Tropical Storm Isaac, Part 2: Isaac Moving Into The Gulf Of Mexico-Hurricane Warnings For Morgan City, LA/MS/AL & FL Panhandle To Destin, FL

[PennyForYourThoughts]

Considering the graphics on the Euro..Does the strength shown on the graphic match up fairly well to what the NHC has forecast for landfall intensity? Around 85mph or so? Or does it look a bit stronger?

[tmagan]

18Z Observations

[andyhb]

Considering the graphics on the Euro..Does the strength shown on the graphic match up fairly well to what the NHC has forecast for landfall intensity? Around 85mph or so? Or does it look a bit stronger?

Probably too early to tell, but I'd say, based on the mid level heights, stronger.

Also...

184130 1647N 07123W 8429 01446 9948 +180 +180 161004 006 017 001 05

[ALhurricane]

Probably too early to tell, but I'd say, based on the mid level heights, stronger.

The Euro has a pressure in the 960s at landfall.

[phil882]

Considering the graphics on the Euro..Does the strength shown on the graphic match up fairly well to what the NHC has forecast for landfall intensity? Around 85mph or so? Or does it look a bit stronger?

Its quite a big stronger... taking the pressure verbatim would suggest Cat 2/3 intensity. Since the ECMWF is a 12 km global model, its has a pretty reasonable representation of TCs. For example, it is showing a similar pressure for Bolavan in the Wpac right now which is a 120 knot Cat 4 TC according to JTWC.

[Dunkman]

Considering the graphics on the Euro..Does the strength shown on the graphic match up fairly well to what the NHC has forecast for landfall intensity? Around 85mph or so? Or does it look a bit stronger?

Yeah the Euro has 94 knots at landfall. Obviously take that number with a huge grain of salt, but it certainly looks stronger.

[EasternUSWX]

Considering the graphics on the Euro..Does the strength shown on the graphic match up fairly well to what the NHC has forecast for landfall intensity? Around 85mph or so? Or does it look a bit stronger?

At 963 or whatever that shows about would probaby be a 100+ storm compared to NHC 85.

[Srain]

EXTENDED FORECAST DISCUSSION

NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD

229 PM EDT FRI AUG 24 2012

VALID 12Z MON AUG 27 2012 - 12Z FRI AUG 31 2012

...ISAAC EXPECTED TO MOVE INTO THE MID-SOUTH BY NEXT FRIDAY...

THE WESTERLIES ACROSS THE NORTHERN PACIFIC ARE NORTH OF WHAT IS

NORMAL FOR LATE AUGUST. THIS FORCES ENERGY TO POOL DOWNSTREAM,

ACROSS THE PACIFIC NORTHWEST, WITH DOWNSTREAM TROUGHING EXPECTED

ACROSS THE NORTHEAST AND A WARM CORE RIDGE EXPECTED TO WAVER IN

POSITION AND STRENGTH BETWEEN THE CENTRAL ROCKIES AND CENTRAL

PLAINS. THE DETERMINISTIC AND ENSEMBLE MEAN GUIDANCE IS IN GOOD

AGREEMENT ON THESE GENERAL IDEAS. THE MODEL CHOICE THIS PERIOD

WAS PARTIALLY CONSTRAINED BY THE LATEST NATIONAL HURRICANE CENTER

(NHC) FORECAST FOR ISAAC, WHICH RESEMBLED THE 12Z UKMET THROUGH

THURSDAY MORNING. A 00Z GFS/00Z ECMWF COMPROMISE WAS USED

THEREAFTER, WITH INPUT RECEIVED FROM NHC REGARDING TROPICAL

SYSTEMS AT 17Z. THE 06Z/12Z GFS AND 12Z CANADIAN SOLUTIONS HAD TO

BE THROWN OUT BY DEFAULT. THIS LED TO REASONABLE CONTINUITY WITH

THE MID-MORNING PRESSURES.

