Isaac makes its final approach towards Louisiana

By: Dr. Jeff Masters , 3:29 PM GMT on August 28, 2012

The winds and water are rising all along the coast from Louisiana to the Florida Panhandle as Tropical Storm Isaac makes its final approach. Two hurricane hunter aircraft in the storm are measuring a steadily lowering pressure and increasing winds aloft, but hurricane-force winds have not yet been observed at the surface. The 8:30 am center fix found a pressure of 976 mb, which is very low for a tropical storm. Top surface winds measured with the SFMR instrument were 70 mph, but the plane measured 102 mph at an altitude of 5,000 feet. It's more typical to see surface winds of 85 mph with a storm with these characteristics. Infrared and visible satellite loops and hurricane hunter reports from this morning have shown that Isaac has developed a 25-mile diameter eye, though the eyewall has not yet formed a full circle around the eye. Heavy thunderstorm activity is lacking on the north side, where light wind shear of 5 -10 knots is still pumping some dry air into the circulation.

Figure 1. Morning satellite image of Isaac. Note how dry air has wrapped into the west side of the storm, causing a lack of heavy thunderstorm activity.

Isaac's rains
One of the most remarkable features of Isaac has been the huge spiral band that parked itself along most of the east coast of Florida and remained there for an entire day, despite the fact the center of the storm moved 400 miles away. This rain band was amplified by a weak trough of low pressure along the East Coast, which pulled away from the coast Monday night, taking the band of heavy rain out to sea (except for a few lingering showers near West Palm Beach.) Isaac's heaviest rains fell along a swath from the east coast of Florida near West Palm Beach to the center of the state, just south of Orlando. The 2-day rainfall total of 9.03" at West Palm Beach brought their monthly rainfall total to 22.28", a new August record (old record: 20.12" in 1995.) Vero Beach's 6.48" of rain was a record for any August day. A possible tornado touched down there, damaging 20 mobile homes. In the Keys, rainfall totals as high as 7.94" (at Upper Matecumbe Key) were measured. 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 24, and two died in the Dominican Republic. The big concern in Haiti is the heavy damage that was done to crops, and the likelihood that the storm's rains will worsen the cholera epidemic that has killed over 7,000 Haitians.

Figure 2. Radar-estimated rainfall from Miami, Florida radar shows that Isaac has dumped a wide swath of 8+ inches of rain (orange colors) across the state. Rainfall amounts in excess of 20" may have fallen just west of West Palm Beach, though the highest amount reported by a rain gauge was 13.10" at Greenacres in Palm Beach County.

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 some differences in what happens after that. Isaac may scoot nearly west-northwest just inland along the coast into Texas, as predicted by the ECMWF model, or head straight inland to the northwest and into Arkansas, as predicted by the GFS model. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over much of Louisiana. It appears likely that Arkansas will see some heavy rains of up to five inches, which would help put a dent in the exceptional drought conditions there.

Intensity forecast for Isaac
Low wind shear of 10 knots or less is likely until landfall, along with very warm ocean temperatures. An analysis of upper level winds from the University of Wisconsin CIMSS shows that Isaac's upper-level outflow is the strongest we've seen, with a solid outflow channel to the south. These conditions favor continued strengthening of Isaac until landfall. However, we've observed in the past many instances of hurricanes suddenly weakening in the final 12 hours before making landfall along the Central Gulf Coast. Katrina, Gustav, Dennis, Ivan, and Rita all did so. A July 2012 paper in the Bulletin of the American Meteorological Society by Rosenfeld et al. titled, AEROSOL EFFECTS ON MICROSTRUCTURE AND INTENSITY OF TROPICAL CYCLONES, theorizes that this may happen because of the impact of small particles that get pulled into the outer circulation of hurricanes, seeding the clouds. These small particles, primarily from air pollution, serve as the seed around which water condenses, increasing the rain in the outer spiral bands. The increase in rain and heat energy at the periphery of the storm comes at the expense of the eyewall and inner core, where the winds tend to weaken. A detailed modeling study by Khain et al. (2010) of Hurricane Katrina in the final day before landfall was able to reproduce the storm's weakening only when this air pollution effect was included. This impact of small particles on hurricanes is not included in any operational hurricane model.

Figure 3. Tide gauge data from Shell Beach, located in Lake Borgne just east of New Orleans. The green line shows the storm surge. The red line is the storm tide, the height of the water above Mean Sea Level (MSL.)

