Tropical Weather Briefing #5, Week of September 18

GEORGE KASICA - 9/22/2006 11:57 AM

Tropical Weather Briefing #5, Week of September 18 

Forecaster: Kasica

Basin: Atlantic

Storm: Hurricane Helene

At 15Z Hurricane Helene is currently located at 34.4N 54.2W and has increased its forward speed and is now moving at 040degrees at 18kts with maximum sustained winds of 75kts.
As stated in the discussion http://www.nhc.noaa.gov/text/refresh/MIATCDAT3+shtml/221441.shtml (limited shelf life warning applies) the inner core of convection is starting to fail and is only clearly visible on the NE Quadrant of the storm. (1km IR and Visible Image attached)

In the NHC Discussion they mention a phase space diagram, which I know is far beyond the scope of 241, but I thought I would include a link to it so that we could all at least look at what the NHC is seeing when they mention the item: http://moe.met.fsu.edu/cyclonephase/gfdl/helene08l/fcst/archive/06092206/6.html shelf life warning applies. For more information if youíre interested you can go to http://moe.met.fsu.edu/cyclonephase/ and thank former 241 alum, Winn Soldani for pointing these out in the Millibar Discussion area a few weeks ago! However we can look at the diagrams and gain some basic information Ė by glancing at the top diagram you can see it places Helene solidly into the Frontal system type of classification, not unexpected as it is currently undergoing extratropical transition. In looking at the 3rd and 4th panels you can see that Helene (or its remnants) is also in a region of very cold water with SSTís of around 17 degrees by the time of that forecast run. 

Lastly you can see in the attached 24 hour mid-layer steering winds forecast run from the E-Wall the trough that is referenced in the last portion of the NHC Discussion.

Discussion topics could be Heleneís continued transition to extratropical status, possibly explore the Phase Space Diagrams a little more to see if there are any changes in the succeeding runs, or analysis of Helene as  affects the area as an extratropical system.

Have a great Friday, 

George

1_2006-09-22.1445Z_IR_1KM_HELENE.75kts-NAmb-338N-550W.jpg
 
2_2006-09-22.1515Z_VIS_1KM_HELENE.75kts-NAmb-338N-550W.jpg  
 
3_2006-09-22_12Z_24HR_MIDLAYER_STEERING.gif
 
 

re: Tropical Weather Briefing #5, Week of September 18
Steve Seman - 9/22/2006 12:32 PM
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.
 
re: Tropical Weather Briefing #5, Week of September 18
GEORGE KASICA - 9/22/2006 01:25 PM
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 :)
 
re: Tropical Weather Briefing #5, Week of September 18
Steve Seman - 9/22/2006 01:29 PM
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!
 
re: Tropical Weather Briefing #5, Week of September 18
GEORGE KASICA - 9/22/2006 02:19 PM
Good point, we're 7 days away from e-port #1 being due.
 
re: Tropical Weather Briefing #5, Week of September 18
MIKE HJERMSTAD - 9/22/2006 03:07 PM
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 SSTís, 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
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re: Tropical Weather Briefing #5, Week of September 18
GEORGE KASICA - 9/22/2006 03:26 PM
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.
 
re: Tropical Weather Briefing #5, Week of September 18
MIKE HJERMSTAD - 9/22/2006 03:45 PM
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 donít know for a fact, but I would think some systems donít 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
 
re: Tropical Weather Briefing #5, Week of September 18
GEORGE KASICA - 9/22/2006 03:47 PM
Good questions all...Lee, Steve????
 
re: Tropical Weather Briefing #5, Week of September 18
PHILIP LUTZAK - Edited 9/22/2006 05:41 PM
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
 
re: Tropical Weather Briefing #5, Week of September 18
MIKE HJERMSTAD - 9/22/2006 06:12 PM
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
 
re: Tropical Weather Briefing #5, Week of September 18
PHILIP LUTZAK - Edited 9/22/2006 06:36 PM
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
 
re: Tropical Weather Briefing #5, Week of September 18
Steve Seman - 9/22/2006 09:52 PM
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.
 
re: Tropical Weather Briefing #5, Week of September 18
BRANDON JONES - 9/22/2006 07:56 PM
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