Little change to Isaac, but storm poses a serious storm surge threat

By: Dr. Jeff Masters , 3:25 PM GMT on August 27, 2012

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Tropical Storm Isaac has changed little in strength or organization this morning, as the storm heads northwest at 14 mph towards the Central Gulf Coast. There are two hurricane hunter aircraft in the storm, and Isaac's central pressure held steady at 989 mb at the 8:30 am and 9:15 am center fixes. Top surface winds remain near 65 mph. Infrared and visible satellite loops show that Isaac is a very large storm, but isn't very symmetric. Heavy thunderstorm activity is lacking on the southeast side, where 10 knots of wind shear is driving dry air into the circulation. The center is surrounded by a ring of echoes now, which was not the case on Sunday. However, the echoes are weak. The 8:30 am center report from the Hurricane Hunters reported about half of a ragged eyewall, but the 9:15 am report did not mention any evidence of an eyewall. Isaac will have to form an eyewall in order to intensify significantly.


Figure 1. Morning satellite image of Isaac. Note the lack of heavy thunderstorm activity on the storm's southeast side, where dry air and wind shear are combining to interfere with development.

Isaac's rains
Isaac's heaviest rains have fallen along a swath from the east coast of Florida near West Palm Beach to the center of the state, just south of Orlando. West Palm Beach received 7.57" of rain from Isaac as of 10 am EDT this morning. A trained spotter in Western Boynton Beach reported 10" of rain from midnight to midnight Sunday. Heavy rains from Isaac are lingering over Cuba but have ended in Haiti and the Dominican Republic; flash floods in Haiti from Isaac's torrential rains killed at least nineteen, and two died in the Dominican Republic.


Figure 2. Radar-estimated rainfall from Melbourne, Florida radar shows that Isaac has dumped a wide swath of 3+ inches of rain (orange colors) across the state.

Latest model runs for Isaac
The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs are fairly unified taking Isaac ashore near Southeast Louisiana late Tuesday night or early Wednesday morning, but continue to show major differences in what happens after that. It is still uncertain if a trough of low pressure passing to the north of Isaac will be able to turn the storm due north, as the ECMWF model is predicting, or bypass the storm, allowing a more west-northwest motion into western Louisiana, like the GFS model is predicting. In either case, Isaac is likely to slow down as it approaches the coast, which will increase the damage potential fro m its wind, storm surge, and rains. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over much of Louisiana. The ECMWF model predicts that these heavy rains will fall more over Mississippi. It appears likely that Arkansas will see some heavy rains from Isaac late in the week, which would help put a dent in the exceptional drought conditions there.


Figure 3. Predicted precipitation for the 8-day period from 2 am Monday August 27 to 2 am Tuesday September 4, from the 2 am EDT August 27 run of the GFS model. This model is predicting a wide swath of 5 - 10 inches of rain (orange colors) will affect portions of Louisiana. Additional very heavy rains are predicted for the Midwest, as moisture from Isaac interacts with a cold front. Image credit: NOAA/NCEP.

Intensity forecast for Isaac
Isaac is currently crossing over a relatively cool eddy of water, which will keep intensification slow today. By tonight, the total heat content of the waters increases, which should aid intensification. Low wind shear of 10 knots or less is likely until landfall. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow to the north is not as strong as yesterday, which should also slow intensification today. The models forecast the upper-level outflow should improve by Tuesday. A storm this large will have trouble undergoing rapid intensification, and Isaac's most likely intensity at landfall will be as a Category 1 hurricane, which is what most of the intensity models are forecasting.


Figure 4. Track of Hurricane Gustav of 2008, which followed a path very similar to that of Isaac's predicted path.