TROPICAL CYCLONE ISAAC

======================

THE 06Z/12Z GFS AND 12Z CANADIAN RECURVE THE SYSTEM QUICKER

THROUGH THE MID-ATLANTIC STATES THAN THE OTHER GUIDANCE, WHILE THE

00Z/12Z ECMWF AND ITS ENSEMBLE MEAN SHOW STRONGER RIDGING ACROSS

THE PLAINS/MIDWEST WHICH KEEPS THE SYSTEM ON A SLOWER AND MORE

WESTERLY TRAJECTORY. THE QUICKER SOLUTION CANNOT BE RULED OUT AS

THE PREFERRED MODEL BLEND DROPS ENERGY FROM THE WESTERLIES AROUND

THE NORTHWEST SIDE OF ISAAC, CLOSING OFF A MID TO UPPER LEVEL LOW

ACROSS TEXAS BY NEXT THURSDAY AND FRIDAY. THIS DEVELOPMENT WOULD

EFFECTIVELY SHIFT THE COL IN THE STEERING PATTERN SO FAR SOUTH

THAT ISAAC WOULD HAVE LITTLE CHOICE BUT TO RECURVE AROUND THE

PERIPHERY OF THE SUBTROPICAL RIDGE, INITIALLY LURED BY A BREAK IN

THE RIDGE ACROSS THE UPPER OHIO VALLEY TUESDAY (SIMILAR TO DEBBY

IN LATE JUNE BUT FARTHER TO THE WEST). HEAVY CORE RAINFALL WITH

AREAL AVERAGE AMOUNTS OF 8-9 INCHES ARE EXPECTED ACROSS SOUTHERN

FLORIDA/THE FLORIDA KEYS/THE FLORIDA PANHANDLE WITH THE SYSTEM

THROUGH WEDNESDAY MORNING PER THE 15Z NHC TRACK, WITH LESSER

AMOUNTS OF 3-5 INCHES POSSIBLE ACROSS THE SOUTHEAST AND MID-SOUTH

BY NEXT FRIDAY NEAR ITS TRACK, ITS DRAPING FRONTAL BOUNDARY ACROSS

THE SOUTHEAST, AND WITHIN ITS MAIN INFLOW BAND FROM THE GULF OF

MEXICO ON ITS EASTERN SIDE. LOCAL AMOUNTS OF 12-16 INCHES ARE

POSSIBLE BOTH WITHIN ITS CENTRAL DENSE OVERCAST (CDO) AND WHERE

ITS MAIN INFLOW BAND SETS UP AND PIVOTS, WHICH COULD BE AS FAR

EAST AS GEORGIA. THE CURRENT SIZE AND FORECAST TRACK FOR ISAAC

RESEMBLE HURRICANE DENNIS OF 2005, THOUGH SOMEWHAT TO THE EAST

AFTER LANDFALL. CENTRAL GEORGIA REMAINS IN AN EXCEPTIONAL,

LONG-DURATION DROUGHT SO THEY COULD USE THE RAINFALL. NORTHEAST

FLORIDA AND SOUTHERN GEORGIA WERE SATURATED FROM BERYL AND DEBBY

AS WELL AS SYNOPTICALLY-DRIVEN RAINFALL EVENTS DURING THE PAST

SEVERAL DAYS, SO THE RAIN WOULD BE PROBLEMATIC IN THAT AREA.

[turtlehurricane]

The cyclonic vorticity being generated in the lee of the mountain may be one of the reasons this is intensifying. Vorticity tends to align so the lee vorticity will be attracted to isaac's vort max.

[Derecho!]

183030 1654N 07050W 8428 01472 9985 +170 +170 167034 036 041 007 01

998.5 mb extrapolated, with no wind shift yet.

Edit - 184130 1647N 07123W 8429 01446 9948 +180 +180 161004 006 017 001 05

There's the center, 994.8 mb. Assuming they dropped a sonde, I'm looking forward to see what it reports!

Almost the 2PM advisory center exactly.

