Tropical Depression Isaac:

Tropical Weather Discussions and Analysis
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djmike
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Hmmm...local Met here in Beaumont just did his weather segment on TV and showed what they think will happen. They showed a graphic of Isaac south of FL come Mon, but by Wednesday, he had Isaac just south of NOLA with the steering currents slim to none. Also showed the ridging building eastward aswell.... Also mentioned (and stressed) that by ALL means, we are not out of the woods just yet (atleast here in Beaumont).... I have a feeling he thinks the EURO may be on to something! .....I kind of have to agree!
Last edited by djmike on Wed Aug 22, 2012 6:50 pm, edited 1 time in total.
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Have to make this quick. No time to post links. Current pattern over area that is likely to change as Isaac enters the Gulf. Models are still shifting westward this evening. I'd expect the NHC to comply.
That is the best answer I can give you at the moment. ;)
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djmike wrote:Hmmm...local Met here in Beaumont just did his weather segment on TV and showed what they think will happen. They showed a graphic of Isaac south of FL come Mon, but by Wednesday, he had Isaac just south of NOLA with the steering currents slim to none. Also showed the ridging building eastward aswell.... Also mentioned (and stressed) that by ALL means, we are not out of the woods just yet (atleast here in Beaumont).... I have a feeling he thinks the EURO may be on to something! .....I kind of have to agree!

was the local met greg bostwick??
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djmike
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weatherrabbit wrote:
djmike wrote:Hmmm...local Met here in Beaumont just did his weather segment on TV and showed what they think will happen. They showed a graphic of Isaac south of FL come Mon, but by Wednesday, he had Isaac just south of NOLA with the steering currents slim to none. Also showed the ridging building eastward aswell.... Also mentioned (and stressed) that by ALL means, we are not out of the woods just yet (atleast here in Beaumont).... I have a feeling he thinks the EURO may be on to something! .....I kind of have to agree!

was the local met greg bostwick??
Yup!
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cool beans! he is still on! good met! I agree about the euro being on to something. like to see the data the G-IV plane has for us tomorrow!
Gene Norman

I'm really NOT liking the model disagreement. NoGaps and NAM look like this:

they put Isaac between a trough in the southeast and the Atlantic ridge late Saturday night and the strength of the trough steers Isaac toward the east coast:
nam_300mb_06Z_Sun.jpg
nam_sfc_06Z_Sun.jpg
But the ECMWF and to a lesser degree the GFS tend to move the trough by before it has a chance to nudge Isaac northward and allow the Atlantic ridge to build westward, thus shunting Isaac into the Gulf where it explodes and maybe heads for LA/MS/AL:
ecm_200mb_12z_Sun.jpg
ecm_sfc_12Z_Sun.jpg
ecm_sfc_12Z_Wed.jpg
Anyone want to make a compelling argument either way? I'm just not seeing where the Atlantic ridge builds so strongly and the trough moves by fast enough to not have a "lifting" influence on Isaac as it emerges over Cuba.
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Well, some model changes today have made for some interesting discussions this evening with Isaac. Not to mention the plane is finding a big mess with multiple centers rotating around a mean circulation...it is large though. Will comment on the models this evening as I see it at this point in time. EURO is well to the west of what was and still is a decent consensus aimed generally at FL (up the west coast, down the middle, or even near the east coast). With that said there has been some differences developing in the longer term trends today Day 4 +. One being the large spread starting to develop in the GFS ENS which does give some pause as they have been fairly well clustered up until this point. I feel the models are starting to trend toward either Isaac being weaker in the near term or the trough weaker in the long term and while these two combined would still likely impart a NW motion at some point, it may not be enough to swing Isaac northward. Additionally the 18Z HWRF is similar to some of the GFS ENS showing a more west turn at the end of its run suggesting the trough leaves Isaac behind and the ATL ridge begins to build back westward.

