Disorganized Leslie headed towards Bermuda

By: Dr. Jeff Masters , 4:18 PM GMT on September 03, 2012

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Tropical Storm Leslie continues to struggle with moderately high wind shear of 15 - 20 knots, due to strong upper-level winds out of the northwest. The shear is keeping heavy thunderstorms confined to the southeast quadrant of the storm. These thunderstorms are as far removed from the center as we've so far with Leslie, as seen on satellite loops. According to the latest SHIPS model forecast, the shear is expected to stay moderately high through Tuesday night, then drop to the low category, 5 - 10 knots, by Wednesday night. At that time, Leslie will be over warm ocean waters of 29°C, and the reduction in shear and warm waters should allow Leslie to intensify into at least a Category 1 hurricane by Friday, as predicted by most of the intensity forecast models. Intensification to a stronger storm may be hampered by its slow motion, which will cause Leslie to churn up cool water from the depths that will slow intensification. Once Leslie begins moving more quickly on Saturday, this effect will diminish, and Leslie could be at Category 2 strength on Saturday, as predicted by the HWRF and LGEM models. Steering currents for Leslie are expected to be weak on Tuesday - Friday, as Leslie gets stuck between two upper level lows. The latest guidance from our top computer continues to show Leslie making a very close pass by Bermuda on Saturday, and that island can expect a 3-day period of rough weather Friday through Sunday. Leslie will stay stuck in a weak steering current environment until a strong trough of low pressure approaches the U.S. East Coast on Saturday. This trough should be strong enough to pull Leslie quickly to the north on Saturday and Sunday, and Leslie may be close enough to the coast that the storm will make landfall in Canada on Monday, September 10. None of the reliable models have shown that a direct hit on New England will occur, but we can't rule that possibility out yet. The most likely long-term fate of Leslie will be for it to miss land entirely and brush by the coasts of Nova Scotia and Newfoundland, but any forecast of what a tropical cyclone might do a full seven days in advance is pretty speculative. Regardless, Leslie will bring an extended period of high waves to Bermuda, the U.S. East Coast, and Nova Scotia and Newfoundland this week. These waves will be capable of causing significant beach erosion and dangerous rip currents.


Figure 1. Morning satellite image of Tropical Storm Leslie. The low-level circulation center is fully exposed to view, thanks to strong northwest winds creating 15 - 20 knots of wind shear.

Invest 99L in the Central Atlantic
A small extratropical low pressure system that got cut off from the jet stream and is now spinning away in the Central Atlantic, near 26°N 42°W, (Invest 99L), is headed west at 10 mph, and has developed a small amount of heavy thunderstorm activity. This storm is not a threat to any land areas, and in their 8 am EDT Tropical Weather Outlook on Monday, NHC gave 99L a 30% chance of developing into a tropical cyclone by Wednesday morning.

Jeff Masters

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1372. MahFL
Quoting TomballTXPride:
A vigorous spin off the coast of SE Florida. If that can close off, could we get a depression out of that later this afternoon as the low transitions down from the upper layers?


No chance at all.
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i am watching an area of disturbed weather,at the back of the tropical wave in the eatl,sw of the cape verde islands. the area is within an elongated area of low pressure within the monsoon trough, near 10n 29.5w. The 850mb vorticity is quite strong. There is good divegence and convergence. although the area is under 20knots of nw shear.the shear is forecast to drop to 5-10knot range the next few days. although none of the global models, at the moment do not develop this area, it is still an area that needs watching,especially there is some form of cyclonic turning within the vicinity.
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Here's an easy system.

1, Start with 1015mb.

2, Find the approximate area covered by each full Millibar of pressure in square kilometers, but not overlapping stronger(lower) pressure values. i.e. the area of each amorphous "donut" of pressure lines around the CoC.

3, For each partition, subtract forecasted landfall intensity in pressure from 1015.

4, Square the result of the pressure subtraction for each partition and multiply by that partition's size.

5, Add all of the partitions.

6, Do the same for historical storms.

7, Compare the results of the forecast to actual values of other real hurricanes.


This would solve both the size and intensity issues by combining them in a multiplicative index directly related to size and pressure.