Looks like the days of multiple center hunting and interpolation may finally be over.

[Srain]

Product: Air Force Vortex Message (URNT12 KNHC)

Transmitted: 24th day of the month at 19:05Z

Aircraft: Air Force Aircraft (Last 3 digits of the tail number are 300)

Storm Number & Year: 09L in 2012

Storm Name: Isaac (flight in the North Atlantic basin)

Mission Number: 12

Observation Number: 02

A. Time of Center Fix: 24th day of the month at 18:43:50Z

B. Center Fix Coordinates: 16°48'N 71°31'W (16.8N 71.5167W)

B. Center Fix Location: 132 miles (212 km) to the SSE (156°) from Port-au-Prince, Haiti.

C. Minimum Height at Standard Level: 1,381m (4,531ft) at 850mb

D. Estimated (by SFMR or visually) Maximum Surface Wind: 55kts (~ 63.3mph)

E. Location of the Estimated Maximum Surface Wind: 49 nautical miles (56 statute miles) to the ENE/E (79°) of center fix

F. Maximum Flight Level Wind Inbound: From 155° at 70kts (From the SSE at ~ 80.6mph)

G. Location of Maximum Flight Level Wind Inbound: 68 nautical miles (78 statute miles) to the ENE (75°) of center fix

H. Minimum Sea Level Pressure: 995mb (29.38 inHg)

I. Maximum Flight Level Temp & Pressure Altitude Outside Eye: 14°C (57°F) at a pressure alt. of 1,524m (5,000ft)

J. Maximum Flight Level Temp & Pressure Altitude Inside Eye: 18°C (64°F) at a pressure alt. of 1,525m (5,003ft)

K. Dewpoint Temp (collected at same location as temp inside eye): 16°C (61°F)

K. Sea Surface Temp (collected at same location as temp inside eye): Not Available

L. Eye Character: Not Available

M. Eye Shape: Not Available

N. Fix Determined By: Penetration, Wind, Pressure and Temperature

N. Fix Levels (sfc and flt lvl centers are within 5nm of each other): Surface and 850mb

O. Navigation Fix Accuracy: 0.02 nautical miles

O. Meteorological Accuracy: 4 nautical miles

Remarks Section:

Maximum Flight Level Wind: 70kts (~ 80.6mph) in the east quadrant at 18:21:30Z

[PennyForYourThoughts]

Thanks guys for all your explanations of land falling intensity. Just getting the biggest jump on preparations to beat the masses just in case we have to bug out farther north. I understand we won't know for sure concerning that for a few more days...but when you wait too long..you end up with motels/hotels being filled to capacity. Anyway, thanks again for your responses.

[Srain]

G-IV mission is underway. Planned drops.

[Connor McCrorey]

New VDM, 994 millibars with a max flight level wind of 70 kts. I don't think hurricane classification is far off at all.

[Superstorm93]

Up 5 knots and down 2mb

AL, 09, 2012082418, , BEST, 0, 168N, 714W, 55, 995, TS

[Superstorm93]

Unless Isaac makes a large jump NW into Hispaniola, it will only scrape the western peninsula.

An extrapolation of the VDM's could argue that it might not even make it that far.

Two regions of high winds within Isaac per recon:

[wxsmwhrms]

Unless Isaac makes a large jump NW into Hispaniola, it will only scrape the western peninsula.

An extrapolation of the VDM's could argue that it might not even make it that far.

Two regions of high winds within Isaac per recon:

Satellite and recon show 290-295 heading last few hours.

[dan88]

Drop indicates it is vertically stacked 850mb-surface, peak winds on the drop were 3kt at flight level (and 2kt at the surface)

[turtlehurricane]

New VDM, 994 millibars with a max flight level wind of 70 kts. I don't think hurricane classification is far off at all.

