According to the 8:00 PM EDT intermediate advisory, the aircraft reconaissance flight currently in Hurricane Dean just reported a minimum central pressure of 920 millibars — down 10 mb from the last reading. Dean is strengthening again. The NHC isn’t reporting any change in the top wind speed yet, which may be because they’re waiting for additional recon data before making the call, or because it’s taking Dean’s winds a while to “catch up” with the falling pressure, or some combination of the two. Either way, I presume there will be a wind-speed bump in the 11:00 PM advisory, if not sooner in a special “update.” If it’s just a 5-mph bump, from 150 to 155, Dean will remain a Cat. 4. Otherwise, he’ll cross into Cat. 5 territory.

Is there a silver lining in this? Maybe. The deepening presumably means the eyewall replacement cycle is over. That’s a little sooner than I thought, and perhaps there will be time for Dean to start another cycle before landfall tomorrow afternoon. That could prevent the calamity of a landfall on the island as a strengthening Category 5 hurricane. But regardless, it ain’t going to be pretty in Jamaica tomorrow.

UPDATE/CORRECTION, 9:43 PM: On the other hand… if I’m reading the reconaissance report correctly, the aircraft is reporting a minimum central pressure of 918 millibars… but a maximum flight-level wind of just 123 knots! That’s 141 mph, which would normally translate to around 127 mph at the surface. Is Dean a 918-millibar Category 3?? I suppose I shouldn’t be so surprised — I remember some of the ‘05 hurricanes went through similar phases while their wind fields were expanding and their eyewalls were cycling.

Speaking of which: the aircraft reports that the strongest winds were in the “outer eyewall.” Apparently the eyewall replacement cycle isn’t over yet. So I guess you can forget my “silver lining” comment. But here’s a different silver lining: meteorologist and fellow weatherblogger Brian Neudorff comments that it “looks like there is some drier air getting into Dean. This seems to be consistent with what Accuweather’s Joe Bastardi was saying in his recent post. ‘Dean may weaken a bit the next 24 hours as dry air from the mountains of Hispaniola is drawn into the storm’.” If I’m not mistaken, hurricanes going through eyewall replacement cycles are especially vulnerable to disruption caused by dry-air entrainment. So that makes sense.

In any event, it will be very interesting to see what the NHC does at 11:00 PM. It appears we have ourselves a deepening, yet weakening, hurricane! Though to be honest, I wouldn’t be surprised if they hold the intensity at 150 mph, not wanting to downgrade it on the basis of a single recon report (which might have missed the strongest winds) only to upgrade it again in a few hours’ time.

UPDATE, 10:47 PM: The 11:00 PM advisory bumped the winds down only slightly, to 145 mph. Waiting for the discussion to post.

UPDATE, 11:06 PM: Here’s what the discussion says about Dean’s intensity:

THE INNER CORE OF DEAN APPEARS TO BE UNDERGOING SOME CHANGES. RADAR IMAGERY ONBOARD THE AIR FORCE RECONNAISSANCE AIRCRAFT DEPICTS CONCENTRIC EYEWALLS AT RADII OF ABOUT 10 AND 20 N MI…WITH THE OUTER EYEWALL RECENTLY BECOMING A LITTLE BETTER DEFINED. FLIGHT-LEVEL WINDS HAVE ALSO REVEALED DOUBLE MAXIMA. WIND DATA AT FLIGHT LEVEL…FROM THE SFMR…AND FROM DROPSONDES DIRECTLY SUPPORT AN INTENSITY OF NO MORE THAN ABOUT 115 KT. THE CENTRAL PRESSURE…HOWEVER…HAS BEEN GRADUALLY DROPPING AND WAS MOST RECENTLY MEASURED AT 918 MB. ASSUMING THAT THE MAXIMUM WIND HAS NOT BEEN SAMPLED…THE ADVISORY INTENSITY IS SET TO 125 KT…BUT THIS COULD BE A BIT ON THE HIGH SIDE. THE AIRCRAFT WILL CONTINUE TO INVESTIGATE DEAN FOR A FEW MORE HOURS TO SEE IF THE WINDS COME BACK UP.

Later, it adds, “THE SHORT-TERM INTENSITY FORECAST IS A BIT UNCERTAIN GIVEN THE DOUBLE EYEWALL STRUCTURE…BUT THE CENTRAL PRESSURE IS QUITE LOW AND THE INNER CORE COULD QUICKLY REORGANIZE AT ANY TIME…SO ANY DIP IN THE INTENSITY WILL PROBABLY BE SHORT-LIVED.”

More significantly, the track forecast keeps Dean just offshore of Jamaica. Hopefully that holds up.

The 5:00 PM advisory is out. Dean remains steady at 150 mph, according to the NHC. The discussion says, “THERE HAVE BEEN NO SIGNIFICANT CHANGES IN THE STRUCTURE OF DEAN DURING THE PAST SEVERAL HOURS.” The forecast, too, remains essentially unchanged, though the track’s “center line” has shifted ever so slightly to the left. Jamaica is still very much in the bull’s eye, however.

Alan Sullivan’s update on Dean is largely text-free. Instead, he’s offering three pictures worth 1,000 words each.

Meanwhile, Eric Berger is trying to give his readers “a sense of the near-historic intensification that Dean could undergo after crossing Jamaica.” He thinks Dean could rival the 2005 trifecta of Katrina, Rita and Wilma — the latter of which set the all-time record for lowest pressure recorded in the Atlantic basin.

P.S. Myself, I’m still worried about the intensification that Dean could undergo immediately before crossing Jamaica. I mentioned yesterday that the waters just east of the island are extremely warm, and “just as the Gulf Stream and the Loop Current tend to supercharge hurricanes that pass over them, I fear Dean could get a final, deadly burst of energy” from that bit of bathtub-like water. Alan Sullivan concurs. And the timing of the current eyewall replacement cycle makes things worse: as Margie Kieper commented earlier, “It appears [the cycle] will have time to complete before the hurricane reaches Jamaica.” Hurricanes generally weaken a bit during replacement cycles (though any weakening of Dean has yet to register in the NHC’s advisories), but they often strengthen once the cycle is over — and if the end of this cycle coincides with Dean’s arrival in the superheated water off Jamaica, the result could be disastrous. It would be sort of like throwing gasoline on a fire just as a gust of wind blows by. Boom!