If it were September I would be more confident that the trough would be deep enough and dig enough to capture. The differences appear to be coming out of the intensity of the short wave which moves across the N Plains and how far this energy digs into the mean east coast trough in place. I cannot help but feel we have been in this position many times before where the models under-estimated the ridging and over-estimated the trough...most notably with Ike.

With all that said, the GFS has been very good this year with Ernesto, TD#7, Helene, and Debby (although Debby was more a likely a fluke with the center relocations) so it is hard to go against its reasoning and the fact it has been steadfast in its runs for days now...very little shifting. At the same time the EURO has been the better model in the past years and is usually one of the first to detect trends... and now with the HWRF showing more of a west bend at the end does lend some support to the EURO.

One thing is for sure...this system will be very large so impacts will be far reaching and if the EURO is anywhere close to correct on intensity a really big threat to the Gulf coast.
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I just wanted to thank the pro mets on the forum here. Your discussions are fascinating and really provide us weather enthusiasts with a "window" into your thinking. I feel privileged to be able to hear your thoughts about Isaac and just wanted to sincerely thank you guys! Please keep it up.
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TROPICAL STORM ISAAC DISCUSSION NUMBER 8
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
1100 PM AST WED AUG 22 2012

AN AIR FORCE HURRICANE HUNTER AIRCRAFT INVESTIGATED ISAAC SEVERAL
HOURS AGO AND FOUND THAT THE CENTER WAS STILL NOT WELL-DEFINED AND
THE INTENSITY ESTIMATE OF 40 KT WAS PROBABLY A LITTLE ON THE HIGH
SIDE. HOWEVER...DROPSONDE DATA FROM A NOAA AIRCRAFT CURRENTLY
INVESTIGATING THE STORM SUGGEST THAT THE CENTER HAS PROBABLY BECOME
A LITTLE BETTER DEFINED AND THAT THE MINIMUM PRESSURE HAS DROPPED A
BIT. THERE HAS ALSO BEEN AN INCREASE IN COLD-TOPPED CONVECTION
NEAR AND OVER THE ESTIMATED CENTER SO THE CURRENT INTENSITY IS HELD
AT 40 KT. ISAAC HAS DEVELOPED AN IMPRESSIVE UPPER-LEVEL OUTFLOW
PATTERN AND DYNAMICAL GUIDANCE SHOWS THAT THE VERTICAL SHEAR SHOULD
REMAIN WEAK DURING THE NEXT SEVERAL DAYS. THEREFORE THE PRIMARY
IMPEDIMENT TO STRENGTHENING SHOULD BE THE INTERACTION WITH THE
MOUNTAINOUS TERRAIN OF HISPANIOLA AND EASTERN CUBA. THE MOST
RECENT RUNS OF THE HWRF AND GFDL SHOW SIMILAR AMOUNTS OF LAND
INTERACTION BUT ARE NOT IN GOOD AGREEMENT ON THE INTENSITY FORECAST
AS THE FORMER MODEL SHOWS MUCH LESS STRENGTHENING THAN THE LATTER.
THE OFFICIAL WIND SPEED FORECAST IS IN REASONABLE AGREEMENT WITH
THE LATEST STATISTICAL-DYNAMICAL GUIDANCE.

AFTER A SLIGHT SOUTHWARD RELOCATION OF THE CENTER...THE INITIAL
MOTION CONTINUES BASICALLY WESTWARD OR 270/17. ISAAC SHOULD
CONTINUE TO MOVE WESTWARD TO WEST-NORTHWESTWARD FOR THE NEXT COUPLE
OF DAYS ALONG THE SOUTHERN PERIPHERY OF THE SUBTROPICAL RIDGE. THE
GLOBAL MODELS CONTINUE TO DEPICT A BREAK IN THE RIDGE IN THE
VICINITY OF FLORIDA IN SEVERAL DAYS...WHICH SHOULD INDUCE A MORE
NORTHWARD COMPONENT OF MOTION. THE ECMWF MODEL SHOWS LESS OF A
BREAK IN THE RIDGE THAN THE GFS SOLUTION. CONSEQUENTLY...THE ECMWF
TRACK IS SOMEWHAT TO THE SOUTHWEST OF MOST OF THE OTHER TRACK
MODELS WHICH WOULD LESSEN THE THREAT TO FLORIDA. THE OFFICIAL
TRACK FORECAST FOR THIS ADVISORY IS QUITE CLOSE TO THE PREVIOUS ONE
AND ALSO VERY CLOSE TO THE DYNAMICAL MODEL CONSENSUS.

