Tropical Storm Isaac, Part 1: Moving Through The Eastern Caribbean Sea

[yoda]

Euro landfall looks to be near Pensacola Wednesday, moving very slowly as the northern branch trough leaves it behind....

Sure does move slowly from its 144 position to 168 hr position... likely some flooding rains down there where they have had enough of that earlier this tropical season with Debby

[ALhurricane]

Well if it comes to MOB/PNS I can give Josh the grand tour of the area. Not sure I want that.....no offense Josh. Anyway, the shift in the guidance is significant tonight. The Euro definitely seems to have had a better short term handle of a weaker storm.

[Superstorm93]

EURO Verbatim: This stays weak for 114 hours until it's NW of Cuba.

Shouldn't be too far off in the short term, but if this only scrapes Hispaniola, then obviously this could be stronger in the medium-long range.

Still a ton of things that are very uncertain with Isaac. I guess these are the "fun" storms.

[U_Thant]

MAX FL TEMP 21 C 327 / 72 NM FROM FL CNTR

Extrap SLP is still falling as the plane heads NNE well away from the last VDM fix.

Whatta Slopgyresque ™ mess...

[phil882]

EURO Verbatim: This stays weak for 114 hours until it's NW of Cuba.

Shouldn't be too far off in the short term, but if this only scrapes Hispaniola, then obviously this could be stronger in the medium-long range.

Still a ton of things that are very uncertain with Isaac. I guess these are the "fun" storms.

Yep.. the first thing is right off the bat, where we will need to see a serious latitude gain over the next 24 hours. Both the GFS and ECMWF are projecting this, but they didn't do well with the short term west-southwest motion the past 24 hours. A little fun trivia fact, Isaac is further south than it was when it was first initiated as a Tropical Depression. That means the long-term motion has been slightly south of west the last 3 days.

[HurricaneJosh]

South of Port Saint Joe and intensifying quickly.

Slows significantly while nearing the coast. This has been consistently shown for at least three runs now.

Euro landfall looks to be near Pensacola Wednesday, moving very slowly as the northern branch trough leaves it behind....

The deceleration near the FL Panhandle is unusual. The hurricanes hitting that area are often accelerating N and coming ashore very briskly. Some examples:

Dennis 2005 - 21 kt

Opal 1995 - 21 kt

Kate 1985 - 13 kt

Eloise 1975 - 20 kt

Well if it comes to MOB/PNS I can give Josh the grand tour of the area. Not sure I want that.....no offense Josh. Anyway, the shift in the guidance is significant tonight. The Euro definitely seems to have had a better short term handle of a weaker storm.

Well, I appreciate that-- I might take you up on it.

[Macintosh]

Euro landfall looks to be near Pensacola Wednesday, moving very slowly as the northern branch trough leaves it behind....

Interesting in that typically storms approaching the northern coast of the GOM tend to start ingesting dry, continental-origin air as they near the coast. If the trough leaves the storm behind, it would cause a stall or very slow movement. But I'm not sure what impact this would have on dry air entrainment and all other things equal, the strength of the system as it crosses the coast.

I guess I just remember the "worst case scenario" as being a slow moving cat 5 on a major metro area, but I wonder with the dry air issue if a storm can be both very strong AND slow moving at the same time. ? Seems difficult to achieve this combination.

[dan88]

850mb center looks to be 14.7N, 63.9W with this pass. The surface center appears to be well NE, with a broad area of 5kt or less SFMR winds along a line from 15.6N 62.9W to 15.3N 63.2W. This area also had the lowest pressures based on extrap, down to 1004.0mb.

[Superstorm93]

Really disorganized.

Whatever low level reflection this has is very weak and very broad.

Still far south of the forecast track though. We'll see what happens tomorrow.

A. Time of Center Fix: 23rd day of the month at 7:51:30Z

B. Center Fix Coordinates: 14°42'N 64°06'W (14.7N 64.1W)

[HurricaneJosh]

Looking at the IR imagery-- which looks awesome-- I would never think the system was so disorganized.

[HurricaneJosh]

The 5 am EDT Discussion is going to have the words "continuity" (to describe how they placed the center) and "generous" (to describe the intensity estimate).

[yoda]

TROPICAL STORM ISAAC DISCUSSION NUMBER 9

NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012

500 AM AST THU AUG 23 2012

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.