The initial values would be very large, but could be scaled down and formatted to manageable numbers by simply dropping the last N digits.

Then you would compare analog landfalls in the public broadcast.

This would give an exact, scientific value to the instantaneous severity of the forecast landfall, rather than "guesswork" on vague notions of size and wind speed.

In this system, a 940mb ring would be given 33% more points than a 930mb ring of the same size.

The score would be expressed in "scientific" units of (mb^2)*(km^2), which has an absolute meaning, rather than vague concepts, and is similar to the IKE value, except it represents an instantaneous severity.


ACE is truly useless for forecasting, and is only useful for ranking a storm after the fact.

Systems like HSI or the one I just made up on the spot, would be excellent as forecasting tools, because they can be predicted within certain margins of error, and they can be related to instantaneous destructive potential, rather than cumulative effects over long times (which truly are meaningless in most cases since most ACE is aquired out over the open water where it effects almost nothing..)
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Quoting GetReal:






It appears that the steering currents are going to take the remnants of Isaac out into the east central GOM...
My guess he'll get sheared apart by the same trough that picks up Leslie.
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Quoting CloudGatherer:


You're absolutely wrong about that.

For starters, a bigger tropical cyclone is far more likely to strike a particular spot on the coastline. If a forecast had Michael coming ashore on the Atlantic seaboard in a week, I wouldn't pay much attention - I'd wait until it got really close. If it showed Leslie a week out, I would - because with a storm the size of Leslie, even if the precise track is way off I'd still be likely to be impacted.

But also, as Isaac showed, 'intensity' is misleading. There are peak winds. Those matter. There's storm surge. That matters too. There're rainfall totals.

The 'maximum potentially severity at their location' is not the only thing that counts. If I get five inches of rain, but they get twenty inches upstream, it matters. If the storm is huge, and breaks a levy dozens of miles away, it can still flood my property - even if I'm lightly hit. If it takes out power over three hundred miles of coastline, it's going to take a hell of a lot longer to get my own lights turned back on than if the effects are relatively concentrated.

Right now, Saffir Simpson tells us about peak winds. It tells us almost nothing about rainfall, and relatively little about storm surge. And it's totally silent on breadth of impact. That's not its fault. It was developed by a structural engineer to provide useful guidance about structural integrity during severe winds - particularly in the third world, where many structures are not designed to withstand sustained winds of hurricane magnitude. As a narrow tool, it's actually quite useful. The trouble is that it's been broadly applied to tell people how much they have to fear from a hurricane. And since the maximum wind loads applied to a given structure are just one component of that broader question, it's often failed spectacularly in that role.


You are so far off base in your response, I don't even know where to begin. I did not say in any way, shape, or form that SS is the ultimate answer. What I did say, and will say again, is that *local* conditions are what ordinary people care about. Yes, the potential for a hit is greater if the storm is larger. Yes, there can be downstream flooding in some areas.

The problem with HSI is that it is yet another obscure, somewhat arbitrary, index that has no meaning for ordinary people. Just look at the words of Impact Weather in their overview of the HSI

Uses for HSI:
* Scheduling post-storm resources
* Objective guidance in implementation of a phased hurricane plan
* A way to quantify offshore wave development potential

This is stuff for professionals, not for convincing individuals to evacuate.
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Quoting bluesydeacon:
Everyone please note...


What in the world wide web are you trying to pull?
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1365. WunderAlertBot (Admin)
JeffMasters has created a new entry.
1364. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 33283
1363. 7544
interesting to see how the ull close to se fla will effect the remains of issasc when he gets in the gulf will it pull him more south n east or will they meet ?
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1362. Grothar
Quoting 7544:
ex issac ?maybe a hint where he will go

Link


Nice.
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Quoting PuntaGordaPete:


No argument about the meaning of HSI. However, local storm surge, rainfall, total duration, maximum wind, etc. are the items that go into a personal decision to prepare and/or evacuate. All of those items are already forecast without resorting to another index. Widespread damage highlighted by HSI potential may be interesting, but it does not drive evacuations. Actually, quite the opposite. It makes evacuation more difficult. Likewise, the potential for widespread power loss does not really drive anyone to evacuate.

