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George, I've got no problem with you introducing the phase-space diagram here in discussion (it does, afterall, graphically show Helene's transition to a cold-core system). But, the phase-space diagram is not a commonly used forecast tool (its more common applications are in research), and thus, it's a tool that is well beyond the scope of Meteo 241. So, folks, I'm not sure I'd spend much time pursuing further discussion regarding the phase-space diagrams in the realm of the weather briefings. I think it's much more worthwhile to continue to apply the growing list of tools you have at your disposal that fall with in the realm of operational forecasting. |
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Steve, exactly, my only reason for showing it was that it was mentioned in the Discussion and it was fairly easy to look at it and see some simple items on the output, I'm no expert on it either - that's Winn's department :) |
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Thanks, George. And, I had no problem with you introducing the concept. I just didn't want the entire forecasting thread to turn into a discussion of a tool that 1) isn't used very often in operational forecasting, and 2) is way beyond the scope of what we're doing here. I didn't want anybody to stress out! |
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Good point, we're 7 days away from e-port #1 being due. |
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Hi George, The tracking of Helen (1)over the next few days definitely puts her in the cooler waters (less than 26 C) as seen in the 36 hr forecast of SST’s, lower right panel.(2) At 12Z it appears Helen is in pretty much solid 26 degree temperatures already. (3). In looking at the IR satellite image you posted it clearly shows the north west quadrant having the better convective activity. I know we will be covering the evolution of hurricanes soon, but when does the official classification of extratropical take place, at a specific latitude or SST or value of wind shear? Mike 1-3day.gif 2-f36.gif 3-sst_922.gif |
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Mike, as they have been stating in the NHC discussions, the exact timing is difficult to call, but my impression is that it becomes extra tropical when it no longer has the characteristics of a tropical system like an eye wall, and the "power" or energy for the storm is coming from what you'd expect on a mid-latitude system as you saw in 101 and 361 (I think you took 361 :)). I don't think it has anything to do with SST, latitude or wind shear. |
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Hey George, I can see having trouble with the timing, but I think the SST, low wind shear, and latitude have a lot to do with a tropical system. And I don’t know for a fact, but I would think some systems don’t even form an eye before they become extratropical. It just seems to me that they must have a criteria list that they would go by to declare a storm to be extratropical. Mike |
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Good questions all...Lee, Steve???? |
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Hey Mike & George, While we're waiting for Steve or Lee to comment, I thought I'd throw this in: The SSTs obviously are important; if the storm moves over cooler waters it will either change to extratropical or dissipate, depending on whether there's any baroclinic zone nearby and/or upper level support. Also, it can go the other way: a mid-latitude low can become extratropical and then tropical if it starts exhibiting warm-core characteristics, for example by moving over warm Gulf waters and sitting there for a few days. Here's something from the NHC: http://www.aoml.noaa.gov/hrd/tcfaq/A7.html And as to when it's considered extratropical, we have to use our knowledge of the cyclone model, especially what we learned about their satellite characteristics and the upper air to figure out how far along they've gone to extratropical. Often the process starts when the tropical cyclone moves close to a baroclinic zone (cold or stationary front and/or trough) and cold air starts to enter the system. We can look for the conveyor belts and expanding wind fields on the satellites. We can look to see if any 500mb trough is going to phase or is phasing with it. We can look for jet streaks, especially right entrance or left exit regions, coming into play. As for the NHC, the advisories and discussions always