Storm surge observations from Isaac
Isaac's storm surge has peaked along the west coast of Florida. As I explain in our Storm Surge Tutorial, we are most interested in the storm tide--the height above Mean Sea Level (MSL) of the tide plus the storm surge. The storm tide is the number given in NHC advisories for how much above ground level the ocean will be at the coast. The storm surge is the extra elevation of the water due to wind blowing on the water, and does not include the action of waves on top of the water, nor the tide. Tide gauges are specially constructed so that transient waves do not impact water level measurements. At Cedar Key on the West Florida coast north of Tampa, a storm surge of 3' and storm tide of 3.8' were observed early this morning. These were the highest water levels measured at any tide gauge along the Florida west coast. Higher storm surges are occurring in the Florida Panhandle. As of 9 am EDT, here were the storm surge/storm tide measurements along the Florida Panhandle:

Apalachicola, FL: 3.5' storm surge, 4' storm tide
Panama City, FL: 2.3' storm surge, 3.3' storm tide
Pensacola, FL: 1.5' storm surge, 2.5' storm tide

A storm surge of 3.5 feet was recorded at 10 am EDT at Shell Beach on the east side of New Orleans in Lake Borgne. This site will have one of the highest surge values during Isaac; a storm surge of 9.5' was measured at Shell Beach during Hurricane Gustav in 2008.

Figure 4. Category 2 Hurricane Gustav of 2008 followed a very similar path to Isaac, and brought a storm tide (the combined effect of the storm surge and tidal levels) of up to 14.5' above ground level to the east side of New Orleans. Isaac's surge may be similar, though probably a little less, than Gustav's.

Figure 5. Track of Hurricane Gustav of 2008, which followed a path very similar to that of Isaac's predicted path.

Isaac: similar to Category 2 Hurricane Gustav of 2008 in destructive power?
Isaac is a huge and slow-moving storm, with tropical storm-force winds that extend out 205 miles from the center. Isaac has cut its forward speed down from 14 mph yesterday to 10 mph today, and a large swath of the coast will be subject to high winds and a large storm surge for an usually long period of time for a hurricane--up to 24 hours. Long duration winds are much more damaging than short duration winds, and a long duration storm surge event allows damage to occur during multiple high tide cycles. The long duration storm event will also allow very high rainfall totals, resulting in greater fresh-water flooding problems than usual. As a result, I expect Isaac's to cause more damage than the typical Category 1 hurricane. The 9:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 2.3 on a scale of 0 to 6, but the destructive potential of Isaacs's storm surge was 4.1 on a scale of 0 to 6. For comparison, the storm surge destructive potential of Category 2 Hurricane Gustav of 2008 was rated at 4.2 on a scale of 0 to 6, and the wind destructive potential was 1.1--which is lower than Isaac's, even though Isaac was just a tropical storm at 9:30 am EDT. Gustav brought a storm tide (the combined height of the storm surge and high tide) of 14.5' to the east side of New Orleans, and 11' to Waveland, Mississippi. However, the destructive potential of Isaac's surge may be overrated by this analysis. Wave heights this morning from buoy and ships in Isaac have mostly been below 15', which is quite unimpressive. One ship report to the SE of the storm had a 19' wave height (thanks to meteorologist Steve Gregory for pointing this out.) With only another 12 - 18 hours over water, Isaac likely won't have time for its slowing increasing winds to build up a storm surge that will reach as high as 14', like Gustav did. The official NHC forecast of maximum storm surge height of 12' looks like a good one. The highest rainfall total observed in Gustav was 21" at Larto Lake, Louisiana, and I expect we'll exceed that for Isaac, since the storm is moving more slowly. Gustav spawned 41 tornadoes--21 in Mississippi, 11 in Louisiana, 6 in Florida, 2 in Arkansas, and 1 in Alabama. The strongest tornado was an EF2 in Evangeline Parish, Louisiana. Isaac will likely produce 10+ tornadoes. The total damage from Gustav in the U.S. was $4.5 billion (2012 dollars.) I expect Isaac's damage total will be in the $500 million - $4 billion range.

Invest 97L in the Middle Atlantic
A tropical wave (Invest 97L) is located in the Middle Atlantic, about 1250 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 40% chance of developing by Thursday morning. None of the reliable models foresee that 97L will be a threat to any land areas.

