Veteran hurricane-watchers know that, sometimes, a storm that seems to have most of the elements needed for intensification working in its favor — high ocean temperatures, low wind shear, good upper-level outflow — just seems unable to “get its act together,” and never gets nearly as strong as feared. Often times, the culprit is a “patch of dry air,” a sort of micro-scale atmospheric feature that the computer models have a tougher time capturing and forecasting than big, “macro” steering elements like big high pressure ridges and upper-level troughs. Yet patches of dry air can be critical in disrupting the core structure of a storm, and without a well-organized core, all the warm water and low shear in the world won’t lead to rapid strengthening. I said yesterday that, under the right circumstances, “a wobble of a few dozen miles in either direction can literally change the course of history.” Well, a patch of dry air can change the course of history, too. Many storms with the potential to become significant hurricanes have failed to live up to that potential because of a well-timed patch of dry air.
On Twitter last night, The Weather Channel’s Jim Cantore raised the possibility that Isaac might be such a storm. He tweeted: “Should #Isaac stay as is b4 it crosses Hispaniola & Cuba Fri/Sat, it will have a hard time strengthening before closing in on south Florida.” He followed up this morning: “Satellite vs RECON data indicate #Isaac is a mess.” The National Hurricane Center’s 5:00 AM advisory confirms that:
AN AIR FORCE RECONNAISSANCE AIRCRAFT INVESTIGATING ISAAC THIS MORNING HAS FOUND A POORLY DEFINED INNER CORE WITH A LARGE AREA OF LIGHT WINDS AROUND A CENTER…WHICH IS SIMILAR TO WHAT AN EARLIER NOAA RESEARCH MISSION INDICATED. RADAR DATA FROM GUADELOUPE AND SAN JUAN ALSO INDICATE A POORLY DEFINED INNER CORE CONVECTIVE PATTERN. RATHER THAN INITIALIZE THE CENTER OF ISAAC WELL TO THE SOUTH OF THE PREVIOUS TRACK…I HAVE OPTED TO USE A BLEND OF THE RECON FIXES…SATELLITE IMAGERY…DATA FROM NEARBY NOAA BUOY 42060…AND A 06Z CONSENSUS FORECAST POSITION FROM THE GFS…ECMWF…AND UKMET MODELS. THE INITIAL INTENSITY WAS ALSO DECREASED TO 35 KT BASED ON DATA FROM THE RECON AIRCRAFT AND NOAA BUOY 42060.
Isaac’s core is so disorganized, forecasters aren’t even sure where its center presently is. And the official wind speed is now down to minimal tropical storm status, 40 mph.
The culprit? A patch of dry air, of course!
UPPER-AIR DATA FROM ST. MAARTEN AT 00Z CONFIRMS THE DRY AIR IN THE MID-LEVELS OF THE ATMOSPHERE BETWEEN 600 AND 300 MB AS ALLUDED TO
IN PREVIOUS DISCUSSIONS. THIS LAYER OF VERY DRY AIR HAS BEEN HINDERING CONVECTIVE DEVELOPMENT IN THE NORTHEASTERN QUADRANT FOR THE PAST 3-4 DAYS. HOWEVER…THE GLOBAL MODELS ARE IN GOOD AGREEMENT ON THE DRY AIR MIXING OUT OVER THE NEXT 36 HOURS AND THE INNER CORE BECOMING QUITE MOIST…WHICH SHOULD ALLOW FOR SOME STEADY INTENSIFICATION TO OCCUR BEFORE ISAAC INTERACTS WITH THE MOUNTAINOUS TERRAIN OF HISPANIOLA. GIVEN THE IMPRESSIVE UPPER-LEVEL OUTFLOW PATTERN THAT THE CYCLONE HAS DEVELOPED…ISAAC IS STILL EXPECTED TO BECOME A HURRICANE BY 36 HOURS.
None of this means Isaac can’t strengthen before reaching Hispaniola and Cuba. But it’s running out of time. In roughly 36 hours, significant interaction with the land mass of Hispaniola will begin. Actually, interaction of the storm’s broader circulation with the mountains of Hispaniola could begin a little earlier than that. Any strengthening needs to happen before land interaction starts. But, as noted above, NHC says the computer models agree that the dry air will “mix out” over the next 36 hours.