Your point appears to be one of finding a scheme to help *individuals* make better decisions in advance of a storm. If so, concentrate on *individual* factors and forget about the billions of dollars in total damage.

One of the more common reactions quoted over the past week has been, "We didn't think it would be this bad." Even though the forecasts called for up to 20 inches of rain over a large area, many folks were apparently basing their expectations on past experience rather than current forecasts.


Most of the world isn't made up of Wundergrounders.

When people are deciding how to prepare for a storm, they don't consult the SLOSH models, or click refresh on the AHPS rainfall map. They turn on the television, and look at the graphic.

You're absolutely right that they reason based on past expectations. What you're missing is how attached most Americans are to the five categories. Storms are complicated; categories are simple. So if they've lived through a number of hurricanes, they'll think back to those storms. If they were fine through a few CAT 1s, got a little flooded in a CAT 2, and got hammered by a CAT 3, that will shape their expectations. They're likely to evacuate for a CAT 3, but not a CAT 1. And that holds true irrespective of the other information flooding in.

In the post-Isaac interviews I saw and read, I heard people say over and over: "We didn't expect this from a Category 1." But actually, the forecasts over the last 48 hours were spot-on - even, truth be told, a little worse than what most actually experienced. People focused on the number, not on the details. And - even more to the point - there's a widespread reaction on the Gulf Coast blaming the NHC for putting the storm intensity too low. Instead of thinking "Golly, there's more to storms than Saffir-Simpson Categories," a surprising percentage of the public is instead complaining, "Wow, that was nothing like previous Category 1s I've lived through, so it must have been a 2 or a 3."

If the NHC is going to put out a single number for a storm, and people are - foolishly, we all agree - going to base their decisions on that number, it should start using a number that's a whole lot more predictive of the experience on the ground.
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1360. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 33283
se.drought.over.now
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1358. GetReal






It appears that the steering currents are going to take the remnants of Isaac out into the east central GOM...
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1357. hydrus
If this were to pan out, it could be serious..Link
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1356. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 33283
Quoting TomballTXPride:

Bingo.

Amazing the lows some will go.

Sick.


What's gong on?
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1353. 7544
ex issac ?maybe a hint where he will go

Link
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Quoting TomballTXPride:
Still think that ULL can become Nadine folks!!


Member Since: September 10, 2007 Posts: 0 Comments: 10577
1351. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 33283
Quoting ncstorm:
remember in the earlier models runs that the Euro was showing a depression forming off the SE coast of florida and interacting with leslie. Remember the NHC said it was one of the players that could influence Leslie's track..of course the Euro dropped it but that dosent mean anything when it comes to the model runs..



visible loop showing the spin


meh.

Combined with the ULL, this might produce a western weakness in the Bermuda High, allowing Leslie to make a left turn after all.

Really needs watching for that reason more than anything at present time.
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...HEAVY RAINFALL POSSIBLE TODAY ALONG THE EAST COAST...
...FLOOD WARNING EAST CENTRAL PALM BEACH COUNTY CONTINUES...
...STRONG TO SEVERE THUNDERSTORMS POSSIBLE THIS AFTERNOON...

THIS HAZARDOUS WEATHER OUTLOOK IS FOR SOUTH FLORIDA.

.DAY ONE...TODAY AND TONIGHT

FLOODING: HEAVY RAINFALL THIS AFTERNOON OVER THE EAST COAST AREAS
COULD LEAD TO ADDITIONAL FLOODING CONCERNS DUE TO ALREADY
SATURATED GROUNDS FROM LAST WEEK.

THUNDERSTORMS: SCATTERED THUNDERSTORMS ARE EXPECTED MAINLY IN THE
INTERIOR AND EAST PORTIONS OF SOUTH FLORIDA TODAY. THE MAIN IMPACTS
FROM THESE STORMS WILL BE FREQUENT LIGHTNING STRIKES, LOCALLY
HEAVY RAIN AND HAIL.

WINDS: WIND GUSTS UP TO 45 TO 60 MPH WILL BE POSSIBLE WITH THE
STRONGEST STORMS THAT DEVELOP.

HAIL: DIME OR NICKEL SIZE HAIL WILL BE POSSIBLE WITH THE
STRONGEST STORMS.

.DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY

SCATTERED THUNDERSTORMS AFFECTING PRIMARILY THE INTERIOR AND EAST
CAN BE EXPECTED. THERE WILL ALSO BE AN INCREASE IN THE RISK OF RIP
CURRENTS ALONG THE EAST COAST BEACHES OF SOUTH FLORIDA THROUGH THE
WEEK AS SWELLS FROM TROPICAL STORM LESLIE MAY AFFECT THE ATLANTIC
COASTAL WATERS.
Member Since: September 10, 2007 Posts: 0 Comments: 10577
Quoting TomballTXPride:

Only 1 comment from you, RTS? LOL

For your information RTS, your comments reset because you began you first blog.

And a good blog it is King!!


Honestly, I mainly only did that as a means to save my posts for later reference.

If anyone wants to read it, they're welcome to do so, obviously.
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1346. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 33283
And the upward trend in the forecast intensity begins with Michael.

FORECAST POSITIONS AND MAX WINDS

INIT 04/1500Z 27.0N 43.5W 35 KT 40 MPH
12H 05/0000Z 27.7N 44.0W 35 KT 40 MPH
24H 05/1200Z 28.5N 44.2W 35 KT 40 MPH
36H 06/0000Z 29.0N 44.0W 35 KT 40 MPH
48H 06/1200Z 29.5N 43.5W 35 KT 40 MPH
72H 07/1200Z 30.5N 43.4W 40 KT 45 MPH
96H 08/1200Z 31.5N 43.5W 40 KT 45 MPH
120H 09/1200Z 33.0N 44.5W 35 KT 40 MPH
Member Since: July 6, 2010 Posts: 108 Comments: 30245
1344. ncstorm
remember in the earlier models runs that the Euro was showing a depression forming off the SE coast of florida and interacting with leslie. Remember the NHC said it was one of the players that could influence Leslie's track..of course the Euro dropped it but that dosent mean anything when it comes to the model runs..



visible loop showing the spin
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1342. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 33283
Quoting RTSplayer:



The HSI affects maximum severity at your own location due to the amount of storm surge, wave setup, duration of winds and rains, etc.

It's not just about what's happening 100 or 200 miles away, it's about how long the storm has to build waves and surge (due to size,) how long YOU will be under the CDO as the storm passes (due to size,) and how big an area it will impact (due to size).

Guess what? Destroying something 100 or 200 miles away CAN AND WILL effect you: power lines and other utilities, dams and levees upstream or downstream, port or oil infrastructure, etc. All of these things are effected just as much by size as peak wind speed. The more damage is done "somewhere else" the longer it takes to fix stuff even near you, or the longer it takes emergency workers to rescue you...

So it works anyway.



No argument about the meaning of HSI. However, local storm surge, rainfall, total duration, maximum wind, etc. are the items that go into a personal decision to prepare and/or evacuate. All of those items are already forecast without resorting to another index. Widespread damage highlighted by HSI potential may be interesting, but it does not drive evacuations. Actually, quite the opposite. It makes evacuation more difficult. Likewise, the potential for widespread power loss does not really drive anyone to evacuate.

Your point appears to be one of finding a scheme to help *individuals* make better decisions in advance of a storm. If so, concentrate on *individual* factors and forget about the billions of dollars in total damage.

One of the more common reactions quoted over the past week has been, "We didn't think it would be this bad." Even though the forecasts called for up to 20 inches of rain over a large area, many folks were apparently basing their expectations on past experience rather than current forecasts.
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1340. yqt1001
Must say, I've seen worse looking tropical storms.

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1338. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 33283
TROPICAL STORM LESLIE DISCUSSION NUMBER 21
NWS NATIONAL HURRICANE CENTER MIAMI FL AL122012
1100 AM AST TUE SEP 04 2012