If only the inner core wasn't almost completely devoid of convection. Some healthy outer bands to the east, but the most intense convection is occurring over the mountain ranges. There is probably some flash flooding going on. http://www.ssec.wisc.../loop_srso.html

[U_Thant]

Quick question - look at the high res RSO and tell me if you don't see some cyclonic curl in the CU line SW of the Tiburon - maybe even a hint of a weak eddy?. Is that a function of the lee side low that's been talked about?

And a comment - even if the center doesn't move across the bulk of Hispanola, I have still seen many cases of serious, serious disruption of TC wind circs in systems that have passed just north or just south. Might be especially true in this case since the winds seem to have always been most robust in the north quad. Yes, not as bad as moving down the spine, but still bad. YOMV

And Isaac had done such a yeoman's job trying to get its act together over the last H24.... *sigh*

[Superstorm93]

Obviously some issues with convection around the center.

Could be dry air or the beginning of interaction with Hispaniola.

[phil882]

Quick question - look at the high res RSO and tell me if you don't see some cyclonic curl in the CU lines SW of the Tiburon. Is that a function of the lee side low that's been talked about?

And a comment - even if the center doesn't move across the bulk of Hispanola, I have still seen many cases of serious, serious disruption of TC wind circs in systems that have passed just north or just south. Might be especially true in this case since the winds seem to have always been most robust in the north quad. Yes, not as bad as moving down the spine, but still bad. YOMV

And Isaac had done such a yeoman's job trying to get its act together over the last H24.... *sigh*

I'd say yes, the cyclonic vorticity you are noticing is a topographically induced effect... notice there is also a strip of higher vorticity that stretches down from that area that has moderate convection firing. This strip of vorticity is not really having a large impact on the storms motion so far. This is probably because its being dominated by the circulation of Isaac which is far more robust. In fact you see the vort max traveling southward with the strip of higher vorticity following suit while Isaac is moving to the wnw... so while the two entities are engaging in binary interaction, Isaac is the driver, and not the other way around.

[turtlehurricane]

Quick question - look at the high res RSO and tell me if you don't see some cyclonic curl in the CU line SW of the Tiburon - maybe even a hint of a weak eddy?. Is that a function of the lee side low that's been talked about?

And a comment - even if the center doesn't move across the bulk of Hispanola, I have still seen many cases of serious, serious disruption of TC wind circs in systems that have passed just north or just south. Might be especially true in this case since the winds seem to have always been most robust in the north quad. Yes, not as bad as moving down the spine, but still bad. YOMV

And Isaac had done such a yeoman's job trying to get its act together over the last H24.... *sigh*

I'm seeing that too, flow is actually northwards in the eastern part of that cyclonic cloud swirl, opposite to Isaac's wind field in the region. Some gravity waves propagating outwards in the swirl too, geostrophic adjustment at work perhaps.

[ALhurricane]

Disappointing discussion. They too often have become a rehash of what the model plots are and too little of the evolution of steering features. This may be banter, but I would appreciate their insight on the evolving synoptic setup.

[Hurriplane]

000

WTNT44 KNHC 242055

TCDAT4

TROPICAL STORM ISAAC DISCUSSION NUMBER 15

NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012

500 PM EDT FRI AUG 24 2012

RECONNAISSANCE AIRCRAFT DATA INDICATE THAT THE CIRCULATION OF ISAAC

HAS BECOME BETTER ORGANIZED THIS AFTERNOON...WITH A WELL-DEFINED

AND MORE CIRCULAR INNER CIRCULATION. THE MAXIMUM 850 MB

FLIGHT-LEVEL WINDS HAVE BEEN 70 KT...THE MAXIMUM RELIABLE SFMR

SURFACE WIND ESTIMATE HAS BEEN 55 KT...AND THE MINIMUM REPORTED

PRESSURE HAS BEEN 994 MB. BASED ON THESE DATA...THE INITIAL

INTENSITY IS INCREASED TO 55 KT. ONE SIGN OF ONGOING ORGANIZATION

ISSUES...HOWEVER...IS THE LACK OF PERSISTENCE OF THE INNER CORE

CONVECTION...WHICH HAS WANED OVER THE PAST FEW HOURS.