Storm surge forecast for Isaac
Storm surge is the primary damage threat from Isaac. Isaac is a huge storm, with tropical storm-force winds that extend out 205 miles from the center. For comparison, Hurricane Katrina at landfall had tropical storm-force winds that extended out 230 miles from its center. Isaac's large size will enable it to set a large area of the ocean into motion, which will generate a large storm surge once the storm approaches land on the Gulf Coast. Water levels at Shell Beach, Louisiana, just east of New Orleans, were already elevated by 1' this morning. Conversely, water levels have fallen by 2' this morning at St. Petersburg, Florida, where strong offshore winds due to Isaac's counter-clockwise circulation have carried water away from the coast. The latest 6:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 0.6 on a scale of 0 to 6, but the destructive potential of Isaacs's storm surge was 2.1 on a scale of 0 to 6. I expect this destructive potential will rise above 3 by time Isaac makes landfall, making Isaac's storm surge similar to that generated by Category 2 Hurricane Gustav of 2008, which followed a path very similar to Isaac's predicted path. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5' to Lake Borgne on the east side of the city. A higher Category 2-scale surge occurred along the south-central coast of Louisiana, and was 12.5' high in Black Bay, forty miles southeast of New Orleans. Recent model runs indicate Isaac may slow down to a forward speed under 5 mph on Tuesday evening and Wednesday morning, close to the coast. If Isaac is just offshore at this time, the coasts of Southeast Louisiana, Mississippi, Alabama, and the Florida Panhandle will be exposed to a large storm surge with battering waves for two high tide cycles. This sort of extending pounding will be capable of delivering more damage than the storm surge of Hurricane Gustav of 2008.

The affect of storm size and angle of approach on storm surge
A 2008 paper by Irish et al., The influence of storm size on hurricane surge, found that large storms like Isaac are capable to delivering a 30% larger storm surge to the coast than a smaller storm with the same maximum wind speeds. The angle with which the storm hit the coast is important, too--a storm moving due north or slightly east of north will deliver a storm surge about 10% greater than a storm moving NNW or NW. Consult our Storm Surge pages for detailed information on what the risk is for the coast. I expect that Isaac's storm surge will be about 30% higher than the typical surge one would expect based on the maximum wind speeds.

Isaac's storm surge will provide the first test of the newly-completed New Orleans levee system upgrade. In the wake of the disastrous storm surge flooding from Hurricane Katrina in 2005, Congress appropriated $14.5 billion to upgrade the New Orleans levee system to withstand a Category 3 hurricane storm surge. Katrina was a Category 3 storm at landfall, but the storm passed far enough to the east of the city that its storm surge was characteristic of a Category 1 - 2 storm at the places where the city's flood walls and levees failed. The new flood defenses were only partially completed in time for the arrival of Hurricane Gustav in 2008, which hit Central Louisiana as a Category 2 storm. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5' to Lake Borgne on the east side of the city. Since that time, the imposing 2-mile long IHNC Flood Barrier has been completed to block off the funnel-shaped pair of canals on the east side of the city. I expect New Orleans' new flood defenses will be able to hold back Isaac's surge, but areas outside the levees are at risk of heavy storm surge damage.


Figure 5. A portion of New Orleans' new $14.5 billion dollar flood defenses, as taken from an Army Corps of Engineers map.

New Orleans flood defense info
Army Corps of Engineers map of the new flood defenses
Army Corps of Engineers video showing the flood defenses
New York Times article, Vast Defenses Now Shielding New Orleans

Invest 97L off the coast of Africa
A tropical wave (Invest 97L) is located in the Middle Atlantic, about 1050 miles west-northwest of the Cape Verde Islands. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 30% chance of developing by Wednesday morning. None of the reliable models foresee that 97L will be a threat to the Lesser Antilles Islands, and high wind shear should begin to tear the disturbance apart on Tuesday.

Another tropical wave that moved off the coast of Africa on Sunday is located just south of the Cape Verde Islands. This disturbance is moving west at 10 - 15 mph, and could arrive in the Lesser Antilles around September 2. Several models develop the disturbance into a tropical depression late this week, and NHC is giving the disturbance a 10% chance of becoming a tropical depression by Wednesday morning.

Angela Fritz will have a new post here by 6 pm EDT. I'm in Atlanta to help out The Weather Channel with their on-air hurricane coverage, and will doing a few 3-minute tropical updates at 30 minutes past the hour between 2:30 - 7:30 pm EDT today.