DEEP CONVECTION HAS DECREASED NEAR THE CENTER OF LESLIE OVER THE
PAST FEW HOURS. ALTHOUGH THERE IS STILL EVIDENCE OF ABOUT 20 KT OF
NORTHWESTERLY SHEAR...THE CONVECTIVE CLOUD STRUCTURE NOW MORE
RESEMBLES A CURVED BAND PATTERN. RECENT MICROWAVE DATA AND VISIBLE
SATELLITE IMAGES INDICATE THAT THE SHEAR HAS CONTINUED TO CAUSE THE
CIRCULATION TO BE VERTICALLY TILTED AND THE LOW-LEVEL CENTER IS
DISPLACED ABOUT 30 N MI TO THE WEST OF THE MID-LEVEL CENTER. THE
INITIAL WIND SPEED IS HELD AT 55 KT...IN AGREEMENT WITH DVORAK
CI-NUMBERS FROM TAFB AND SAB...BUT THIS COULD BE GENEROUS. LITTLE
CHANGE IN STRENGTH IS FORECAST FOR THE NEXT 36 HOURS DUE TO THE
POOR VERTICAL ALIGNMENT OF LESLIE AND THE FORECAST SHEAR...WHICH IS
EXPECTED TO REMAIN HIGH DURING THAT TIME. IN A COUPLE OF DAYS...THE
SHEAR IS EXPECTED TO LESSEN AND THE NUMERICAL MODELS SHOW A MUCH
MORE FAVORABLE UPPER-LEVEL WIND PATTERN FOR LESLIE TO GAIN
STRENGTH. THEREFORE...THE OFFICIAL FORECAST REMAINS IN LINE WITH
THE PREVIOUS FORECAST AND IS BETWEEN LGEM AND HWRF GUIDANCE.

LESLIE IS MOVING VERY SLOWLY NORTHWARD AT ABOUT 3 KT AND IS LOCATED
IN WEAK STEERING CURRENTS. THE STEERING IS EXPECTED TO REMAIN WEAK
FOR THE NEXT COUPLE OF DAYS...THEREFORE LESLIE IS FORECAST TO
CONTINUE DRIFTING NORTHWARD DURING THAT TIME. BY THE END OF THE
WEEK...THE MODELS ARE IN FAIR AGREEMENT IN SHOWING A MID-LEVEL
RIDGE BUILDING TO THE NORTHEAST OF LESLIE. THIS SHOULD CAUSE THE
CYCLONE TO FIRST TURN NORTHWESTWARD AND THEN ACCELERATE
NORTH-NORTHWESTWARD IN 4 TO 5 DAYS AS IT MOVES ALONG THE WESTERN
PERIPHERY OF THE RIDGE. THE OFFICIAL FORECAST IS SIMILAR TO THE
PREVIOUS ONE AND CLOSE TO THE CONSENSUS AIDS...TV15 AND TVCA.

DUE TO THE LARGE SIZE OF LESLIE AND ITS EXPECTED SLOW MOTION...
LARGE SWELLS PROPAGATING AWAY FROM THE TROPICAL CYCLONE WILL
CONTINUE TO AFFECT BERMUDA AND NEARLY ALL OF THE U.S. EAST
COAST FOR MUCH OF THIS WEEK.

FORECAST POSITIONS AND MAX WINDS

INIT 04/1500Z 25.0N 62.5W 55 KT 65 MPH
12H 05/0000Z 25.5N 62.5W 55 KT 65 MPH
24H 05/1200Z 26.0N 62.5W 55 KT 65 MPH
36H 06/0000Z 26.5N 62.5W 55 KT 65 MPH
48H 06/1200Z 26.8N 62.6W 60 KT 70 MPH
72H 07/1200Z 27.5N 63.3W 70 KT 80 MPH
96H 08/1200Z 29.2N 64.6W 80 KT 90 MPH
120H 09/1200Z 33.0N 65.5W 90 KT 105 MPH

$$
FORECASTER CANGIALOSI/ZELINSKY
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1335. hydrus
Quoting yqt1001:
THE DEPRESSION IS NOW TROPICAL STORM
MICHAEL...THE 13TH NAMED STORM OF THE 2012 SEASON. ONLY 2005 AND
2011 HAD THE 13TH NAMED STORM FORM EARLIER THAN 4 SEPTEMBER.
Good thing The approach of El-Nino slowed things down a bit ...Lol
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1334. LargoFl
lots of flood warnings in alabama today..........BULLETIN - EAS ACTIVATION REQUESTED
FLASH FLOOD WARNING
NATIONAL WEATHER SERVICE BIRMINGHAM AL
923 AM CDT TUE SEP 4 2012

THE NATIONAL WEATHER SERVICE IN BIRMINGHAM HAS EXTENDED THE

* FLASH FLOOD WARNING FOR...
LOWNDES COUNTY IN SOUTH CENTRAL ALABAMA...
SOUTHEASTERN DALLAS COUNTY IN SOUTH CENTRAL ALABAMA...

