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 1529. Tropicsweatherpr:
Pouch 12L is about to emerge into the water. Look at the high moisture content.


Pouch 11 could be thinning out the SAL for what's to come for the season.JMO.
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1541. ackee
Quoting 1524. thunderstromsmike:
later today they are having a march rally for george zimmerman....
I must say I look forward for each of your post its quite entertaining to me
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1540. Grothar
Quoting 1509. kingcane:
been lurking on this blog for a few years, Grothar it seems that many of the bloggers respect your perspective. There are so many bloggers that admittedly I can get lost in the dialogue. Would you be willing to share your top ten bloggers so I can really key in on their input. I know that seems like an odd request, but it seems like the post change like the tides as the different GFS models come out. Using the context clues, Grothar, Levi, Cyberteddy, washingtoniam, neapolitan, and pottery seemed to be the frequent posters.


I certainly appreciate that. However, I do not think I would be able to be impartial enough. There are many bloggers on here with vast knowledge in different areas who contribute enormously. I am sure you will able to discern between the bloggers who are really good information and the others. I am usually the comedy relief, but when I make a statement about weather, it is based on fact and observation. Just read everybody and enjoy the blog. We all contribute something.
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Quoting 1529. Tropicsweatherpr:
Pouch 12L is about to emerge into the water. Look at the high moisture content.




Yep this is going to be large if it develops that why I don't think SAL will be a big deal plenty of moisture to work with.

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Link
Per the gfs this has a very excellent shot to become a ts near 45 - 55w...then perhaps weaken as it gets to the Bahamas but still remain a very well defined vort.
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Quoting 1534. stormpetrol:


Nice looking TW , probably the best of the season so far. Has that look to it too.


Stormpetrol what does the conditions look like in the path ahead of it ?
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Quoting 1517. GTstormChaserCaleb:
There was a TS and TD that year. Could you imagine a season with no activity? The year of "No Storms."


for us Bloggers that would be awful if we had a year of NO activity lol....we would have to find other things to talk about.
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Quoting 1520. VR46L:


I always maintain its no coincidence that there are more storms since Satellite ...also sub tropical storm only were recently named and recognised ...
I remember when sub-tropical storms were named.Some people had a fit.lol.
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Nice looking TW , probably the best of the season so far. Has that look to it too.
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Quoting 1509. kingcane:
been lurking on this blog for a few years, Grothar it seems that many of the bloggers respect your perspective. There are so many bloggers that admittedly I can get lost in the dialogue. Would you be willing to share your top ten bloggers so I can really key in on their input. I know that seems like an odd request, but it seems like the post change like the tides as the different GFS models come out. Using the context clues, Grothar, Levi, Cyberteddy, washingtoniam, neapolitan, and pottery seemed to be the frequent posters.


To be honest, most of us are all very good with sharing information so there is no real "best blogger." If you were to go the guys I personally respect the most, I'd honestly say Levi32, Stormchaser2007, Grothar, Pottery, hydrus, Skyepony, Drakeon, TropicalAnalysiswx13, hurricane23 and MiamiHurricane09 are your best bet simply because of both seniority and knowledge of resources to track cyclones with.
Member Since: July 8, 2005 Posts: 259 Comments: 24972
1530. VR46L
Quoting 1516. thunderstromsmike:
does anyone have any insight on how the easter ilsand heads play a role in weather?


Probably as much of a role as a groundhog does !
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Pouch 12L is about to emerge into the water. Look at the high moisture content.


Member Since: April 29, 2009 Posts: 75 Comments: 15663
1528. hydrus
I wanted to post this so some folks can see what strange weather we have been having in this part of the world....By the way, we are already soaked..


For Middle TN..

A VERY ODD PATTERN IS IN STORE FOR THE MID-STATE REGION OVER THE
NEXT 7 DAYS. SUMMERTIME IS HERE...BUT IT SURE ISN`T ACTING LIKE
IT. TEMPERATURES AND DEW POINTS ARE LOOKING LIKE THEY WILL BE
SEASONAL...BUT THE PRECIPITATION THAT IS EXPECTED OVER THE NEXT 5-7
DAYS SURE ISN`T.


SHOWERS AND A FEW THUNDERSTORMS BEGAN SEVERAL HOURS AGO AND CONTINUE
AT FORECAST TIME. THOUGH WEAKENING DUE TO A VERY DIFFUSE UPPER
LEVEL PATTERN...PLENTY OF BOUNDARY LAYER MOISTURE AND A FEW
MID-LEVEL SHORTWAVES HAVE SPARKED OUR OVERNIGHT ACTIVITY. THIS IS
JUST THE BEGINNING THOUGH. WHILE WE MAY SEE A LULL DURING THE
REMAINDER OF THE MORNING HOURS...PRECIP COVERAGE ON THE WHOLE WILL
INCREASE OVER THE NEXT 24-48 HOURS AS MULTIPLE SHORTWAVES TRAVERSE
THE OHIO AND TENNESSEE VALLEYS. FORTUNATELY...WITH SOUTHWEST
SURFACE FLOW AND NO SURFACE LOWS TO DEAL WITH...MOST OF THE
CONVECTION WE WILL EXPERIENCE THROUGH TUESDAY SHOULD REMAIN BELOW
SEVERE CRITERIA. I WOULD NOT BE SURPRISED TO SEE A SIGNIFICANT
WEATHER ALERT OR TWO ANY OF THE NEXT 3 AFTERNOONS...BUT WITH TYPICAL
SUMMERTIME WARM AIR ALOFT...THE ONLY THING STORMS WILL HAVE TO FEED
OFF OF WILL BE THE HEAT OF THE DAY AND LOTS OF BOUNDARY LAYER
MOISTURE. WE WILL ALSO NEED TO REMAIN VIGILANT TO THE LOCALIZED
FLASH FLOOD POTENTIAL AS PW VALUES OF 2 PLUS INCHES THROUGH MONDAY
COULD RESULT IN VERY EFFICIENT RAINFALL RATES WITH ANY STORMS THAT
MAY BE EXPERIENCED.