ISAAC HAS MADE ITS EXPECTED TURN TOWARD THE NORTHWEST AND THE

INITIAL MOTION IS NOW 315/14. DURING THE FIRST 48-60 HR...THE

TRACK GUIDANCE MADE AN EASTWARD SHIFT SINCE THE LAST ADVISORY.

THIS WAS LED BY THE GFS...WHICH CHANGED TO A TRACK INTO THE

SOUTHEASTERN GULF OF MEXICO TO LANDFALL IN SOUTHEASTERN FLORIDA.

THE ECMWF AND UKMET ARE NOW ON THE LEFT EDGE OF THE GUIDANCE...

SHOWING A TRACK ALONG THE NORTH COAST OF CUBA. THIS PART OF

THE FORECAST TRACK IS SHIFTED EASTWARD BUT STILL LIES A LITTLE TO

THE LEFT OF THE CENTER OF THE GUIDANCE ENVELOPE AND THE CONSENSUS

MODELS. THERE REMAINS A SPREAD IN THE MODEL GUIDANCE AFTER 48

HR. THE GFS-BASED GUIDANCE SHOWS A NORTHEASTWARD MOTION AFTER

LANDFALL ON THE NORTHERN GULF COAST...WHILE THE UKMET AND ECMWF

SHOW A NORTHWARD MOTION. THE LATTER PART OF THE FORECAST TRACK IS

SHIFTED EASTWARD CLOSE TO THE CONSENSUS MODELS.

THE MAIN CONVECTIVE BAND AND STRONGEST WINDS NORTHEAST OF THE CENTER

ARE MOVING INLAND AT THIS TIME. BASED ON THIS AND THE CURRENT LACK

OF CONVECTION NEAR THE CENTER...LITTLE CHANGE IN STRENGTH IS

EXPECTED BEFORE LANDFALL. THE INTENSITY FROM 12-48 HR WILL BE

DETERMINED BY THE AMOUNT OF LAND INTERACTION. THE CURRENT FORECAST

IS BASED ON THE PREMISE THAT ISAAC WILL REMAIN OVER LAND LONG

ENOUGH TO ALLOW GRADUAL WEAKENING. HOWEVER...ANY CHANGE IN THE

TRACK THAT ALLOWS IT TO REMAIN OVER WATER FOR A LONGER TIME COULD

ALLOW IT TO BE STRONGER THAN FORECAST. AFTER 48 HR...THE

ENVIRONMENTAL CONDITIONS APPEAR GENERALLY FAVORABLE FOR ISAAC TO

STRENGTHEN INTO A HURRICANE OVER THE GULF OF MEXICO...AND THE

INTENSITY FORECAST REFLECTS THIS. CONVERSELY...A LONGER TRACK OVER

LAND MAY DISRUPT ISAAC ENOUGH TO BE UNABLE TO TAKE ADVANTAGE OF

THESE FAVORABLE CONDITIONS.

IT IS IMPORTANT NOT TO FOCUS ON THE EXACT TRACK DUE TO THE

UNCERTAINTIES IN THE FORECAST AND THE FACT THAT ISAAC HAS A LARGE

AREA OF TROPICAL STORM FORCE WINDS ASSOCIATED WITH IT. THE TROPICAL

STORM WATCH FOR SOUTH FLORIDA AND THE FLORIDA KEYS MAY HAVE TO BE

UPGRADED TO A HURRICANE WATCH IF THE INTENSITY FORECAST IS REVISED

UPWARD.

