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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Caribbean Disaster Emergency Management Agency

Link


Grenada Met. Office
Link
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Quoting sunlinepr:
Preparing link:

Trinidad & Tobago Met. Office

Link

That's good.
But enter 'piarco' in the search box, top of this page, for a preferred site.
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Quoting TOMSEFLA:
levi update today?
Yes..
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230. Aniah
Dr. Masters, this is my favorite blog post EVER. I am so impressed!
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levi update today?
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Quoting afj3:

Don't systems that form from tail ends of fronts need more time than normal to form (not to mention this time of year)?
The Gulf blob has a lot of dry air to contend with and water temps are only marginal at best. Cyclones that form from old frontal boundaries take longer because of shear. One of the weather maps I saw had a gale center forming in the Gulf .
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Preparing link:

Trinidad & Tobago Met. Office

Link
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Quoting hurristat:


I dunno if a Jig is good...

A different kind of Jig, I had in mind!
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Quoting pottery:

LOL, no sulking allowed!
Get up and do a Jig.
It will be Cocktail hour eventually.......


I dunno if a Jig is good...
Member Since: October 15, 2008 Posts: 11 Comments: 2305
Quoting NRAamy:
no no no no no no no!!!!!!

( sulks in corner )

LOL, no sulking allowed!
Get up and do a Jig.
It will be Cocktail hour eventually.......
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Quoting smartinwx:


My point was that in the 6 hours between advisories, it would have been in the LO category.

Gotcha. It would have had the characteristics of a low, but it would not ever have been designated as such. I think we're good now... ;-)
Member Since: November 8, 2009 Posts: 4 Comments: 13296
Quoting hurristat:


In the long run, .7 is only about 60 miles...

Storms have been known to influence South America

Have not checked your link, but heavy rains in the mountains around here have resulted in many grim stories.
The feeder bands from Ivan killed several people in Tobago...
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A circulation is definitely becoming better defined with 91L and it will be gaining latitude, so I do not think it being so far south will have any sort of impact on development.
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if 91L continues to move that fast, depending on when the dip in the jet comes into the southern and eastern US, it may miss the trough and get steered westward by the high
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Quoting pottery:

Good to be precise.
But academic really.
The thing is very far south, and not many systems have been able to do much, down here.
There have been some though, that have turned into Beasts....


In the long run, .7 is only about 60 miles...

Storms have been known to influence South America
Member Since: October 15, 2008 Posts: 11 Comments: 2305
Quoting NRAamy:
;)

pott.... just making sure I hadn't been banned.... again....


You have been misbehaving, AGAIN?
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;)

pott.... just making sure I hadn't been banned.... again....

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Quoting NRAamy:
check check check.... 1 2 3....

4, 5, 6,...
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I think that 90L has another shot at becoming named when it crosses this area of wind shear -- it managed to survive 60 kt when it was less organized, it should be able to hang on in 40 kt until at least sunday or monday, when it emerges in less-sheared skies.
Member Since: October 15, 2008 Posts: 11 Comments: 2305
Quoting hurristat:


whoops... thought isidore was at 7.9N

Good to be precise.
But academic really.
The thing is very far south, and not many systems have been able to do much, down here.
There have been some though, that have turned into Beasts....
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check check check.... 1 2 3....
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Quoting Hurricanes101:


1990 Isidore and that is not true, Isidore formed at 7.2N, this invest is already north of that


whoops... thought isidore was at 7.9N
Member Since: October 15, 2008 Posts: 11 Comments: 2305
208. afj3
Quoting hydrus:
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 ...

Don't systems that form from tail ends of fronts need more time than normal to form (not to mention this time of year)?
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207. 7544
91L is moving at 18mph
Member Since: May 6, 2007 Posts: 0 Comments: 6618
duplicate post...
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Quoting Neapolitan:

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!


My point was that in the 6 hours between advisories, it would have been in the LO category.
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Quoting JRRP:
That would be a close caLL FOR US jRRP! Looks like Hortense coming from the south east in 1996
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Quoting TropicalMan2010:

Showing some decent Vort. too!
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Quoting ILwthrfan:


Yes, I agree too, though I'm no expert it still looks FAR better than many other invests of the season. Whats the hold up on it. Has not enough convection around it's center? Or is it center still elongated or not vertically stacked yet?

Persistence.
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Quoting stormpetrol:
I think 91L is best looking Invest I've seen so far this year, personally I think this very close TS status, if not a TS already!



please noooooooooooooooooooooooo!!!!!!!!!!
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Quoting ILwthrfan:


Yes, I agree too, though I'm no expert it still looks FAR better than many other invests of the season. Whats the hold up on it. Has not enough convection around it's center? Or is it center still elongated or not vertically stacked yet?

Hard for anything to achieve a dominant center, this far south.
Rotation is needed for a true 'center' to be apparent.
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Quoting hurristat:
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)


1990 Isidore and that is not true, Isidore formed at 7.2N, this invest is already north of that
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Quoting stormpetrol:
I think 91L is best looking Invest I've seen so far this year, personally I think this very close TS status, if not a TS already!


Yes, I agree too, though I'm no expert it still looks FAR better than many other invests of the season. Whats the hold up on it. Has not enough convection around it's center? Or is it center still elongated or not vertically stacked yet?
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Quoting stormpetrol:
I think 91L is best looking Invest I've seen so far this year, personally I think this very close TS status, if not a TS already!

Yes indeed.
Unfortunately, we plan to attend an open-air Pan concert Friday night, and a Birthday party on Monos Island off the North West peninsula for the weekend.
Could be fun, with rough seas and torrential rains....
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Trinidad pressure now, 1007hPa, falling.
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I think 91L is best looking Invest I've seen so far this year, personally I think this very close TS status, if not a TS already!
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"The Weather Channel's preliminary count of tornadoes totalled 42 on Tuesday and 14 on Wednesday, making it the second-largest October outbreak on record. In Oct. 2007, 62 tornadoes were recorded."

(SPC says 46 Tuesday and 15 yesterday, for 61. But we'll see whether and how the numbers change after all the preliminary data are sifted through.)
Member Since: November 8, 2009 Posts: 4 Comments: 13296
Good afternoon...
So, after a period of squally weather with isolated heavy showers over parts of the Island this morning, things have become humid and still.
Looking at 91L, which looks like a system that should be further north (by appearances), it seems likely that we will get some heavy weather out of this, starting tomorrow night.
A 1006 pressure is pretty impressive.
The current appearance is very good, considering it is just about DMin right now.

Based on upper level wind directions, a more northern track than the ones predicted, is my guess.
In any case, Barbados,Trinidad&Tobago, Grenada, St. Vincent and beyond need to watch out for heavy rains.
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I really enjoyed reading Dr. Masters' post. Thanks.
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Quoting TropicalMan2010:
182:unlikely
time will tell
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Quoting hurristat:
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)

That's interesting. I suppose it would be because the ITCZ has sunk farther south as it always does this time of year, while conditions are still unusually ripe for cyclogenesis in that area.

FWIW, Ivan became a tropical storm just south of 10N, a major hurricane at 10.2N, and a Cat 5 at just 13.8N.
Member Since: November 8, 2009 Posts: 4 Comments: 13296

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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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