FORECAST POSITIONS AND MAX WINDS

INIT 23/0300Z 15.8N 63.0W 40 KT 45 MPH
12H 23/1200Z 16.2N 65.4W 45 KT 50 MPH
24H 24/0000Z 16.9N 68.1W 55 KT 65 MPH
36H 24/1200Z 17.5N 70.5W 65 KT 75 MPH
48H 25/0000Z 18.6N 72.7W 55 KT 65 MPH
72H 26/0000Z 21.5N 77.0W 60 KT 70 MPH...ON NORTH COAST OF CUBA
96H 27/0000Z 24.0N 80.5W 65 KT 75 MPH
120H 28/0000Z 27.0N 83.5W 70 KT 80 MPH

$$
FORECASTER PASCH
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I have been looking at the 00z GFS run and a few key things (noted by Jeff and other mets earlier) have been showing up differently in this run compared to the 12z. The shortwave that the 12z run was showing is not nearly as deep or impressive as earlier but the 00z run does transition this energy across the plains slower as shown below:

12z:
gfs_atlantic_102_500_vort_ht 12z.gif
00z:
gfs_atlantic_090_500_vort_ht 00z.gif
It seems like the gfs is still having issues determining the speed and depth of this and being 90 hours out it could continue to have troubles. This will be key for the overall strength of the trough and how much it affects Isaac.

Next the 00z continues to show lack of organization in the short-term which would match up well with current observations. Looking at 850mb vort, multiple vort max show up and the energy does not look vertically stacked.
gfs_atlantic_033_850_vort_ht.gif
This could be a bad thing though, allowing the storm to "survive" better once it moves over the islands. With a lack of vertical alignment higher geography will not affect it as much ( like we saw with Ike). That will be one key thing to watch. As we head into the medium range the gfs starts to intensify Isaac as conditions according to the gfs look more favorable. How close Isaac stays to land and the shape it is after it crosses land will be key to how quickly he intensifies. With such a large system this could take a while and it could delay the intensification. Looking at the longer range mid level ridging starts to collapse as the trough stretches across the central and eastern part of the U.S. but lower level (850mb) ridging does start to build back in relatively fast. If Isaac does not strengthen enough or if the trough is weaker than expected then the turn could be less dramatic.

Looking at the 00z euro in the short range, it seems to be too strong to me. With so much land interaction depicted by the Euro it will be hard for any type of organization until it enters the gulf. In the longer range it does deepen Isaac relatively quickly in the gulf as it comes in for a central/eastern gulf landfall. That looks realistic enough and the gfs and Euro seem to be coming together on a target area of Central to Eastern part of the gulf. I think the key here will be placement of Isaac once it gets in the gulf and the vector of movement. The turn to the North may not be as prominent as once thought and that could put more of the gulf at risk. There are a lot of factors to consider in the next couple of days and plenty of time to watch things unfold.
00zeurotropical850mbVortSLP168 (1).gif
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Both the NHC and the GFS are smoking crack this early morning. Back to bed until somebody can get it right.
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Oh BB...~sigh~
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PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD
338 AM EDT THU AUG 23 2012

