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Grading NHC's Tropical Weather Outlook

By: Dr. Jeff Masters , 4:33 PM GMT on June 06, 2007

The National Hurricane Center's Tropical Weather Outlook (TWO) is a text-only product that rates the potential of disturbed areas of weather to turn into tropical depressions or tropical storms. The outlooks are issued four times daily, at 5am, 11am, 5pm, and 11pm EDT. I've found them to be an excellent guide to what to watch out for. But how accurate are these outlooks? To find out, Jamie Rhome and Dan Brown, who are two of the hurricane specialists that write the Tropical Weather Outlook, verified the accuracy of all the outlooks issued in 2005 and 2006. They used a three-tiered classification of threat based on the following language appearing in the TWO:

High: "A tropical depression could form tonight or the next day."

Medium: "Some slow development is possible."

Low: "Tropical storm formation is not expected."

These forecasts were then graded by looking at the "best track" database of Atlantic hurricanes and seeing if a tropical depression formed within 48 hours of each TWO issued. The results, shown below, reveal that for the Atlantic in the years 2005 and 2006:

-When the TWO said, "A tropical depression could form tonight of the next day," a depression formed within 48 hours 53% of the time.

-When the TWO said, "Some slow development is possible," a tropical depression formed within 48 hours about 20% of the time.

-When the TWO said, "Tropical storm formation is not expected," a tropical depression formed within 48 hours only 3% of the time.

Figure 1. Verification of the Atlantic and Eastern Pacific Tropical Weather Outlooks issued in 2005 and 2006. Image credit: Jamie Rhome and Dan Brown, National Hurricane Center.

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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2084. SouthernLady
10:09 PM CDT on June 10, 2007
Moonlight, where do you live in Ms.???
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2082. Miamiweather
3:03 AM GMT on June 11, 2007
i just wanted to know your opinions on the Carribean outlook
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2081. moonlightcowboy
3:01 AM GMT on June 11, 2007
...yeah, FLBoy, think maybe so. When John Anderson sang "seminole wind"...I think he was actually referring to the unusual smell sometimes generated there during hurricane season!
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2080. weatherblog
3:02 AM GMT on June 11, 2007
I remember last year when we had Chris brewing we did. I think we reached over 60 pages.
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2079. RL3AO
10:00 PM CDT on June 10, 2007
Whens the last time we hit 2000 comments?
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2077. PalmBeachWeatherBoy
2:49 AM GMT on June 11, 2007
R.i.P 93L
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2076. KYhomeboy
2:47 AM GMT on June 11, 2007
Thanks moonlight and cane. I wasn't sure if it was shear or not cause I just found it strange that the deeper convection actually maintained its circular shape. I assumed the convection affected by shear would take on an angular or flattened shape instead. Now I know! thanks.
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2075. moonlightcowboy
2:39 AM GMT on June 11, 2007

...the "smell factor" could be a good, new tool in casting this year. Never know when these things are going to be around??? Conditions appear to be getting more favorable, so it's likely we'll get a sightings report soon!

KYhomeboy, that's kind of what I saw, too with 93L. The Navy has NOT taken it down yet and I know it's not likely going to hold on; but, it was sneaky to begin with...may still be sneaking around way out there, trying to find the Caribbean. lol, we'll see!
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2074. TheCaneWhisperer
2:39 AM GMT on June 11, 2007
Very Possible KY, it's called destructive shear! Storm tops are being pushed off the circulation, happens all the time!
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2073. KYhomeboy
2:33 AM GMT on June 11, 2007
It looks almost as if 93L's low level clouds are keeping their signature but racing ahead of the deeper convection (although still limited). That appears to be what I'm seeing. Is that possible? lol
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2072. TheCaneWhisperer
2:21 AM GMT on June 11, 2007
What 93L Contributes is, a little insight to the abnormally warm sst's observed off the African Coast. This bothers me a bit because, when conditions are ripe in the EATL and CATL, there's nothing to stop it. Good thing is the weakness in the CATL is still there, could be alot of recurving storms this year! Bermuda/Azors high starts anchoring into place at the end of this month so we'll see what happens with that. The fly in the ointment is the dust but, that is observed on a case by case basis. Quiet now, just the way we like it right, nahhhhhhhhhhhhhhhh!
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2071. moonlightcowboy
2:27 AM GMT on June 11, 2007
...lol, FLBoy, yeah, I think something's been smelling in the Caribbean since yesterday morning. The "big fellow" may just be right.

93L shook us up a bit, off guard to see an invest that early way out there. The Caribbean and the GOM these past few days have looked better for potential -- lower shear and SST's.

May have to make some calls to see if there's been any local sightings of the big fellow. You know a report a week or so ago was from the Keys...and guess what, we had Barry, despite most of the logic.
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2070. TheRingo
10:18 PM EDT on June 10, 2007
although the bam medium and deep show it going south later. maybe they are seeing some ridging filling in.
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2067. BahaHurican
10:05 PM EDT on June 10, 2007
Back to Coriolis for a second.

I found this at wikipedia.com.