The critical question is, when in those 36 hours will the “mixing” finish up? Here’s where I insert the caveat that I’m not a meteorologist — I’m just an 30-year-old attorney who’s been following hurricanes with intense interest, but as a layman, since I was 7 years old — but as I understand it, the answer is really hard to divine. We’re talking about the evolution and organization of the storm’s inner core, and this is where the science of hurricane intensity forecasting becomes pretty dicey. We’ve gotten better and better at forecasting hurricanes’ tracks; 5-day forecasts today are more accurate than 3-day forecasts were just a decade or two ago. But intensity remains more mysterious, in large part because predicting the behavior of tropical cyclones’ inner cores is very, very difficult. So when I see a storm like Isaac, which might have a window of anywhere between 0 and 24 hours to gets its core structure together, I see a storm that could reach Hispaniola as anything from a middling tropical storm to a well-developed hurricane. It’s very hard to say.
Isaac’s intensity — and, perhaps more importantly, the integrity of its core structure — when it reaches Hispaniola is critical, because land interaction with Hispaniola and Cuba will inevitably prevent strengthening, and indeed, induce weakening, for 36 to 48 hours on Friday and Saturday. Isaac will have time re-intensity before a U.S. landfall once it pops off the coast of Cuba, but much will depend on the storm’s state when it does that. Will it have diminished winds but an intact inner core? Or will its structure be, as Cantore put it, a “mess” yet again? The answer could determine the fate of Isaac, and maybe of the Republican National Convention in Tampa.
A weakened storm, when it’s over warm water and under weak shear and good outflow, can “wind up” into a much stronger storm very quickly — provided it has a well-structured inner core to begin with. But if the core is a disorganized “mess” when it emerges over those warm waters, the process of reorganization takes some time. It’s very hard to say exactly how much time — and that uncertainty could further complicate Tampa Mayor Bob Buckhorn’s tough call about the RNC — but it can certainly take a day or more. Only after the structural reorganization finishes can what we usually call “strengthening” (increasing wind speeds, plummeting barometric pressures) begin in earnest.
Depending on Isaac’s track, the time needed for its inner core to recover after Hispaniola and Cuba might mean it won’t have time to significantly strengthen before reaching the U.S. coast. If the core never really gets its act together before reaching Hispaniola and Cuba, that’s even more likely. But it’s also possible Isaac will kick that dry air soon, get its act together quickly, and maintain its core reasonably well as it passes over the islands. If so, watch out, Florida.
We just don’t know yet which it will be. Today and tonight will be critical, both because of the storm’s evolution and because NOAA and the Air Force are throwing the whole data collection arsenal at the storm, to try and get the best information possible into those computer models. Hopefully by tomorrow morning, we’ll have a clearer idea of how this thing is going to turn out.
UPDATE: While I was writing this, the NHC’s 11:00 AM advisory came out. The discussion confirms that Isaac’s core problems haven’t resolved yet:
THE SATELLITE APPEARANCE OF ISAAC HAS IMPROVED SOMEWHAT THIS MORNING…WITH MULTIPLE CONVECTIVE BANDS DEVELOPING AROUND THE CENTER. HOWEVER…REPORTS FROM AIR FORCE RESERVE AND NOAA HURRICANE HUNTER AIRCRAFT INDICATE THAT THE CENTRAL CORE OF ISAAC REMAINS POORLY ORGANIZED…WITH MULTIPLE LOW-LEVEL VORTICITY CENTERS ROTATING AROUND A LARGE AREA OF LIGHT WINDS. IN ADDITION…THE CENTER IN AIRCRAFT DATA IS NORTH OF THE CENTER SUGGESTED BY THE SATELLITE IMAGERY.
The discussion also articulates the core forecasting dilemma:
IN THE SHORT TERM…THE STRUCTURE OF ISAAC SUGGESTS THAT ONLY SLOW STRENGTHENING IS LIKELY. HOWEVER…THE ENVIRONMENTAL CONDITIONS ARE FAVORABLE FOR RAPID INTENSIFICATION…WHICH COULD OCCUR IF THE CYCLONE DEVELOPS A WELL-DEFINED INNER CORE.
When will that happen? Will it ever happen? We simply don’t know.