An evening shift at NHC: A Shary situation

By: Dr. Jeff Masters , 2:57 PM GMT on October 28, 2010

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We have a rare late October triple threat in the Atlantic this morning, three "Invests" with a decent chance of developing. The most serious threat is Invest 91L, a tropical wave centered near 7N 49W, about 950 miles east-southeast of the Lesser Antilles Islands. 91L is moving west to west-northwest at 15 - 20 mph, and will spread heavy rains and gusty winds to the northern coast of South America and the southern Lesser Antilles Islands beginning on Friday night. The system is under low wind shear less than 10 knots, but is too close to the Equator to spin up very rapidly. The storm will also have difficultly developing due to land interaction with South America this weekend. However, several models are indicating the possibility that 91L could develop into a tropical depression in the Central Caribbean by the middle of next week. NHC is giving 90L a 20% chance of developing into a tropical cyclone by Saturday.

A low pressure system (Invest 90L) centered near 27N 42W in the middle Atlantic Ocean has developed a broad and elongated circulation. Heavy thunderstorms on its east side are generating tropical storm-force winds. However, the circulation of 90L has become increasingly stretched out this morning, and the storm is not as well organized as it was last night. NHC is giving 90L a 50% chance of developing into a tropical storm by Saturday.

Finally, a low pressure system (Invest 92L) centered 700 miles south-southeast of Bermuda is developing a surface circulation, and appears very close to tropical depression status. NHC is giving 92L a 60% chance of developing into a tropical cyclone by Saturday. The only land area that might be affected by 92L is Bermuda.


Figure 1. A rare late-October triple threat in the Atlantic: three areas of disturbed weather listed by NHC as areas of interest (Invests) worth running forecast models on. Image credit: National Hurricane Center.

A quiet Tuesday evening shift at NHC
Tuesday evening was a quiet shift at the National Hurricane Center, where I've spent the week as a participant in their visiting scientist program. Each week during hurricane season, NHC invites a hurricane researcher or forecaster in academia, government, or private industry to spend a week shadowing the NHC forecasters as they prepare their forecast products. The evening shift is chosen, since it is less of a zoo, and the presence of the visiting scientist will present less of a distraction to the forecasters.

There was only one area of interest (Invest 90L) on Tuesday. 90L was a disorganized low pressure system in the middle Atlantic that had gotten tangled up with an upper-level low pressure system that was bringing dry air and disruptive wind shear. I worked with senior hurricane specialist Dan Brown, who cheerfully analyzed 90L with me, but confided that this storm was barely worth keeping as an Invest. He lowered its chances of development to 10%, but did order one more run of the various forecast models, so I could see how that was done. He also pointed out two other systems he thought might turn into "Invests" worth watching later in the week, and noted in particular that the large tropical wave approaching South America was unusually vigorous for this time of year, and might be something to be concerned about if it managed to avoid South America and penetrate into the southern Caribbean.

Since there wasn't much else to see on the hurricane end of their operation, I spent the rest of the evening working with NHC's marine forecasting branch. The National Hurricane Center is responsible for preparing weather analysis charts and marine forecasts for the tropical Atlantic and Eastern Pacific, and I worked with meteorologist Felix Garcia of NHC's Tropical Analysis and Forecast Branch (TAFB). He prepared the 8pm Tropical Weather Discussion, and the 00Z tropical analysis chart. I'm highly envious of the software tools NHC has to prepare forecasts and make analysis charts! I want an NAWIPS and ATCF workstation like these guys have, which allows one to zoom, pan, overlay, and quickly change speeds of animations. I'm proud to say that I am responsible for a portion of the 1016 mb isobar on the 00Z tropical Atlantic surface analysis map for Tuesday night, which I drew using the fantastic map drawing software at NHC.

Wednesday evening: A Shary situation
Wednesday evening was a bit more interesting. Invest 90L had been joined by Invest 91L and Invest 92L, and odds for development of 90L had been bumped up to 30%. I spent the first portion of the shift working with TAFB forecaster Wally Barnes, who made the intensity and position estimates of the three invests based on infrared satellite imagery. This task is accomplished using the Dvorak technique, a system of classifying cloud patterns of tropical cyclones based on how cold the cloud tops are, how much spiral banding is present, and other factors. Wally let me determine where the center of 90L was at 00Z last night, and enter the fix into the official database. I am now forever responsible for a tiny piece of the 2010 Atlantic hurricane data base--an awesome responsibility! (It's my first addition to the cause since I sent in my final Hurricane Hunter VORTEX report from the eye of Hurricane Hugo on September 15, 1989, complaining about hitting 5.7 G's of acceleration.) We classified 90L as a T2.0, which is respectable, and meant the system might be on its way to status as Tropical Storm Shary. Wally had to do the analysis for the large, ill-defined tropical wave (Invest 91L), since his eye was much more highly trained to pick out subtle motions in the satellite animations that indicated where the most likely center of circulation might be trying to develop.