* UNTIL 1230 PM CDT

* AT 921 AM CDT...THUNDERSTORMS WITH VERY HEAVY RAINFALL CONTINUED
TO MOVE SOUTHEAST ACROSS THE AREA. THIS RAIN WILL BE FALLING ON
AREAS ALREADY EXPERIENCING FLOODING FROM RAINFALL EARLIER THIS
MORNING. ADDITIONAL RAINFALL WILL CAUSE FLOODING TO CONTINUE OR
POSSIBLY WORSEN DURING THE NEXT ONE TO TWO HOURS.

* RUNOFF FROM THIS EXCESSIVE RAINFALL WILL CAUSE FLASH FLOODING TO
OCCUR. SOME LOCATIONS THAT WILL EXPERIENCE FLOODING INCLUDE...FORT
DEPOSIT...HAYNEVILLE...MOSSES...BEECHWOOD...GORDON VILLE...LOGAN AND
LOWNDES COUNTY AIRPORT.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

DO NOT DRIVE YOUR VEHICLE INTO AREAS WHERE THE WATER COVERS THE
ROADWAY. THE WATER DEPTH MAY BE TOO GREAT TO ALLOW YOUR CAR TO CROSS
SAFELY.
Member Since: August 6, 2011 Posts: 4 Comments: 33283
1332. 7544
Quoting LargoFl:
boy you folks down there just cannot catch a break huh
or more

lol thats for today then they have to deal with more rain from ex issac by friday if he heads sse
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Quoting CloudGatherer:


You're absolutely wrong about that.

For starters, a bigger tropical cyclone is far more likely to strike a particular spot on the coastline. If a forecast had Michael coming ashore on the Atlantic seaboard in a week, I wouldn't pay much attention - I'd wait until it got really close. If it showed Leslie a week out, I would - because with a storm the size of Leslie, even if the precise track is way off I'd still be likely to be impacted.

But also, as Isaac showed, 'intensity' is misleading. There are peak winds. Those matter. There's storm surge. That matters too. There're rainfall totals.

The 'maximum potentially severity at their location' is not the only thing that counts. If I get five inches of rain, but they get twenty inches upstream, it matters. If the storm is huge, and breaks a levy dozens of miles away, it can still flood my property - even if I'm lightly hit. If it takes out power over three hundred miles of coastline, it's going to take a hell of a lot longer to get my own lights turned back on than if the effects are relatively concentrated.

Right now, Saffir Simpson tells us about peak winds. It tells us almost nothing about rainfall, and relatively little about storm surge. And it's totally silent on breadth of impact. That's not its fault. It was developed by a structural engineer to provide useful guidance about structural integrity during severe winds - particularly in the third world, where many structures are not designed to withstand sustained winds of hurricane magnitude. As a narrow tool, it's actually quite useful. The trouble is that it's been broadly applied to tell people how much they have to fear from a hurricane. And since the maximum wind loads applied to a given structure are just one component of that broader question, it's often failed spectacularly in that role.



Bravo.

Almost the same exact valid arguments as me.

Thank God someone else sees this.
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1330. yqt1001
THE DEPRESSION IS NOW TROPICAL STORM
MICHAEL...THE 13TH NAMED STORM OF THE 2012 SEASON. ONLY 2005 AND
2011 HAD THE 13TH NAMED STORM FORM EARLIER THAN 4 SEPTEMBER.
Member Since: Posts: Comments:
1328. LargoFl
Quoting SFLWeatherman:
boy you folks down there just cannot catch a break huh
Member Since: August 6, 2011 Posts: 4 Comments: 33283
Tropical Storm LESLIE NESDIS Satellite | NDBC Obs | Storm Archive
...DISORGANIZED LESLIE DRIFTING NORTHWARD...
11:00 AM AST Tue Sep 4
Location: 25.0°N 62.5°W
Moving: N at 3 mph
Min pressure: 994 mb
Max sustained: 65 mph
Member Since: May 23, 2012 Posts: 0 Comments: 4207
Quoting PuntaGordaPete:


The HSI is close to useless for ordinary people. It is a measure of financial impact for the insurance industry and a measure of assistance needed for government and non-government agencies. Ordinary folks, those who need to prepare and evacuate, don't really care if someone else 200 miles away is also impacted. They care about maximum potential severity at their location.


