NHC's Tropical Weather Outlook: How Accurate are its Predictions?

By: Dr. Jeff Masters , 2:46 PM GMT on July 20, 2013

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Every Internet-savvy tropical weather enthusiast is familiar with the National Hurricane Center's Tropical Weather Outlook (TWO), which details potential threat areas that might become a tropical cyclone. (Tropical depressions, tropical storms, and hurricanes are all tropical cyclones.) The Tropical Weather Outlook is issued four times per day during hurricane season, and beginning in 2010, NHC began issuing 48-hour forecasts of the probability that specific threat areas identified in the TWO could develop into a tropical cyclone. Their Graphical Tropical Weather Outlook now color-codes each threat area depending upon how likely development is expected to be. A yellow circle is drawn for 0%, 10%, and 20% chances; orange for 30%, 40% and 50% chances, and red for 60% and higher odds. For example, the Saturday morning, July 20 TWO gave a 0% chance of development for an area of disturbed weather near the Florida Panhandle (Figure 1.)


Figure 1. NHC's Tropical Weather Outlook for 8am EDT July 20, 2013, showed an area of disturbed weather over the Gulf of Mexico being given a 0% chance of development in 48 hours. In 2012, 8% of all disturbances being given a 0% chance of development actually did develop.

How accurate is NHC's Tropical Weather Outlook?
So the big question is, how good are these forecasts? When NHC gives a 30% chance that an "Invest" will become of tropical cyclone, does this happen 30% of the time? Well, according to the 2012 National Hurricane Center Forecast Verification Report, NHC should have drawn fewer yellow circles and more orange circles and red circles during 2012, as there was a tendency to under-predict when a threat area might develop. For example (Figure 2), for the 46 forecasts where a 30% chance of development was given, 50% of the threat areas actually developed. Every disturbance that was given a 70% and higher chance of development ended up developing. This under-prediction tendency in 2012 is in contrast to the results from 2011, when the genesis forecasts were closer to the mark. For example, the 59 forecasts for a 30% chance of development resulted in a 31% "hit" rate of the storm actually developing in 2011. So far in 2013, there seems to be a tendency to under-predict again. For example, 48 hours before Tropical Storm Barry developed, NHC was carrying just a 10% chance of development.




Figure 2. NHC did predictions on new formation of a tropical depression or tropical storm (cyclogenesis) beginning in 2010. The forecasts are expressed in the Tropical Weather Outlook in 10% probability increments, and in terms of categories (“low”, “medium”, or “high”) for a tropical cyclone forming within a 48-hour period. These genesis forecasts had a low (under-forecast) bias in the Atlantic basin during 2012 (top). For example, for cases where a 30% chance of formation was given, the actual percentage of storms that formed was 50%. However, there the 2011 forecasts showed no systematic bias, and were closer to the mark (bottom.)

Expansion of the Tropical Weather Outlook to 5 Days
By August of the 2013 hurricane season, NHC is planning to begin including information about a system’s potential for development during the following five-day period. This will supplement the 48-hour probabilistic formation potential already provided in the Tropical Weather Outlook. NHC is currently developing a corresponding five-day genesis potential graphic that might also be available in 2013.

Quiet in the Atlantic
There are no other tropical cyclone threat areas in the Atlantic to discuss today. Saturday morning's 06Z run of the GFS model predicted that a tropical wave expected to move off the coast of Africa on Monday will develop later in the week. None of the other reliable models develop this wave, though the unreliable Canadian (CMC) model also suggests that the wave could develop.

Have a great weekend, everyone, and I'll be back Monday with a new post.

Jeff Masters

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Quoting 789. CybrTeddy:


Looks rather large can't wait too see how it transition to the water.
Member Since: May 25, 2012 Posts: 0 Comments: 2043
Quoting 767. Grothar:
300 hours



360 hours
So I am planning to take a boat from Fort Lauderdale to Freeport, Bahamas on July 30, returning on August 2. Wonder if I should hold off on booking for a couple days......
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24 more hours to go unit its hit water
Member Since: March 12, 2013 Posts: 138 Comments: 123246
Member Since: July 8, 2005 Posts: 259 Comments: 24682
788. JLPR2
Quoting 786. Gearsts:
Can you post that image?


