Isaac approaching hurricane strength

By: Angela Fritz , 9:14 PM GMT on August 27, 2012

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Isaac is walking the line of hurricane status this afternoon after a hurricane hunter mission investigated the storm and found winds of 80+ mph with the SFMR instrument, which looks down at the surface from the plane and estimates what wind speeds are. This instrument has a notoriously rough time in doing so when there's heavy rain, and since the strongest winds were recorded coinciding with the strongest rain, you can imagine that this region of high wind speed could be suspect. The hurricane hunter mission is still in the storm, so I imagine they will issue a special update if needed. Currently the best estimate of wind speed within the storm is 70 mph. Isaac's pressure has been dropping today as well and is now 981 mb. Isaac is moving northwest at 12 mph--no change since this morning. Satellite loops show that Isaac remains large, though asymmetric, with most of the strong thunderstorm activity on the west and southwest side. Isaac's southeast side continues to struggle with dry air and wind shear, which could help to moderate Isaac's intensity as it approaches the coast.

An oil platform in the northern Gulf of Mexico is reporting sustained winds from the north-northeast at 40 mph this afternoon. A buoy west of Tampa, Florida is recording sustained winds around 30 mph, and platforms south of Louisiana are recording winds from 35-40 mph. The widespread heavy rain of yesterday has lightened up in Florida, but a strong line of thunderstorms in one of Isaac's outer bands is training northward along and offshore of the east coast of Florida, affecting everyone from Miami to Jacksonville.

This afternoon the AP reported that Isaac's death toll in Haiti jumped to 19, which puts Isaac's total death count at 21. It appears most of the deaths in Haiti were due to collapsing structures.


Figure 1. Satellite imagery of Tropical Storm Isaac around 3pm EDT on Monday.

Track forecast:
Models seem to be coming into better agreement today on where Isaac will make landfall, closing in on Louisiana and New Orleans as most likely landfall point. The ECMWF, HWRF, and UKMET all suggest New Orleans as the landfall location. The GFS is only slightly west of that. The GFDL is the farthest west, predicting landfall near the Louisiana-Texas border. Landfall timing remains Tuesday night. Beyond landfall, Isaac is expected to move north toward the Midwest through the rest of this week, however, models are showing that the system will likely slow down around landfall time, prolonging impacts like surge and inland flooding.

Intensity forecast:
The closer Isaac gets to landfall without having formed an eye, the better it is for intensity at landfall. Isaac has strengthened only modestly in the past 24 hours, and is still struggling with a less-than-conducive atmospheric environment. The HWRF remains on the high end of the intensity spectrum, suggesting Isaac will be a weak category 2 upon landfall. Other models suggest it will be a strong category 1, but the difference is splitting hairs. The National Hurricane Center's official forecast is for Isaac to continue strengthening over the next day, reaching category 2 at landfall.


Figure 2. Tide gauge data from St. Petersburg, Florida. The green line shows the storm surge. As Isaac's counterclockwise winds blew offshore this morning, water levels feel two feet at St. Petersburg. The winds switched to onshore this afternoon as the center of Isaac moved more to the northwest, bringing a storm surge of two feet to the city.

Storm surge observations from Isaac
This morning, as Isaac's counter-clockwise winds brought offshore winds to the Tampa/St. Petersburg area, ocean waters fell two feet along the coast. This afternoon, winds have shifted to blow onshore, and a two foot storm surge has been observed at Naples, Fort Meyers, and St. Petersburg on the west coast of Florida. Water levels have also begun to rise along the coast of Louisiana, with a storm surge of 1.5 feet already occurring at Shell Beach on the east side of New Orleans in Lake Borgne.

Angela and Jeff

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Quoting GetReal:



Has anyone on the blog currently ever seen an ATL TS, in the month of August or Sept, with a 981mb pressure reading that wasn't a strong Cat 1, or low end Cat 2???

I know that I haven't until today, after 40 years of observing ATL storms.


Yes it is rather fascinating, however anything is possible with mother nature. I think this year more than ever we have seen that.
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You can see what happened to the trough that was supposed to pick up Isaac, very weak and already lifting out. GOES-EAST CONUS VISIBLE
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I'm still holding onto my thought that the USA will be impacted by a major hurricane this season. Ernesto missed it by four days. Isaac will likely miss it due to dry air. It seems we're getting closer though (unfortunately) as we head farther into the season.
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Quoting forecaster1:
Heres a crazy thought. There is no where for this storm to go but north east slightly then north all the way to Atlanta then east out into the Atlantic? the only other option is to stall then go east with the next boundry....... My 2 Cents still calling a north route.
..if it sat for a day in arkansas for a day soaking both it and oklahoma..i think the folks up there would appreciate all the rain huh
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Quoting GTcooliebai:
I agree that pesky ULL that was over Eastern Cuba the other day moved into the Yucatan and is still there. Which is a good thing, the weaker the better. Now the rain and surge will still be bad especially if he slows to a crawl near the coast.