The U.S. Gulf Coast has been spared the worst of several recent hurricanes, including Katrina, because of a combination of cooler water near shore and well-timed eyewall replacement cycles. With Dean and Jamaica, by contrast, the cycle timing is just awful, and the water near shore is anything but cool. I don’t want to be unduly alarmist or feed hysterical hype. But I fear the worst. As I said yesterday, “The only thing worse than a Category 5 hurricane making landfall is a strengthening Category 5 hurricane making landfall.” That’s precisely what I fear Dean will be for the people of that island.

In Sullivan’s words, “What can one say, except pray?”

[UPDATE, 4:43 PM: Welcome, InstaPundit readers! My newest post on Dean is here. All my latest posts on Dean, as they appear, can be found here.]

Just a quick update on Hurricane Dean this afternoon. The storm appears to be going through an eyewall replacement cycle, and may weaken a bit as the cycle continues. The 2:00 PM advisory is calling for “fluctuations in intensity” during the next 24 hours. For now, Dean is still at 150 mph with a 930 mb central pressure.

Some folks, like Dr. Jeff Masters, believe Dean has “peaked” for now, while others, like Alan Sullivan, think intensification is likely once the current cycle ends. Sullivan predicts that Dean will become Cat. 5 overnight tonight. Myself, I continue to worry about a final burst of intensification tomorrow, right before it hits Jamaica, due to the extremely warm waters just east of the island.

Whether or not Dean has “peaked” for the time being, a second “peak” is likely in between Jamaica and the Yucatan, as the waters are warmer in the western Caribbean than in the eastern Caribbean. (Indeed, as Sullivan noted yesterday, Dean’s development so far is remarkable, given its location; hurricanes don’t usually get this strong in the eastern Caribbean.) The official forecast calls for slight weakening over Jamaica, followed by strengthening to a Cat. 5 with 160 mph winds in 48 hours, en route to a second landfall near Cancun.

The extent of weakening over Jamaica is a big deal, though, and highly unpredictable. Because the island is so mountainous, the weakening could be significant. It all depends on Dean’s exact track over the island. The NHC is back to forecasting a tip-to-tip direct hit on the island, but as I said yesterday, it will likely be impossible to predict the storm’s exact course vis a vis Jamaica until landfall is imminent or occurring, because of last-minute “wobbles” and such. Sullivan notes that the island itself could cause the storm to wobble: “A spinning coin on a flat surface will bounce off an obstacle. Hurricanes do this too, sometimes, though such wobbles are less likely with a fast moving storm. … Will Dean march straight over Jamaica, or swerve just enough to reduce the destruction ashore?” And later: “Dean appears to be aiming directly at Jamaica; but the Blue Mountains are like a great wedge, and they may deflect the core to one side or the other.”

The question of Dean’s exact path over (or past) Jamaica is relevant not just to the destruction on the island, and not just to Dean’s future intensity, but to Dean’s future track as well. Again, I quote Sullivan:

If Dean wobbles south of Jamaica, then it will likely cross Yucatan and head for NE Mexico, as most models predict. If Dean wobbles north of Jamaica, it will pass much closer to Cuba, perhaps even crossing the island’s western tip. In that case Texas and Louisiana will be at risk later, as a few models indicate.

The models actually came into much better agreement overnight on a final landfall between Tampico, Mexico and Corpus Christi, Texas. Even the pesky GFDL agrees now. The improved consensus is probably a result of the fact that Dean is now moving in tandem with the upper low near Florida, as you can see on the satellite loop. The hurricane slowed down, the low sped up, and now they’re in sync. This makes things a bit easier for the computers to predict. The only northern “outlier” is CLP5, which isn’t really a forecasting model at all, as explained here. All in all, the central Gulf looks much safer today than yesterday. Indeed, the Houston-Galveston area and points east are “outside the cone.”

Still, as Sullivan notes, small wobbles can have big implications, and it would behoove everyone in the Gulf to keep paying attention. It was, after all, a wholly unexpected leftward wobble over the Florida peninsula that sealed New Orleans’s fate in 2005, sending Katrina hurtling toward the central Gulf instead of the panhandle.

One final note: I’ve added some new links to my “Dean blogroll” at right. Easily the three best sources for updates, though, are the first three on the list: Jeff Masters, Eric Berger and Alan Sullivan. So if you’re looking for the latest on Dean, and I haven’t updated my blog in a while, I highly recommend their sites. As you may have noticed, at least two-thirds of my information is stolen quoted from them anyway. :)

P.S. An interesting little tidbit from the 5:00 AM discussion:

NEAR [1:00 AM EDT]…AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT REPORTED [177 MPH] FLIGHT-LEVEL WINDS IN THE NORTHERN EYEWALL OF DEAN…AND ALSO REPORTED THAT THE CENTRAL PRESSURE FELL TO 930 MB. THE FLIGHT-LEVEL WINDS COULD HAVE JUSTIFIED CALLING DEAN A CATEGORY FIVE HURRICANE. [177 mph flight levels winds would normally translate, using the standard 90% reduction formula, to 159 mph surface-level winds. -ed.] HOWEVER…DATA FROM DROPSONDES…THE STEPPED-FREQUENCY MICROWAVE RADIOMETER…AND SATELLITE INTENSITY ESTIMATES DID NOT SUPPORT THAT STATUS. THE INTENSITY WAS THUS SET [as of the 2:00 AM intermediate advisory -ed.] TO A SLIGHTLY MORE CONSERVATIVE [150 MPH]. THERE HAS BEEN LITTLE CHANGE IN THE SATELLITE APPEARANCE OF DEAN SINCE THE AIRCRAFT LEFT…SO [150 MPH] IS THE INITIAL INTENSITY FOR THIS ADVISORY.