VALID 12Z MON AUG 27 2012 - 12Z THU AUG 30 2012

THE JET STREAM IS EXPECTED TO REMAIN ACROSS CANADA THROUGH THE
MEDIUM RANGE PERIOD...WITH A SMALLER MOSTLY SEPARATED BRANCH
ACCOMPANYING A CLOSED LOW DRIFTING INTO THE PACIFIC NORTHWEST
...FASTER IF YOU BELIEVE THE 00Z ECMWF. THIS LONGWAVE FLOW FAVORS
AN UPPER RIDGE OR WEAK UPPER HIGH PREVAILING OVER THE
SOUTHWEST...AND A MEAN LONGWAVE TROUGH ALONG THE EAST COAST.
MODEL AGREEMENT IS NEAR AVERAGE UNTIL ABOUT DAY 5/TUE...WHEN THE
00Z GFS MOVES TOWARD THE SLOW EDGE OF THE GUIDANCE WITH THE ABOVE
MENTIONED UPPER LOW...WHICH IN TURN RESULTS IN MUCH FASTER FLOW
LYING TO ITS NORTH AND ULTIMATELY A VERY DIFFERENT SHORTWAVE
PATTERN FORMING ACROSS CANADA BEGINNING WITH DAY 6/WED COMPARED TO
THE MAJORITY OF OTHER GUIDANCE. THUS...THE GENERAL PREFERENCE IS
TO DISCOUNT THE GFS BEGINNING WITH DAY 5. OTHERWISE...THE 00Z
ECMWF AND ITS PRIOR ENSEMBLE MEAN ARE THE MOST PLAUSIBLE SOLUTIONS
TO USE GIVEN THEIR PHASE AGREEMENT AND SUPPORT FROM OTHER NON-GFS
SOLUTIONS. FOR TC ISAAC...THE GFS/ECMWF/UKMET ARE QUITE CLOSE TO
THE LATEST NHC TRACK FORECAST...WITH THE ECMWF A LITTLE TO ITS
SOUTH AS ISAAC POSSIBLY NEARS SOUTHERN FLORIDA BY DAY 4/MON.
CONFIDENCE IS CONSISTENT WITH THE SOLUTION SPREAD THIS
CYCLE...AVERAGE THROUGH DAY 5...THEN BELOW AVERAGE
THEREAFTER...DUE TO THE COMPLICATED FLOW ACROSS CANADA AND
POSSIBLE INTERACTIONS OF MULTIPLE STREAMS.

JAMES
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G-IV Flight Mission scheduled for today...this is a high altitude sampling of the upper air environment for ingest into future model guidance...

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Although Isaac has remained rather unorganized, it is doing about what has been expected for days via guidance when models suggested a weak, non vertically stacked cyclone trekking W. The intensification stage was never really expected unto Isaac exits Hispaniola/Cuba where some rather rapid intensification could begin over the very warm Gulf Stream waters and the Gulf of Mexico.
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12Z Tracks & Intensity:
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08232012 12Z Intensity Isaac aal09_2012082312_intensity_early.png
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Morning e-mail from Jeff:

Disorganized tropical storm moving across the eastern Caribbean Sea.

Discussion:
USAF plane overnight investigating Isaac has found multiple centers rotating around a large mean circulation…a disorganized tropical system. Convection continues to struggle to consolidate and maintain itself and this is likely due to dry air shown in the Leeward Island soundings in the mid to upper levels occasionally entering into the fragile inner core of the system. This along with the large size and the multiple centers has prevented Isaac from much intensification. Isaac continues toward the W, but has slowed in the last several hours.

Track:
I have spent a lot amount of time this morning reviewing the various global models and their handling of the upstream features over the US that will ultimately determine the final track and landfall of Isaac on the US coast. For the next 24-48 hours it appears the system will continue to move toward the W to WNW on the south side of the sub-tropical high over the central Atlantic. This will take the system toward the southern coast of Haiti, but if Isaac does not starting gaining some latitude it is very possible the system will pass south of Haiti.

