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

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Quoting 2540. Kyon5:
The high pressure is very strong too. I see it hard that the tropical wave breaks the ridge. Unless there's a weakness, the wave will keep moving westward, IMO.
Talk about walking between a fine line and a dangerous one.The northern movement is why the GFS doesn't really develop it as well.But when new information gets fed into the models I'm sure they will have a better handle on the situation.
Member Since: August 14, 2010 Posts: 12 Comments: 22379
2541. pcola57
A warm pathway awaits..

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2540. Kyon5
Quoting 2539. washingtonian115:
So it's not like the models don't like our wave (sort of).It's that northern movement that leads to it's demise.If it stays south with that ULAC and can more at 10-15 mph then we could be looking at a tropical storm Vs. a weak open wave.I see the ECWMF is on board with development but it sends it north to fast in my opinion.
The high pressure is very strong too. I see it hard that the tropical wave breaks the ridge. Unless there's a weakness, the wave will keep moving westward, IMO.
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So it's not like the models don't like our wave (sort of).It's that northern movement that leads to it's demise.If it stays south with that ULAC and can more at 10-15 mph then we could be looking at a tropical storm Vs. a weak open wave.I see the ECWMF is on board with development but it sends it north to fast in my opinion.
Member Since: August 14, 2010 Posts: 12 Comments: 22379
2538. Kyon5
Quoting 2537. GeoffreyWPB:


Will do! But do they come in any other colors? Blue just doesn't do my house justice.
They come in brown.
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Quoting 2534. Grothar:


Not yet, Geoff. But I would still buy those blue tarps if I were you.


Will do! But do they come in any other colors? Blue just doesn't do my house justice.
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LAKESHORE HAZARD MESSAGE
NATIONAL WEATHER SERVICE CHICAGO IL
352 AM CDT MON JUL 22 2013

ILZ006-014-INZ001-002-221700-
/O.NEW.KLOT.LS.S.0001.130723T2000Z-130724T1500Z/
LAKE IL-COOK-LAKE IN-PORTER-
352 AM CDT MON JUL 22 2013

...UNSEASONABLY LARGE AND BATTERING WAVES LIKELY TUESDAY
AFTERNOON THROUGH WEDNESDAY MORNING...

STRONG NORTHERLY WINDS BEHIND A STRONG SUMMER COLD FRONT ARE
EXPECTED TUESDAY AFTERNOON INTO TUESDAY NIGHT OVER THE LAKE AND
NEAR THE LAKE SHORE. THE STRONG WINDS BLOWING DOWN THE FULL LENGTH
OF THE LAKE AND COOLER AIR MOVING OVER THE WARMER LAKE WATERS WILL
RESULT IN UNUSUALLY LARGE AND BATTERING WAVES FOR SUMMER. CURRENT
INDICATIONS ARE THAT WAVES COULD BUILD UP TO OR SLIGHTLY OVER 10
FT...PARTICULARLY FROM CHICAGO SOUTH INTO THE NORTHWEST INDIANA
SHORE LINE. THESE LARGE WAVES WILL INUNDATE THE TYPICALLY LARGE
WAVE VULNERABLE LOCATIONS ALONG THE SHORE...INCLUDING A FEW SPOTS
ALONG THE CHICAGO LAKE FRONT BIKE PATH. DO NOT VENTURE OUT ONTO
BREAK WALLS OR TOO CLOSE TO THE BREAKING WAVES AS OCCASIONAL ROGUE
WAVES COULD REACH UP TO AROUND 15 FEET AND POTENTIALLY SWEEP YOU
OUT INTO THE LAKE!

IN ADDITION...THE LARGE WAVES WILL MAKE VENTURING INTO THE LAKE
VERY DANGEROUS...EVEN FOR EXPERIENCED SWIMMERS. PLEASE REFER TO
THE SURF ZONE FORECAST (CHISRFLOT) AND BEACH HAZARD STATEMENTS
(CHICFWLOT) FOR UPDATED INFORMATION ON THE ANTICIPATED LARGE
WAVES.