Jeff Masters

Tropical Storm Isaac (chelina)
View from the north side of the Sunshine Skyway Bridge.
Tropical Storm Isaac
our street at noon today (seflagamma)
Aug 26, 2012: Isaac floods our street really bad. Now nearly 11
our street at noon today

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I wonder when we are going to have a real hurricane again. I mean, really? It seems like the past few years it's been just sort of under-par compared with 2004-2005 storms. Heck, in 2005 Wilma seemed like a weaker storm compared to Katrina, Rita, Ivan, etc. because it was "just" a minimal category 3 in South Florida, and now the best 2006-2012 has cranked out are less than well developed category 2's that just never can seem to get there cores together. Gustav was forecast to be a category 3 at landfall, but the center just could never get organized, as it was just a garbled mess throughout the Gulf, and then it weakened to a category 2 at landfall. Then Ike had "high expectations" by everyone to be a doomsday storm with it being huge wiping out Houston/Galveston and it seemed like it had a favorable atmosphere, but the same thing seemed to happen to Ike. As with Gustav, the center seemed like it had marbles mixed in it that prevented it from winding up in the Gulf. It took almost the ENTIRE Gulf of Mexico for the wind to rise just about 30 miles per hour from the time it left Cuba. Now I am not saying this because I am a fear mongering, storm wanting, destruction wanting teen who just wants to get out of school to enjoy seeing destruction and a real disasters on TV, but what I am saying is that THESE STORMS COULD BE MUCH WORSE!!! The era of the 2004-2005 storms seems like a memory from another time, another age, when disasters happened around every corner. And now the fear mongering, storm wanting, destruction wanting people (TWC, News Stations, etc.) are just craving more of that, and the reason they are is because their businesses thrived on those disasters. The point is, the storms we have had in recent years since 2005 are not up to par with what we were experiencing during 2004-2005.

It is true, we are entering a major hurricane drought if none happen for the rest of this year, as it seems Isaac is plagued by the same ailments Gustav and Ike were.

And another thing. All the storms since 2005 have made landfall in the Central Gulf Westward. No more Florida, no more Tampa Bay disaster, no more Miami disaster, no more Pensacola disaster. The Florida hurricane experience since 2005 has become obsolete and seems to be continuing to do so. All the storms do nowadays are go out to sea, and if they just happen to make it into the Gulf of Mexico, they go West, West, West. Resident's of the Eastern Gulf's memories are fading, it will happen, slowly but surely. We cannot forget what major hurricanes do!

It is puzzling of why these weaker storms in recent years don't strengthen as much, and it is unknown of when we will have a "REAL" (Major) hurricane again. But this is a word of warning. The Major Hurricane Drought will not last forever!!!

Member Since: May 12, 2012 Posts: 11 Comments: 2728
Quoting PalmBeachWeather:
The weather in Palm Beach county is unbelieveable... One of the distant bands from Isaac has continued since very early this morning to ride up west of the turnpike to east of I-95. It is still over us now. It's hard to imagine Iasaac is so far away. Many flooding problems here.


Still bad in Boynton/Delray Beach area - had to run an essential errand and most major streets are now flooding (clogged drains), numerous accidents on the congress corridor.

Luckily our community re-did our drainage system this summer - looks to be holding up for now. (can't imagine a few more hours of this stuff)
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Quoting divdog:
all the way in. Some seem to get in this position and jog east. Still a scenario or nw all the way. Like your opinions. Hailing from destin florida


What I am interested in seeing is not so much the next center pass but the third and fourth passes. I think they will show winds over Cat one threshold.
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Quoting jeffs713:
JB also says things to get attention.



Joe Bastardi is the biggest wishcaster of all times. That guy cannot get one thing right from this system and probably many others he has tried to forecast. That guy is a joke.
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Quoting Dsntslp:
Good idea, thanks for that. The parking lot at Northport School here has water at waist level. Kids are being taken out to parents by golf cart using another route.