Actually I think she's about to get rid of that dry air. It does look a bit elongated tho.

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Quoting blsealevel:




It's a pretty nice consensus.
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Quoting mojofearless:


Are you personally hosting a lot of New Orleans visitors? Or are you referencing our Katrina evacuees your town so graciously opened its arms to in 2005? Hard to read your tone online.

Yours cordially from New Orleans

PS: Traffic looks fine.


Man, y'all come to Houston.. bring some gumbo and you can stay at my house!
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Quoting LargoFl:
GFS at 336 hours..storm #3
if this verifies,florida could see some of this
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OH NO NOT KIRK! NOT PENSACOLA. :(
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Quoting MAweatherboy1:
He's in huge trouble now... Watch the last frame or two... convection has weakened significantly in the N/NE quadrant, worse than it already was... I think he's swallowed a ton of dry air... Anything more than a minimal hurricane seems unlikely to me now. Feel free to bash me, but that's what I'm seeing.



Funny HH just found 65+kt winds n the NE Quad....

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GFS at 336 hours..storm #3
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Heres a crazy thought. There is no where for this storm to go but north east slightly then north all the way to Atlanta then east out into the Atlantic? the only other option is to stall then go east with the next boundry....... My 2 Cents still calling a north route.
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Quoting MAweatherboy1:
Landfall. Boom.

Now that is a Major Hurricane that the GFS is showing right there.
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Rains have started in South Walton County, FL
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is he still moving NW?
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Quoting mojofearless:


Are you personally hosting a lot of New Orleans visitors? Or are you referencing our Katrina evacuees your town so graciously opened its arms to in 2005? Hard to read your tone online.

Yours cordially from New Orleans

PS: Traffic looks fine.




No, was just more curious than anything. Just wondering in light of K what the evac response has been. I have memories of the NOLA visitors but also a few weeks later sitting in the traffic from Rita.
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Product: Air Force Vortex Message (URNT12 KNHC)
Transmitted: 27th day of the month at 22:25Z
Aircraft: Air Force Aircraft (Last 3 digits of the tail number are 306)
Storm Number & Year: 09L in 2012
Storm Name: Isaac (flight in the North Atlantic basin)
Mission Number: 27
Observation Number: 18
A. Time of Center Fix: 27th day of the month at 22:11:50Z
B. Center Fix Coordinates: 26°31'N 86°19'W (26.5167N 86.3167W)
B. Center Fix Location: 257 miles (414 km) to the WSW (247°) from Tampa, FL, USA.
C. Minimum Height at Standard Level: 1,272m (4,173ft) at 850mb
D. Estimated (by SFMR or visually) Maximum Surface Wind: 57kts (~ 65.6mph)
E. Location of the Estimated Maximum Surface Wind: 15 nautical miles (17 statute miles) to the WNW (303°) of center fix
F. Maximum Flight Level Wind Inbound: From 29° at 47kts (From the NNE at ~ 54.1mph)
G. Location of Maximum Flight Level Wind Inbound: 15 nautical miles (17 statute miles) to the WNW (303°) of center fix
H. Minimum Sea Level Pressure: 982mb (29.00 inHg)
I. Maximum Flight Level Temp & Pressure Altitude Outside Eye: 19°C (66°F) at a pressure alt. of 1,523m (4,997ft)
J. Maximum Flight Level Temp & Pressure Altitude Inside Eye: 22°C (72°F) at a pressure alt. of 1,538m (5,046ft)
K. Dewpoint Temp & Sea Surface Temp (collected at same location as temp inside eye): Not Available
L. Eye Character: Open in the east
M. Eye Shape: Elliptical (oval shaped)

M. Orientation of Major Axis in Elliptical Eye: 60° to 240° (ENE to WSW)
M. Length of Major Axis in Elliptical Eye: 30 nautical miles (35 statute miles)
M. Length of Minor Axis in Elliptical Eye: 20 nautical miles (23 statute miles)
N. Fix Determined By: Penetration, Radar, 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: 0.5 nautical miles
Remarks Section:
Maximum Flight Level Wind: 74kts (~ 85.2mph) in the southwest quadrant at 19:52:30Z
Maximum Flight Level Wind Outbound: 51kts (~ 58.7mph) in the east quadrant at 22:17:40Z
Maximum Flight Level Temp: 22°C (72°F) which was observed 6 nautical miles to the NW/NNW (326°) from the flight level center
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Quoting MAweatherboy1:
He's in huge trouble now... Watch the last frame or two... convection has weakened significantly in the N/NE quadrant, worse than it already was... I think he's swallowed a ton of dry air... Anything more than a minimal hurricane seems unlikely to me now. Feel free to bash me, but that's what I'm seeing.

Remember how Nate look.
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Quoting Thing342:
But the surface winds associated with them are barley above TS force.

The National Hurricane Center used flight level winds to go with an initial intensity of 55 knots two nights ago even though surface winds were lower.
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wow GFS showing Explosive Intensification with Kirk. didnt show that for isaac. long range but a realistic possibility
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Landfall. Boom.



