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.

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Quoting 1937. Naga5000:


That looks like an equipment failure at San Diego Airport (KSAN), right before 11 it stopped reporting Air Pressure and Wind Speed.


Between you and Brian, you guys are like forensic weather detectives. Good catch.
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1941. Thrawst
.
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Quoting 1937. Naga5000:


That looks like an equipment failure at San Diego Airport (KSAN), right before 11 it stopped reporting Air Pressure and Wind Speed.


Perhaps no temperature being recorded was interpreted as zero Celsius, or 32F!
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Quoting 1936. BaltimoreBrian:
I've assumed that wunderground gets its data from the NWS. And that whenever there's some sort of error like that it triggers their record-reporting algorithm. They happen occasionally.

According to threadex the coldest temperature in San Diego in July is 54, set on July 4 1880 and July 16 1884. And for Los Angeles downtown, 49 on July 12 1888 before there was a heat island.


Thanks for looking further into it. BTW, 'threadex' link didn't work.
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AMAZING Weather Facts

Where is the driest place on earth, you ask? Look no further than Antofagasta in Chile! The place is so dry due to receiving less than 0.1mm of rainfall per year. And if you think that's bad, some years Antofagasta doesn't receive any rainfall whatsoever!
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Quoting 1930. LAbonbon:


Kudos to you for digging deeper. Did you see that temp graph? It went from 70 to 32 in a vertical drop. Perhaps they meant 72?? Where does wu get their data (serious question)?


That looks like an equipment failure at San Diego Airport (KSAN), right before 11 it stopped reporting Air Pressure and Wind Speed.
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I've assumed that wunderground gets its data from the NWS. And that whenever there's some sort of error like that it triggers their record-reporting algorithm. They happen occasionally.

According to threadex the coldest temperature in San Diego in July is 54, set on July 4 1880 and July 16 1884. And for Los Angeles downtown, 49 on July 12 1888 before there was a heat island.
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If it makes anybody feel any better I do think this wave will develop. Time will tell. It looks very healthy right now
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Things Everyone Should Know

Switch your phone to "airplane mode" before you charge it. Your phone won't search for a signal while its charging and you'll get to full bars much more quickly.
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Quoting 1918. Envoirment:
Also, I really don't get why a lot of people are down casting the entire season based on not seeming to have anymore storms in July. Although there is the possiblity for 1-2 more given the right conditions.

The middle of August is when the activity really starts, so just be patient until then. :)
Well some people went into this July thinking we were going to see something on the lines like 05 or on a more realistic level but certainly far fetched 2008 type of thing.Climatology has done it's thing.However I'm on the look out Aug-Oct to see if the Atlantic tries to throw anything at us.2011 also had some good activity..but those were mostly from trough splits.
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Quoting 1927. WEATHERFANATIC5:
i was actually gonna take that troll person up and take his bet that we dont see any storms for the next 4 to 6 weeks, but after reading up more i am seeing alot of people are saying the same thing that he has been saying...now people are posting models that say there wont be anything until september... maybe he was a pro weather person


He said on the blog yesterday that he's in the 8th grade (or may have been 9th?).
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Quoting 1921. BaltimoreBrian:



It does to me also. Especially since it happened at 11:00 a.m. However the NWS page reports a low of 65. Which to believe....


Kudos to you for digging deeper. Did you see that temp graph? It went from 70 to 32 in a vertical drop. Perhaps they meant 72?? Where does wu get their data (serious question)?
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Quoting 1925. galvestonhurricane:


I don't have any cookies to give you.
That's okay ;).I'm cooking dinner so I don't want to spoil it.
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Afternoon thunderstorms are literally forming right over me in central west palm beach. It's not raining anywhere but my house lol.
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Quoting 1919. washingtonian115:
Well I'm still sticking to my forecast for July with little activity expected.So far the prediction has held strong.

I think August and September might be like 2010 when all the real activity started.


I don't have any cookies to give you.
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To add to what Teddy said,it doesn't matter how many systems form even if it turns out that the activity is not as numerous as the experts predicted earlier because without El Nino around it guarantees an active season (IMO between 13-15 named storms) and we have three in the books already. And it only takes one hit to do all the harm to a place so be prepared for anything that mother nature has instored.
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Quoting Camille33:
Cfs model has no storms through september 2 to note,it has 2 tropical depression that it.


Season's over then I guess..
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I'm honestly not sure how much simpler this could be made.

It's July.

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Quoting 1915. LAbonbon:


Since you posted about Cali - I saw that San Diego had a record low of 32 (wu's main page). Their all-time record low (per Wiki) is 25, set 100 yrs ago in January. Seems freakish to me that they hit 32 in July.



It does to me also. Especially since it happened at 11:00 a.m. However the NWS page reports a low of 65. Which to believe....
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a EL Nino from 1997 would be nic
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Well I'm still sticking to my forecast for July with little activity expected.So far the prediction has held strong.