Sure.

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This week is also a Good time to assess ones Hurricane Plan and Kits for any items to complete it.

Also, ready ones Evac items like papers,Pet needs etc.

What you do now, will only make a exit easier then.
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Quoting 781. JLPR2:


After seeing the GFS develop it 4 times in a row I'm pretty confident something will try to form. Since Irene I trust the GFS till 180hrs or so. I have an image saved that showed Irene hitting PR 147hrs out.

But of course nothing is certain until a trackable low forms.
Can you post that image?
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Quoting 772. CybrTeddy:
Here comes our wave. Ignore the models out beyond 144, remember how they had Hurricane Dean going north of the islands instead of trucking west like it did?


Also, here's some better images of the Atlas V launch from yesterday I took.


I hope this is not the type of wave that have us all excite and when it hits water it frizzles.
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Quoting 739. PedleyCA:
I was trying to put up a TV antennae today and I get Thunder and it starts to sprinkle. Figured it might, 20% chance. We weren't on the roof. Just adding the
unit already attached to 10' Pole to a pole that was already in concrete. We out it
on there and clamped it down and ran for it. Rained for about 45 minutes and got a good cell in there for a bit. PWS up the hill reported .08, Airport (KRAL) showed .10..... If ya want rain, wash your car, go golfing or do what I did today. You will get something.
Hey Joe, you get anything down your way? How about you Tom?


A TV Antennae? Were you wearing your high button shoes?
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I look at the GFS that far out as a joke.
Member Since: August 14, 2010 Posts: 10 Comments: 18856
781. JLPR2
Quoting 774. Relix:
One week away is still fantasy land. I don't think anything will pop up from that wave. Maybe a chantal type system.


After seeing the GFS develop it 4 times in a row I'm pretty confident something will try to form. Since Irene I trust the GFS till 180hrs or so. I have an image saved that showed Irene hitting PR 147hrs out.

But of course nothing is certain until a trackable low forms.
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100% agreed dont pay attention to the model 144+ hours as the 12z had it going into Northern Mexico and the 18z had it going to NYC. But it has been very consistent with development on a less reliable 18z run. Our wave may just be the first big time system to watch it has about 24 hours over land before it touchdown into the Atlantic. This would be devastating to PR if this was to pan out they already having record rainfall this summer.
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The weather is going to do what it's going to do
Member Since: August 14, 2010 Posts: 10 Comments: 18856
Quoting 774. Relix:
One week away is still fantasy land. I don't think anything will pop up from that wave. Maybe a chantal type system.


Patience young Jedi, given that the GFS was holy unenthusiastic about Chantal and is wanting to spin this up into a near-hurricane, that makes me believe this will have a far better chance. I think we're all just a little sore from Chantal.
Member Since: July 8, 2005 Posts: 259 Comments: 24682
Twilight zone WCOSS GFS solution a little further west


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775. Relix
On a sidenote... that wave looks massive o.0
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774. Relix
One week away is still fantasy land. I don't think anything will pop up from that wave. Maybe a chantal type system.
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Quoting 767. Grothar:
300 hours



360 hours
one place we do not wanted a hurricane is the northeast!!
Member Since: March 12, 2013 Posts: 138 Comments: 123246
Here comes our wave. Ignore the models out beyond 144, remember how they had Hurricane Dean going north of the islands instead of trucking west like it did?


Also, here's some better images of the Atlas V launch from yesterday I took.


Member Since: July 8, 2005 Posts: 259 Comments: 24682
Quoting 739. PedleyCA:
I was trying to put up a TV antennae today and I get Thunder and it starts to sprinkle. Figured it might, 20% chance. We weren't on the roof. Just adding the
unit already attached to 10' Pole to a pole that was already in concrete. We out it
on there and clamped it down and ran for it. Rained for about 45 minutes and got a good cell in there for a bit. PWS up the hill reported .08, Airport (KRAL) showed .10..... If ya want rain, wash your car, go golfing or do what I did today. You will get something.
Hey Joe, you get anything down your way? How about you Tom?


