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| If Helene is to follow the predicted path
(attachment 1) to the north east of her current position, I would
think that the ridge of high pressure (attachment 2) that Helene will
encounter will aid in the development of anitcyclonic relative
vorticity. Attachment 3 illustrates the 500mb relative vorticity.
The increase in anticyclonic vorticity should accelerate Helene along
her projected path.
1_3Day_Cone_145035W_sm.gif
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| You lost me a bit here, Shawn. The ridge of high pressure will aid in the development of anticyclonic relative vorticity--which will accelerate Helene? How will vorticity accelerate a tropical cyclone's forward speed? | |
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| Shawn, Again, mid-level winds help to steer storms (the mean wind in the layer from 850-mb to 200 mb). 500-mb winds are a proxy for steering currents. |
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| As a parcel of air approaches a ridge, does it not acquire a anticyclonic relative voricity as it accelerates around the curved heights? Am I off basis with my thinking? | |
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| Yes, individual parcels do as they travel through a ridge in a relatively quiescent atmosphere. But, you can't apply parcel-theory to an entire weather system--which spans hundreds of miles. As Lee said, the mid-tropospheric winds are the key for steering. | |
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| The attached satellite picture from 1915Z
shows a nice picture of Helene about to run into the cold front
slightly to the west. With Helene's western cloud presentation already
beginning to deteriorate, I would expect the westerly shear shown in
the attachment (1200Z NAVY (NOGAPS) model below to do do even more
damage to the storm's circulation over the next 24 hrs. However, the NHC guidance continues to show its intensity holding just below hurricane force at 60kts, even once it becomes extratropical ! It is only at 70kts now. (1500Z advisory) Of course, it also shows it accelerating from 29N.6N to 39.6N in 48 hrs. (10 deg N lat *69 mi/deg lat /48 hrs )= 14.4 mph for just the northerly component. From the 1500Z NHC Forecast advisory for Helene: HURRICANE CENTER LOCATED NEAR 29.6N 56.9W AT 21/1500Z POSITION ACCURATE WITHIN 25 NM PRESENT MOVEMENT TOWARD THE NORTH OR 5 DEGREES AT 11 KT FORECAST VALID 23/1200Z 39.6N 46.6W...BECOMING EXTRATROPICAL MAX WIND 65 KT...GUSTS 80 KT. 50 KT...100NE 100SE 100SW 100NW. 34 KT...225NE 225SE 200SW 200NW. So, my question is: Does the fast acceleration due to the steering currents at 500mb, keep the hurricane or extratropical storm at that point from losing its intensity even though it is entering colder waters ? If not, why is the intensity forecast to still be a minimal hurricane at latitude 39N.6N on Saturday morning ? Link to NAVY (NOGAPS) model for 1200Z below - I would have used the GFS but the data past 12 hrs was missing. http://www.meteo.psu.edu/~gadomski/NGPTROPATL_12z/nogapsloop.html Thanks, Bill helene_and_cold_front.gif |
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| Bill, As a cyclone becomes extratropical, it no longer operates like a hurricane. In other words, it doesn't rely on warm ocean waters for its intensity. Intensity, if you're measuring it by maximum sustained wind speed, is really a function of pressure. Maintaining a low central pressure will allow the storm to maintain a strong pressure gradient, and thus, strong winds. Check out the 48-hour forecast for Helene from the GFDL. What do you think could be causing the cyclone to remain "deep" (low central pressure, strong pressure gradient)? |
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| Hi Bill, I think this might help you with
your question. The models show Helene holding quite a bit of her strength as the system becomes extratropical. The transition to extratropical occurs when the hurricane begins to stop relying on warm water to sustain itself. Extratropical systems feed off of the temperature contrasts between air masses as the storms poleward displacement increases. An expansion of the wind field also occurs when hurricanes become extratropical. |
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| Hi William, Yes, I agree with Brandon. As they become extratropical, they can still have hurricane force winds, so you'll see the NHC still referring to it that way sometimes. I think what's important at this transition stage is the fact that it's becoming more and more like a mid-latitude cyclone, so we have to start thinking of it more in terms of the cyclone model. So if you look at the NOGAPS and GFS maps you have there for Saturday, the upper right panel shows that she's interacting with a 500mb trough and cold pool, which will provide new energy to keep her pressure down and winds up. In addition, there's strong divergence coming over her from the jet stream. I've attached a divergence map from CIMMS showing the area of divergence at 150-300mb already starting to interact with her. As long as these two factors integrate with her, she can hold her intensity due to strong divergence and thus lift aloft. I also remember that we had a similar case with Florence. Take a look at last week's dicussion in briefings number 1 & 2 from September 11th. |
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| Excellent responses, Brandon and Phil! What do you think the source of divergence aloft will be fore Helene? (See GFDL prog I attached for Bill). | |
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| Hi Steve, It'll be right under the right entrance region of a powerful (140-160 knot) jet streak. That'll provide lots of upper level divergence. |
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| Hit the nail right on the head, Phil. | |
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| Hey Steve, Strong winds aloft at 300 millibars will provide significant divergence. The winds at this level in the vicinity of Helene are expected to be exceeding 100 mph. | |
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| The fast winds are definitely the culprit, Brandon--more specifically, see Phil's response for "how" the divergence is being created. | |
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| Thanks Steve, Brandon, and Phil Steve, I stared at that GFDL map and couldn't get it until about a minute ago when I realized I placed Helene incorrectly on the map. Now, I get it. Thanks for your patience guys. |
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