Figure 2. "My boat is right here!" Forecaster Wally Barnes of NHC's Tropical Analysis and Forecasting Branch (TAFB) shows where he suspects the center of rotation of Invest 91L might be at 00Z on October 28, 2010.

Wally and I printed out the fix information we'd come up with for 90L, and took it over to Dan Brown, who was working the evening shift again over at the hurricane side of NHC.

"What, you're giving this a T2.0?" Dan good-naturedly hassled us, as we presented the fix info. "You're just trying to get something going for Jeff here so he can see some advisories get issued." Wally defended our analysis, pointing out how the heavy thunderstorms of 90L were pushing closer to the center of circulation, and how the cloud tops had gotten much colder. Dan agreed that 90L really was worthy of more attention, and commented that there was a good chance one of our three invests would probably develop into something NHC would have to issue advisories on before my final shift at NHC ended on Friday night. His prediction was that it would be 92L, the system a few hundred miles north of Puerto Rico.

An hour later, Dan wasn't so sure that 90L wouldn't beat 92L to the title of Tropical Storm Shary. The European ASCAT satellite had just sent in an image of the surface winds over 90L, and ASCAT was showing that the storm had a closed circulation and a respectable area of 40 mph tropical storm-force winds. He gave a call to James Franklin, the head of the hurricane specialist unit at NHC, who was at home. I listened in.

"Hey, I just got ASCAT," said Dan. "It's 35 knots. You can see the center, and the convection is about 130 miles to the northeast. I'm thinking of starting it as a tropical storm, but I hate to start it now, since the convection started at 21Z, and I'd like to see it persist. The ASCAT pass shows the circulation is a bit elongated, and the most recent microwave images are also showing that."

After discussing whether or not to initiate advisories on Tropical Storm Shary for a few more minutes, Dan hung up, then told me the scoop. "This is one of the most difficult parts of the job. It's a real judgment call whether or not to name a storm, when it's such a borderline situation like this. What we're going to do is issue a Special Tropical Weather Outlook mentioning that 90L has gale-force winds, bump the probability of development up to 50 or 60%, watch it for a few more hours, then re-assess." Dan then proceeded to call his replacement, Eric Blake, who was due to work the night shift, to tell him to come in as planned, since it looked like there could well be a Tropical Storm Shary to deal with. Dan then proceeded to write the Special Tropical Weather Outlook and send it out.


Figure 3. "The one that got away was this big!" Wally Barnes tells hurricane specialist Dan Brown what he thinks of 90L's recent burst of heavy thunderstorm activity.

Next update
I'll have an update on Thursday morning from the National Hurricane Center on the latest from the tropics.

Jeff Masters

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gee whiz another possible invest in the gulf? looks like windshear has picked up this afternoon over the basin as for 91. slu you ready?
Member Since: September 11, 2010 Posts: 1 Comments: 4335
Quoting smartinwx:


And his point was that the next STOP (i.e. level of strength is not TD, but LO). No storm SKIPS a strength, it may go up 2 strengths between advisories, but it will be a LO at some point before it's a TD. even if for only 10 minutes.

No, I'm afraid that wasn't his point. The OP stated that 91L was only a wave. I pointed out his error. He admitted the error. End of that story.

To your larger point, though: not every system is designated as every possible stage of development, and that's what we're talking about here: arbitrary designations made by ATCF. If ATCF says, for example, that 91L is suddenly a TS at 5:00 PM, you can rest assured that they likely didn't surreptiously classify it as both a LO then a TD before then. In effect, they would have indeed skipped a classification category. Two, in fact...