"Coriolis effect

Infrared image of Cyclone Monica near peak intensity, showing clockwise rotation due to the Coriolis effect.The Earth's rotation imparts an acceleration known as the Coriolis Effect, Coriolis Acceleration, or colloquially, Coriolis Force. This acceleration causes cyclonic systems to turn towards the poles in the absence of strong steering currents.[35] The poleward portion of a tropical cyclone contains easterly winds, and the Coriolis effect pulls them slightly more poleward. The westerly winds on the equatorward portion of the cyclone pull slightly towards the equator, but, because the Coriolis effect weakens toward the equator, the net drag on the cyclone is poleward. Thus, tropical cyclones in the Northern Hemisphere usually turn north (before being blown east), and tropical cyclones in the Southern Hemisphere usually turn south (before being blown east) when no other effects counteract the Coriolis effect.

The Coriolis effect also initiates cyclonic rotation . . ."

^ Baum, Steven K. (January 20, 1997). The Glossary: Cn-Cz.. Glossary of Oceanography and the Related Geosciences with References. Texas A&M University. Retrieved on 2006-11-29.

So whoever it was who said that the Coriolis effect might cause 93L to move northward was not actually wrong, according to this information.
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2066. TheRingo
10:12 PM EDT on June 10, 2007
Baha in about 4-5 hours it will be sunrise for 93L. Looks like it made a northward jog lately though. When it hits 40w it will probably recurve.
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2064. Bamatracker
2:09 AM GMT on June 11, 2007
well im out for the night....you all have fun!!
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2063. moonlightcowboy
2:06 AM GMT on June 11, 2007
Baha, it doesn't look as organized; but, if I'm seeing this right, infrared does seem to have some low level spin, or spin not directly associated to the little remaining convection. (prolly missed this one, too...lol) But, I'm with you, gonna still take a peak in the a.m.

L8R, BAMA, JP...have a good one!
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2062. BahaHurican
9:56 PM EDT on June 10, 2007
Looking at that image kman posted, I'd say that 93L looks about the same as it did when I was here a few hours ago. I need to look at a loop to see if it's losing spin, though.

I still say a wave this strong can potentially hold on to its identity long enough to limp across the central atlantic. It is also the only wave far enough north so far to be in a position to do damage. I'll surely be getting up to take a look at it before I head to work in the morning. I'm interested to see if the southward bobble it seems to have been taking will be enough to allow it to fight off the dry air and dust it has been dealing with all day today.

That area of the ATL should be hitting diurnal max time in 3 -4 hours, right?
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2061. Bamatracker
1:59 AM GMT on June 11, 2007

Well im not saying it wont or can't happen. (heck, i didnt expect to have two storms already this season) What I am saying is that probability is against a definitive yes answer that we will see an early cape verde storm.

Of course it could happen...we already had a tropical storm (barry) that possibly wasnt a true tropical system.
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2058. KoritheMan
1:59 AM GMT on June 11, 2007
Good night, StormW.
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2057. KoritheMan
1:57 AM GMT on June 11, 2007
BTW, a question for everyone: When do you think we will see the next storm of the season?

I'm guessing around June 20th to June 30th, with the 25th on up being the most likely time.
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2055. KoritheMan
1:56 AM GMT on June 11, 2007
If more than one model picks up on it in the next few days, believe a storm may form. Till then, just use your own analysis.
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2054. kmanislander
1:56 AM GMT on June 11, 2007
Have a few things to do but will check back before calling it a day
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2053. KoritheMan
1:55 AM GMT on June 11, 2007
Bamatracker: Remember Bertha in 1996? One of the earliest Cape Verde storms ever recorded, and it reached Category 3 strength, albeit briefly. In June, I don't expect anything like 93L again, but in early July, I'd expect to see some sort of tropical storm develop relatively far east like Bertha or Dennis did. How strong that'll be, I don't know.
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2051. Bamatracker
1:51 AM GMT on June 11, 2007
poor cavs...they look lost tonight.

as for earlier cape verde storms i wouldnt think so. 3 things keep them from forming right now if im right

1--they are froming from the itcz which is still too far south for the most part.
2-- wind shear is more aggressive this time of year
3-- sst's are cool.

all three of these come together in late july normally and more intense waves wouldnt change that.

this is just my thinking. Im sure im wrong though
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2045. moonlightcowboy
1:48 AM GMT on June 11, 2007
FLBoy, would you say that means more Verde storms earlier?
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2044. kmanislander
1:46 AM GMT on June 11, 2007
You were right on Storm !
Even in Cape Verde season it takes almost near perfect conditions for a wave to make it to the Caribbean or the E Coast of the US when it develops a clearly defined circulation that far east.
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2040. kmanislander
1:40 AM GMT on June 11, 2007
93L will be history tomorrow. Very little left of it now

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2039. Bamatracker
1:40 AM GMT on June 11, 2007
as for the models i always try to wait for them to show something for 3 or 4 consecutive runs before I take it too serious. Seems like alot of times they just get a crazy idea and spit it out there.
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2038. Bamatracker
1:39 AM GMT on June 11, 2007
93L is about to see davie jones locker....arggggggh!
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2037. stormhank
1:39 AM GMT on June 11, 2007
thanks storm w.. i was noticing the NAM has some sort of system south of cuba out 84 hrs? Link
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2035. moonlightcowboy
1:35 AM GMT on June 11, 2007
93L doesn't seem to have the spin it did a few hours ago...lookin' more like a fish!
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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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