[UPDATE, 2:00 AM: Now Dean is at 150 mph, with 930 mb of pressure. Will it be a Category 5 by the time I wake up in the morning? I wouldn’t be at all surprised.]

As of 11:00 PM, Hurricane Dean has maximum sustained winds of 145 mph, and a minimum central pressure of 937 mb — and he’s not done strengthening. The headline on the public advisory is “CATEGORY FOUR DEAN INTENSIFYING OVER THE EASTERN CARIBBEAN,” and the discussion predicts that Dean will reach 155 mph (1 mph short of Cat. 5 status) in 24 hours.

Personally, I’d bet even money that Dean will become a Cat. 5 sometime tomorrow. Keep in mind, it has increased from 100 mph to 145 mph today, so jumping another 11 mph tomorrow (or 15, really, since they always round to the nearest multiple of five) isn’t exactly a huge stretch. Nor would it be surprising if the NHC’s current forecast is slightly downplaying the potential for further intensification. Because of the difficulty inherent in forecasting the internal dynamics of intense hurricanes, the NHC tends to be rather conservative in its intensity forecasts at this stage of a storm’s life. So just because the forecast has Dean hitting Jamaica in roughly 42 hours with “only” 155 mph sustained winds, it doesn’t necessarily follow that that’s the most likely scenario. I’d wager on 165 mph, myself, but as always, I’m just a layman and that’s just a guess.

The forecast track has shifted ever-so-slightly to the left, and now calls for the eye to rake the south shore of Jamaica instead of crossing the middle of the island from east tip to west tip. Of course, the whole island is easily within the “cone of uncertainty,” and the possibility of small but crucial last-minute “wobbles” means it will probably be impossible to predict Dean’s precise course vis a vis Jamaica until very close to landfall.

What seems fairly certain is this: Jamaica is going to be hit, and hit hard. A direct hit is distinctly possible — and if it’s a direct hit from a Category Five, that would be a first in the island’s history, according to Wikipedia — but even a “glancing blow” from a Cat. 4-5 hurricane would be quite bad. Much like the monster hurricanes of 2005, Dean is growing geographically larger, its wind field expanding as it intensifies. Still, Jamaica’s best hope at this point is for the storm’s eye to track far enough south of the island (or north, but south seems more likely) that the eyewall, or at least the inner portion of the eyewall, doesn’t come ashore. Otherwise, Drudge is right: it will indeed be a “HISTORIC HELL STORM.”

Dean’s exact intensity between now and landfall in Jamaica will depend mostly on eyewall replacement cycles, the dynamics of which meteorologists really don’t fully understand and can’t reliably predict. So another thing to hope for, if Dean does hit land, is a well-timed cycle right before landfall that brings the eyewall ashore during a weakening phase.

Alas, however, there is reason to fear that Dean might come ashore during a strengthening phase. Take a look at the Tropical Cyclone Heat Potential map of the waters surrounding Jamaica:

That white-hot color on either side of the island represents the most dangerously warm waters in the whole Atlantic basin, and just as the Gulf Stream and the Loop Current tend to supercharge hurricanes that pass over them, I fear Dean could get a final, deadly burst of energy from the patch of pink and white just off the eastern shore. That would be a very bad thing, because major hurricanes are worse when they are strengthening than when they’re stable or weakening; the greater instability of the intensification cycle leads to higher wind gusts and such. In other words, the only thing worse than a Category 5 hurricane making landfall is a strengthening Category 5 hurricane making landfall.

It’s probably too late to get the hell out of Jamaica now; I’m told all outbound flights are booked through Sunday afternoon, at which point the storm will be hitting. But if there’s any way to get out, do it, as Alan Sullivan says:

If you are a tourist on Jamaica, get off the island tomorrow — even if you have to go to Cancun. You’ll have a extra day to get out of there. Seriously, you don’t want to be trapped in a hurricane-ravaged Third World country. There will be no utilities, no untainted water or food, scarcely even a secure place to sleep, in areas exposed to the full force of a category four or five storm. Do whatever you have to do, pay whatever it costs, and get out. Jamaica residents should of course rush all preparations to completion. They live there. One hopes they know what to do for their own security.

For those who can’t leave, Jamaican residents and tourists alike, now is the time to make prudent preparations (like filling bathtubs with water) and get ready to hunker down for the storm… then ride out the aftermath. And it’s also a good time for the rest of us to keep them in our thoughts, and pray for them if we’re so inclined. It’s going to be a very rough few days in paradise.

After Jamaica? The computer models continue to disagree about where Dean will go, and there will be plenty more time to talk about that over the weekend. For now, the only thing I want to say about the storm’s post-Jamaica future is that Dean’s path over the island could be quite significant to the long-term intensity forecast. The more interaction between the hurricane and the mountains, the more likely it is to weaken. Conversely, the further offshore it stays, the better for Jamaicans but the worse, potentially, for residents of the Yucatan Peninsula, who would then be more likely to bear the full brunt themselves. Ultimately, though, it may not matter, since the waters between Jamaica and the Yucatan are plenty warm, and Dean will have plenty of time to get its act back together even if it does weaken over Jamaica. Two Cat. 5 landfalls are certainly not out of the question. In fact, the NHC, which is holding Dean to a high Cat. 4 in its predicted Jamaica landfall (and not specifically predicting any weakening from land interaction), is forecasting it to reach minimal Cat. 5 status in 72 hours, and make landfall as such near Cozumel late Monday or early Tuesday.