try to tell you as close as they can. For example, today's 5PM discussion on Helene: FORECAST POSITIONS AND MAX WINDS INITIAL 22/2100Z 35.4N 53.0W 65 KT 12HR VT 23/0600Z 36.8N 50.1W 60 KT 24HR VT 23/1800Z 38.4N 45.3W 65 KT...EXTRATROPICAL 36HR VT 24/0600Z 40.3N 40.5W 65 KT...EXTRATROPICAL 48HR VT 24/1800Z 42.2N 36.1W 60 KT...EXTRATROPICAL 72HR VT 25/1800Z 47.0N 28.0W 55 KT...EXTRATROPICAL 96HR VT 26/1800Z 51.0N 20.0W 45 KT...EXTRATROPICAL 120HR VT 27/1800Z 55.0N 14.0W 40 KT...EXTRATROPICAL It seems to me there's often going to be a fuzzy period of a few hours to even days when the storm is in between stages and nobody can say for sure what it is. I think thats why they usually talk about a transition period, so that all bases are covered. By the way, the currrent (5PM 09-22-2006) discussion on Helene is particularly interesting as it covers a lot on this very topic: her incipient transformation to extratropical. It also clearly shows how difficult it is to analyze, and how even the experts can have trouble pinning it down. Here's the (short shelf life) link: http://www.nhc.noaa.gov/text/refresh/MIATCDAT3+shtml/221441.shtml |
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Hi Philip, That clears up most of the question of when a storm might become extra-tropical and how difficult it is to pin point the timing. The reference to the core temperature, elevation of maximum sustained winds, and frontal structure was reinforcing your mention of conveyor belts and other mid latitude cyclone features that may be seen on satellite imagery. The concept of mid latitude cyclones going tropical is interesting, I wonder how often something like that occurs. Mikee |
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Hi Mike, Thanks for the comments. I could take a stab at your question about how often we see mid-latitude storms going tropical, since I'm a rabid hurricane watcher and have been following every season for many years. In all of the years I've been watching, I'd say that you see at least one or two in most seasons, so it's actually not that rare. They are often from a cold-core low forming on the end of a dying cold front off the south or southeast US coasts, and then transitioning to tropical as they sit over very warm water. But I'm sure we will be covering this in detail later in the course, and I wouldn't want to guess on all of the physics going on there before we learn about it. But I would dare say that that's the most common case. By the way, Beryl of this year came close to that type of storm, although she developed out of a decaying cold front/trough rather than a fully formed cold-core low. But I guess you could still say she developed from a cold-core feature. Here's the NHC wrap-up: http://www.nhc.noaa.gov/pdf/TCR-AL022006_Beryl.pdf |
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Thanks, Phil. Excellent response.
You hit on a lot of important points about extratropical transition. It's a process--not something that happens at the flick of a switch. Forecasters are watching the evolution of internal storm-data and satellite presentation, as well as the overall synoptic pattern. The classification is also somewhat subjective. During the transition period, there's often some uncertainty about how to exactly classify the storm. |
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The latest from the National Hurricane
Center indicates Helene is entering the early stages of the
extratropical transition. You can see that the convection continues to
erode especially on the southern half of the system. (See attachment
IR). The erosion of the convection means that Helene's strength will
began to rely on temperature contrasts for energy, rather than warm
SST's. The NHC also shows Helene strengthening as an extratropical
system, see excerpt from the 5 PM disscussion. FORECAST POSITIONS AND MAX WINDS INITIAL 22/2100Z 35.4N 53.0W 65 KT 12HR VT 23/0600Z 36.8N 50.1W 60 KT 24HR VT 23/1800Z 38.4N 45.3W 65 KT...EXTRATROPICAL 36HR VT 24/0600Z 40.3N 40.5W 65 KT...EXTRATROPICAL 48HR VT 24/1800Z 42.2N 36.1W 60 KT...EXTRATROPICAL 72HR VT 25/1800Z 47.0N 28.0W 55 KT...EXTRATROPICAL 96HR VT 26/1800Z 51.0N 20.0W 45 KT...EXTRATROPICAL 120HR VT 27/1800Z 55.0N 14.0W 40 KT...EXTRATROPICAL Dry air also continues to filter into the storm as it becomes more involved with the frontal system. These are also signs that the extratropical transition is occuring. (See Atlantic surface analysis and water vapor images attached) Atlantic_Surface_Map.gif Helene_IR_1.jpg Helene_WV_1.jpg |