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

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

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


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

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down to 1004mb on here!! its that a tropical d or tropical storm!!
Member Since: December 31, 1969 Posts: Comments:

Likely to form into a hurricane.
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The consistency of the GFS is rather disturbing, the storm and the ridge.

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1087. will40
dropped it at 204 hrs last run be interesting to see how far it keeps it this run
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I see two lows on here 1009 mb and 1011 mb
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1085. Thrawst
7th model run in a row GFS has developed this wave. Will be watched.
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1084. Gearsts
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Quoting 1017. opal92nwf:
These are entertaining...
img src="">

Even that it not expected much storm during the peak of the season.
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1082. will40
1003 at 102 hrs still a lil further south than last run it looks like
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I think I got the right computer modeles now.
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1080. nigel20
Quoting Tropicsweatherpr:


Hi nigel. That is correct.

Thanks much!
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Quoting 1073. Astrometeor:


Nah, didn't see it, thanks for pointing that out. I really hope to see a tornado one of these days, one of those stay out in farmland 'naders. Was close to an EF3 in 4th grade at school, wasn't allowed to go out and see it though. That was a bummer...


There was a tornado early last year. I was at work, and it touched down literally across the street from where I live; I found that out later. Needless to say, I was pissed.
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Quoting 468. GTstormChaserCaleb:
Why am I more inclined to believe the GFS more than any model, I guess since Debby it has earned my respect. We will see but I do believe we will get Dorian.

GFS is the best about the cyclogenesis as the Europeen model are not well about it however they are both the most reliable ever.
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1077. will40
Quoting 1076. hurricanes2018:
I cant not find the 00 run



Link
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Quoting 1069. Tropicsweatherpr:


That is the 18z run not the 00z.
I cant not find the 00 run
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55w is > sst.
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78 hrs

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Quoting 1065. LAbonbon:


Not sure if you saw it, but there was a blogger on earlier regarding a tornado S of Fredericton, New Brunswick (close to the Maine border). He figured it was an EF-1 as well.


Nah, didn't see it, thanks for pointing that out. I really hope to see a tornado one of these days, one of those stay out in farmland 'naders. Was close to an EF3 in 4th grade at school, wasn't allowed to go out and see it though. That was a bummer...
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Quoting 1070. nigel20:

That would take it south of the SAL. Right?


Hi nigel. That is correct.
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300 hours watch out!!
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1070. nigel20
Quoting CybrTeddy:
Develops it much quicker on the 00z, shows a 1008mb low by 48 hours, also further south by a few degrees.

That would take it south of the SAL. Right?
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Quoting 1067. hurricanes2018:


That is the 18z run not the 00z.
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1068. will40
1005 at 72 hrs
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Develops it much quicker on the 00z, shows a 1008mb low by 48 hours, also further south by a few degrees.
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Quoting 1059. Astrometeor:


Thanks! I'm more attracted to severe weather here in the states than the tropics.

EF-1 I heard, mother was reading about the tornado off a page from Yahoo.


Not sure if you saw it, but there was a blogger on earlier regarding a tornado S of Fredericton, New Brunswick (close to the Maine border). He figured it was an EF-1 as well.
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Here we go!!!
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Quoting 1061. wxchaser97:
GFS will likely develop the wave again.

It just formed already,now the question how strong where it goes.
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1062. Grothar
Look what is behind the first wave



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GFS will likely develop the wave again.
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Quoting 1047. LAbonbon:


News networks are reporting a tornado hit Ursuline College in Ohio this morning.

BTW, love that you post this daily.


Thanks! I'm more attracted to severe weather here in the states than the tropics.

EF-1 I heard, mother was reading about the tornado off a page from Yahoo.
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Quoting 1025. sunlinepr:


The good about it is that although it places the system over PR... and that's 178 hrs from now.... The probabilities are quite low that such trayectory happens...
⚡ i know
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Hello everyone 0z GFS at 36 hours

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1056. Gearsts
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Quoting 1053. Camille33:

Thas is 18z wrong run
its that old run
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At 36 hours

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Quoting 1049. hurricanes2018:
120 hours!

Thas is 18z wrong run
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Quoting 1049. hurricanes2018:
120 hours!
That is the 18z run. 
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0z GFS full-res out 21 hours 

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Quoting 1042. whitewabit:


Should not have been posted !!

EDIT .. Please remove it !
I did say "don't be surprised if 1023 get shot down, lol". I knew it was matters of time.
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120 hours!
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Quoting 1040. Astrometeor:


News networks are reporting a tornado hit Ursuline College in Ohio this morning.

BTW, love that you post this daily.
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Quoting 1038. Hurricanes305:

Thanks bro! I was getting tired when I though about it maybe I'll try another one later in the season. This got me pumped for the late night runs.

I am a little late buy I vote for ecmwf.
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36 hours 1009 mb..
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Alright folks now that we have got that out of our system the 0z run has started

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Quoting 1036. Bluestorm5:
1023 is brilliant, though!


Should not have been posted !!

EDIT .. Please remove it !
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Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather

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