And with that, I’m going to bed. I’ll try my best to keep posting storm updates through the weekend, though I also continue to have a ton of errands and housework I desperately need to get done, and it seems like this hurricane-blogging thing very quickly becomes all-consuming. :) As soon as I’m done with one lengthy post, something else happens and I feel the urge to start a new one! And, as I told Becky earlier, all this extra web traffic is like a drug: it makes me want to blog more and more! After all, if lots of people are visiting my blog, I want to give them good, timely information, and not leave them disappointed with what’s here! So I blog and blog and blog. And blog. … Anyway, I’ll do my best to balance the blogging with my other responsibilities over the next few days, so that I can keep giving y’all updates without totally screwing the pooch on everything else (like cleaning our massively untidy house for our houseguest who is arriving in less than a week… AAAHH!!!).

Anyway, g’nite all. Pray for Jamaica. I leave with you with another scary satellite image, from a few hours ago:

P.S. Interesting aside: a blogger in Jamaica notes that the island nation has an general election scheduled for August 27. All campaigning has stopped as the storm approaches, and it is possible the election may need to be postponed, depending on the extent of the damage.

[FINAL UPDATE: I’ve just finished a new post with all the latest on Dean. … Thanks for the links, Glenn and Hugh! … P.S. If anyone wants to bookmark, or link to, a stable URL that will always point to my most recent Dean-related posts, you can use my 2007 Hurricane Season category.]

[UPDATE, 10:47 PM: I missed it till just now, but the NHC issued a special 9:30 PM update further bumping Dean’s official top wind speed to 145 mph. I thought it was stronger than 135 mph. … The forecast as of 5:00 PM predicted that it would max out at 150 mph, two days from now. What will the 11:00 PM discussion predict? The NHC is usually pretty conservative when predicting further intensification of major hurricanes, so I sort of doubt they’ll bring it to Cat. 5 status (156+ mph) in the forecast; they might call for it to max out at 155, followed by “fluctuations in intensity.” But that’s just a guess. … Anyway, the full 11pm advisory, with discussion, should be out any minute now. I’ll discuss it in a new post above.]

[UPDATE, 8:01 PM: Dean has been bumped to a Category 4 hurricane as of the 8pm intermediate advisory, with 135 mph winds. The NHC’s decision is apparently based on preliminary recon data. Looking at the satellite, I wouldn’t be at all surprised if there’s a further “bump” at 11:00 PM, based on additional recon data and additional strengthening over the next three hours — but as always, take my opinions with a grain of salt. Dammit, Jim, I’m a law clerk, not a meteorologist!]

On the satellite loop, Dean really looks like he’s getting his act together. See here, too. The hurricane hunter aircraft isn’t expected to arrive until 8:00 PM, which is when the next intermediate advisory from the NHC is due out, so I assume they won’t have any new recon data in time for the advisory, and thus will probably hold off on any major intensity changes until 11:00 PM. I wouldn’t be at all surprised to see a significant bump in strength then — or even perhaps in an “update” in between 8 and 11, if the data justifies it. Nor would I be surprised to wake up tomorrow and find out that Dean went through a rapid deepening cycle overnight and is officially a monster by morning. Check out the visible shot at sunset:

Amid all the talk about computer models and Dean’s future track, let’s not forget Jamaica. Alan Sullivan sums up the situation vis a vis that tropical isle in the bullseye:

I’m impressed with the progress Dean has made today; it’s starting to justify the hype. But the hurricane has not yet made direct contact with the shear field of the upper low in its path. As the distance closes, that should happen tomorrow, unless the low speeds up, Dean slows down, or both. Once these features match speeds, then there will no longer be much likelihood of shear interfering with the storm. In that case, Jamaica will really take a terrible hit. …

Bottom line here: deadly menace to Jamaica. Locals have one nice day to prepare. On Sunday it will start to get stormy. Sunday night, well, pray Dean’s core slips a bit north or south of the island.

[UPDATE, 9:05 PM: The GFDL has shifted back south again, and now shows a 165 mph landfall between Corpus Christi and Houston, rather than a 155 mph hit on central Louisiana. The GFS has also shifted south, and is now predicting landfall in Belize (not the Yucatan) with no significant re-emergence over water until reaching the Pacific — in other words, no threat to the United States. The NOGAPS and UKMET remain essentially unchanged, both taking the storm into Mexico and keeping it away from the U.S. You can view all of the “big four” models here, and even more model tracks here.]

The debate over Hurricane Dean’s future track is boiling down to a simple question: Does the GFDL computer model know something that all the others don’t? Over the last several model runs, the GFDL has been trending northward, while the other models have been trending southward. Take a look at Weather Underground’s model map as of 2:00 PM… the blue line is the GFDL, while the white, purple and red lines represent the other three of the “big four” dynamical models (UKMET, GFS and NOGAPS):

As you can see, the GFDL is stubbornly taking Dean into Louisiana, while the other models are leaning increasingly toward a Mexico landfall. So, what are we to make of this stark disagreement? Dr. Jeff Masters has an excellent post asking the question, “Which model do you trust?” After posting and analyzing a couple of charts of model accuracy, he concludes that “the official NHC forecast outperforms all the individual models, particularly at long ranges. Looking at the individual model plots can be helpful to determine the uncertainty in the forecast, but it’s tough to beat the NHC. In the case of Dean, where one model is an outlier from the rest, it is usually better to believe the consensus of the other models.”

Usually. But what if the GFDL is seeing something the other models are missing? Recall Alan Sullivan’s post from this morning, in which he wrote, “In the real world, as opposed to the world of computer models, I see something happening” — namely, a “mighty sluggish” upper low off Florida defying forecasts that it will quickly get out of the way — that could nudge Dean’s track to the right. Perhaps the GFDL is seeing the same thing Sullivan saw. (It should be noted here that Sullivan, like me, isn’t a meteorologist; he’s a layperson with an intense interest in, and a good deal of lay knowledge about, hurricanes. He’s also a self-described “contrarian.” None of which necessarily means he’s wrong.)

