Invest 91L more organized, but has little time to develop

By: Dr. Jeff Masters , 2:31 PM GMT on April 22, 2011

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Hurricane season is more than a month away, but we have a tropical disturbance (91L) typical of what one might see in June or November. 91L is spinning over the waters a few hundred miles south of Bermuda, and has improved considerably in organization since yesterday, thanks to a drop in wind shear. The latest SHIPS model output is showing shear of 40 - 55 knots over 91L, but shear analyses from the University of Wisconsin CIMSS group is showing lower shear values of 20 - 30 knots over the main circulation center and to 91L's north, where the heaviest thunderstorms are. The system has a warm core at low levels, but a trough of low pressure lies over the storm at upper levels, and this trough is pumping cold, dry air into 91L, making it not completely tropical. One characteristic of subtropical systems like 91L is the presence of the main band of heavy thunderstorms removed several hundred miles from the circulation center, and 91L fits that description. 91L has two centers of circulation competing to be dominant, and this competition is slowing the storm's development. The storm was headed north at 5 - 10 mph early this morning, but that motion has halted, and 91L appears to be moving more south-southwesterly now, away from Bermuda. Sea surface temperatures are 23°C, which are very cold for a tropical storm to form in, but could support development of a subtropical storm.

As 91L moves south today, shear will steadily rise, and the storm likely has only until Friday night before shear grows too high to permit development. NHC is giving 91L a 20% chance of developing into a subtropical or tropical depression, which is a reasonable forecast. There has been only been one named April storm in the Atlantic since 1851, Tropical Storm Ana of 2003. The formation of a tropical disturbance at this location this time of year is unusual, but is not a harbinger of a active season ahead. Had this been going on in the Caribbean, that would be a different story.


Figure 1. Morning satellite image of the Atlantic tropical disturbance 91L. Note the two centers of circulation competing to be dominant. I expect the northern center will become dominant.

Jeff Masters

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The GFS is still chasing ghosts. However, the Caribbean is likely to be an active place this May, so pre-season activity will have to be watched for down there.

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154. unf97
Quoting NICycloneChaser:
If it's going to be classified, I'd expect another TWO at 2pm EDT to up the chances, they're not going to go from 20% to a depression or storm. If we get nothing at that time, 91L ain't getting a name.


I agree with you on this. Although the system does look rather good on satellite imagery currently, I don't think NHC will upgrade unless ship reports verify that sustained winds are or just over gale force. Slim chance for that to happen currently with only about a 12-24 window for an upgrade.
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Quoting ShenValleyFlyFish:
Only way to cut out subjectivity is to cut out any human factor. Happens to be that intuition (a totally subjective entity) is what separates the great from the also-rans in most human endeavors, especially science.


The human factor would still be everywhere present in the forecasting process. The only issue that I think could use more objective methodology is the initial classification.
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Quoting Drakoen:
Interesting:




It's certainly not every day that we see the wind max close to the center of a subtropical cyclone.
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Quoting Levi32:


It's not a problem now, but it has been a big problem in the past when a storm is sitting near our coastline impacting people.

It was a problem in September, 2008 when a 991mb low was sitting off of the Carolinas looking like this, and the NHC did not name it:





It had a 7C difference between the inside of the core and the outside, as measured by recon, but for an undeclared reason they didn't classify it as even subtropical. People that don't see a name tied to a storm during the summer time usually don't see it as a threat, and often do not even know it is there. The impacts can be quite negative in that case if the storm is potent.

To avoid these problems, a more clearly-defined and straight-forward method of when to classify these things should be devised. One can easily cut out the subjective part to make things easier and less confusing/controversial.
Only way to cut out subjectivity is to cut out any human factor. Happens to be that intuition (a totally subjective entity) is what separates the great from the also-rans in most human endeavors, especially science.
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Quoting Drakoen:
Interesting:



ASCAT finally got a bull-eye on a storm.
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Try scrolling your pointer across the stream at the bottom of Google's Earthday logo.
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Anyone else have this problem?
NRL Monterey web pages and data are currently inaccessible due to a technical outage. This home page will be updated as information becomes available.

Any other Navy site available?
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Interesting:


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Just a reminder....

I've created a blog entry, If anyone is interested. Feel free to write your location on this blog. Just so as to keep track of where people are in the event of severe weather/hurricane.

Just go here
Thanks

P.S Finally changed my Avatar, thanks Pat and Levi.
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Good Friday greetings all,

Too bad the anemometer isn't working with Buoy Station 41049, which is a bit NE of 91L... Appears wave heights recently reached a peak of 12.8 ft with the 1450Z obs... and min pressure of 29.77" at 1250Z...

91L certainly looks a lot better today since the now weakening upper trof - that was sitting about 5 degrees west yesterday inducing high shear / pumping dry air - has shifted EWD back over the fairly well stacked low-mid circ (per GFS model guidance of upper evolution)...