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300 hrs. I think Kirk has the best chance to become this season's first Major Hurricane, which is kind of amazing we haven't gotten one yet.

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I see that the new HWRF is coming out even as I re-typed that post.

It is over-doing it by 12mb, about the same as last time, so again even though the initialization is bad, the TREND is actually correct.

Let's see how this pans out.
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Why does it look like it's going NNW?
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Quoting Thing342:
But the surface winds associated with them are barley above TS force.


Barely above TS force? you meant Hurricane I suppose. Next update he will be a hurricane.
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300- vicious RI, we're in fantasy land now but still interesting to watch.

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Quoting MAweatherboy1:
He's in huge trouble now... Watch the last frame or two... convection has weakened significantly in the N/NE quadrant, worse than it already was... I think he's swallowed a ton of dry air... Anything more than a minimal hurricane seems unlikely to me now. Feel free to bash me, but that's what I'm seeing.

I agree that pesky ULL that was over Eastern Cuba the other day moved into the Yucatan and is still there. Which is a good thing, the weaker the better. Now the rain and surge will still be bad especially if he slows to a crawl near the coast.
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276:

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Quoting FSUstormnut:


does the Euro develop this to?

Yup, it develops it but sends it out to sea.
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WTC said 24+Hrs of TS conditions over the warning areas
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Quoting TropicalAnalystwx13:

Flight level winds are running 70-80 knots. That's sufficient for an upgrade.
But the surface winds associated with them are barley above TS force.
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Quoting Fishinnfever05:
Man there is a lot of activity from mobile tonight. I am here off of cottage hill!

Do you. Think we will really get that much rain?


I have seen various forecasts from 10"-22". But i guess time will tell
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We probably have a closed eye now.

It went from dry slot to a hot tower over the former dry slot within about a 1 hour time frame.


Crazy storm has been deepening about 1mb per hour all day long, so at that rate it could shed another 24 to 30 or more mb before landfall.

If you correct the HWRF by 11mb to fix the bad initialization, you get 957mb at land fall.

If you subtract one millibar per hour between now and the official forecast landfall, you'd get 961mb at lower plaquemines and about 940mb at NOLA, since the model actually says it will keep intensifying for a few hours after passing Plaquemines. So basically split the difference on that, and you'd still maybe have 950mb at NOLA coordinates, and this ends up in the same ballpark as the model itself.

Overall, HWRF may be doing a good job on intensity, it just started out a tad too strong.


GFDL is about 3 to 6 mb too high, so we need to subtract that from the results, giving 960 to 963mb as it approaches landfall on the other side of the state.


Of course, those model runs are about to update, so we'll see if they corrected their mistakes and had better initializations this time around.

Anyway, reality has not been too far behind HWRF's rate of intensification once you correct for the bad inititialization, indeed, reality has actually beaten the average rate of intensification of the HWRF...
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18z GFS 228 hrs.

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Quoting hurrtracker1994:


There isn't any indication from recon that we are there yet. I would agree we are close to a hurricane, but we are not there yet.

Flight level winds are running 70-80 knots and Dvorak intensity estimates from SAB and TAFB are T4.5/77 knots. That's sufficient for an upgrade.
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He's in huge trouble now... Watch the last frame or two... convection has weakened significantly in the N/NE quadrant, worse than it already was... I think he's swallowed a ton of dry air... Anything more than a minimal hurricane seems unlikely to me now. Feel free to bash me, but that's what I'm seeing.

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Quoting washingtonian115:
I don't think Isaac will go through RI.

So get that out of your heads.

I completely agree with that, only a steady strengthening as there are still some factors that prevent RI.
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Quoting wxchaser97:

Kirk could be pretty bad and that wave definitely needs to be watched. GFS also shows Isaac bringing me some rain which would be good.


does the Euro develop this to?
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i just seen a chevy truck loaded down with A/C units headed to LA. hopefully they will stay cool
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357. HCW
CBS news said that the NE quad of isaac was the most dangerous part.. Did they bother to look at a Sat image ? Major fail
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New convection starting to wrap around the west/southern part of the eyewall working itself around.
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Quoting houstonstormguy:
What is the traffic like getting out of NOLA right now?
Is there any panic to leave town? I live in Houston, should I prepare for lots of out of town visitors to Htown?


Are you personally hosting a lot of New Orleans visitors? Or are you referencing our Katrina evacuees your town so graciously opened its arms to in 2005? Hard to read your tone online.

Yours cordially from New Orleans

PS: Traffic looks fine.
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Quoting hurrtracker1994:


There isn't any indication from recon that we are there yet. I would agree we are close to a hurricane, but we are not there yet.



Has anyone on the blog currently ever seen an ATL TS, in the month of August or Sept, with a 981mb pressure reading that wasn't a strong Cat 1, or low end Cat 2???

I know that I haven't until today, after 40 years of observing ATL storms.
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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.

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