Another tropical wave that moved off the coast of Africa on Sunday is located just southwest of the Cape Verde Islands, and is moving west at 15 mph. 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 Thursday morning. The disturbance could begin to affect the Northern Lesser Antilles as early as Saturday night, though our two best models, the GFS and ECMWF, predict the center of the disturbance will pass a few hundred miles north of the islands. The disturbance could be a long-range threat to Bermuda.

Jeff Masters

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1075. JeffM
I hate the idiots that insist on walking back and forth and waving while the TWC people are giving live field reports.

People have to act like idiots when a camera is on them. See the same idiotic behavior while watching sporting events.
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ok I'm going to leave and comeback let me see what there when I comeback
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(click to enlarge)

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1072. Patrap
Member Since: July 3, 2005 Posts: 439 Comments: 137203
Quoting Bluestorm5:
Heavy flooding in Charleston, SC. Picture of people kayaking through famous mile long market in Charleston:

yep...been a wet day
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10-12 foot waves reported on the beaches in Alabama.

Those are on top of the surges reported 3-4 ft.
Member Since: August 18, 2008 Posts: 0 Comments: 628
WTNT34 KNHC 281816 RRA

100 PM CDT TUE AUG 28 2012


LOCATION...28.4N 88.7W
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1068. Jax82
Todays MODIS True color Image of Isaac!

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1067. Patrap
Quoting HrDelta:

Yeah, same thoughts. You're the fire department, help move them.

One cant get up US 1 now. They will shelter in Place in a Grand Isle Bldg most likely.
Member Since: July 3, 2005 Posts: 439 Comments: 137203
Quoting LuvsStorms:
Vernon Parish here on the western side of the state. Winds are 16mph with gusts up to 32mph. Didnt think we would feel anything yet.

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AL, 11, 2012082818, , BEST, 0, 237N, 434W, 30, 1008, TD, 34, NEQ, 0, 0, 0, 0, 1014, 160, 100, 0, 0, L, 0, , 0, 0, ELEVEN, S,

Jose, that you? I see ATCF has td11 and floater page has eleven.
Member Since: March 16, 2012 Posts: 127 Comments: 7984
Heavy flooding in Charleston, SC. Picture of people kayaking through famous mile long market in Charleston:

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Wobblin' west now.....don't want to be on the east side of that.
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Isaac's surge coming into Waveland MS. Live mobile video:
Live Mobile Video, Waveland MS
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Quoting 7544:
we have td 11 ???

Yes it got renumbered.
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Quoting LostTomorrows:

Is that a west wobble? Could mean more time over warm waters. Bad news, with the way Isaac is pulling out all of the stops now.

Looks like it
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Quoting LuvsStorms:
Vernon Parish here on the western side of the state. Winds are 16mph with gusts up to 32mph. Didnt think we would feel anything yet.

Ya, his entirety is pretty large...we've already felt some windshift in texas with a gyst every now and then.
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1058. Patrap
Well The Saints Helmet will be on if we have to venture out Now.

Son has a Secure AB Hard hat as well.
Member Since: July 3, 2005 Posts: 439 Comments: 137203
1057. HrDelta
Quoting freeroam:
Fire Chief Chaisson of Grand Isle on WWL now says some are staying on the island because they are elderly! WTF.
Why not help move them, I don't get that.

Yeah, same thoughts. You're the fire department, help move them.
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Quoting RitaEvac:

Is that a west wobble? Could mean more time over warm waters. Bad news, with the way Isaac is pulling out all of the stops now.
Member Since: August 26, 2008 Posts: 0 Comments: 743
ok I leave and come back and find Isaac is soon to landfall 97L looks to be TD11 by 5pm the evening and we should have 98L soon if not now

hmm some strong wording from NHC about 97L but low on precentage(50%)

PRE-98L up to 20%
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Lake Pontchartrain is a-rising at the west end. (The dotted black line is for same-day 2005.)


Also, Mississippi Sound at Grand Pass:

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which huuricane is this!? katrinaa or issac
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1052. 7544
we have td 11 ???
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1051. guygee
Quoting GetReal:
There maybe something to that theory of the northern circulation interacting with the friction of the coast, causing an increase in convection as Isaac makes landfall. The radar is indicating an increase in coverage and intensity of the convection as it is being crushed up against the coast of MS and LA. Those stronger winds found in the upper levels of Isaac could very well be force to the surface.
Definitely getting a lot more convection from speed convergence as the inner band starts coming ashore.