WEDNESDAY LOOKS LIKE A TRANSITION DAY FOR TENNESSEE AS MEDIUM RANGE
GUIDANCE IS SUGGESTING A TROUGH WILL AMPLIFY OVER THE EASTERN HALF
OF THE U.S. AND PUT US INTO A NORTHWEST FLOW. IN FACT...WE COULD
SEE SOME SLIGHTLY DRIER AIR PUSH INTO THE REGION FOR THURSDAY AND
FRIDAY...DROPPING DEW POINTS BACK INTO THE LOW 60S. HAVE BACKED OFF
POPS BOTH THURSDAY AND FRIDAY BECAUSE OF THIS.

OUR UNUSUAL SUMMER PATTERN COULD VERY WELL CONTINUE INTO NEXT
WEEKEND. RETURN FLOW STARTS ON FRIDAY AND A DECENT SHORTWAVE TROUGH
COULD BE HEADED OUR WAY FOR SATURDAY. YES...THIS MEANS A SURFACE
LOW COULD BRING ENOUGH WIND SHEAR INTO THE STATE TO BE CONCERNED
ABOUT SEVERE WEATHER. HOW ABOUT THAT FOR WEIRD?
HAVE GONE SLIGHTLY
ABOVE GUIDANCE FOR POPS FRIDAY NIGHT AND SATURDAY AS LONG RANGE
GUIDANCE IS ABOUT 12 HOUR APART ON ARRIVAL...BUT BOTH ARE NOW
SHOWING THIS FEATURE. OBVIOUSLY I WILL REFRAIN FROM PUTTING THIS
IN THE HAZARDOUS WEATHER OUTLOOK FOR NOW...BUT KEEP THIS IN MIND
AS WE GO THROUGH THE UPCOMING WEEK.

UNGER
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1526. VR46L
Quoting 1514. hydrus:
Once in a while, a small hurricane like Michael will form without any of the models picking it up. I have actually seen this quite a few times.


Yes , also shows the models are not the be all and end all ... they dont catch everything !
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Quoting 1515. Tropicsweatherpr:


In other words the models are not a gospel but only guidance. History tells that things can form without model support but that is the minority of the time.
Chantal didn't have model support either as a wave until we had a LLC.So we had to use our eyes and knowledge to predict it's next move.Since it stayed on a southern route away from SAL it was able to develop.If this wave is to develop it has to take a southern route in order to survive.The reason why the storm was dropped last model run was because of a northern bias.
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Quoting 1514. hydrus:
Once in a while, a small hurricane like Michael will form without any of the models picking it up. I have actually seen this quite a few times.
Hydrus, do you remember what the models were like for Charley, I have always been curious about that. I do remember News Channel 8 in Tampa would always show their VIPER model.
Member Since: June 30, 2013 Posts: 12 Comments: 9235
were about 1  to 1 1/2 hours away from the initialization of the 12Z GFS, everyone seems to be on edge at this point
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1520. VR46L
Quoting 1513. washingtonian115:
Their were probably storms that formed in the northern atlantic like Grace (2009) for a example that were never caught.


I always maintain its no coincidence that there are more storms since Satellite ...also sub tropical storm only were recently named and recognised ...
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1518. MPI88
Quoting 1506. Grothar:
Repost from the other day. This was all the activity for the 1914 season.



Prior to the satellite era many fish-storms went unreported. What was the meso-scale pattern in 1914?
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Quoting 1506. Grothar:
Repost from the other day. This was all the activity for the 1914 season.

There was a TS and TD that year. Could you imagine a season with no activity? The year of "No Storms."
Member Since: June 30, 2013 Posts: 12 Comments: 9235
Quoting 1505. washingtonian115:
He was a pleasant surprise.The models also missed Chris and Kirk for the most part as well.They to were never suppose to reach hurricane status let alone even develop..


In other words the models are not a gospel but only guidance. History tells that things can form without model support but that is the minority of the time.
Member Since: April 29, 2009 Posts: 75 Comments: 15663
1514. hydrus
Quoting 1499. VR46L:


Ah my favourite in the past couple of years Michael never seen by any model

Once in a while, a small hurricane like Michael will form without any of the models picking it up. I have actually seen this quite a few times.
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Quoting 1506. Grothar:
Repost from the other day. This was all the activity for the 1914 season.