I do not know if there is any flooding in the school itself yet.
this is turning out to be some weather story alright, someone else reported kids trapped inside the schools because of all the florida there, im sure its happening up and down the east coast of florida..rain just wont stop..im surprised they didnt close the schools there, like they did over here today
Member Since: August 6, 2011 Posts: 4 Comments: 39309
Quoting NSB207:
The blob over Florida is breaking away...notice the pull of this break a way system toward the North Northeast. It will become part of a system from mid-atlantic and move out of Florida.
Probably just a wobble and not a trend yet. I think
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Quoting Chicklit:


That rainbow IR makes me think of some old 60's song...'do the tighten' up'...looks like he's really trying to anyway
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so can baton rouge expect power outages? just trying to figure out what to expect over here. not as bad as Gustav, but for sure no electricity for a day or so??
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Quoting RitaEvac:


WTH yall do during strong cold fronts?


I was wondering that myself- I've lived most of my life on the eastern side of the Alleghenies, and I can't remember buses not running because of wind. Not even during nor'easters. And trust me, there is no wind on earth like a strong nor'easter storm blowing Alleghany gnats through every layer of clothes you can find to put on :) We don't know tornados, and we have very few brushes with hurricanes, but we can do snow and wind like none other.
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Quoting katharsiss:


Yeah, the folks in New Orleans are tired of it too.


Feel bad for you guys. Stay safe and hope you and yours come out okay. One hurricane in a lifetime is MORE than enough!
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Quoting RitaEvac:


It's no wonder some models have this going back west after landfall, that high over the plains supposed to shift east keeping it stuck or going west as the flow around it pushes it away



Or it gets pick up and shoots between the two highs - which is probably what Joe B is referring to.
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Massive wind field for such a compact convective structure.

Member Since: September 2, 2009 Posts: 130 Comments: 21194
847. CJ5
Quoting TallyWeather15:
What is the chance that Tallahassee will get 30 mph winds tommorow? School busses can't drive in it


Pretty good
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846. Skyepony (Mod)
Quoting tennisgirl08:


How accurate are the 12Z CMC and HWRF? Good models? The CMC has a VERY scary scenario. Stalling off MS Coast as hurricane.


CMC hasn't done great with ISAAC with ave error in nm 0hr, 24hr, 48hr & etc..
CMCI 0 72.1 125.8 195.7 297.8 488.4

HWRF is a little better..
HWRF 6.9 75.6 81.7 80.6 110.2 164.9

But those are still average 70 nm error on the 24hr forecast..
Member Since: August 10, 2005 Posts: 175 Comments: 38212
845. Caner
Quoting tennisgirl08:


How accurate are the 12Z CMC and HWRF? Good models? The CMC has a VERY scary scenario. Stalling off MS Coast as hurricane.


All of the major models are good, per se.

Some of them are better at certain scenarios than others.

All but one of the models are in agreement on a stall.

So that's a fairly safe bet.

good resource, if you want to view major models:

Link
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844. FOREX
Quoting presslord:


Let me guess...Up to New York?


He didn't specify. lmao.
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Quoting GetReal:


wobble into the deepening convection... Anyone that has been on the WU for a while has seen this many times...


Agreed but wobbles also creat a big difference in landfall give or take 100 miles
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Quoting jeffs713:

I wouldn't be concerned about that - it is being sustained by the upper-level flow caused by Isaac, combined with the lower-level inflow of moisture, also caused by Isaac.

Right now, a vast majority of the focus is on Isaac.


Yeah, I hear you. It's being fed by all sorts of Isaac "stuff," for sure, it's just also looking increasingly like it's splitting off and playing with stuff to _its_ NE. The whole thing is all connected, of course.

And I get that everybody is focused on Isaac proper, especially as he's starting to try to get organized (again, but maybe for real now.) But that thing over there is really hammering parts of FL, and it doesn't look like it's going to lighten up anytime real soon -- if it's splitting off and being influenced elsewhere, I'm concerned about it staying semi-stationary and dropping heaps of rain from any moisture source it can get.