Coastal Convergence - Meteorological Physical Background
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TD 11's convection has absorbed the llc, shear looks to be dying down enough for continued strengthening into a TS.
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Quoting Patrap:
Nearest Analog I can think of,..from 65, my first eyewall dance.

This image shows Hurricane Betsy in the Gulf of Mexico in September of 1965 as taken by the TIROS VIII weather satellite.

Date 1965 September 4
Source spac0010, NOAA In Space Collection
Author NOAA Photo Library

Mine to! 1 Scary night.
Member Since: June 2, 2008 Posts: 0 Comments: 225
Fire Chief Chaisson of Grand Isle on WWL now says some are staying on the island because they are elderly! WTF.
Why not help move them, I don't get that.
Member Since: September 1, 2007 Posts: 0 Comments: 249
Mobile,AL Springhill Area Power Out ALREADY. Of course, it went out Sunday for 3 hours in the sunshine. I think I'll take my business elsewhere.
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1046. Patrap
Well we just found out what that was.. neighbor had Put a Plywood Patch 4 by 8 on a Slate pitched roof some time back,"Rental Property' and it went well..up and into our back yard.

He obviously pocketed the Gustav insurance,and well did dat.

Member Since: July 3, 2005 Posts: 439 Comments: 137203
Quoting Patrap:
Land friction will begin to spin down those spinners at some point

On radar it looks like it is moving due west
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So much for RIPing 97L which is now TD 11 lol!
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Vernon Parish here on the western side of the state. Winds are 16mph with gusts up to 32mph. Didnt think we would feel anything yet.
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hold up we mite have a potential another storm in the GOM by late next week?
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Quoting nola70119:

On the North or South Shore?

Bucktown, south shore. Wind has been steady since morning, about 25 to 35 MPH, just guessing. Whitecaps on the lake, but still well below where I have sen it before during similar events. We do have a long way to go, however.
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Doc's on!!
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Just saw this tweet from the weather channel...

@twc_hurricane: 24 pumps in Orleans Parish pumping more than 50,000 cubic feet per second. #Isaac @wdsu

This sounds like it's going to come down to simple math...can the pumps pump more than the rain/surge puts in?
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At Providence Hospital in Mobile. Very windy, and waiting for this squall line to come through. Looks to be moving quickly this way. Everyone stay safe!
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1035. HrDelta
Quoting WalkingInTheSun:

What I think is odd, however, is the way some of the storms this season have been wettest on the left side (as looking in direction the storm is going)...when the "dirty" side of the storm is usually on the right. I wonder what the deal is on that.

The deal is that it entrained dry air and is going to the very dry air mass that has caused the drought.
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97L TD 11 ???? wow
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Dr. Masters wrote in his post that

"This impact of small particles on hurricanes"--ones that explains the weakening of Hurricane Katrina right before U.S. landfall--"is not included in any operational hurricane model", the small particles "primarily" being "from air pollution".

This caught my attention for two reasons.
--One, if air pollution can influence hurricane strength, what else is it doing?
--Two, WHY do the models NOT include this critical piece of intensity information in their forecasts?

Dr. Jeff Masters, I've been reading your blog for probably two years, and I'm still learning more and more about hurricanes, how they form, what influences their track, etc. My interest in them is quite high (I usually go to your site first before anywhere else just to see if you've updated your blog). Your blogs are great! I'm glad The Weather Channel values your insight on things.
Thank you for being so detailed in your blogs.
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Another band about to move into New Orleans... after that you're pretty much into the main part of the storm.
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This thing suddenly looks scary. I'll feel a lot better when it's inland!
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@twc_hurricane: 24 pumps in Orleans Parish pumping more than 50,000 cubic feet per second.
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I just want all of you in the path of Isaac to know that your are in my thoughts and prayers. I appreciate any updates from the area as I have family in NOLA. Good luck everyone!
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those are "storm trails" (fig. 2).. i must emphasize that.
it is not a radar image, that is precipitation data from storms passing over, accumulated.
the precipitation events over east Florida were directly a result of the amount of precipitable moisture shedding off the main circulation, as the overall moisture field of Isaac approaching that area was enormous.
passage to be read with an extremely irritated tone...
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Thanks a lot Shepard Smith.."If you put this storm over D.C the impacts could be felt all the way up to N.Y"
Member Since: August 14, 2010 Posts: 12 Comments: 20737
Quoting hydrus:
Godspeed to all..keep us here posted when possible..:)
I sure will do hydrus
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Dr. Masters (r) co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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