A typical problem for subtropical systems, even as we see shear lessened with better co-location of vorticity layers, dry air contamination has severely entrained into 91L, limited it's ability to sustain convection around it's low-mid center... The conflicting secondary spin-off sure hasn't helped with consolidation either...
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The names for this year:
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138. IKE
12Z GFS @ 132 hours showing a cold-front moving through the SE USA next week...




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91L is a nice looking system for this time of year. Still competing with two centers although the northern center looks like it is trying to dominant. Water vapor imagery shows the upper level confluent flow, associated with an upper level trough east of the United States, approaching the system. The window of opportunity is very small for significant development into a subtropical cyclone.


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Quoting IKE:
Dry air getting to it?


...big time,soon it will be a inverted trough,already starting to have that appearance on WV
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Quoting IKE:
Dry air getting to it?




Been doing so throughout its life time.
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Quoting RandomText:


What I don't get is that visually, when you look at the satellite imagery, I agree with you that if the storm generally moves SW to WSW then that would give it the best opportunity to develop.

My impressions was that even though the air is much drier in that region, the shear is much lower, or so I thought anyway...

But when you look at the shear map Sammywammy posted on post 28, it seems to show the exact opposite of what you'd expect from the satellite images. Based on that map, the shear is weakest due south of the storm, even though it certainly doesn't look that way on satellite.


And additionally, the maps don't agree with one another because one is showing calmness and the other is showing like 70kts wind shear.


Remember, wind shear is the difference in winds between the upper and low levels of the atmosphere. Sometimes its hard to distinguish wind shear when all that exists in a region is a stratus and/or cumulus cloud regime since those clouds do not rise into the upper levels of the atmosphere, thus you would not see wind shear occurring.
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132. IKE
Dry air getting to it?


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Quoting KEEPEROFTHEGATE:


Hmm, seems that the shear to the south has been decreasing. Though it would need to decrease an awful lot to stop it destroying 91L.
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Quoting sammywammybamy:


URGENT - FIRE WEATHER MESSAGE NATIONAL WEATHER SERVICE MIAMI FL 853 AM EDT FRI APR 22 2011 GLADES-HENDRY-INLAND PALM BEACH-INLAND COLLIER-INLAND BROWARD- INLAND MIAMI DADE-MAINLAND MONROE- 853 AM EDT FRI APR 22 2011 ...RED FLAG WARNING IN EFFECT FROM NOON TODAY TO 8 PM EDT THIS EVENING FOR RELATIVE HUMIDITIES BELOW 35 PERCENT AND TRANSPORT WINDS AROUND 15 MPH FOR THE INTERIOR AREAS OF SOUTH FLORIDA EXCEPT FOR SOUTHERN MIAMI-DADE COUNTY... THE NATIONAL WEATHER SERVICE IN MIAMI HAS ISSUED A RED FLAG WARNING...WHICH IS IN EFFECT FROM NOON TODAY TO 8 PM EDT THIS EVENING. DRY AIR HAS MOVED INTO THE SOUTH FLORIDA THIS MORNING FROM THE ATLANTIC WATERS AHEAD OF AN UPPER LEVEL LOW PRESSURE SYSTEM EAST OF THE BAHAMA ISLANDS. THIS WILL ALLOW FOR HIGHS TO WARM UP INTO THE LOWER TO MID 90S...AS DEW POINTS FALL INTO THE LOWER TO MID 50S OVER MOST OF THE INTERIOR AREAS OF SOUTH FLORIDA THIS AFTERNOON. SO THE RELATIVE HUMIDITIES WILL BE ABLE TO FALL INTO THE MID 20S TO LOWER 30S THIS AFTERNOON INTO EARLY THIS EVENING. THE TRANSPORT WINDS WILL ALSO BE AROUND 15 MPH THIS AFTERNOON INTO EARLY THIS EVENING OVER THE INTERIOR AREAS OF SOUTH FLORIDA...DUE TO THE PRESSURE GRADIENT BETWEEN THE HIGH TO THE NORTH AND THE LOW PRESSURE TO THE EAST. THEREFORE...A RED FLAG WARNING HAS BEEN ISSUED FOR MOST OF INTERIOR SOUTH FLORIDA FROM NOON TO 8 PM EDT THIS EVENING EXCEPT FOR SOUTHERN MIAMI-DADE COUNTY. PRECAUTIONARY/PREPAREDNESS ACTIONS... A RED FLAG WARNING MEANS THAT CRITICAL FIRE WEATHER CONDITIONS ARE EITHER OCCURRING NOW...OR WILL SHORTLY. A COMBINATION OF STRONG WINDS...LOW RELATIVE HUMIDITY...AND WARM TEMPERATURES WILL CREATE EXPLOSIVE FIRE GROWTH POTENTIAL.


Poor Florida and Poor Texas....


Yeah Texas is pretty crispy right now. Reading Texas hurricane history we've apparently been in this spot oh for about forever. Repeatedly it mentions tropical drought busters. Like this on from the 1886
LAST Indianola hurricane.

