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Re: "Potential" tropical cyclones

Posted by Chris in Tampa on 6/20/2017, 2:04 am

It's all about watches and warnings. There were some instances where storms developed close to land and there wasn't a lot of time between when it officially had advisories started and landfall. A watch can be issued 48 hours in advance and a warning 36 hours in advance. But if there is no advisories, it could get really close to landfall before developing. Of course usually a depression/storm that forms is unlikely to be too powerful in terms of wind, but there are times when small storms can strengthen very quickly. Of course there are other threats too, like from rain. It is nice to have the full amount of warning.

But it's definitely going to be harder to forecast something without a well defined center. If it does eventually form, the center could also reform. And the track could be very different, shifting those early watches and warnings they issue. I suppose there is a balance. If there was a lot of uncertainty they might hold off on calling it a potential tropical cyclone. If they issues watches and warnings and the storm was so unorganized that the center reformed a hundred miles away and then that ended up allowing the storm to go in a new direction, that would have those watches and warnings not where you wanted them. Always possible that happens, but certainly a greater chance for something not very well organized. For Bret, perhaps they wanted to give people some warning it case the small storm developed more perhaps. Not a lot of ways to observe it and it was better defined. There's a small chance it could have developed more. But also, a tropical storm watch/warning is what it is. Whether it is 40mph or 70mph, you issue the same watches and warnings consistently. Just because it was on the weaker side didn't mean it wasn't worth issuing. As for Three in the Gulf, rain is a bigger threat from it, though that is not the requirement for a tropical storm watch or warning. They think 40mph wind is possible (watch) / expected (warning).




For Three in the Gulf recon didn't find what I would call a well defined center. From NHC discussions...

At 4pm CDT:

"The ASCAT data and visible
satellite imagery show that the center is not well defined, and in
fact multiple low-level swirls are evident in the latest imagery."

At 10pm CDT:

"Earlier this afternoon, an Air Force Reserve reconnaissance
aircraft investigating the central Gulf of Mexico disturbance found
that the multiple swirls that this system possessed had consolidated
into a single low-level circulation center with a pressure of about
1000 mb. However, since the circulation was and still is elongated
north-to-south, the large low pressure system is being maintained as
a potential tropical cyclone for this advisory cycle."

The recon mission did seem to show some improvement while they were out there. If you wanted to see the mission you could view it here and view a map:
http://hurricanecity.com/recon/recon.cgi?basin=al&year=2017&storm=Three&product=hdob

The wind barbs do show a circulation that is better than the start of the mission when they were seeing multiple wind shifts in different spots. I wasn't watching at the time so I didn't see if it was multiple swirls or if it was very elongated, more like a trough. But NHC said swirls. Later it was better defined. Half way through the mission they found a bit less of a wind shift north of where they later released a vortex message for. After that the wind barbs show a counter clockwise circulation that isn't too bad. It's hard to see because there is a lot wind barbs, some old and some new if viewing in the web based recon display where you can't hide some of the data.

I guess the best way to see how well the NHC thinks it can pinpoint a center is in the "Forecast
Advisory". In the first advisory for TWO in the Atlantic the "POSITION ACCURATE WITHIN" was "40 NM". (nautical miles) For the first one with THREE it was "60 NM". For the second one for THREE in the Gulf it was 45 NM. It went from multiple swirls to elongated and they estimated the accuracy of the center position better.



Some definitions from the NHC glossary are below. Sometimes they are loose with "organized deep convection" which is rather subjective. "well-defined center" is a bit less subjective and usually they get rather strict about that. It's better to have what they have rather than call something a tropical cyclone when it really doesn't meet the criteria. I suppose it might a little more confusing at first, but it's new. Hopefully people will get more familiar with it, most especially when we get a small storm that develops rapidly into a hurricane, perhaps within as little as a day, right before landfall. While weaker storms in terms of wind can have other serious impacts, it will be most useful I think for the rarer instances.

I do hope there's not too many of the potential tropical cyclones though. Especially ones that don't develop. They need to do things fairly standard, but there is more wiggle room with these I think.

You can read about the new policy change here in this PDF file:
http://www.nhc.noaa.gov/news/20170309_pa_2017SeasonChanges.pdf
Item 2.

They mention about how forecasts are getting better.

"Advances in forecasting over the past decade or so, however, now allow the confident prediction of tropical cyclone impacts while these systems are still in the developmental stage. For these land-threatening "potential tropical cyclones", NHC will now issue the full suite of text, graphical, and watch/warning products that previously has only been issued for ongoing tropical cyclones."

Models, forecast tools and the forecasters too are getting better. Track especially. I am curious though about whether the track error they calculate post-season will include these potential tropical cyclones. I would imagine track error is going to be worse. This could increase the error, though maybe if it does it would still not be enough to increase the average track error at various hour forecasts given improvements with tracking other storms in the year. 5 year average error determines the cone size too and it is nice to see that shrink each year. But maybe these will not be included. Maybe just while it was a depression or higher perhaps.



"Potential Tropical Cyclone:
A term used in NWS advisory products to describe a disturbance that is not yet a tropical cyclone, but which poses the threat of bringing tropical storm or hurricane conditions to land areas within 48 hours."

"Tropical Cyclone:
A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center. Once formed, a tropical cyclone is maintained by the extraction of heat energy from the ocean at high temperature and heat export at the low temperatures of the upper troposphere. In this they differ from extratropical cyclones, which derive their energy from horizontal temperature contrasts in the atmosphere (baroclinic effects)."

"Tropical Storm Watch:
An announcement that sustained winds of 34 to 63 knots (39 to 73 mph or 63 to 118 km/hr) are possible within the specified area within 48 hours in association with a tropical, subtropical, or post-tropical cyclone."

"Tropical Storm Warning:
An announcement that sustained winds of 34 to 63 knots (39 to 73 mph or 63 to 118 km/hr) are expected somewhere within the specified area within 36 hours in association with a tropical, subtropical, or post-tropical cyclone."

From: http://www.nhc.noaa.gov/aboutgloss.shtml

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"Potential" tropical cyclones - Beachlover, 6/20/2017, 12:47 am

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