I think August and September might be like 2010 when all the real activity started.
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Also, I really don't get why a lot of people are down casting the entire season based on not seeming to have anymore storms in July. Although there is the possiblity for 1-2 more given the right conditions.

The middle of August is when the activity really starts, so just be patient until then. :)
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Just give it time.
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Quoting 1904. HurricaneHunterJoe:
Moisture flowing into Soo Cal



Since you posted about Cali - I saw that San Diego had a record low of 32 (wu's main page). Their all-time record low (per Wiki) is 25, set 100 yrs ago in January. Seems freakish to me that they hit 32 in July.
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Bottom line (tropical wave)

- Threat to develop
- needs to be watched carefully
- Intensification probability into a TS 50% (my guess)
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Tomorrow is expected to be the hottest day of the year here, with temperatures as high as 33-34C. Humidity will be increasing too, which will trigger off thunderstorms later in the evening. Then everything will start to cool down, with temperatures falling back to about average later next week.

So glad this hot spell of weather isn't set to last much longer and we'll be getting some rain! :)
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Quoting 1907. CybrTeddy:
2010 didn't get their 4th named storm, Hurricane Danielle, until August 21st. That season produced 19 named storms, 12 hurricanes, and 5 major hurricanes. Stop being impatient and wait and see. Just because the GFS dropped a ghost storm doesn't mean the entire season is a bust.
your right kyle. 2010 was a la nina and conditions didnt become favorable until mid august
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Here is a good paper by Dr. Kossin for folks to read on the numerous issues, and factors, impacting hurricane genesis and track. Good discussion on pg. 3068 on African Waves and development, or problems developing, based on their relative location (north or south flank) to the African Easterly Jet........Good read as related to the current wave folks are looking at; it might not be in the most favorable position for development.

Link
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notting here to!! 78 hours..
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Quoting 1908. Camille33:
Cfs model has no storms through september 2 to note,it has 2 tropical depression that it.



Interesting for sure.
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Cfs model has no storms through september 2 to note,it has 2 tropical depression that it.
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2010 didn't get their 4th named storm, Hurricane Danielle, until August 21st. That season produced 19 named storms, 12 hurricanes, and 5 major hurricanes. Stop being impatient and wait and see. Just because the GFS dropped a ghost storm doesn't mean the entire season is a bust.
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1906. xcool
hmm active hurricane season not sure anymore
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1905. Skyepony (Mod)
Quoting 1888. CaribBoy:


Honestly I would love to see a LA NINA.

It a little early for me to hope full blown La Niña.. I'd expect a return to the cooler side of neutral, maybe La Nina conditions a bit. Favors more rain for you..
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Moisture flowing into Soo Cal

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Quoting 1899. Tazmanian:






don't be so sure on that
I tend to find that when someone over analyze the tropics when there is no storms make you think it will last as soon as you give it a decent break and come back we see doomcasting everywhere. Tropics love playing tricks on us (Chantal) lol.
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1902. LargoFl
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Quoting 1899. Tazmanian:






don't be so sure on that

Danielle didnt form until late august 2010,plenty of time till.
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Quoting 1895. floridaboy14:
you guys still expecting an active august and sepmteber?


So far yes
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Quoting 1897. Camille33:

Yes and multiple landfalls.






don't be so sure on that
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Quoting 1895. floridaboy14:
you guys still expecting an active august and sepmteber?


Yes. I'm sticking with 16-6-3
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Quoting 1895. floridaboy14:
you guys still expecting an active august and sepmteber?

Yes and multiple landfalls.
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Quoting 1895. floridaboy14:
you guys still expecting an active august and sepmteber?




at this point in time am not sure any more
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you guys still expecting an active august and sepmteber?
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Quoting 1856. Sfloridacat5:
Actually Monroe county (Key West) has a 36.7% chance of a a hurricane making landfall in the county. That's by far the greatest probability in the U.S. mainland.


I don't mean to sound arrogant, but anyone who gives you an exact percentage of a hurricane landfall is lying to you.
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Got a 5 minute deluge yesterday afternoon! Looks like more on the way. PW values are like 2.1", current temp and humidity are 80 and 52%, hoping for more rain today. Looks like some rain got dumped on the 24,000 acre fire by Idylwild CA. Thats good news....

Weather Conditions for:
Sunshine Summit, CA (SSSSD)
Elev: 3244 ft; Latitude: 33.344; Longitude: -116.732

Current time: Sun, 21 Jul 11:14 am (PDT)
Most Recent Observation: Sun, 21 Jul 11:00 am PDT (PDT)
Time Temp. Dew Relative Wind Wind Quality
Point Humidity Direction Speed Control
(PDT) (f) (f) (%) (mph)
21 Jul 11:00 am PDT 80 61 52 S 8G16 OK
21 Jul 10:50 am PDT 79 60 52 SSE 9G16 OK
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Here it is the 585 circle now.
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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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