAD A CLOSED CIRCULATION THIS MORNING…THANKS TO DATA FROM AN AIR FORCE HURRICANE HUNTER PLANE CURRENTLY INVESTIGATING CHANTAL…WE WERE ABLE TO LOCATE ENOUGH OF A CIRCULATION TO MAINTAIN ADVISORIES. INITIAL INTENSITY IS 40 KNOTS [45 MPH] BUT GIVEN THE STRONG SHEAR THAT IS EXPECTED TO PREVAIL ALONG THE PATH OF CHANTAL…ALONG WITH THE INFLUENCE OF SURROUNDING LAND…THE OFFICIAL FORECAST CALLS FOR GRADUAL WEAKENING. CHANTAL WILL LIKELY DISSIPATE IN 3 DAYS…OR PERHAPS MUCH EARLIER.

Some meteorologists disagree with the NHC’s decision not to downgrade Chantal or declare her dissipated. But regardless, she remains officially a Tropical Storm, for now.

Chantal is so weak, however, that lower-level steering currents are dominating her movement, which has resulted in more of a continued west-northwest track instead of the expected northwest turn — as I mentioned was possible last night. The official forecast has caught up with this conventional wisdom, and now has the storm’s center missing Hispaniola altogether.

Beyond that, the computer model forecast tracks are all over the map, literally:

It remains possible that, once Chantal reaches more favorable conditions in the Bahamas (or maybe the Gulf??), she could restrengthen, either from her current barely-holding-on state, or even from a remnant low or tropical wave if she does eventually dissipate. But that’s very speculative. For now, it’s hard to draw many conclusions, except that this storm is really weak at the moment, and that forecasters don’t know if it will ever amount to anything (and, if so, where it will go).

As I say at the end of each post: I’ll keep updating Weather Nerd when I can. In the mean time, follow me on Twitter (@brendanloy) for more frequent storm updates (largely in the form of retweets), as well as tweets about various other, unrelated topics. Another good resource for the latest information on Chantal is Amy Sweezey’s “Wx Tweeps” Twitter list.

Chantal has probably dissipated

July 10th, 2013 - 8:35 am

It appears that Tropical Storm Chantal, torn apart by wind shear and by her own rapid forward motion, has probably dissipated to a “tropical wave” — a disorganized collection of thunderstorms with no closed circulation. Here’s the current satellite view this morning:

vis0

The National Hurricane Center’s “intermediate” advisory at 8:00 AM Eastern Time (in between the full advisories, with detailed meteorological discussions, at 5am and 11am) said this:

…CHANTAL MOST LIKELY A TROPICAL WAVE…
…RECONNAISSANCE PLANE EN ROUTE TO VERIFY…

So, unless the reconnaissance plane finds something unexpected, it is likely that Chantal will be downgraded to a dissipated former tropical cyclone at 11:00 AM Eastern, if not sooner. Stay tuned. I’ll update Weather Nerd when I can, and in the mean time, follow me on Twitter (@brendanloy) for more frequent updates. Another good resource for the latest information is Amy Sweezey’s “Wx Tweeps” Twitter list.

Cuba, not Hispaniola?

July 10th, 2013 - 4:00 am

Might Tropical Storm Chantal miss altogether its expected fateful encounter with Hispaniola, which was supposed to determine its fate? The latest computer models suggest maybe so:

chantal-shift

John Morales, chief meteorologist at NBC’s Miami affiliate, discussed this possibility earlier Tuesday night: “How do you stop the #Chantal train? Moving at 26mph it’d be hard for it to turn sharply NW. If it misses Hispaniola, all bets are off.”

By saying “all bets are off,” Morales was referring to the forecast for weakening or even dissipation caused by a direct hit on mountainous Hispaniola. Squeezing between eastern Cuba and Jamaica would be an entirely different scenario — although Morales was quick to add: “it should also be pointed out that #Chantal’s survival isn’t entirely up to Hispaniola’s mountains. It will encounter shear.”

Morales then RT’d this tweet by David Bernard, the Chief Meteorologist at CBS’s Miami affiliate, showing that shear:

Meanwhile, models and forecasts aside, the simple fact is that right now, Chantal is looking ragged and disorganized. Its winds are down to 50 mph, and its forward speed is up to a remarkable 30 mph.

Stay tuned, as they say. I’ll keep updating Chantal here on Weather Nerd for as long as the storm remains a potential threat to the U.S. coastline. For even more frequent updates, follow me on Twitter (@brendanloy). Another good resource for the latest information is Amy Sweezey’s “Wx Tweeps” Twitter list.