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The photo above shows an eye-popping complex of waterspouts I observed over the Adriatic Sea on a boat trip to Brindisi, Italy. As we departed, the weather was very summer like -- some humidity, hot and sunny. Cumuliform clouds developed during our excursion, but the weather didn't appear threatening. In fact, the atmospheric pressure was stable at 1024 millibars. Suddenly, we saw a line of funnel clouds straight in front of our boat! The photo shows the most recently formed waterspout in the foreground; the oldest spout, in the background, would disappear in a few seconds. Our boat actually passed through the scary funnels. The spouts were spaced about 1/3 nautical mile from each other. I asked the boat's captain if he thought cruising past the spouts would be dangerous, but apparently, he wasn't bothered much by their proximity. Nevertheless, waterspouts can generate winds of over 70 mph (F0 on the enhanced Fujita Scale) and can be hazardous to boaters. Othoni Island is at left center.
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2534. Grothar
Quoting 2531. GeoffreyWPB:


So it's safe to say we don't know if this is coming over my house?


Not yet, Geoff. But I would still buy those blue tarps if I were you.
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06z GFS wants to develop it but droped it as it starts to move WNW



A mid level trough may try to weaken the ridge a little and the SE side allow our AOI to move N of due west or WNW into the subtropical jet and a little more SAL thus killing it. This will probably be a matter of track and the difference of a strengthening TS or a remnant low (if it develops). I still believe the model is underplaying the ridge a little and the flow is more of a zonal trade wind flow with bits of north tug if it reaches a slight weakness before the High rebuild and force it back into the MDR. 12z should be interesting now that its officially over Maritime climate.
Member Since: May 25, 2012 Posts: 0 Comments: 2043
Quoting 2473. Tropicsweatherpr:
Wave off Africa gets 10%. That is interesting.


Let's said very
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Quoting 2525. Grothar:



You mean we can't post the Hebert box or talk about the Fujiwhara effect yet? Darn.


So it's safe to say we don't know if this is coming over my house?
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2530. pcola57
Good Morning All..
Well well well..
This looks interesting..





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2529. KEEPEROFTHEGATE (Mod)
Quoting 2520. Waltanater:
No, actually I answered a zodiac sign question first, THEN I told him to post somewhere else! Let me get this straight, so you'll be ok with what I do if I follow the same rules myself? Alright, but they will be my rules, not yours. You should be banned for insighting an argument with me, assuming the MODS are awake.
iam awake watching ya nitpik over every comment since like last week

whats up with that
Member Since: July 15, 2006 Posts: 196 Comments: 63760
Quoting 2525. Grothar:



You mean we can't post the Hebert box or talk about the Fujiwhara effect yet? Darn.


Yes you can but AFTER the wave enters the Hebert Box...
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Quoting 2496. hurricanes2018:
I am happy to see this tropical wave moving west at 15 mph...not 30 mph.

Yes by now however if it keep moving westward it will accelerate
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2526. Matt74
Quoting 2516. weathermanwannabe:
Good Morning. 1 in 10 chance of development of the wave at this point. Plenty of time to watch it and not the time to speculate about formation into a storm and much less track..............It would not be a good bet at this point.

A reminder from the NOAA Mariners Guide to Tropical Cyclones. Please do not assume that this isolated wave, surrounded by dry air and with marginal conditions, is "the one":

Each hurricane season approximately 60 of these waves cross the tropical North Atlantic. strong>Although the majority of these waves pass through the basin without any significant tropical cyclone development, passage of these waves is often accompanied by squally weather with brief periods of higher sustained winds.
You always put out good info. Certain ones on here need to read this and stay calm and level headed.
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2525. Grothar
Quoting 2516. weathermanwannabe:
Good Morning. 1 in 10 chance of development of the wave at this point. Plenty of time to watch it and not the time to speculate about formation into a storm and much less track..............It would not be a good bet at this point.

A reminder from the NOAA Mariners Guide to Tropical Cyclones. Please do not assume that this isolated wave, surrounded by dry air and with marginal conditions, is "the one":

Each hurricane season approximately 60 of these waves cross the tropical North Atlantic. strong>Although the majority of these waves pass through the basin without any significant tropical cyclone development, passage of these waves is often accompanied by squally weather with brief periods of higher sustained winds.