Not everyone thinks the GFDL is on to something. Earlier this afternoon, Eric Berger wrote that the GFDL “continues to jump all over the map, this time from a Galveston landfall [which it was predicting at 8:00 AM] to central Louisiana. This indicates one of two things: [either] 1) the model has not properly initialized Dean and is rendering its path incorrectly, or 2) the model is detecting a trend in the upper atmosphere the other models have missed. Given its inconsistency I’d lead toward the former answer, but there’s no way to know for sure.”

But one man’s “inconsistency” is another man’s “trend.” For the last 24 hours, the GFDL has been trending more to the right, or north, with each model run. At 8:00 PM yesterday, the GFDL showed Dean’s track pointing toward Corpus Christi; at 2:00 AM this morning, an area just south of Houston/Galveston appeared to be the target; at 8:00 AM, Houston/Galveston was in the bull’s eye; and then at 2:00 PM today, Central Louisiana was favored. If that’s a genuine trend — and with only four data points, I admit it’s a somewhat shaky contention — it would seem consistent with Sullivan’s observation that the upper low off Florida is continually refusing to move as predicted. It will be interesting to see what the model comes up with at 8:00 PM.

Berger, though, is looking at a different “trend” — the trend toward a more southward track in the other model runs:

The hurricane center’s afternoon update appears to have nudged the five-day forecast only very slightly southward. As noted below, the models have made a more significant move south. What is happening here is that the hurricane center doesn’t want to have its forecasts, in the wise words of Jeff Masters, have a “windshield-wiper-like” effect.

Therefore if this evening’s model runs show a similiar southward movement, as this morning’s did, I would expect a southward shift in the 10 p.m. forecast track.

As for the GFDL, he writes, “The key feature here is an upper-level low pressure system…that’s moving to the west ahead of Dean. The GFDL model (the outlier at this point) handles the movement of this system differently than the other forecast models. I’ll have more on this tomorrow morning if the models have not agreed upon how to handle the upper-level low, which could steer Dean into Texas or even Louisiana.”

The bottom line continues to be: Stay tuned. There’s still a lot more ink (er, and pixels) to be spilled on this storm before we figure out precisely where it’s going.

UPDATE: In comments, Sullivan writes:

I see the bifurcation in models as part of the same trend. Dean could also turn left if the upper low refuses to move. In effect the hurricane would be trying to sneak underneath the opposing circulation.

In other words, I think all the models may be seeking ways to resolve a single fact: that the hurricane is overtaking the upper low.

I had a similar thought rolling around in my head, but wasn’t sure quite how to express it, or whether it made sense. But yeah. Thanks, Alan.

UPDATE 2: Now Alan has a new post on his blog fleshing out that same thought. Excerpt:

Some [of the computer models] stay with the Jamaica, Yucatan, upper Mexico run — almost a straight line. Others are shifting left toward Nicaragua or even further south. And some are shifting right, into the Gulf.

I have been thinking about the righthand shift all day. The leftward shift in some models surprised me — but only for a moment. Then I had an insight. What if all these diverging models are responding to the same phenomenon — namely the sluggish movement of the upper low near Florida, which the hurricane is steadily overtaking? For Dean could also avoid the encounter by turning left, in effect bouncing off the periphery of the other system’s circulation.

Two effects, one cause…but which way will Dean turn? Or will it simply blow through the opposing system and stay on the straight course?

P.S. Bryan Woods makes an important point about the intensity forecast for Dean, post-Jamaica: “If, as currently indicated, Dean tracks directly over Jamaica, we will have to reevaluate everything. Jamaica has quite large mountains in the center of the island and Hurricanes do very poorly when passing over high elevations. Jamaica could do quite a number on our storm and knock his strength down significantly with the right conditions. Not only will the elevation condense out a lot of water from Dean’s circulation, unstable flow and wave breaking over mountains can cause a redistribution of potential vorticity, which can create several types of lateral instability in the flow.”

Okay, that’s my last Howard Dean joke (well, in a headline anyway), I promise. Because this is deadly serious stuff: the National Hurricane Center issued a special advisory at 1:45 PM upgrading Hurricane Dean all the way from a 105 mph Category 2 to a 125 mph Category 3. It is now expected to have 150 mph winds when it reaches Jamaica in 48 hours. And check out the satellite image:

Yikes.

UPDATE: Okay, okay, one more Howard Dean joke:

Heh.

If you live in Texas, it’s far too early to think about evacuating from Hurricane Dean — he’s at least five days away yet, and could potentially go anywhere from Belize to Louisiana, or perhaps even further east. But Eric Berger offers some good advice to Texans, particularly in the Houston-Galveston area: review the relevant disaster plans and maps, and decide if you’ll eventually need to evacuate, if Dean heads your way. That advice works well for the whole Gulf coast, actually. As Scar from The Lion King would say: “Be Prepared!”

…leave!

Glenn Reynolds linked this morning to my post last night advising people in Jamaica and the Yucatan to “get the hell out” if they can. My phraseology was, of course, a throwback to my famous Katrina post in the same vein. And I mean it: in Jamaica especially, there is absolutely no reason to keep waiting for further information before making a decision — and by doing so, you risk delaying too much and becoming unable to leave.

Hurricane Dean will hit Jamaica, or pass very near Jamaica. The “macro” track forecast for the next 48 hours, which is the relevant period of time as far as Jamaica is concerned, appaers to be pretty much set in stone at this point. Only the “micro” track details remain to be determined (e.g., will it wobble a handful of miles to the right, and make a direct hit, or a handful of miles to the left, and deliver only a glancing blow?), and those details won’t be determined until it’s too late to evacuate anyway.

Likewise, on the intensity front, it remains to be seen whether Dean will live up to its potential (which, as I noted yesterday, is to become “Gilbert, the sequel“) or whether its growth will be stunted by dry air or shear, as Alan Sullivan suspects. However, again, that won’t be known for certain until it’s too late to evacuate. If the hurricane puts on a burst of rapid intensification 12 hours before landfall and goes from a Cat. 3 to a Cat. 5 in that time — which is entirely plausible — you won’t be able to get out once you realize for certain how bad it’s going to be.