There were probably storms that formed in the northern atlantic like Grace (2009) for a example that were never caught.
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Quoting 1452. hurricanes2018:
I hate it to..

Gfs didn't drop it...it has a very well defined vorticity,12z gfs will show a big storm again.
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been lurking on this blog for a few years, Grothar it seems that many of the bloggers respect your perspective. There are so many bloggers that admittedly I can get lost in the dialogue. Would you be willing to share your top ten bloggers so I can really key in on their input. I know that seems like an odd request, but it seems like the post change like the tides as the different GFS models come out. Using the context clues, Grothar, Levi, Cyberteddy, washingtoniam, neapolitan, and pottery seemed to be the frequent posters.
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Thunderstormikey is very uneducated and just trolling the blog. Don't listen to this bozo.
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1507. GatorWX
Quoting 1494. Hurricanes305:


Excellent post I forgot about the TUTT if this is able to gather a ULAC the location of the TUTT relative to the track of the system will make matters much more interesting and thus allow this to intensify. Overall, it has a much better shot than Chantal. Which develop in Marginally favorable conditions (not including the roaring trades).


I like reading your posts gt. I tend to agree most of the time. I know I've been noticing a similar scenario. You don't need a chart to see the position of the a-b high, look at the track. It'd be nice to know the upper air and surface conditions over the continent of NA.

Sorry that was for gt, but I think the same about your thoughts as well. It shall get interesting at some point, I tend to think sooner than later, ugh, anticipation. Every year, it never fails. Every year the blog gets less and less encouraging. There's still those here and I appreciate them to the fullest. They know who they are.
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1506. Grothar
Repost from the other day. This was all the activity for the 1914 season.

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Quoting 1499. VR46L:


Ah my favourite in the past couple of years Michael never seen by any model

He was a pleasant surprise.The models also missed Chris and Kirk for the most part as well.They to were never suppose to reach hurricane status let alone even develop..
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Our "thing" could do good by strengthening and stirring up the waters before a real monster shows up. SSTs are not at their height and won't be till probably September something.

Models are very wacky until we have a LLCOC.

Having a strong wave this early before the usual Cape Verde season start of about August 15th is bad sign for the downcasters.
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Quoting 1495. thunderstromsmike:
a man of my own words..... but can that be true they did forcast a wild and above average number of storms and i trust them


In 2010 we went all the way down the list to the T named storm. It was a very active year with very LITTLE impact. In 2009 we had 1 storm. Ida and it only had 9 named storms....in 2011 a lot of storms 1 hit the US. Irene. In 2012 once again to the T named storms. Major event Sandy.....regardless of the seasonal forecast all it takes is 1....even if it is homegrown development.
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1499. VR46L
Quoting 1489. washingtonian115:
Good points you've brought up.Speaking of storms their were last year that reached hurricane status without the models say so last year.They've been doing terrible this year as well.


Ah my favourite in the past couple of years Michael never seen by any model

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The EURO has a more pontent ridge than the GFS. #downcasting

168 hrs

192 hrs
Member Since: June 20, 2012 Posts: 0 Comments: 2178
1497. hydrus
Quoting 1477. Grothar:
One important item many people do not include in their hurricane preparedness are blue tarps and string to cover their roofs. They are not that expensive and trying to get some after a storm is difficult. With Wilma, we were fortunate not to have roof damage but many of our neighbors had severe damage. We gave them ours and saved a lot of damage.
Good advice. After Charley, both house and cottage had severe roof damage. Blue tarps did a great job keeping water out, even when Francis came through. All was lost after Jeanne tho..Jeanne was horrible..People were trying to put back the few things they had left and ended up losing them too.
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the dry air near the wave honestly looks to be going away on the rainbow loop. May just be me though


Link
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Quoting 1484. GTstormChaserCaleb:
A tropical wave moved off the coast of Africa on August 14, and organized into Tropical Depression Three on August 16 while located about halfway between the Windward Islands and the coast of Africa. It moved to the west-northwest, and strengthened into Tropical Storm Andrew on August 17. After reaching winds of 50 mph (85 km/h), strong southwesterly shear weakened the storm, and by August 20 it weakened to a minimal storm with a pressure of 1,015 mbar (30.0 inHg). It bypassed the Lesser Antilles completely, and turned to the west in response to the building of a high pressure system to the north. Upon turning to the west, a trough of low pressure positioned to the southwest of Andrew created an environment with little vertical shear and well-defined outflow.

Comparison to the environment now:
1) Strong ridge to the north
2) TUTT over Hispaniola

Not saying the storm is going to be as strong, I certainly hope not, but just using this to compare the environment. I wish I had surface charts for when Andrew formed so I can compare it to now.


Excellent post I forgot about the TUTT if this is able to gather a ULAC the location of the TUTT relative to the track of the system will make matters much more interesting and thus allow this to intensify. Overall, it has a much better shot than Chantal. Which develop in Marginally favorable conditions (not including the roaring trades).
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Quoting 1492. thunderstromsmike:
lets talk about the polar ice thats always entertaining


Melting away fast.
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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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