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Dolly in limbo; will its center re-form to the north?

July 20th, 2008 - 10:02 pm

The National Hurricane Center’s 11:00 PM EDT discussion on Tropical Storm Dolly begins with this unusual and intriguing statement:

DATA FROM A NOAA P3 AIRCRAFT INDICATE THAT DOLLY HAS NOT BECOME ANY BETTER ORGANIZED THIS EVENING. INDEED…THE SYSTEM PROBABLY DOESN’T HAVE A CLOSED SURFACE CENTER RIGHT NOW. HOWEVER…DOLLY IS MAINTAINING VIGOROUS CONVECTION AND THE STRONGEST WINDS…AS ESTIMATED BY THE SFMR…HAVE INCREASED TO 45 KT. SINCE DOLLY COULD REGENERATE A CENTER AT ANY TIME…NO GOOD WOULD BE SERVED BY HANGING ON A TECHNICALITY.

A closed circulation, rotating around a defined center, is one of the foundational, definitional criteria for a tropical cyclone. Indeed, it was the failure to form a closed circulation that prevented Dolly’s precursor tropical wave, Invest 94L, from being designated as a tropical depression or storm until this morning. So it’s unusual to see the NHC admit that, in essence, Dolly is presently a “tropical storm” without a center or a closed circulation.

That said, the decision to maintain the storm’s designation seems quite sensible, as Dolly is indeed likely to regenerate, and soon — and you don’t want to give the public whiplash by naming and then quickly un-naming storms. Closed circulation or not, Dolly remains a real threat.

In fact, the uncertainty surrounding Dolly’s center may be increasing the threat, depending on the all-important issue of where Dolly regenerates. The NHC’s discussion alludes to — though it does not predict — the possibility that the center could re-form to the north of its previous location, within the deep convection (i.e., the heavy thunderstorms). If this happens, it would have several crucial implications, none of them good news.

First, and most immediately, re-forming under the convection would give Dolly an immediate opportunity for an intensity boost. A storm with a circulation center under its convection is always more potent, and has more potential for rapid intensification, than one with its center elsewhere.

Secondly, the resulting change in Dolly’s short-term track would allow her to potentially avoid the Yucatan Peninsula altogether, thus preventing the weakening that inevitably comes with land interaction. Also, relatedly, that same track shift would put Dolly over warmer waters than expected.

Finally, and most speculatively, a more northerly re-formation of the center could place Dolly on a longer-term track a bit to the north of what’s currently being forecast — which could put Texas, rather than Mexico, in the line of fire. I emphasize: this is all hypothetical and speculative. But suppose Dolly’s center re-forms right under the heart of the convection, and the storm then moves in roughly the expected direction. The result would be something like the white line below, as opposed to the currently predicted black line:

dolly-reform.jpg

Put it all together — an immediate intensity kick-start, avoidance of land interaction, warmer waters, and a more northerly track — and it’s safe to conclude that re-formation to the north would definitely not be good news for Texas.

It’s anyone’s guess whether this will happen, and I’m not predicting that it will. But it’s definitely worth watching to see how things develop overnight.

P.S. I’m reminded a little bit of the Thursday night in 2005 when Hurricane Katrina, during its first landfall on the Florida peninsula, made an unexpected left turn. Up until that point, Katrina had been expected to weaken significantly over the peninsula, then re-emerge over the Gulf, slowly re-strengthen, and turn right toward the Florida panhandle. But the left turn changed all that. Katrina re-emerged sooner than expected, after a short, swampy trek over land that barely weakened it at all — and she also re-emerged further south than expected, which, when you extrapolated the pre-turn forecast tracks to reflect the new, post-turn reality, put New Orleans squarely in danger. As I wrote at the time: “What makes me nervous is that Katrina‚Äôs southwestward turn and refusal to weaken makes a New Orleans doomsday scenario considerably more plausible than it seemed just a few hours ago.”

The possibility of Dolly’s center re-forming under the deep convection makes me feel much the same way about the scenario of a major hurricane potentially hitting Texas next week. So let’s hope the center stays to the south, away from those heavy thunderstorms and on track to weaken over the Yucatan.

P.P.S. Regardless of where the center re-forms, it seems quite likely that Dolly will become a hurricane, and possibly a strong one, once it gets out over the open Gulf of Mexico. The discussion states:

ONCE THE SYSTEM ENTERS THE GULF OF MEXICO…ALL GLOBAL GUIDANCE SUGGESTS A VERY FAVORABLE ENVIRONMENT FOR STRENGTHENING…WITH A LARGE ANTICYCLONE ALOFT…NO SHEAR…AND WARM GULF WATERS.

The official forecast brings Dolly up only to Category 1 hurricane status, but that may not mean much. The NHC virtually never predicts rapid intensification in advance, for the simple reason that scientists don’t yet have the ability to precisely forecast when such cycles will occur. However, the conditions just described are ideal for rapid intensification, and it wouldn’t be at all surprising to see Dolly suddenly ramp up to a major hurricane at some point in the next three days.

Oh, and one final note, unrelated to Dolly. The 8:00 PM EDT Tropical Weather Outlook said the following about the tropical wave over Africa: “A VIGOROUS AND WELL-DEFINED TROPICAL WAVE IS LOCATED OVER WESTERN AFRICA A FEW HUNDRED MILES EAST OF DAKAR SENEGAL. THIS SYSTEM HAS THE POTENTIAL TO BECOME A TROPICAL CYCLONE VERY QUICKLY AFTER IT EMERGES INTO THE EASTERN ATLANTIC ON TUESDAY.” That’s unusually strong langage for the NHC in describing a wave that isn’t even over water yet. Sounds like they really do think we may have a proto-Edouard on our hands.

UPDATE: Back to Dolly. The 2:00 AM advisory says:

SATELLITE IMAGES…SURFACE OBSERVATIONS…AND RADAR DATA FROM CANCUN MEXICO INDICATE THAT DOLLY LACKS A WELL-DEFINED CENTER OF CIRCULATION AT THIS TIME. HOWEVER THERE ARE INDICATIONS THAT THE CENTER IS REFORMING A LITTLE TO THE NORTH OF THE PREVIOUS TRACK.

But how far to the north? The morning will reveal much.

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