THE INITIAL MOTION ESTIMATE IS AN UNCERTAIN 270/12 KT. THE 00Z

GFS...ECMWF...AND UKMET MODELS DID AN OUTSTANDING JOB PREDICTING

THE RECENT SOUTHWESTWARD JOG OR REFORMATION OF THE CENTER OF ISAAC.

THOSE SAME MODELS ARE ALSO FORECASTING ISAAC TO MAKE A SHARP JOG TO

THE NORTHWEST OVER THE NEXT 12-24 HOURS...AND THEN STEADY OFF ON A

GENERAL WEST-NORTHWESTWARD MOTION THROUGH 48 HOURS. AFTER THAT...

THE GLOBAL AND REGIONAL MODELS HAVE COME INTO BETTER AGREEMENT ON

ISAAC MOVING NORTHWESTWARD TOWARD A BREAK IN THE SUBTROPICAL RIDGE

THAT IS FORECAST TO DEVELOP AFTER 72 HOURS ACROSS FLORIDA AND THE

EASTERN GULF OF MEXICO. THE ECMWF MODEL HAS CONTINUED ITS EASTWARD

SHIFT OVER THE PAST 36 HOURS...AND EVEN THE LATEST NOGAPS AND

CANADIAN MODEL RUNS HAVE SHIFTED WELL TO THE WEST ACROSS THE

FLORIDA PENINSULA ARE NOW NO LONGER CONSIDERED TO BE OUTLIERS. DUE

TO LESS SPREAD IN THE LATEST NHC MODEL GUIDANCE...AND THE FACT THAT

ECMWF HAS SHIFTED CLOSER TO THE PREVIOUS FORECAST AT DAYS 4 AND

5...THE OFFICIAL FORECAST TRACK IS JUST AN UPDATE OF THE PREVIOUS

ADVISORY TRACK...AND IS CLOSE TO THE DYNAMICAL CONSENSUS MODELS

TVCA AND TV15.

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. AFTER THAT...HOWEVER...

THE THE PRIMARY IMPEDIMENT TO STRENGTHENING WILL BE THE INTERACTION

WITH THE LAND MASSES OF HISPANIOLA AND EASTERN CUBA. HOW MUCH THE

INNER CORE OF ISAAC IS DISRUPTED WHILE OVER LAND WILL DETERMINE

JUST HOW MUCH RE-STRENGTHENING WILL OCCUR ONCE THE CENTER MOVES

BACK OVER WATER AFTER 72 HOURS. THE OFFICIAL INTENSITY FORECAST IS

SIMILAR TO THE PREVIOUS FORECAST AND IS IN REASONABLE AGREEMENT

WITH THE DECAY-SHIPS AND LGEM INTENSITY MODELS.

FORECAST POSITIONS AND MAX WINDS

INIT 23/0900Z 15.3N 64.0W 35 KT 40 MPH

12H 23/1800Z 16.2N 65.9W 40 KT 45 MPH

24H 24/0600Z 16.9N 68.4W 50 KT 60 MPH

36H 24/1800Z 17.8N 71.1W 65 KT 75 MPH

48H 25/0600Z 19.1N 73.4W 55 KT 65 MPH

72H 26/0600Z 21.8N 77.4W 60 KT 70 MPH...INLAND

96H 27/0600Z 24.4N 80.9W 65 KT 75 MPH...OVER WATER

120H 28/0600Z 27.4N 83.7W 70 KT 80 MPH

$$

FORECASTER STEWART

[jcwxguy]

Looking at the IR imagery-- which looks awesome-- I would never think the system was so disorganized.

seems like the last few systems have looked good as well, but looks have been deceiving each time.

[HurricaneJosh]

Ugh. It's one of these cyclones that's just really going to take some patience. I wish I could just ignore it for the next two days. Tracking it through the Greater Antilles is going to be a big chore.

[thewxmann]

The bursting pattern is linear and convection, though strong, is not persistent. The outflow pattern is beautiful, but really not much else.

[NaoPos]

http://tropic.ssec.wisc.edu/real-time/storm.php?&basin=atlantic&sname=09L&invest=NO&zoom=4&img=1&vars=11111000000000000000000&loop=0

Just going off this loop, it seems that the western half of Isaac is keeping the convection and cooler cloud tops more persistent than the eastern half. I'm guessing that's associated with where the mid level center is located?

[Snow_Miser]

According to the current 5 AM advisory, Isaac has slowed down significantly since yesterday. Yesterday it was moving quickly at around 20 mph. Now it is moving at 12 mph.

This slow down could allow for the circulations to finally consolidate and vertically stack.