In the longer term (after 48 hours) the guidance spread increases with the ECMWF on the left (west) edge of the clustering and the CMC on the right (east) edge of the clustering. The ECMWF has been shifting eastward in its last few runs while the CMC has been shifting westward and the GFS has been holding strong for the past several runs with very little shifting. It is interesting that at the 120-144 hr points the GFS shows a classic recurve over Isaac over the SE US while the CMC and ECMWF drive the system inland and do not show a hard recurve.

The differences are in the handling of the troughing that currently extends from the NE US into the Gulf of Mexico. This trough will begin to lift over the next 24 hours, but a weakness or break in the ridge over the SW Atlantic and a building high over the southern plains is left behind over the SE US. This weakness appears to be enough to induce a NW turn of Isaac near the eastern or central Cuban coast. At this point the weakness begins to fill some as a zonal flow briefly develops over the US. However a strong shortwave digs down from Alaska which helps pump up (build) the southern plains ridge resulting in troughing digging across the SE US toward early next week in a weak omega pattern. This pattern allows Isaac to be captured or tugged by the trough and brought NNW and then N across the eastern Gulf toward the FL panhandle. Each of the global models has the same solution, but each differ some on the intensity and amplification of the southern plains ridging and the resultant downstream trough. The ECMWF has a weaker trough which allows Isaac to move further west while the GFS has a deeper trough and curves the system nearly up the FL west coast and then into SC. The GFS has been very consistent on its track and the ECMWF has been slowly trending toward its solution over the past two model runs although the ECMWF ensembles are well west of the operational track toward the LA coast.

There is also the potential for short term center relocations to the south of the current position as supported by the aircraft fixes, and this could have some longer term track implications.

With all that said, the consensus is still in very good agreement through the Day 4-5 period with the system track toward the FL Keys and then NW along the west coast of FL.

Intensity:
Isaac is continuing to suffer from dry air intruding into the inner core and the lack of a well defined center. While the potential has been and continues to be there for intensification, the internal dynamics of the inner core continue to be lacking which is preventing organized deep convection. We need a well defined center to take shape and to flush the dry air out of the inner areas of the system allowing deep convection to develop and maintain itself. Another factor is the large size of the system as larger systems tend to take time to consolidate. Isaac has a well defined outflow pattern established with high level cirrus expanding outward in all directions helping to vent the circulation. When the inner core finally does consolidate then the system should begin to intensify given warm waters and good upper level outflow. The next item of concern with intensity will be how much the system interacts with the islands of Hispaniola and Cuba. While the track takes the system across western Haiti and eastern Cuba, I am starting to consider that the system may pass south of both of these land areas and over the water which would have the potential to produce some significant intensity differences compared to the current forecast. Due to the uncertainty of the consolidation of the inner core and how long it is going to take and the possibility of land interaction will follow along with the NHC on the intensity, but if Isaac stays a little more to the south it could be stronger in the middle and latter part of the forecast. Also there is data suggesting Isaac is currently suffering from the passage of an upper level Kelvin wave which is helping to induce unfavorable conditions across this part of the basin in the face of otherwise favorable conditions. Isaac should come out from under the influence of this feature in the next 48 hours or so.

NHC Forecast Track and Error Cone:
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I know to never say never, but if Im reading correct that the euro shifted well east and FL continues to have a very good model consensus, I'm almost inclined to believe Isaac will be FLs storm. Latest models also look to have shifted eastward a bit also. :( Plus I have a feeling NHC track will be millimetered east aswell. As much as I wish Isaac would move close enough for us to get some beneficial rains, Im losing hope fast. I guess the only thing right now to hope for is that Isaac stays weak and the weakness in the ridge won't be strong enough which will promote a more westerly track....(Not asking Isaac to pay us a visit or anything, certainly dont need the destruction and winds and so forth...just want some good tropical rains)
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TROPICAL STORM ISAAC DISCUSSION NUMBER 10
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
1100 AM AST THU AUG 23 2012

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. A COMBINATION OF FLIGHT-LEVEL...DROPSONDE...AND
SFMR WINDS SUGGEST THAT THE INITIAL INTENSITY IS NEAR 35 KT.