You mean we can't post the Hebert box or talk about the Fujiwhara effect yet? Darn.
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2524. VR46L
Quoting 2519. Grothar:


These things have a habit of dumping on you over there the past few years. eh?


Yes they do !

But due to the Large high we have had no rain to speak of for nearly a month ... reduced water into houses etc so could do with a fish
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Quoting 2488. Tazmanian:





I wish you guys would stop spaming the blog with twos all ready



its been posted more then once if you need too see it this look back in one of the commets or go too the nhc site but plzs stop spaming the blog with the twos
I agree with TAZ on this one.
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Quoting 2487. biff4ugo:
It would be great if they could develop an intuitive standard dashboard for tropical systems, like a cars dashboard.

Where Sea surface temps would be gas in the tank, warning lights for shear and SAL, and other indicators to denote the speed, expanse of the system, and environment in the direction of travel.
That's a good idea!
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2519. Grothar
Quoting 2513. VR46L:


Hi Gro !

Hope you are keeping well

I am hoping this wave develops into a fish ... could do with the moisture at the moment that a fish would generate for here ...

Yes I know , the irony of me looking for rain !




These things have a habit of dumping on you over there the past few years. eh?
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Quoting 2509. weatherlover94:


Maybe its only Marginally favorable because it's so close to land.
Its only marginal mainly because of SAL and maybe a bit of cool SST near the CV ilslands but if you at the wave out ahead of this it is actually moistening up to path for our AOI as long as it keep west and dont gain too much latitude before the Islands it should have a good chance to develop.



SAL is really thinning out so if this was to develop it wont have much of any issues with dry air which could mean a large (size) system to track. We will see how it adjust to the water today and tomorrow before looking at its chances tho.
Member Since: May 25, 2012 Posts: 0 Comments: 2043
Good Morning. 1 in 10 chance of development of the wave at this point. Plenty of time to watch it and not the time to speculate about formation into a storm and much less track..............It would not be a good bet at this point.

A reminder from the NOAA Mariners Guide to Tropical Cyclones. Please do not assume that this isolated wave, surrounded by dry air and with marginal conditions, is "the one":

Each hurricane season approximately 60 of these waves cross the tropical North Atlantic. strong>Although the majority of these waves pass through the basin without any significant tropical cyclone development, passage of these waves is often accompanied by squally weather with brief periods of higher sustained winds.
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Quoting 2511. SouthernIllinois:

Who?


Last page not going point fingers
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Wave about to leave the coast, as Grothar shows also .. development to be slow .
Evening Aussie...!



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2513. VR46L
Quoting 2508. Grothar:


Yes, it is. Hey VR!


Hi Gro !

Hope you are keeping well

I am hoping this wave develops into a fish ... could do with the moisture at the moment that a fish would generate for here ...

Yes I know , the irony of me looking for rain !


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be back on later
Member Since: September 8, 2010 Posts: 0 Comments: 2250
TROPICAL WEATHER DISCUSSION
NWS NATIONAL HURRICANE CENTER MIAMI FL
805 AM EDT MON JUL 22 2013

TROPICAL WEATHER DISCUSSION FOR NORTH AMERICA...CENTRAL
AMERICA...GULF OF MEXICO...CARIBBEAN SEA...NORTHERN SECTIONS OF
SOUTH AMERICA...AND ATLANTIC OCEAN TO THE AFRICAN COAST FROM THE
EQUATOR TO 32N. THE FOLLOWING INFORMATION IS BASED ON SATELLITE
IMAGERY...WEATHER OBSERVATIONS...RADAR...AND METEOROLOGICAL
ANALYSIS.

BASED ON 0600 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH
0952 UTC.

...TROPICAL WAVES...
A TROPICAL WAVE IS OVER THE EAST TROPICAL ATLC WITH AXIS
EXTENDING FROM 18N20W TO 08N21W. IT IS EMBEDDED WITHIN A
MODERATE TO HIGH MOIST ENVIRONMENT AND ITS AXIS IS MOVING NEAR
10 KT. SCATTERED MODERATE/ISOLATED STRONG CONVECTION IS FROM 07N-
14N E OF 23W. ISOLATED MODERATE CONVECTION IS WITHIN 170 NM W OF
THE AXIS S OF 18N.
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Quoting 2503. Hurricanes305:
NHC wording about marginally favorable is just right for now... as its located in a moist environment. And the wave in front of it is moistening up the track for it even more. It has a ULAC to help spin a low underneath. If it acquires invest status we should get more accurate runs out of the models


Maybe its only Marginally favorable because it's so close to land.
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2508. Grothar
Quoting 2438. VR46L:


Maybe its what has just left Africa that the euro is seeing..