[40/70 Benchmark]

EURO Verbatim: This stays weak for 114 hours until it's NW of Cuba.

Shouldn't be too far off in the short term, but if this only scrapes Hispaniola, then obviously this could be stronger in the medium-long range.

Still a ton of things that are very uncertain with Isaac. I guess these are the "fun" storms.

If the dry air doesn't abate and the system doesn't better align itself, I don't think that the land interaction is going to be the main factor driving intensity.

I think some folks are obsessing over rhe impending land influence, when the reality is that it simply won't matter very much is this remains an amorphous, ill-organized system.

[wxsmwhrms]

If the dry air doesn't abate and the system doesn't better align itself, I don't think that the land interaction is going to be the main factor driving intensity.

I think some folks are obsessing over rhe impending land influence, when the reality is that it simply won't matter very much is this remains an amorphous, ill-organized system.

Completely agree with this.

I think today will be a fairly critical day in seeing how and where Isaac is going to end up. It is currently well to the left of where NHC and the models (particularly GFS/UK) were saying it would be 24-48 hours ago. The 06Z GFS actually shifted back east a little this run and in the long term is back to running it up just west of the FL coast and into the Big Bend area. However, it continues to insist on a more rightward movement in the shorter term. With the UK now more supporting the EC (or at least its a lot farther west than it had been in days 5-6), and the EC ensemble mean implying a fairly large west to east spread across the Gulf in the longer term (looks like anywhere from LCH to AQQ) if Isaac continues to go farther west and remains weak it will be interesting to see how the models react to that.

[winterymix]

Ugh. It's one of these cyclones that's just really going to take some patience. I wish I could just ignore it for the next two days. Tracking it through the Greater Antilles is going to be a big chore.

Forecaster Stewart suggests that the system could spike intensity just as it nears Hispaniola. Tiny changes of track could yield unusually large implications when it re-organizes over open water again. Isaac's persistence has been to resist turning to the right. Josh: any prev. analogs come to mind? This should be fun to watch but a nail-biter for someone wanting to chase. I wouldn't discount the EURO idea of just west of Mobile.

[CandymanColumbusGA]

Here's the flight path for this afternoon's G-IV mission: http://noaahrd.files.wordpress.com/2012/08/2012082400_al092012_n49track.gif

Can't wait to get that data ingested in the 00Z models. That's when the fun should REALLY begin.

[Scott747]

Here's the flight path for this afternoon's G-IV mission: http://noaahrd.files...12_n49track.gif

Can't wait to get that data ingested in the 00Z models. That's when the fun should REALLY begin.

The G-IV is scheduled to fly a synoptic surveillance mission this afternoon, with a takeoff time of 1730 UTC (1:30pm Eastern) from MacDill AFB and land back there too. They will fly north of the Greater Antilles. The G-IV crew will be sampling the subtropical ridge north of Isaac as well as getting the northern portion of the circumnavigation around the storm. Here’s their track.

[phil882]

Radar from San Juan looks markedly improved... Although It's difficult on radar to tell if we're looking at the low level circulation or a vortex at a higher level.

[Derecho!]

Radar from San Juan looks markedly improved... Although It's difficult on radar to tell if we're looking at the low level circulation or a vortex at a higher level.

Even harder to tell but that radar center seems to be gaining latitude as forecast.

I THINK it's the advisory LLC. trying to correlate it to satellite and the last low level recon.

Update: P3 dropsonde east of the center had 35 kt winds at the surface, so the (justified) pretending it's a TS for the last 12 hours or so (when it probably technically wasn't) is over.

[40/70 Benchmark]

Completely agree with this.

I think today will be a fairly critical day in seeing how and where Isaac is going to end up. It is currently well to the left of where NHC and the models (particularly GFS/UK) were saying it would be 24-48 hours ago. The 06Z GFS actually shifted back east a little this run and in the long term is back to running it up just west of the FL coast and into the Big Bend area. However, it continues to insist on a more rightward movement in the shorter term. With the UK now more supporting the EC (or at least its a lot farther west than it had been in days 5-6), and the EC ensemble mean implying a fairly large west to east spread across the Gulf in the longer term (looks like anywhere from LCH to AQQ) if Isaac continues to go farther west and remains weak it will be interesting to see how the models react to that.

I'm already proclaiming the EURO as the victor with regard to this system....weak, and west has been the way to go.....I'm increasingly confident that if this ever becomes a signifcant cyclone (it may never), it won't be until its in the central GOM.