You can’t wait for absolute certainty. It’s already certain enough. Now is the time for action, not deliberation. If you have no choice but to ride out the storm on the island, fine: figure out what is the safest place for you to be (hint: a well-constructed building, able to withstand very high winds, at a location far enough removed from the coast that it’s not vulnerable to storm surge, but not so high in the mountains that it’s vulnerable to mudslides, and not in an area that’s vulnerable to flash flooding) and make plans to go there. But if you can leave, then leave! This isn’t a difficult decision! There is a very serious possibility — not a certainty, but certainty is too much to ask — of a Category 4 or 5 hurricane hitting Jamaica in two days’ time. Choosing to ride out such a monster anywhere, but especially in a third-world country, is a very bad choice.

When Hurricane Gilbert slammed Jamaica in 1988, it killed 45 people and caused $4 billion of damage. It destroyed houses, roads and small aircraft. It “severely damaged all but two medical facilities and 50% of the water supply.” So even if you survive the hurricane, the aftermath will be ugly, if it hits as badly as it could. Seriously — there’s no reason to risk it. Get the hell out.

(If you’re in the Yucatan, you might have an extra day to mull things over. But otherwise, the rest of the above applies equally to you. And personally, if I were your position, I wouldn’t mull; I’d leave ASAP.)

P.S. A commenter going by “HangGlider” left a comment on the previous thread saying: “My daughter and her husband left for Jamaica last Sunday for their honeymoon and have (foolishly - in their Father’s opinion) decided to stay on the island through the hurricane. I just hope the resort people have enough experience to get their customers to a safe location to ride out the storm.” I responded via e-mail, making a plea for him to try again to convince his daughter to change her mind, and I’ve reprinted that e-mail after the jump for anyone who might be in a similar situation.

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Hurricane Dean grazed the southern tip of Martinique overnight. Thankfully, he weakened a bit while passing by, probably lessening the blow suffered by the islands. But now he’s strengthening again: 105 mph and counting, as of 11:00 AM.

Unfortunately, there’s some bad news on the computer-model front. Last night I noted that confidence in the forecast might improve this morning because of newly available data from the NOAA jet. It hasn’t worked out that way. Indeed, the 11:00 AM discussion states, “IT SHOULD BE NOTED THAT GUIDANCE FOR DAY 4 AND 5 IS MORE UNCERTAIN TODAY THAN YESTERDAY.” According to Dr. Jeff Masters, “The NOAA jet mission did not help at all with narrowing down the uncertainty in the computer forecasts for the 4-5 day period, which remain divergent.” See for yourself here.

Why this disagreement? In Dr. Masters’s words, “each model has a different solution for the behavior of an upper-level low pressure system expected to be over the Gulf of Mexico early next week, and there is no is currently no way to guess which model will be right.” It also seems like each weather blogger has a different interpretation of the models. For instance, AccuWeather’s Henry Margusity wrote this morning that “South Texas [is] now becoming the only spot in the U.S. where Dean may hit.” On the other hand, his colleague Jesse Farrell wrote, “This morning’s Model Spread seems to shift the tracks up the coast from southern Texas to evenly cover the entire state.” FLhurricane.com notes the same thing: “the track models have begun to shift a bit north… This means, now more than ever, that the entire Gulf should be watching [Dean].”

Eric Berger, in Houston, looks at the scenarios and their implications for Texas. He sounds worried, and it’s not hard to see why. As Farrell notes, and shows graphically, the GFDL is showing a 916 mb Category 5 monster offshore of Houston on Wednesday. That would be, um, bad. The model image is rather scary:

For his part, however, Alan Sullivan wonders whether we should be trusting the models at all. Looking at “the real world, as opposed to the world of computer models,” he “see[s] something happening that could notch down those intensity forecasts” and alter Dean’s track. Specifically, and I quote his analysis almost in its entirety:

I’m watching the upper low off Florida. It looks mighty sluggish. It’s hesitating in response to a deepening polar trough over eastern Canada. As usual this summer, that polar feature is growing abnormally strong, and digging a bit further south than expected. By holding the cut-off low longer, it changes Dean’s prospects in two ways.

First, intensity: Dean is moving very swiftly. It is overtaking the upper-level cyclonic flow that wraps round the base of the cutoff into the northern Caribbean. Unless the low speeds up, its flow will begin to shear Dean. Already we can see a hint of shear in the storm’s shape. Dean’s envelope of outflow is becoming subtly elongated along a NE-SW axis as the NW flank of its circulation begins to encounter the shear-field. At this point I think Dean will encounter enough adversity in the Caribbean to prevent further intensification until it nears Jamaica. Meanwhile it may even weaken back to category one for a time. OK, this is a wild guess, but I’m a contrarian. (Make sure to include that last sentence if you quote me, Brendan!)

Second, track: Dean will probably continue on an unchanged course as it bucks the shear, but as it gets further west it may begin responding to the pressure by edging right. There is more land to the right. All that much-touted deep warm water means little if the hurricane declines to pass over it. Also, should Dean hit mountainous Jamaica head on, or go even further right and skims near the south coast of Cuba, its intensity will be affected by friction with land. A course close to Cuba would eventually take Dean over the western end of the island and onto the Loop Current in the eastern Gulf. Intensity would probably surge there, and Dean could in time present a serious threat to New Orleans or points east.

Meanwhile, watch that upper low. Run the NW Atlantic vapor loop… That will show you how the Florida low is interacting with the polar flow over the northeastern US. Don’t trust the models. They do not seem to see what is happening.

[NOTE: A lengthier “get the hell out” advisory to folks in Jamaica can be found here.]

I’m breaking my Dean-silence 80 minutes early to report that the storm is now a Category 2 hurricane, with 100 mph winds, rapidly approaching the Lesser Antilles. Indeed, the outer bands are already affecting the islands, and the eye will pass through overnight tonight and tomorrow morning. A new advisory is due out within the next few minutes, and I’ll post an update after it’s released.

