Can someone explain the "probability" percentages?

[Persepone]

When you look at the "probability" percentages of whether hurricane center will pass within 65 natutical miles of a given location, the percentages are very low--even though the places ultimately get "nailed."

Even allowing for the fact that the winds affect areas 100+ miles from the center, the percentages still seem very low...

What am I not understanding here?

[gulfcoaster53]

You have to look at the time frame of the probabilities. For example, the center is likely to pass within only a few miles of Venice or Tampa, but not within the time frames being analyzed in the current tables. As the storm gets closer to FL, the percentages will rise dramatically.

[Josephine96]

Thanks Gulf.. I was kinda wondering the same thing

[quickychick]

The strike probs come out looking something like this:

CHANCES OF CENTER OF THE HURRICANE PASSING WITHIN 65 NAUTICAL MILES
OF LISTED LOCATIONS THROUGH 8AM EDT MON SEP 13 2004

LOCATION A B C D E LOCATION A B C D E

18.3N 78.0W 47 X X X 47 MARATHON FL X X 7 11 18
19.2N 79.0W 28 4 1 X 33 MIAMI FL X X 3 12 15
20.5N 80.2W 1 20 5 1 27 W PALM BEACH FL X X 1 11 12
MTCA 183N 738W 3 X X X 3 FT PIERCE FL X X X 10 10
MKJP 179N 768W 64 X X X 64 COCOA BEACH FL X X X 8 8
MKJS 185N 779W 47 X X X 47 DAYTONA BEACH FL X X X 6 6
MWCG 193N 814W 1 14 5 2 22 JACKSONVILLE FL X X X 3 3
MUGM 200N 751W 6 2 X X 8 KEY WEST FL X X 7 12 19
MUCM 214N 779W 1 16 3 1 21 MARCO ISLAND FL X X 2 13 15
MUCF 221N 805W X 7 15 2 24 FT MYERS FL X X 1 13 14
MUSN 216N 826W X 1 14 5 20 VENICE FL X X X 13 13
MUHA 230N 824W X X 12 8 20 TAMPA FL X X X 10 10
MUAN 219N 850W X X 2 10 12 CEDAR KEY FL X X X 7 7
MMCZ 205N 869W X X X 4 4 ST MARKS FL X X X 3 3

COLUMN DEFINITION PROBABILITIES IN PERCENT
A IS PROBABILITY FROM NOW TO 8AM SAT
FOLLOWING ARE ADDITIONAL PROBABILITIES
B FROM 8AM SAT TO 8PM SAT
C FROM 8PM SAT TO 8AM SUN
D FROM 8AM SUN TO 8AM MON
E IS TOTAL PROBABILITY FROM NOW TO 8AM MON
X MEANS LESS THAN ONE PERCENT


So, like gulfcoaster53 said, you have to look at the time frame for which the percentage is given.

Edit: ok, I knew that would come out looking crappy. So let's look at
this line:

KEY WEST FL X X 7 12 19

That means less than 1% chance from now to 8 AM Saturday
Less than 1% chance from 8 AM Saturday to 8 PM Saturday
7% chance from 8 PM Saturday to 8 AM Sunday
12% chance from 8 AM Sunday to 8 AM Monday
19% total chance from now to 8 AM Monday that the center will pass, etc.

[Persepone]

Thanks to all for the explanation.

But why 19% for Key West? Is that still a time issue and Ivan is not expected to get there until later in the week, at which time the percentage will go up?

[Jupiter]

Why would column D be 12 for Key west and 11 for West Palm...I've been wondering but didn't ask because I figured the answer was obvious and I was the only one who didn't know...I understand the time thing, but I would think the chance of it hitting Key West by the time stated inthat column would be significantly greater than the chance it would hit West Palm in that time frame.
Thanks

[quickychick]

Thanks to all for the explanation.

But why 19% for Key West? Is that still a time issue and Ivan is not expected to get there until later in the week, at which time the percentage will go up?

Yes, it's a time issue--it's simply the sum of all the percentages in the A, B, C, and D columns (0+0+7+12). Not many people agree with the mathematical soundness of summing them up like that though.

So yes, it's just saying that between now and Monday, Key West's total chances of having the center pass near it will have increased by (supposedly) that amount.

Jupiter, it's not the chance that it will hit that city, it's the chance that the center of the storm will pass within 65 nautical miles of that city. Big difference. If the eye of Ivan ran north alongside the west coast of Florida, you would see the percentages for individual cities increase and then decrease as it passed by (kind of like a wave of increased percentages moving northward along the coast).

[Jupiter]

I understand that, what I don't understand is if it went along the west coast or anywhere in the gom then it would not pass within 65nm of west palm, that is why I didn't understand the chances being the almost the same for key west as they were for west palm. Key west is more south and more likely to be within the 65nm range within that time period, west palm seems less likely to be within that 65nm range and not very likely within that time frame since it would take longer to get to west palm....
I'm sorry I guess I just still am a little confused...maybe when they go all the way to monday I will probably understand because there will be a significant difference between places on the gom side and places on the ec...
I am just learning, I have been only following hurricane sites for about 1 1/2 weeks.
(had to buy an external mouse for my laptop...at first when my finger started to hurt from moving it on the track pad to much I would change fingers, then I changed and used all the fingers on my left hand, now they all hurt except my thumbs so I finally gave up and bought a regular usb mouse)