FORECAST POSITIONS AND MAX WINDS

INIT 24/2100Z 17.2N 71.9W 55 KT 65 MPH

12H 25/0600Z 18.7N 73.6W 50 KT 60 MPH

24H 25/1800Z 20.7N 76.2W 50 KT 60 MPH...INLAND

36H 26/0600Z 22.4N 78.6W 45 KT 50 MPH...NEAR COAST OF CUBA

48H 26/1800Z 23.9N 80.7W 55 KT 65 MPH...OVER WATER

72H 27/1800Z 26.5N 84.0W 65 KT 75 MPH

96H 28/1800Z 29.5N 86.0W 75 KT 85 MPH

120H 29/1800Z 32.0N 86.0W 45 KT 50 MPH...INLAND

[Ground Scouring]

What is most interesting is that the ECMWF shows a nearly NNW movement for the first 24 hours, and then turns Isaac nearly due WNW along the N coast of Cuba. Given the very large envelope of Isaac, and particularly given the N-S vorticity stretching and momentum transfer by the lee cyclone, such a turn back to the WNW seems less likely than some movement between NNW and NW over the next 24 hours, even if someone might argue that any such NNW-ish movement is unlikely to be prolonged. In the short term, and to some degree after 48 hours, I would give more weight to the GFS than to the ECMWF, owing to the size of Isaac combined with the weak trough split. The same pattern might well result in a slower movement over the next two days before turning than the NHC shows. Such a trend, plus a GFS-type track, would maximize time over the Bahamas for more than two days. Thoughts?

Edit: By the way, while central convection is meager, the structural appearance of the storm has improved dramatically, and I would guess that the lee cyclone has actually helped the low-level vorticity organize in consort with the MLC, resulting in a steady pressure drop.

[Ground Scouring]

Quick question - look at the high res RSO and tell me if you don't see some cyclonic curl in the CU line SW of the Tiburon - maybe even a hint of a weak eddy?. Is that a function of the lee side low that's been talked about?

And a comment - even if the center doesn't move across the bulk of Hispanola, I have still seen many cases of serious, serious disruption of TC wind circs in systems that have passed just north or just south. Might be especially true in this case since the winds seem to have always been most robust in the north quad. Yes, not as bad as moving down the spine, but still bad. YOMV

And Isaac had done such a yeoman's job trying to get its act together over the last H24.... *sigh*

Normally, I would agree, but the latest reconnaissance and satellite data suggest that the lee cyclone has greatly aided Isaac in forming a small, well-defined center that may be less vulnerable to disruption should the the recent WNW movement continue.

[thewxmann]

Quick question - look at the high res RSO and tell me if you don't see some cyclonic curl in the CU line SW of the Tiburon - maybe even a hint of a weak eddy?. Is that a function of the lee side low that's been talked about?

And a comment - even if the center doesn't move across the bulk of Hispanola, I have still seen many cases of serious, serious disruption of TC wind circs in systems that have passed just north or just south. Might be especially true in this case since the winds seem to have always been most robust in the north quad. Yes, not as bad as moving down the spine, but still bad. YOMV

And Isaac had done such a yeoman's job trying to get its act together over the last H24.... *sigh*

Isaac will probably get pummeled by Haiti. But yeah, at its current state, we could give Isaac 3 days in perfect conditions and the inner core would still be too loose for it to intensify to >Cat 1 status. So might as well get a reboot and see what happens.

[Ground Scouring]

Isaac will probably get pummeled by Haiti. But yeah, at its current state, we could give Isaac 3 days in perfect conditions and the inner core would still be too loose for it to intensify to >Cat 1 status. So might as well get a reboot and see what happens.

Latest satellite images do show a secondary band developing near the center. As the lee cyclone weakens and the UL divergence shifts away, I think the system will shed the large outer band overnight. Anyway, someone has to be positive!

[PSUBlizzicane2007]

Isaac will probably get pummeled by Haiti. But yeah, at its current state, we could give Isaac 3 days in perfect conditions and the inner core would still be too loose for it to intensify to >Cat 1 status. So might as well get a reboot and see what happens.

Isaac isn't going over the worst part of Haiti. While the SW peninsula is mountainous, it isn't nearly as bad as the central cordillera... plus it is very narrow and Isaac is moving at a decent speed. So while Isaac will likely weaken, it will not get "pummeled".