Tropical entities are so touchy.....at times they can absolutely go to town more than anyone had imagined, and at others remain inhibited by what superficially seems to be the most trivial of atmospheric nuances...and this case, the sliver of mid level dry air, and to a greater degree IMO, the titled structure of the low pressure, seem to be the double-headed culprit.

Factors like that are very difficult to overcome for these systems, and I'd play the conservative card for the forseeable future with regard to intensity.

[wxsouth]

Radar from San Juan looks markedly improved... Although It's difficult on radar to tell if we're looking at the low level circulation or a vortex at a higher level.

Looks like there is a mid-level (24-25kft) circulation possibly developing on radar around 15.2N 65.7W. This is interesting since it's much closer to the apparent low-level center. The P3 data seems to support this as it flew just west of this feature winds going from southerly to southwesterly. Also, satellite imagery seems to suggest the deep convection is seems to be starting to rotate around a point just north of 15N. Who knows if this will finally become the dominant center, but interesting none the less.

[wxmx]

It's also interesting because it shows a farther south dive at the start before turning NW towards Hispaniola. It looks like the southwestern dive is occurring, at least the convection is distinctly moving that way, probably as it tries to get stacked with the mid level circulation to it's south, then it probably will tumble around with a more poleward motion.

This is my take on what has happened with models and Isaac.

My thoughts last night (which I quoted) were reflected in the 5am advisory. Models did forecast the southwest jog while crossing the Lesser Antilles and a bit after that...and for a at least a couple of days (at least the Euro and GFS, and the Ukie and GFDL since at least yesterday). Not just the 700/500mb vortices aren't aligned with the 850 vortex, but the anticyclone is a bit to the NW of the latter vortex, and is impinging some light Nerly shear, probably helping dry air to get into the NE quad, and aiding convection downshear (SW quad). The lower level vortex most of the times win in this situation, but as some posters have theorized, the very strong convection on the SW quad, might be hurting any convection on the NE with cold downdrafts. This, in my opinion, is taking a bit longer than what models had forecasted, and the mid level vortex is dragging the whole circulation a bit south...and it will be that way until we get a stronger low level vortex with persistent convection...then it will start to gain some latitude (WNW). So far, I think, this will have the implication to diminish the disruption by Hispaniola down the road, and maybe shift the forecasted path a bit left in subsequent model runs towards the western FL panhandle to New Orleans.

[LakeEffectKing]

I'm already proclaiming the EURO as the victor with regard to this system....weak, and west has been the way to go.....I'm increasingly confident that if this ever becomes a signifcant cyclone (it may never), it won't be until its in the central GOM.

Tropical entities are so touchy.....at times they can absolutely go to town more than anyone had imagined, and at others remain inhibited by what superficially seems to be the most trivial of atmospheric nuances...and this case, the sliver of mid level dry air, and to a greater degree IMO, the titled structure of the low pressure, seem to be the double-headed culprit.

Factors like that are very difficult to overcome for these systems, and I'd play the conservative card for the forseeable future with regard to intensity.

Great points. And climatologically speaking, this area of the E/C Carribean is notorious for these "nuances", and most of the models have been pretty decent at keeping this fairly weak....now get this south of Cuba and from there forward, and it's a whole different ballgame.

[40/70 Benchmark]

Great points. And climatologically speaking, this area of the E/C Carribean is notorious for these "nuances", and most of the models have been pretty decent at keeping this fairly weak....now get this south of Cuba and from there forward, and it's a whole different ballgame.

Yes!

Tropical systems are still very poorly understood, at least relatively speaking, and this is why I think climo (aka the ability to draw upon past experience) needs to play a crucial role in forecasting.

Even if you can't rationalize something 100% scentifically speaking, you have to go with your gut, so to speak......and the e Carribean seems to be an area, which for whatever reason, seems to temper these systems at least excuse imaginable.

As you alluded to, the nw Carribean (yes, I agree that it will end up there) is on the opposite end of the spectrum.

[tornadotony]

Even harder to tell but that radar center seems to be gaining latitude as forecast.

I THINK it's the advisory LLC. trying to correlate it to satellite and the last low level recon.

Update: P3 dropsonde east of the center had 35 kt winds at the surface, so the (justified) pretending it's a TS for the last 12 hours or so (when it probably technically wasn't) is over.

It absolutely is NOT the LLC. The radar is scanning at about 26kft at that location (above 500mb). It is about .3 degrees S of the advisory LLC position.