Please let me know if you'd like further examples; there are several in the ATCF archives... Cheers!
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If 91L were to form right now, it'd be the furthest south a storm has formed in the Atlantic, beating Isidore a few years back (not the 2002 one, a different Isidore)
Member Since: October 15, 2008 Posts: 11 Comments: 2305
Quoting 7544:
91L is getting huge at this hour if this all comes together it could take up alot of realestate wherever it goes

whats that blob in the gom could something try to form from the digging front ?
Well. It would not be the first time a tropical cyclone formed on the tail end of a front. If it hangs around long enough, it might spin something up...Alot of dry air around ...
Member Since: September 27, 2007 Posts: 1 Comments: 20509
Is it possible that 90L could develop once it gets on the other side of the shear in front of it?
Member Since: October 15, 2008 Posts: 11 Comments: 2305
175. 7544
91L is getting huge at this hour if this all comes together it could take up alot of realestate wherever it goes

whats that blob in the gom could something try to form from the digging front ?
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Quoting Neapolitan:

In a perfectly linear world, yes. But any particular system can skip one or more categories if need be--as we've already seen several times this year. The point of my post was that 91L was no longer a wave as the OP wrote...and that's an inarguable point.


And his point was that the next STOP (i.e. level of strength is not TD, but LO). No storm SKIPS a strength, it may go up 2 strengths between advisories, but it will be a LO at some point before it's a TD. even if for only 10 minutes.
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eventhough it is early, looks like the dynamic models you posted for 91L have shifted a bit westward. maybe the models see a stronger high above it
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Quoting cat5hurricane:
18z

Click To Enlarge


Looks like an anti-cyclone is developing right over 91L
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Quoting hydrus:
Shows up in a big way on the MIMIC-TPS...

Especially when compared to the other invests!
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Quoting serialteg:


this baby looks promising
Shows up in a big way on the MIMIC-TPS...
Member Since: September 27, 2007 Posts: 1 Comments: 20509
Quoting VAbeachhurricanes:


Actually, it goes DB, the LO, then TD

In a perfectly linear world, yes. But any particular system can skip one or more categories if need be--as we've already seen several times this year. The point of my post was that 91L was no longer a wave as the OP wrote...and that's an inarguable point.
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164. JRRP
Member Since: August 16, 2007 Posts: 0 Comments: 5354
ATCF Summary:

90L: 30 knots | 1010mb | 25.9N/41.4W


91L: 30 knots | 1006mb | 07.7N/50.4W


92L: 30 knots | 1009mb | 24.9N/60.6W
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Quoting serialteg:


this baby looks promising
It does not lack anything in the presentation department either. It is a large system with a lot of energy..
Member Since: September 27, 2007 Posts: 1 Comments: 20509
Quoting wunderkidcayman:
yes PensacolaDoug it won't have a GOM Ivan track but it may very well have a Central/Western Caribbean Ivan track then make a sharp Right turn and head into Cuba and Bahamas


How's that High looking over FL?
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Quoting Neapolitan:

ATCF has it classified as a "disturbance", a step above a wave. Next stop: TD.

AL, 91, 2010102812, , BEST, 0, 72N, 486W, 30, 1006, DB, 34, NEQ, 0, 0, 0, 0, 1009, 150, 120, 0, 0, L, 0, , 0, 0, INVEST, S,


Actually, it goes DB, the LO, then TD
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91L has some very nice structure to it. I can't see why this won't develop into a TD by at least tonight or tomorrow.
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157. unf97
Quoting Jax82:
Extreme drought for me now along the jacksonville coast. Need rain! Didnt get any this week after our biggest chance in weeks. May get some tonight but it wont be much, just a 40% chance, then sunny and mid-70s the next 5 days. Doh!



Just checked the latest radar and it looks as if there is a solid batch of showers and thunderstorms along the southern end of the cold front from Tifton, GA southwestward through Tallahassee. However, my fear is that there is not much activity forming yet along the front across Central GA as of yet. Hopefully, some showers and thundershowers will develop just ahead of the front late this afternoon and during the evening hours as the front passes through the Jax metro area. My goodness, we are in dire need of rain. I have yet to measure a drop of rain at my home in 28 days now and counting as I am located a few miles outside of where the NWS Jax office is located.

