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How low can Dean go?
Posted by on Sunday, August 19, 2007 at 11:59 pm

FYI - I’m heading to Nashville in the morning for a two-day State of Tennessee law-clerk seminar thingy, so blogging will be extremely sporadic tomorrow and Tuesday, I’m afraid. The timing sucks, as tomorrow will probably be one of those exciting days for a weatherblogger, filled with amazing and terrifying images of a rapidly intensifying Category 5 hurricane… but what can ya do? Duty calls. At least I should be able to blog from my hotel room tomorrow night as Dean approaches the Yucatan. And I’ve again asked my hurricane guestbloggers to help fill in when I’m indisposed. (Speaking of which, thanks, Brian, for your help today!)

Anyway, Dean has now officially cleared the western tip of Jamaica. It never actually made landfall; according to the 11:00 PM discussion, the closest approach occurred when the center of the eye “came within about [23 miles] of Portland Point of the south-central coast.” Tomorrow, we’ll start to find out what kind of damage Dean did. Hopefully it wasn’t too severe, though I’m sure it wasn’t a cakewalk on the island, especially the south shore.

The downside of Dean’s eye failing to hit land is that it didn’t weaken over said land. So now the conditions are ripe for some potentially historic intensification tomorrow. That Dean will become a Category 5 hurricane tomorrow is, I think, almost a foregone conclusion, given the favorable atmospheric conditions, the lack of land interaction, and the extremely warm water:


The brown line is Dean’s approximate track so far; the black line is the approximate forecast track.

The heat potential isn’t quite as high as it would be if Dean were trekking a bit further north, but even so, it’s plenty warm enough to support rapid intensification; indeed, it’s significantly warmer than the water (between the two small black vertical lines near the lower right-hand corner of the map above) in which Dean exploded from a low-end Cat. 2 into a high-end Cat. 4 on Friday. And because of his failure to make landfall in Jamaica, Dean starts his trek into these bathtub-like Western Caribbean waters with a top wind speed of 145 mph and a minimum pressure of 925 millibars. So the question, I think, is not whether he’ll strengthen into a Cat. 5; the question is, how low can he go? Pressure-wise, that is. Gilbert, following a similar track 19 years ago, held the Atlantic-basin record (888 mb) until Wilma surpassed it in 2005 with a mark of 882 mb. Can Dean rival these numbers? I’m not predicting it, but I certainly don’t think it’s out of the question. He dropped 46 millibars in 24 hours on Friday, where the heat potential was, on paper, not even quite high enough to support rapid deepening. Another 46-millibar drop tomorrow, in these far more conducive waters, would break Wilma’s record by 3. I’m just sayin’.

Can anything hold Dean back? Maybe another eyewall replacement cycle… but you’d think that wouldn’t happen for a while, now that the lengthy cycle which continued throughout Dean’s approach and passage of Jamaica is finally over. We’ll see.

In any event, the good news is that Dean now appears to be headed for an area of the Yucatan Peninsula that’s much more sparsely populated than the Cancun/Cozumel area. Hopefully the people who do live along the targeted coast are taking the proper precautions and getting themselves out of harm’s way, because regardless of whether Dean approaches any meteorological records, the bottom line is that they are going to be hit very hard.

Anyway, here’s what the 11:00 PM discussion has to say about Dean’s track and intensity:

DEAN IS HEADING TOWARD 280 DEGREES [i.e., just north of due west] AT ABOUT [20 MPH] WITH STEERING PROVIDED BY A STRENGTHENING MID-LEVEL RIDGE OVER THE SOUTHEASTERN UNITED STATES. THE MODELS ALL AGREE THAT THE RIDGE WILL GET EVEN STRONGER AND MOVE A LITTLE WESTWARD WITH DEAN…MEANING THAT DEAN WILL PROBABLY MOVE IN A RATHER STRAIGHT LINE UNTIL FINAL LANDFALL IN MEXICO. OVERALL THE MODELS HAVE AGAIN EDGED A LITTLE SOUTHWARD…AND SO HAS THE OFFICIAL FORECAST. …

ALL OF THE OBJECTIVE GUIDANCE CALLS FOR STRENGTHENING IN THE NORTHWESTERN CARIBBEAN WHERE THE SHEAR WILL REMAIN WEAK AND THE OCEAN HEAT CONTENT VERY HIGH. THE OFFICIAL FORECAST FOLLOWS SUIT AND INDICATES THAT DEAN COULD REACH CATEGORY FIVE STATUS DURING THE NEXT DAY OR SO. THE HURRICANE WILL WEAKEN WHILE OVER YUCATAN IN PROPORTION TO JUST HOW LONG IT SPENDS OVER LAND. THE OPPORTUNITY FOR RESTRENGTHENING OVER THE SOUTHWESTERN GULF LIKEWISE DEPENDS ON THE EXACT TRACK…AND THAT WINDOW HAS BEEN SHORTENING WITH THE SOUTHWARD SHIFTS IN TRACK. ALTHOUGH NOT EXPLICITLY INDICATED IN THE NEW OFFICIAL FORECAST…IT IS STILL POSSIBLE THAT DEAN COULD RESTRENGTHEN AND AGAIN REACH MAJOR HURRICANE STATUS PRIOR TO FINAL LANDFALL ALONG THE COAST OF MAINLAND MEXICO.

P.S. In other tropical news, Brian Neudorff notes a tropical storm over Oklahoma — well, the remnants of Erin, anyway, looking mighty spiral-y! — and a potential proto-Felix.

UPDATE, 2:00 AM: Dean’s pressure went up a millibar, to 926… but its top wind speed is up too, to 150 mph.

UPDATE, 7:13 AM: Another eyewall replacement cycle already?? According to the 5am discussion, “OBSERVATIONS FROM THE LAST AIR FORCE HURRICANE HUNTER MISSION AGAIN INDICATED A CONCENTRIC EYEWALL STRUCTURE.”




4 Comments on “How low can Dean go?”

  1. WX-MAN’s Perspective - » What’s Next For Dean? Says:

    […] eye stayed intact and wasn’t affected by land. So the question as Brendan put it, How Low Can Dean Go? (please visit his post he goes into more detail and says pretty much what I was already thinking) […]

  2. Gideon Says:

    There’s still fair amount of stuff going on south of Cancun, but I agree, it’s better there than directly in Cancun. It’s a shame, because a lot of those beaches took a beating a couple years back, and many haven’t recovered yet.

    I wonder what all of those cruise ships who normally dock in Cozumel do when this happens? I suppose they have high tailed it for the upper gulf.

  3. Rebecca Loy Says:

    There has to be some nerd out there with a formula that predicts the statistical liklihood of a given storm to enter an ERC at X point in its development. Of course, such formulas have limited utility and applicability because every storm is different, but it’s potentially interesting stuff.

  4. Fresh Bilge » Erin Lives Says:

    […] Via Brendan Loy, I learn that Erin — remember Erin? — is not quite dead. Sometimes tropical systems can endure a long while over land. There are two requirements. The land must be flat, and it must be wet. Oklahoma is always flat, but rarely so wet as it has been this year. No more rain was needed. It came anyway. […]


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