You're absolutely wrong about that.

For starters, a bigger tropical cyclone is far more likely to strike a particular spot on the coastline. If a forecast had Michael coming ashore on the Atlantic seaboard in a week, I wouldn't pay much attention - I'd wait until it got really close. If it showed Leslie a week out, I would - because with a storm the size of Leslie, even if the precise track is way off I'd still be likely to be impacted.

But also, as Isaac showed, 'intensity' is misleading. There are peak winds. Those matter. There's storm surge. That matters too. There're rainfall totals.

The 'maximum potentially severity at their location' is not the only thing that counts. If I get five inches of rain, but they get twenty inches upstream, it matters. If the storm is huge, and breaks a levy dozens of miles away, it can still flood my property - even if I'm lightly hit. If it takes out power over three hundred miles of coastline, it's going to take a hell of a lot longer to get my own lights turned back on than if the effects are relatively concentrated.

Right now, Saffir Simpson tells us about peak winds. It tells us almost nothing about rainfall, and relatively little about storm surge. And it's totally silent on breadth of impact. That's not its fault. It was developed by a structural engineer to provide useful guidance about structural integrity during severe winds - particularly in the third world, where many structures are not designed to withstand sustained winds of hurricane magnitude. As a narrow tool, it's actually quite useful. The trouble is that it's been broadly applied to tell people how much they have to fear from a hurricane. And since the maximum wind loads applied to a given structure are just one component of that broader question, it's often failed spectacularly in that role.
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Member Since: May 23, 2012 Posts: 0 Comments: 4207
Quoting PuntaGordaPete:


The HSI is close to useless for ordinary people. It is a measure of financial impact for the insurance industry and a measure of assistance needed for government and non-government agencies. Ordinary folks, those who need to prepare and evacuate, don't really care if someone else 200 miles away is also impacted. They care about maximum potential severity at their location.



The HSI affects maximum severity at your own location due to the amount of storm surge, wave setup, duration of winds and rains, etc.

It's not just about what's happening 100 or 200 miles away, it's about how long the storm has to build waves and surge (due to size,) how long YOU will be under the CDO as the storm passes (due to size,) and how big an area it will impact (due to size).

Guess what? Destroying something 100 or 200 miles away CAN AND WILL effect you: power lines and other utilities, dams and levees upstream or downstream, port or oil infrastructure, etc. All of these things are effected just as much by size as peak wind speed. The more damage is done "somewhere else" the longer it takes to fix stuff even near you, or the longer it takes emergency workers to rescue you...

So it works anyway.

This is why Isaac did so much damage in some locations, and smashed inland flooding records on some creeks and bayous in S. Tangipahoa and S. Livingston parishes: a combination of near-record rainfall plus above average storm surge reflected by it's very large size, although slow movement isn't accounted for in any system, size still effects rainfall totals and rainfall's wide spread impacts, since it takes longer for a large storm to move past given the same forward speed. Ten inches of rain over a few square miles might not hurt YOU, but ten inches of rain over half the state probably will hurt YOU or someone near you...

The HSI did (or would have done) a very good job of showing just how much worse Isaac was than it's mere 80mph official wind speed implied.

It really did have surge impacts in isolated locations which were typical of category 2 or 3 hurricanes, and it had far more rainfall impacts both locally and regionally than any category 3 in anyone's memory, or for that matter, any U.S. TC in anyone's memory except possibly Allison...
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We have eight more names between the most active month of the hurricane season that usually breeds 4-6 named storms, a slightly less active month that usually breeds 2-4 named storms, and a quiet month that usually breads 1-2 named storms.
Member Since: July 6, 2010 Posts: 108 Comments: 30245

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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.