Tropical Weather Briefing #3, Week of September 18  

GEORGE KASICA - 9/20/2006 11:13 AM

Tropical Weather Briefing #3, Week of September 18 

Forecaster: Kasica

Basin: Atlantic

Storm: Hurricane Helene

As Gordon has been downgraded to a tropical storm and there is no activity in the Pacific Basin at this time I’m going to switch back to Hurricane Helene for the briefing today.
At 15Z Hurricane Helene is currently located at 26.0N 56.3W and moving at 315degrees at 10kts with maximum sustained winds of 90kts.

As stated in the discussion http://www.nhc.noaa.gov/text/refresh/MIATCDAT3+shtml/201436.shtml limited shelf life warning applies) the intensity decrease was not expected given the environment around the storm and the forecasters at the NHC seem to be a bit puzzled by this. 

The latest water vapor satellite image (attached) shows that Helene has a restriction on outflow to the southwest as mentioned in the discussion. 

I’m also attaching the mid and upper level shear products from UW-Madison CIMSS as they are referenced as well in the discussion above, and in looking at them you can see the shear they are referring to as well.

There are a number of items to pursue here from what I can see in the discussion: Additional weakening from shear, the large trough coming off the US East Coast, an upper level jet in about 72 hours possibly as Helene is transitioning to an extratropical system, and lastly the track and intensity forecasts as they are now recording some effects of the storm at several east coast observing locations.

Have a great Wednesday, 

George

1_2006-09-20_1415Z_WV_1km.08LHELENE.95kts-958mb-256N-560W.jpg  
2_2006-09-20_12Z_MID_LEVEL_SHEAR.gif
 
 
3_2006-09-20_12Z_UPPER_LEVEL_SHEAR.gif
 
 

 
re: Tropical Weather Briefing #3, Week of September 18
PHILIP LUTZAK - Edited 9/20/2006 04:12 PM
We’ve finally got a vortex message; I found it at the CIMMS page on Helene:
http://cimss.ssec.wisc.edu/tropic/real-time/atlantic/storm/storm3.html

Here’s the text:

WXTLIST WMO=URNT12 MATCH=HELENE
URNT12 KWBC 201739                                              2006263 1749
VORTEX DATA MESSAGE
A. 20/1717Z
B. 25 DEG 50 MIN N
  56 DEG 42 MIN W
C. 500 MB 5567M
D. NA
E. NA
F. 306 DEG  58 KT
G. 229 DEG  28 NM
H. 960 MB
I. 2 C/5136 M
J. 6 C/5185 M
K. NA/NA
L. POORLY DEFINED
M. NA
N. 12345/5
O. 1/1 NM
P. NOAA2 WXWXA HELENE04 OB 12  AL082006
  MAX FL WIND 58 KT SW QUAD 1705Z
  POOR EYEWALL RADAR PRESENTATION
WXTLIST: done

From the looks of this, Helene isn’t in good shape right now. Although the pressure is a quite respectable 960mb, the max flight level (in this case, 500mb) winds found were only 58 knots in the SW quadrant. Also we can see in “L” that the eye is poorly defined. I haven’t been able to find the flight plan anywhere yet, so I’m guessing that they’re still flying around in there and we’ll be getting more data; it seems unlikely that the highest winds are only 58 knots. But we can see why there has been weakening. Look at the water vapor image from 1715 in attach 01. There’s dry air getting pulled into the circulation on the south and west sides, and you can see the strong south-southwesterly flow associated with the approaching trough and cold front starting to engage with the storm, which will soon add a lot of shear. Also, attach 02 shows how her Dvorak t-numbers have dropped considerably. They were around 6.4 late last night.

01_2006-09-20_1715_sat_wv_Helene.JPG    
02_2006-09-20_1915_Dvorak_ADT.JPG
 

 
re: Tropical Weather Briefing #3, Week of September 18
GEORGE KASICA - 9/20/2006 04:45 PM
WOW! The storm is REALLY falling apart quickly....I'm suprised to see it happen this fast, but it did weaken fast overnight as well.
 
re: Tropical Weather Briefing #3, Week of September 18
BARBARA DECLERCK - 9/20/2006 04:44 PM
Based on the QuikScat data at 18Z on 20 Sep, both the upper level winds (Helene_mid_level_winds.gif) and lower level winds (Helene_low_level_winds.gif) indicate that the storm is moving into a saddle point (light and variable winds) in both layers.  I think this indicates that the storm will lose its punch, slowing and getting caught up in the front approaching from the west.