UPDATE: Dean is still at 100 mph as of 11:00 PM. According to Dr. Jeff Masters, the storm’s strengthening has stalled because “dry air on the storm’s northwest side…is getting wrapped into the storm. This dry air will persist through at least Friday, and should act to prevent Dean from undergoing rapid intensification until it clears the Lesser Antilles Islands.”

Once Dean gets into the Caribbean, however, it’ll be a different story. “Dean is steadily moistening the environment around it, and may be able to overcome the dry air on Friday and put on a burst of rapid intensification,” Masters writes. “I expect Dean will become a large and extremely dangerous major hurricane by Saturday.” And by Monday night, when it’s expected to reach the Yucatan Peninsula, “The GFDL and HWRF intensity forecasts both project Dean will be a Category 5 hurricane.” In fact, according to Eric Berger, the GFDL predicts that Dean will have 180 mph winds by Monday. Berger thinks “that’s almost certainly too high,” but I’m not sure why — as Dr. Masters noted earlier today, the Western Caribbean’s “ocean heat content is near the record levels observed during 2005.” Frankly, I’ll be surprised if Dean doesn’t reach at least 165 mph at some point between now and Monday night. It’ll probably take an unexpectedly severe run-in with either wind shear or dry air to prevent such an eventuality. (The NHC’s 11pm discussion provides a glimmer of hope on that front: “THE UPPER FLOW COULD BE A LITTLE LESS FAVORABLE IN THE WESTERN CARIBBEAN…PARTICULARLY IF THE UPPER LOW LAGS A BIT.”)

As for Dean’s future track, Berger says the latest model runs show “some hints that Dean might skirt the Yucatan peninsula and come into the Gulf between there and Cuba,” which would be the “worst-case scenario” that I diagrammed yesterday. But that’s four days away yet, and anyway most models still show the storm hitting the Yucatan.

After that? The current consensus is a final landfall somewhere between the Mexico-Belize border and the Texas-Louisiana border. Dr. Masters said earlier today that he’d “be surprised to see Dean make a turn northwards in the Gulf of Mexico towards Louisiana or points further east, as there are no strong troughs of low pressure coming across the U.S. until late next week.” That statement still appears to hold, for the most part, although the GFDL is now taking Dean toward western Louisiana.

In any event, rather than dissecting the current model runs too closely, the best course of action is probably to take a deep breath and wait until tomorrow morning. As Dr. Masters points out, “Tonight marks the first flight of the NOAA jet, and we’ll have a much more reliable set of model runs Friday morning. Hopefully, this will narrow down the uncertainty of what will happen when Dean reaches the Gulf of Mexico.” Berger agrees, saying it will be “more prudent to seriously consider the models tomorrow morning, when they have aircraft data for the first time and hopefully a better handle on the atmospheric dynamics that will guide Dean early next week.”

So keep an eye out for those 12Z (8:00 AM) model runs. The more “clustered” they look, the more confidence we can have in their predictions (though, as always, don’t make life-or-death decisions purely on the basis of computer models). When they’re available, you’ll be able to see them here and here and here and here.

P.S. If anybody in Jamaica or the Yucatan Peninsula is reading this, now would be a good time to, ahem, get the hell out. Or at least start making plans to get the hell out soon, because Dr. Masters believes the airports in Jamaica will close on Sunday, and the ones in the Yucatan will follow suit on Monday. And personally, I wouldn’t want to be in the path of Gilbert, the sequel if I could avoid it.

As for the U.S. Gulf Coast, it’s far too early to think about evacuating, but definitely keep an eye on this thing through the weekend. That goes doubly for Texas. Hard decisions may await early next week.

UPDATE, 12:54 PM: In a new post, I’ve written much more about the wisdom of evacuating if you’re in Jamaica or the Yucatan (especially Jamaica).

Oh, and the latest specs and track speculation on Dean can be found here.

From the National Hurricane Center:

DEAN HAS CONTINUED TO STRENGTHEN THIS MORNING…WITH OCCASIONAL GLIMPSES OF AN EYE IN INFRARED SATELLITE IMAGERY AND CLOUD TOPS COLDER THAN -70C NEAR THE CENTER. SATELLITE INTENSITY ESTIMATES ARE 65 KT FROM TAFB AND SAB…SO DEAN IS UPGRADED TO THE FIRST HURRICANE OF THE 2007 HURRICANE SEASON.

According to the latest forecast, Dean is looking like he likes the more Southern route, taking him right over Mexico and away from the Gulf. Of course, this is bad for Mexico, but good that he won’t reach the warm waters of the Gulf of Mexico.

11 AM ET UPDATE:

Dean is gaining strength quickly, and is almost a Category 2 already. From the NHC advisory:

MAXIMUM SUSTAINED WINDS ARE NEAR 90 MPH…150 KM/HR…WITH HIGHER
GUSTS. DEAN IS A CATEGORY ONE HURRICANE ON THE SAFFIR-SIMPSON
SCALE. STRONGER WINDS…ESPECIALLY IN GUSTS…ARE LIKELY OVER
ELEVATED TERRAIN. ADDITIONAL STRENGTHENING IS FORECAST DURING THE
NEXT 24 HOURS. AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT WILL
INVESTIGATE DEAN THIS AFTERNOON.

Dean will be a Cat 2 hurricane when it reaches wind speeds of 96 mph. I presume when the next advisory comes out that will be a Cat 2.

Okay, so, I have a ton of errands to run, tasks to accomplish, and house-cleaning to do this week, and it occurs to me that my typical obsessive blogging about Tropical Storm Dean is seriously impeding my progress. So, I’m declaring a personal period of Dean-silence for the next 23 hours and 8 minutes. :) If the guestbloggers want to post updates on Dean here, they are obviously welcome to do so, but I won’t be doing so myself.