Thanks for the response.
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NASPen starting to bus out the students.
No "official" word on base closure as yet...
Pensacola State College closed until Thursday. UWF closed also.
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Quoting bwi:
Most recent satellite pictures show the storm not looking so squashed on the east side, even expanding back east a little? Looking more symmetrical to my eyeballs.


Agreed,the centre is taking on a domed appearance

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Quoting FOREX:


Joe Bastardi is reporting that Isaac will become a CAT2 just before landfall then stall, re-emerge over the Gulf and get picked up by a trough coming Friday, taking Isaac Eastward.


wonder why he is not at accuweather anymore...
Member Since: April 23, 2011 Posts: 104 Comments: 14873
Quoting kmanislander:


The storm is between two highs that can be seen in the image below. The Atlantic high is driving it to the NW generally and the high to West is in a blocking position which should prevent any real motion to the WNW. IMO a set up like this should result in a NW track continuing .

all the way in. Some seem to get in this position and jog east. Still a scenario or nw all the way. Like your opinions. Hailing from destin florida
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Quoting Mikla:
Issac continues to dump rain on So. FL. Lake behind my house is up 2ft with more rain to come. Several areas are flooded up to mailbox height.
..omg just wont stop raining there..this is going to be a disaster if it keeps up much longer..we said it days ago.,.this storm is making weather history
Member Since: August 6, 2011 Posts: 4 Comments: 39309
Quoting NICycloneChaser:
Recon now heading straight north and will come back to the centre fro the NE.

This is the interesting bit.
I agree.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
Quoting tennisgirl08:


How accurate are the 12Z CMC and HWRF? Good models? The CMC has a VERY scary scenario. Stalling off MS Coast as hurricane.


Well the CMC is most definitely not a preferred model.
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Quoting CajunCrawfishhunter:


Ok so then why are the last 2 points on 26.1 but 86.3 and then 86.9 this would be west


wobble into the deepening convection... Anyone that has been on the WU for a while has seen this many times...
Member Since: July 4, 2005 Posts: 204 Comments: 8896
Quoting FOREX:


Joe Bastardi is reporting that Isaac will become a CAT2 just before landfall then stall, re-emerge over the Gulf and get picked up by a trough coming Friday, taking Isaac Eastward.


Let me guess...Up to New York?
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Quoting LargoFl:
Folks..IF you are flooding or see bad flooding..dont assume your local govt knows about it..phone it in and let them know whats happening ok...
Good idea, thanks for that. The parking lot at Northport School here has water at waist level. Kids are being taken out to parents by golf cart using another route.

I do not know if there is any flooding in the school itself yet.
Member Since: August 31, 2011 Posts: 0 Comments: 70
Quoting Skyepony:


They use like a 6 hr average so if it's going west for just 2 hrs now they will wait instead of change with every wobble.


Except tha each .6 wobble represents a different landfall area by a bit. Too many of them represents a pretty big change.
Member Since: September 11, 2008 Posts: 0 Comments: 153
this is what could happen with Isaac

Irene as a Cat 2...weak hurricane for a cat 4 pressure


Irene as a TS over my town Danbury, CT.. weak storm for a cat 2/3 hurricane pressure
Member Since: April 23, 2011 Posts: 104 Comments: 14873
Quoting Caner:


Link

Link


How accurate are the 12Z CMC and HWRF? Good models? The CMC has a VERY scary scenario. Stalling off MS Coast as hurricane.
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Quoting FOREX:


Joe Bastardi is reporting that Isaac will become a CAT2 just before landfall then stall, re-emerge over the Gulf and get picked up by a trough coming Friday, taking Isaac Eastward.
JB also says things to get attention.
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Quoting kmanislander:


The storm is between two highs that can be seen in the image below. The Atlantic high is driving it to the NW generally and the high to West is in a blocking position which should prevent any real motion to the WNW. IMO a set up like this should result in a NW track continuing .