Last time I painted the house in South Florida it was just before the 2005 Hurricane season.

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Lol.TMC's (TWC') it could happen scenario at play here?.lol.

Member Since: August 14, 2010 Posts: 10 Comments: 18856
Quoting 767. Grothar:">


Tempted to hit "minus" just because I dislike that picture so much!
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300 hours



360 hours
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Tonight in my Blog I'll be posting the Moonwalk as it Aired on TV 44 years at the exact time it happened.



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Quoting 759. ncstorm:






I guess Dorian wanted to give us a welcoming present.
Member Since: August 14, 2010 Posts: 10 Comments: 18856
Quoting 753. JLPR2:


Yes it basically disintegrated after it passed PR and the rain was focused in my area with the west side of the island getting one inch or nothing. It was as if the wave wanted to specifically flood my area.





The convective mass covered all PR/VI, so it's very surprising to see little rainfall in western PR... but also in the Virgin Islands.

Looks like what happened to us in the N Leewards, also happened in the Virgin Islands, West PR and T&C islands.

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Quoting 751. CaribBoy:
GFS 18Z XD

lol
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Quoting 753. JLPR2:


Yes it basically disintegrated after it passed PR and the rain was focused in my area with the west side of the island getting one inch or nothing. It was as if the wave wanted to specifically flood my area.



nice post :)
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Quoting 732. JLPR2:


Actually much farther north than Chantal.

**More like Irene (2011) part II.
I agree looks like Irene.
Quoting 716. washingtonian115:
Here comes Hispaniola to the rescue again..
Chantal almost hit Hispañola this will be the second if it happens.I just hope it doesn`t continue for the rest of the season,Haiti continues to be in a lot of trouble after the quake.
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Quoting 736. washingtonian115:
I didn't like Irene either.She was a drama queen.


She was more than a drama queen up here -- we are still trying to recover. And we've already had lots of flooding so far here this summer, so these runs (fortunately still in the "fantasy range") showing a hit to the NE really make my stomach sink.

Of course, even a storm hitting the northern gulf can bring us more water than we are able to handle this year.
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i dont like the looks of this run it head straight to puerto rico and bigger than chantal and father north
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word of the day: snowball
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Found it!

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Quoting 721. JLPR2:
Well now, that would be very bad for my area. After the 9inches of rain that fell on Wednesday a significant rain event like a TS would be pretty serious.



Oh boy.
Member Since: April 29, 2009 Posts: 75 Comments: 15281
the only thing i care about is consistency and the GFS is showing it. long term track is a big ? considering the wave isnt even over water
Member Since: July 25, 2011 Posts: 0 Comments: 1102
753. JLPR2
Quoting 734. CaicosRetiredSailor:


After watching the severe rains you had from that Tropical Wave, I was VERY surprised that we in the Turks and Caicos got NO rain at all from it.
What was left after it hit you, did go west and dissipate.


Yes it basically disintegrated after it passed PR and the rain was focused in my area with the west side of the island getting one inch or nothing. It was as if the wave wanted to specifically flood my area.



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I like to count my chickens after they hatch
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GFS 18Z XD

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Quoting 737. MAweatherboy1:
Looks like it grazes the Carolinas, then comes for the Northeast:



Edit: Makes a pretty sharp right turn though so just a brush with the south coast of New England.


I dont see any "grazing"..NCEP has it riding the entire coast of NC in that run..
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Quoting 730. VR46L:
US Landfall





And SC looks already drenched by the time it falls.
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Looks like Irene to me.
Member Since: July 2, 2013 Posts: 0 Comments: 1523
Similar "TRACK"?
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Dorian wants to be retired, I can feel it.
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Quoting 701. GTstormChaserCaleb:
Oh my!

what in helll is that thing aproaching the island and pr ?¿?¿
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A decent hurricane snips N.C.Hey be original Dorian.
Member Since: August 14, 2010 Posts: 10 Comments: 18856
total precip up to 384 hours
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Definitely a different atmosphere than what it was like with the first few pages on this blog..

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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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