|We’ve finally got a vortex message; I
found it at the CIMMS page on Helene:
Here’s the text:
WXTLIST WMO=URNT12 MATCH=HELENE
URNT12 KWBC 201739 2006263 1749
VORTEX DATA MESSAGE
B. 25 DEG 50 MIN N
56 DEG 42 MIN W
C. 500 MB 5567M
F. 306 DEG 58 KT
G. 229 DEG 28 NM
H. 960 MB
I. 2 C/5136 M
J. 6 C/5185 M
L. POORLY DEFINED
O. 1/1 NM
P. NOAA2 WXWXA HELENE04 OB 12 AL082006
MAX FL WIND 58 KT SW QUAD 1705Z
POOR EYEWALL RADAR PRESENTATION
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.
|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.|
|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.
Where did you get that term "saddle point"? I've never heard of it.
|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...
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.
|Thanks for that Barbara. Yes it is very descriptive, and apt. I like it.|
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...
|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...|
|I concur, David. Nice observation about upwelling.|
|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.|
|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.
|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
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.
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?