One of the most positive aspects of the tempest was the rain it brought. A serious drought
had developed across the region. In Galveston, water was being sold for 10¢ a bucket, twice
the going rate for beer. It was so bad at Corpus Christi that residents had to give water to the
poor, lest they die of dehydration. The drought was considered broken at Stephenville. Heavy rains led to flooding of streams in central Texas.
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Quoting nrtiwlnvragn:


I guess their interpretation of the definition is different than yours.


Right. There is no line about the requirements for convective organization, and thus there can be no interpretation of it by anybody. The NHC goes on subjective criteria that aren't written down, and it seems that if they want a certain level of convective organization, they should state so in their official definition of a subtropical cyclone. Otherwise, it becomes a confusing guessing game. Their definitions should actually be their definitions.

That is hardly an unreasonable request.
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Quoting Levi32:


If that slide was created by the NHC, then they are not including something in their glossary, which certainly mentions nothing about having organized convection in their definition of a subtropical cyclone. In fact, it even mentions that most subtropical cyclones have a disorganized convective pattern.

A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones. This system is typically an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles or more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection.


I guess their interpretation of the definition is different than yours.
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Quoting RastaSteve:
91L is beginning to look very well! Very surprised that this isn't atleast a STD. Also I'm seeing this strong shear in the Bahamas over by PR and DR yes there is very strong shear but not in the Bahamas. This may have to be watched despite what some think on here. It looks to me that if it goes SW then it would avoid the worst of the shear. Any thoughts? Kinda reminds me of Jeanne which many thought it would be ripped to shreads but instead pulled a fast one and intensified as it moved closer to FL. On another not the SST's in the Bahamas and FL are atleast 80.

By the I just used Jeanne as a comparisonthat's all as the track seems similar.


As it moves southwest, it will be encountering an increasing wind shear environment as shown with the latest wind shear analysis product from CIMSS-Wisconsin.


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Quoting RastaSteve:
91L is beginning to look very well! Very surprised that this isn't atleast a STD. Also I'm seeing this strong shear in the Bahamas over by PR and DR yes there is very strong shear but not in the Bahamas. This may have to be watched despite what some think on here. It looks to me that if it goes SW then it would avoid the worst of the shear. Any thoughts? Kinda reminds me of Jeanne which many thought it would be ripped to shreads but instead pulled a fast one and intensified as it moved closer to FL. On another not the SST's in the Bahamas and FL are atleast 80.

By the I just used Jeanne as a comparisonthat's all as the track seems similar.


Confluence aloft will cause the low to fill rapidly before it reaches the Bahamas.
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120. IKE
91l......


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Lawdy,..LOL
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Quoting nrtiwlnvragn:


Bullet point three specifically mentions subtropical.


If that slide was created by the NHC, then they are not including something in their glossary, which certainly mentions nothing about having organized convection in their definition of a subtropical cyclone. In fact, it even mentions that most subtropical cyclones have a disorganized convective pattern.

A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones. This system is typically an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles or more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection.
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116. beell
Moderate Risk

click graphic for full outlook

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XX/INV/91L
MARK(CORRECTED POSITION)
26.65N/63.75W
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Quoting Patrap:
It matters NOT if a system is named anyway.

I mean,,who gives a care?

One cant quantify everything,,esp in April.


Invest in time preparing for the Season as that's the only hedge bet one can do now,,for downstream later.


It's not a problem now, but it has been a big problem in the past when a storm is sitting near our coastline impacting people.

It was a problem in September, 2008 when a 991mb low was sitting off of the Carolinas looking like this, and the NHC did not name it:





It had a 7C difference between the inside of the core and the outside, as measured by recon, but for an undeclared reason they didn't classify it as even subtropical. People that don't see a name tied to a storm during the summer time usually don't see it as a threat, and often do not even know it is there. The impacts can be quite negative in that case if the storm is potent.

To avoid these problems, a more clearly-defined and straight-forward method of when to classify these things should be devised. One can easily cut out the subjective part to make things easier and less confusing/controversial.
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A little diversion from the 91L, facination but:-

I think this is about a mile from the Nuclear plant and 5.5.


Magnitude

5.5



Date-Time

Friday, April 22, 2011 at 15:25:19 UTC

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Quoting Levi32:


That is a well-known criteria for tropical cyclones. A subjective one, but at least it is written down. We're talking about subtropical cyclones, for which a different set of criterion exists.


Bullet point three specifically mentions subtropical.
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Earth Day images
Earth Day Graphics
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It matters NOT if a system is named anyway.

I mean,,who gives a care?

One cant quantify everything,,esp in April.


Invest in time preparing for the Season as that's the only hedge bet one can do now,,for downstream later.
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Quoting nrtiwlnvragn:
In my opinion, bullet point three is most likely the reason no designation on 91L.




That is a well-known criterion for tropical cyclones. A subjective one, but at least it is written down. We're talking about subtropical cyclones, for which a different set of criterion exists, and which mentions nothing about convective organization requirements.
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Quoting j2008:
I don't get this, it should at least be a subtropical-depresion based off of the way it looks combined with its wind speed of 30-35 mph.

I agree it looks nice on satellite nice counter-clockwise spin!
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In my opinion, bullet point three is most likely the reason no designation on 91L.


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.
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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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