Yes, it is. Hey VR!
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Why do the regional sever weather maps cut Lake County, Florida in half? I don't see other large counties bisected.
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Early morning troll activity already?
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we are going to see invest soon..
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ECMWF takes it off to the Northwest and then dissipates

60 hours



132 hours
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NHC wording about marginally favorable is just right for now... as its located in a moist environment. And the wave in front of it is moistening up the track for it even more. It has a ULAC to help spin a low underneath. If it acquires invest status we should get more accurate runs out of the models
Member Since: May 25, 2012 Posts: 0 Comments: 2043
Quoting 2484. weatherlover94:


I think they will bump it up at 2:00 or 8:00.
its only moving at 15 mph..
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2501. Grothar
This is why there has been confusion as some tried to explain last night. The wave given the circle is very close to the African coast. There is another pouch or feature which emerged a few days ago. This has not been designated yet. But if it hold together, it may be. Although the conditions do not appear that favorable at this time.

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Quoting hurricanes2018:
I am happy to see this tropical wave moving west at 15 mph...not 30 mph.


So happy you said it twice within 3 blog comments. Hmmmm
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Quoting 2469. Waltanater:
Leo.
Quoting 2471. Waltanater:
This is not a "real-time" earthquake blog. It is a weather blog! Post this somewhere else.


You tell someone to post earthquake information somewhere else, but you answer a question about zodiac signs which isn't even completely close to weather. Earthquakes are more related to weather than zodiac signs.

Don't tell people to do something if you won't follow the same rules yourself. This blog is open discussion for all things related to weather, and geography. (why do you think this site updated earthquakes).
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it's got a good little ULLAC over head of it to.
Member Since: September 8, 2010 Posts: 0 Comments: 2250
Quoting Grothar:


Aussie, we may take away your paint brushes if you keep coloring those globes.

10% is not bad to start with. We all thought that last night.

But we all know, You saw it 1st and Taz saw it 1st too. LOl
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I am happy to see this tropical wave moving west at 15 mph...not 30 mph.
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ULAC is not too bad either lets see how this evolves as time go on.

Member Since: May 25, 2012 Posts: 0 Comments: 2043
one thing good about this tropical wave is only moving west 10 to 15 mph..its will help this tropical wave out!! its not going at 25 mph this time again.. big key here
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2493. Grothar
Quoting 2474. AussieStorm:



Meteosat 0 degree Convection Western Africa



Meteosat 0 degree Dust Western Africa


Meteosat 0 degree Atmospheric Motion Vectors Western Africa


Meteosat 0 degree Multi-Sensor Precipitation Estimate Western Africa


Meteosat 0 degree Cloud Top Height


Meteosat 0 degree Tropospheric Humidity


A TROPICAL WAVE HAS EMERGED OFF OF THE WEST COAST OF AFRICA ABOUT
500 MILES EAST-SOUTHEAST OF THE CAPE VERDE ISLANDS. ENVIRONMENTAL
CONDITIONS ARE EXPECTED TO BE MARGINALLY CONDUCIVE FOR SOME
DEVELOPMENT OF THIS DISTURBANCE OVER THE NEXT FEW DAYS AS IT MOVES
WESTWARD AT 10 TO 15 MPH. THIS SYSTEM HAS A LOW CHANCE...10
PERCENT...OF BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS.


Aussie, we may take away your paint brushes if you keep coloring those globes.

10% is not bad to start with. We all thought that last night.
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Quoting 2490. AussieStorm:
Don't people read back on the blog before they post the same information that's already been posted?

Find more about Weather in Rydalmere, New South Wales, Australia
My Current Weather, Click for 15 day Forecast






I guss they do not
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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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