If you want the latest on Dean — which will almost certainly become a hurricane today — please visit Dr. Jeff Masters, Eric Berger, Alan Sullivan, and the other fine sites linked at right — including, of course, the National Hurricane Center itself.

Tropical Storm Dean’s winds are up to 70 mph. One more bump, and he’ll be a hurricane. Also, Hurricane Watches are in effect for much of the Lesser Antilles. Map here.

Meanwhile, the tea-leaf-reading continues with regard to Dean’s eventual track. Alan Sullivan is reading some leaves that might suggest a more northerly course than expected:

This evening an upper low is closing off at the base of the unseasonal polar trough that has dipped all the way to the latitude of the Bahamas. This jet-level system is centered several hundred miles ENE of Abaco. It is supposed to move southwest then west into the Gulf, opening the way for Dean to make its way westward at a lower latitude. But I see no sign of the predicted movement. Instead the center of the Abaco low is lifting slightly northeast this evening, as though it means to go out to sea. This is why a few models still draw Dean onto a more northwest course. Florida and points north are not quite safe yet. We need to see how that upper low moves, and how the growing tropical cyclone responds.

Eric Berger, on the other hand, sees leaves pointing in the opposite direction — southward:

I’ll have an full analysis of the overnight models in the morning, but there seems to be some hope that the system might now remain south of the Gulf of Mexico. That would be bad news for Mexico, however, as Dean might cross the Yucatan and Bay of Campeche.

Tropical Storm Dean’s winds are up to 65 mph as of 5:00 PM, which puts it pretty darn close to hurricane status. The official forecast calls for it to become a Category 1 hurricane tomorrow, a Category 2 hurricane by landfall in the central Antilles on Friday, and a Category 4 hurricane in the Western Caribbean on Monday. Here’s the forecast track:

Here’s what the 5:00 PM discussion had to say about Dean’s future intensity:

BASED ON WHAT WE THINK WE KNOW ABOUT INTENSITY CHANGE…THERE DO NOT SEEM TO BE TOO MANY INHIBITING FACTORS TO A FUTURE INTENSIFICATION OF DEAN. GLOBAL MODELS UNANIMOUSLY DEVELOP A LARGE UPPER-LEVEL ANTICYCLONE NEAR THE CENTER OF THE HURRICANE AS THE SYSTEM TRAVELS INTO THE CARIBBEAN SEA. IN COMBINATION WITH VERY DEEP WARM WATERS…THIS PATTERN WOULD FAVOR THE DEVELOPMENT OF A POWERFUL HURRICANE. THE OFFICIAL INTENSITY FORECAST IS HIGHER THAN THE PREVIOUS NHC FORECAST AND IS IN GOOD AGREEMENT WITH A CONSENSUS OF THE SHIPS…GFDL…HWRF…AND LGEM MODELS.

Now, tropical track predictions are notoriously unreliable beyond five days out — indeed, even between three and five days, there is a huge error rate, which explains the wide “track cone” in the forecast graphic above — but even so, Dean’s expected course obviously implies the possibility that it will reach the Gulf of Mexico in roughly six days. What then? Alas, there is no reason to believe it couldn’t maintain monster hurricane status, or even strengthen, in the Gulf. Eric Berger, the Houston Chronicle’s “SciGuy,” examines the water temperatures in the Gulf, and concludes that it “looks pretty bad.” The sea-surface temperatures are “bath-tub-like” — indeed, they’re warmer than the waters in 2005 that fueled Katrina, Rita and Wilma. Thankfully, the tropical cyclone heat potential, which measures the depth of the warm water as well as its surface temperature, its somewhat lower than in ‘05. Still, there is plenty of warm water to fuel an intense hurricane. Berger writes:

The worst scenario would be for Dean to track into the Western Caribbean, south of Cuba, and move into the Gulf across warm, deep waters in the Yucatan channel. This would bring an undiminished hurricane into the warm Gulf where it would have a chance to strengthen further — although probably less explosively than in 2005. Wind shear is a wild card.

Let us hope it does not come to that.

Here’s a rough graphical representation of that “worst scenario,” drawn by yours truly based on the basinwise heat potential map. Please note, this is not a forecast; it’s just a drawing of the worst-case scenario in terms of the potential, based on oceanic heat, for explosive development:

Again, I’m not predicting that will happen; indeed, I have no reason to believe that such a sharp recurvature is likely. I’m just saying that’s the scenario we should root against. Or perhaps I should say, one of the scenarios to root against — here’s another one, involving less recurvature and slightly more land interaction, but still plenty of deep, bathtub-like water to feed the beast:

Yeah. Those scenarios would be bad. Hopefully something else happens… or wind shear intervenes… or the Saharan Sand Fairy sprinkles some more of her dust in the storm’s path. Because otherwise: yikes.

(I reiterate again: the above graphics are not forecasts.)

P.S. With regard to the official forecast track, and the age-old “line” vs. “cone” debate, Dr. Jeff Masters says that at least in the short term (i.e., the next 48 hours), we’re safe focusing more on the “line” and less on the “cone”:

The cone of uncertainty Friday morning, when Dean is expected to pass through the islands, covers a wide stretch of ocean from Grenada to Antigua. We can expect that this uncertainty cone is too wide, since the steering currents are relatively stable right now and Dean is well-formed. This puts the islands of Martinique, Dominica, and Guadaloupe in the bulls-eye.

As an aside, I’d love to see the NHC start tailoring its “track cones,” or “cones of uncertainty” — whatever you want to call them — to the actual level of uncertainty for each individual storm, instead of relying purely on historical error rates. There is no question that some forecasts can be relied upon with more confidence than others, depending on computer-model agreement, steering-pattern stability, and so forth. Yet judging from the NHC’s graphics, you’d think that all track projections are created equal. Moreover, sometimes the NHC will choose to place the official forecast track in the left or right portion of the “guidance envelope”; when they do this, shouldn’t the uncertainty cone be fatter on one side than on the other? Yet it never is. I’d love to see this change.