THE OVERALL CIRCULATION OF ISAAC IS A LITTLE FARTHER SOUTH THAN 12
HR AGO. HOWEVER...IT SEEMS TO HAVE RESUMED A WESTWARD MOTION AT
ABOUT 13 KT. THE TRACK MODEL GUIDANCE IS IN GOOD AGREEMENT THAT
ISAAC SHOULD MOVE WEST-NORTHWESTWARD FOR THE NEXT 36-48 HR...
STEERED BY THE SUBTROPICAL RIDGE TO THE NORTH. AFTER THAT...THE
STORM SHOULD APPROACH A WEAKNESS OR BREAK IN THE RIDGE OVER FLORIDA
AND THE EASTERN GULF OF MEXICO. THE NEW FORECAST TRACK IS SHIFTED A
LITTLE TO THE WEST OF THE PREVIOUS TRACK BASED ON CURRENT POSITION
AND LIES DOWN THE MIDDLE OF THE GUIDANCE ENVELOPE NEAR THE
CONSENSUS MODELS. IT SHOULD BE NOTED...THOUGH...THAT NEAR THE 96
HR POINT THE GUIDANCE MODELS ARE SPREAD BETWEEN SOUTHEASTERN
FLORIDA AND WESTERN CUBA...AND AT 120 HR THEY ARE BETWEEN
NORTHEASTERN FLORIDA AND THE CENTRAL GULF OF MEXICO. THUS...
SIGNIFICANT UNCERTAINTY REMAINS ABOUT THE THREAT ISAAC POSES TO
FLORIDA.

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. BASED ON THIS...THE
INTENSITY FORECAST CONTINUES TO SHOW HURRICANE STRENGTH BEFORE THE
SYSTEM REACHES HISPANIOLA IN ABOUT 36 HR. AFTER THAT...PASSAGE OVER
HISPANIOLA AND CUBA SHOULD CAUSE SOME WEAKENING...FOLLOWED BY
LITTLE CHANGE OF STRENGTH FROM 48-72 HR. THE INTENSITY FORECAST
SHOWS A LITTLE MORE STRENGTHENING OVER THE GULF OF MEXICO DUE TO
THE FORECAST TRACK BEING A LITTLE FARTHER FROM THE FLORIDA
PENINSULA THAN BEFORE. THE INTENSITY FORECAST IS BASED ON A BLEND
OF THE PREVIOUS FORECAST AND THE SHIPS/LGEM MODELS...AND IT IS WELL
ABOVE THE INTENSITY CONSENSUS.


FORECAST POSITIONS AND MAX WINDS

INIT 23/1500Z 15.6N 65.4W 35 KT 40 MPH
12H 24/0000Z 16.1N 67.4W 40 KT 45 MPH
24H 24/1200Z 16.9N 70.0W 50 KT 60 MPH
36H 25/0000Z 18.0N 72.4W 65 KT 75 MPH
48H 25/1200Z 19.3N 74.6W 55 KT 65 MPH
72H 26/1200Z 22.0N 78.5W 55 KT 65 MPH...INLAND
96H 27/1200Z 24.5N 81.5W 65 KT 75 MPH...OVER WATER
120H 28/1200Z 27.5N 84.5W 75 KT 85 MPH

$$
FORECASTER BEVEN
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08232012 11AM EDT Isaac 145525W5_NL_sm.gif
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000
WTNT44 KNHC 231453
TCDAT4

TROPICAL STORM ISAAC DISCUSSION NUMBER 10
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
1100 AM AST THU AUG 23 2012

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. A COMBINATION OF FLIGHT-LEVEL...DROPSONDE...AND
SFMR WINDS SUGGEST THAT THE INITIAL INTENSITY IS NEAR 35 KT.

Not very impressive
JMS
SR. ENSC.
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