Seeming to the contrary, the SST (SST_20Sep18Z.gif) shows abundant warm water both to the west and north of Helene’s location.  I believe the warm water will continue to support convection, but the storm winds will not.  If you look at the satellite image provided by George, the storm appears lopsided, with the vast majority of the convection in the NNW clockwise through S quadrants.
Helene_low_level_winds.gif    
Helene_mid_level_winds.gif
 
 
SST_20Sep18Z.gif
 
 
re: Tropical Weather Briefing #3, Week of September 18
PHILIP LUTZAK - 9/20/2006 04:51 PM
Hi Barbara,
  Where did you get that term "saddle point"? I've never heard of it.
 
re: Tropical Weather Briefing #3, Week of September 18
PHILIP LUTZAK - 9/20/2006 04:57 PM
It was as I suspected in my last post; the P-3 is still investigating the storm, so there must be winds higher than 58 knots somewhere in Helene. I assume they'll find the highest winds in the right front quadrant when they get there. Here's an excerpt from the 5PM discussion:

A NOAA P-3 EQUIPPED WITH THE SFMR INSTRUMENT HAS BEEN SAMPLING HELENE THIS AFTERNOON.  THUS FAR...THE DATA CONFIRMS THAT THE EYEWALL IS POORLY DEFINED AND THE SYSTEM HAS A LARGE WIND FIELD. AS A RESULT...THE INTENSITY SHOULD BE SLOW TO RESPOND TO CHANGES IN THE ENVIRONMENT. WHILE THE DATA FROM THE SFMR AND THE LARGE WIND FIELD ARGUE AGAINST A 90 KT HURRICANE...AN EYEWALL DROP REPORTED AN
IMPRESSIVE 960 MB.  IN LIGHT OF THE PRESSURE AND THE FACT THAT THE PLANE HAS NOT YET SAMPLED THE ENTIRE STORM...THE INITIAL INTENSITY IS HELD AT 90 KT.  IF THE PLANE DOES NOT FIND HIGHER WINDS IN SUBSEQUENT PASSES...THE INTENSITY WILL NEED TO BE LOWERED LATER.

I guess we'll just have to wait and see...
 
re: Tropical Weather Briefing #3, Week of September 18
BARBARA DECLERCK - 9/21/2006 01:42 AM
Philip--

A saddle point is the intersection of two ridges and two troughs.  So, a constant pressure surface looks like a saddle (high in front and back, low on each side).  It's a very descriptive term, wouldn't you agree?  Saddle points tend to be more easily identified on streamline charts and are typically characterized by light and variable winds.
 
re: Tropical Weather Briefing #3, Week of September 18
PHILIP LUTZAK - 9/21/2006 02:57 PM
Thanks for that Barbara. Yes it is very descriptive, and apt. I like it.
 
re: Tropical Weather Briefing #3, Week of September 18
DAVID WILLIAMS - 9/20/2006 06:12 PM
Hi, everyone.

One factor that will hasten Helene's demise over the next several days is sea surface temperature.  Currently, the storm is at a latitude of 26.6N, and the corresponding SST is 28.10C.  However, cold waters loom just to the north, as the critical 26C isotherm is at approximately 35N.

Helene will move into the colder waters over the next 2-3 days, as the corresponding SST is forecast to be 23.65C by 60 hours...

...also, note how close Helene is forecast to move to colder waters in the wake of Gordon (upwelling?) during the next 24-36 hours... 

recent_na_sst.jpg  
 
re: Tropical Weather Briefing #3, Week of September 18
PHILIP LUTZAK - 9/20/2006 07:27 PM
Nice point, David, about the upwelling. Especially interesting is the pool of cooler water around 30N and 52W, where we see low to mid twenties C water temps showing up in the midst of more typical warm water for that region (mid to high twenties). And it just "happens" to be where Gordo stalled out for a while. Hmmm...
 
re: Tropical Weather Briefing #3, Week of September 18
Steve Seman - 9/20/2006 09:47 PM
I concur, David.  Nice observation about upwelling.
 
re: Tropical Weather Briefing #3, Week of September 18
SHAWN O'LEARY - 9/20/2006 11:02 PM
The 5PM NHC discussion mentions a poor defined eye wall.  Attached is the TMI wind image from 21Z 9-20-2006, to illustrate the decomposing eye.  With the projected path curving off to the north east, cooler sea surface temperatures to the north of Helene, and the eye wall breaking down it doesn’t seem like Helene can maintain or deepen her intensity.