Looks as if we may have our best chances of rain in quite awhile hopefully by the middle part of next week. The models are advertising a strong Upper Low to develop over the GA -SC area, so if that materializes, spokes of energy rotating around the Low would enhance the moisture profile over NE Florida. The models should have a better handle on this hopefully as time progresses.
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Oh Noooooo !!!!!! 91L looks like a Tropical Storm !!!!! please no, go away from us !!!!!
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It's almost november so a recurve in the carribean cant be ruled out
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yes PensacolaDoug it won't have a GOM Ivan track but it may very well have a Central/Western Caribbean Ivan track then make a sharp Right turn and head into Cuba and Bahamas
Member Since: June 13, 2009 Posts: 2 Comments: 11048
Check out this rotating wall cloud/ Funnel cloud pic coming out of Mechanicsville VA(a suburb of Richmond)There were 3 or 4 reports of tornadoes in the richmond area, 2 of them were confirmed, one near the racetrack (Richmond International Raceway) and one that you see right below me. There was also a confirmed report of a tornado in Mecklenburg County (50-60 miles south of Richmond) near South Hill. It was an EF0 tornado. Typical Virginia tornado.


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Quoting sunlinepr:


Let me guess, this one (91L) will enter the carribean, everyone will think it will hit FL but goes into Mexico, same deal all year.
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I remember IVAN got it's start way down there where 91L is now. Of course it won't follow follow IVAN's track into the GOM.
Member Since: July 25, 2006 Posts: 0 Comments: 551
147. JRRP
Quoting JLPR2:


Scary...
XD

yea but you know that always HWRF exaggerate the intensity
jejeje solo para mantenernos ocupados
Member Since: August 16, 2007 Posts: 0 Comments: 5354
146. JLPR2
Quoting JRRP:


Scary...
XD
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145. JRRP
mmmmmmmmm
Link
Member Since: August 16, 2007 Posts: 0 Comments: 5354
144. JRRP
Member Since: August 16, 2007 Posts: 0 Comments: 5354
143. Jax82
Extreme drought for me now along the jacksonville coast. Need rain! Didnt get any this week after our biggest chance in weeks. May get some tonight but it wont be much, just a 40% chance, then sunny and mid-70s the next 5 days. Doh!

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Quoting hydrus:
They do not get many cyclones there. Probably just being cautious. Still quite far away from the islands...


this baby looks promising
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Quoting islander101010:
canadian not good for hispanola
NCEP at 144 hours...
Member Since: September 27, 2007 Posts: 1 Comments: 20509
140. JLPR2
Quoting Vincent4989:

Only upside down.


What do you mean? Olga had all the weather to the north when she was Subtropical, that's why PR never got a TS warning.
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canadian not good for hispanola
Member Since: September 11, 2010 Posts: 1 Comments: 4335
12z Invest92 Early Cycle NHC model tracks
Invest92
Statistical/Simple Models (CLIPER,BAMs,LBAR,other Statistical Models)




Dynamic Models (More sophisticated models)





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Quoting JLPR2:
92L reminds me of STS Olga in 07

Only upside down.
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Quoting PensacolaDoug:
91L is coming right along. Hard to believe they only have it at 20%
They do not get many cyclones there. Probably just being cautious. Still quite far away from the islands...
Member Since: September 27, 2007 Posts: 1 Comments: 20509
12z Early Cycle NHC model tracks
Invest91
Statistical/Simple Models (CLIPER,BAMs,LBAR,other Statistical Models)




Dynamic Models (More sophisticated models)




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12z Early Cycle NHC model tracks
Invest90
Statistical/Simple Models (CLIPER,BAMs,LBAR,other Statistical Models)





Dynamic Models (More sophisticated models)




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Quoting PensacolaDoug:
91L is coming right along. Hard to believe they only have it at 20%



2pm TWO

A VIGOROUS TROPICAL WAVE LOCATED OVER THE TROPICAL ATLANTIC ABOUT
850 MILES EAST-SOUTHEAST OF THE WINDWARD ISLANDS IS PRODUCING A
LARGE AREA OF SHOWERS AND THUNDERSTORMS. THIS SYSTEM HAS BECOME
BETTER ORGANIZED TODAY...AND ENVIRONMENTAL CONDITIONS APPEAR TO BE
FAVORABLE FOR SOME SLOW DEVELOPMENT OF THIS DISTURBANCE DURING THE
NEXT COUPLE OF DAYS. THERE IS A MEDIUM CHANCE...30 PERCENT...OF
THIS SYSTEM BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS AS
IT MOVES WESTWARD OR WEST-NORTHWESTWARD AT 15 TO 20 MPH.

Member Since: June 9, 2007 Posts: 4 Comments: 15777

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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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