It's no wonder some models have this going back west after landfall, that high over the plains supposed to shift east keeping it stuck or going west as the flow around it pushes it away

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826. Mikla
Issac continues to dump rain on So. FL. Lake behind my house is up 2ft with more rain to come. Several areas are flooded up to mailbox height.
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825. FOREX
Quoting weathermanwannabe:
Isaac is just a few clicks away on the current forecast points from where he was supposed to reach hurricane strength so this part of the forecast appears to be on track based on satt loop appearance.


Joe Bastardi is reporting that Isaac will become a CAT2 just before landfall then stall, re-emerge over the Gulf and get picked up by a trough coming Friday, taking Isaac Eastward.
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824. bwi
Most recent satellite pictures show the storm not looking so squashed on the east side, even expanding back east a little? Looking more symmetrical to my eyeballs.
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Thanks to Largo and Charmeck -- I've been kinda figuring it's one of those "nobody quite knows what to think about it at the moment" things, but what I'm seeing going on there kinda concerns me. That's some nasty over a substantial part of FL, and it looks to me like it's really more like spawn-of-Isaac than just a "band" at this point.

But again, I watch this stuff, that doesn't mean I know what I'm talking about. :P

Back to lurk mode for now, but thanks for noting it, too. Stay safe out there, y'all.
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I have to give credit to my local city and county people, months and months ago they started replacing most of those old storm drains and replacing them with much bigger ones,around my immediate area also, they doubled the size of the storm retention lake and the storm drains, and this last week before issac they had crews going all around, cleaning out the drains and sweeping the streets, making sure the drains do not clog with debris..my hats off to those workers and planners..a job well done in my view...folks in other counties and cities..if this hasnt been done..hmmm start asking them..why?..look at all the flooding etc
Member Since: August 6, 2011 Posts: 4 Comments: 39309
Quoting JustSouthofEquator:


Ice crystals in the upper atmosphere? The cirrus overcast from ISAAC?


Dunno. But it was amazing as heck. Never seen anything like that before. Really intense colors too initially. The ring is still there now but the colors are not present.
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Quoting RitaEvac:


WTH yall do during strong cold fronts?


Right.....a 30mph wind gust doesn't really change much for bus routes here...what kinda busses do yall have?
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Someone took a dump and flushed the toilet...

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Quoting keisa90:
Correct me if I am wrong, but wouldn't Isaac have to move NNW at this point to make it to NOLA?

I was just thinking the same thing
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Quoting RitaEvac:


WTH yall do during strong cold fronts?

Skip school... like TallyWeather15 is apparently hoping to do.
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Quoting tennisgirl08:


KMAN, your thoughts on track?


The storm is between two highs that can be seen in the image below. The Atlantic high is driving it to the NW generally and the high to West is in a blocking position which should prevent any real motion to the WNW. IMO a set up like this should result in a NW track continuing .

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The weather in Palm Beach county is unbelieveable... One of the distant bands from Isaac has continued since very early this morning to ride up west of the turnpike to east of I-95. It is still over us now. It's hard to imagine Iasaac is so far away. Many flooding problems here.
Member Since: October 3, 2008 Posts: 0 Comments: 5888
813. Skyepony (Mod)
Quoting TheDewd:
Link

Looks like W motion to me. Anyone care to help me understand why it's WNW or NW?


They use like a 6 hr average so if it's going west for just 2 hrs now they will wait instead of change with every wobble.
Member Since: August 10, 2005 Posts: 175 Comments: 38212
The blob over Florida is breaking away...notice the pull of this break a way system toward the North Northeast. It will become part of a system from mid-atlantic and move out of Florida.
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Quoting GetReal:


You are probably observing the fast expanding CDO, and convection mentioned earlier quickly developing on the west side of Isaac, giving the illusion of a west move.


Ok so then why are the last 2 points on 26.1 but 86.3 and then 86.9 this would be west
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L. Eye Character: Not Available
M. Eye Shape: Not Available

Isaac can't get it together.

I do however, think Isaac will become a weak Cat 1 overnight tonight.
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Quoting TallyWeather15:
What is the chance that Tallahassee will get 30 mph winds tommorow? School busses can't drive in it


WTH yall do during strong cold fronts?
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.