  20060920.2120.trmm.x.wind.08LHELENE.90kts-NAmb-259N-568W.40pc.jpg.jpeg  

 
re: Tropical Weather Briefing #3, Week of September 18
BRANDON JONES - 9/20/2006 11:28 PM
The latest information from the NHC isn't too different from the 5 PM advisory package. Helene continues to fight some westerly shear. I believe the main problem for Helene right now is the dry air that has been pulled in to the storm (see attachment of a water vapor image). 

I looked at an experimental product used at the CIMMS called Morphed Intergrated Microwave Imagery. Here is the link to the page. Check out the loop of Helene, it will take a moment to load. The images show the deterioration of the eyewall of Helene very well. 

(http://cimss.ssec.wisc.edu/tropic/real-time/marti/2006_EIGHT/webManager/displayGifsBy12hr_13.html)

  Helene_Water_Vapor.jpg  

 
re: Tropical Weather Briefing #3, Week of September 18
FRANK DEMPSEY - 9/20/2006 11:58 PM
I followed George’s suggestion about looking at the large trough moving eastward over eastern North America and the effect on the path of Helene. No question that it causes the path to curve northward and then northeastward – as expected for meeting the SW flow east of the big trough. (Attachment 1 shows the 500 mb sequence to 66 hr with the storm nearly attached to the base of the trough.) But a very interesting detail about the steering winds is that the hurricane or storm not only gets influenced by the steering winds, but also affects and becomes part of the overall steering pattern (at least in the model forecasts). In this case, by 72 hours, the storm is definitely absorbed in the SW flow between the trough and a big mid-Atlantic ridge but by 96 hours the storm (or whatever status it has by then) is moving eastward through the ridge, and not along the flow that might have been expected (Attachment 2). The attached charts are mostly sticking with the GFS but also the European ECMWF model charts attached (attachments 3-6) show the forecast 500 mb pattern for the northern hemisphere, and the 144 hr forecast shows the remains of Helene heading toward Great Britain in a week, having cut through the middle of the ridge, and also Gordo’s remnant has moved across Spain.
Anyway I’m not sure of any relevance to storm motion toward the eastern USA but in the bigger picture it seems to be an interesting case of the storm following the mid and upper-level steering currents to the mid-Atlantic and then moving through a strong ridge instead of along the “steering winds” flowing northward around the ridge.
Cheers,
Frank 

1_Sept_20_gfs_500_066m.gif    
2_Sept_20_gfs_96h_mean_wind.gif
 
 
3_Sept_20_EC_72h_500_mb.gif
 
 
4_Sept_20_EC_96h_500_mb.gif
 
 
5_Sept_20_EC_120h_500_mb.gif
 
 
6_Sept_20_EC_144h_500_mb.gif
 

 
re: Tropical Weather Briefing #3, Week of September 18
PHILIP LUTZAK - Edited 9/21/2006 01:54 AM
Hi Frank,
  Although it looks like the storm is pushing through the middle of the ridge, so to speak, I don't think it actually is. 
Just to cover all the bases, as far as her current northward movement, you can see the weakness she is moving through in the middle of the Atlantic ridge that extends from the southeast US to north central Africa in attach "entire_UA" from the Ocean Prediction Center. 
  But when she gets far enough north and reaches the westerlies, they will pick her up and she'll move along with them towards Europe. If you look at your 4-panel attach 2, upper left, you can clearly see the steering winds (in this case the westerlies), with Helene riding in the middle of the them. She'll continue to ride them as far as she can go to the east before dissipating. The other diagrams (3-6) don't show it as well. They make it look like she is more embedded in the ridge, but she's actually in the westerlies along the north side of the ridge. Look also at this attached more detailed GFS forecast map f96. She is clearly embedded in the westerlies rather than cutting through the ridge, although there is an associated trough beneath her passing through the northern side of the ridge. Does that make sense?

2006-09-24_1200_forecast_GFS_f96.gif    
entire_UA.gif