The Steering of Dorian

By: Lee Grenci , 3:54 PM GMT on July 26, 2013

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The 5 A.M. discussion from the National Hurricane Center indicated that Tropical Storm Dorian "lost organization" as it encountered southwesterly wind shear and middle- to upper-tropospheric dry air (one of the traditions I learned from the late John Hope was to never use "he" or "she" to describe a named tropical cyclone). NHC's discussion also focused on low- to middle-tropospheric winds associated with the the Atlantic subtropical high-pressure system (06Z GFS model analysis of 700-mb heights early this morning) as the primary steering currents for Tropical Storm Dorian (see the 06Z GFS model analysis of 700-mb heights and 700-mb streamlines below (larger image). At the time, Dorian was moving to the west-northwest at 17 knots.



The 06Z GFS model analysis of 700-mb heights and 700-mb streamlines on July 26, 2013. 700-mb wind speeds are color-coded in knots. Larger image. Courtesy of Penn State.

When I was a young forecaster (a long, long time ago), I typically looked at mid-tropospheric winds as a proxy for the general movement of tropical cyclones. That's because mid-tropospheric winds serve as a rough approximation for the mean airflow in the troposphere. More specifically, old timers like me looked at the winds between 700 mb and 500 mb at a radius of approximately five to seven degrees latitude from the center of the storm (one degree latitude equals 60 nautical miles). As it turns out, winds in the layer from 700 mb to 500 mb often tend to correlate best with the movement of tropical cyclones (at these radii, environmental winds are essentially unaltered by the circulation associated with the tropical cyclone).

Obviously, my approach as a young forecaster was old school. Nonetheless, my simple method had some merit. Indeed, research has shown that a deep-layer mean flow (between 1000 mb to 100 mb, for example) can be used as a tool to assess steering currents (this technique captures the spirit of my old-school approach).

Subtropical highs are not the only features that steer tropical cyclones. Indeed, mid-latitude systems (500-mb troughs, for example) can also steer tropical cyclones as they move poleward from the Tropics. At times, two tropical cyclones can steer each other, assuming that they're close enough for their circulations to interact (the Fujiwhara effect...a topic for a future blog). Finally, tropical cyclones contribute to their own steering, especially when steering currents are rather weak (the Beta effect, which is fodder for another future blog).


The variation of the steering layers for tropical cyclones with minimum central pressure. Larger image. Courtesy of CIMSS and Dr. Chris Velden.

That's all well and good, Grenci, but why did NHC specifically reference "low- to mid-tropospheric winds in their 5 A.M. discussion today? Experience gained from the careful observations of operational forecasters eventually prompted further research aimed at establishing the connection between the minimum pressure of a tropical cyclone and the corresponding depth of the steering layer. The bar graph above (larger image), which displays the minimum pressure of tropical cyclones versus the depth of their steering layers in the Atlantic basin, supports the notion that the steering layer for a tropical depression is shallower and resides lower in the troposphere. In contrast, the steering layer for strong hurricanes is much deeper. The simple physical connection for you to take away after reading my blog is that a weak tropical cyclone (like Dorian) is usually associated with a shallow vortex. Thus, the mean wind in a correspondingly shallow and low-level layer serves as the steering current. As a general rule, the deeper the vortex, the deeper the layer mean that steers the tropical cyclone.


The 09Z analysis of the streamlines designating the mean wind in the layer from 850 mb to 700 mb on July 26, 2013. Larger image. Courtesy of CIMSS.

To get a better sense for the movement of Dorian, focus your attention on the first layer on the left of the bar graph above (central pressure between 1000 mb and 1010 mb). The steering current for Dorian and other similarly weak tropical storms boils down to the mean wind between 850 mb (roughly 5000 feet) and 700 mb (10000 feet). The 09Z analysis from CIMSS (above; larger image) indicates the streamlines of the mean wind in the layer between 850 mb and 700 mb. Wind speeds are color-coded in knots.

At the other end of the spectrum, note the deep steering layers for strong tropical cyclones whose central pressures are lower than 940 mb or range from 940 mb to 949 mb.

I should point out that these results do not include the impact of the Beta effect on the movement of a tropical cyclone. Moreover, other factors such as season, latitude, easterly versus westerly environmental flow, the rates at which the intensity of tropical cyclones changes with time, etc., probably can skew these results a bit, but, as a general rule, the bar graph above will get you in the ballpark in all the ocean basins.

We've come a long way since I was a young forecaster.

Lee

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Issued: Jul 26, 2013 8:00 pm HST

For the central north Pacific, between 140°W and 180.

1. The National Hurricane Center in Miami Florida is issuing advisories on tropical storm Flossie, centered about 1220 miles east-southeast of Hilo Hawaii. Flossie is moving west near 20 mph, and is expected to cross 140°W into the Central Pacific Hurricane Center area of responsibility on Saturday.

Public advisories on tropical storm Flossie are now being issued under WMO header wtpz31 KNHC and under AWIPS header MIATCPEP1. Forecast/advisories on Flossie are now being issued under WMO header WTPZ21 KNHC and under AWIPS header MIATCMEP1.

Otherwise, no tropical cyclones are expected through Sunday evening.

DONALDSON
Member Since: July 21, 2011 Posts: 83 Comments: 7167
Quoting 1569. plywoodstatenative:
Kori, shows how long I have been gone, and how inactive the seasons have been.


Last year was very active.
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Quoting 1565. scott39:
Im working on trying to learn how to read maps better. I thought some of you would be open to a lesson on them.

From my perspective...I don't look at the tons of "conventional" maps out their to understand what's going on....

I usually generate my own each day during the hurricane season....and post them on my blog. The two maps (one I call atmospheric and the other I call thermodynamic) I put out on my blog help me understand what is going on. My latest blog post was 6 AM yesterday morning...so my next map will come out tomorrow morning.
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Quoting 1571. Camille33:

Now explain to me how it is coming back? This is diurnal max in where the ocean is warmer than the air temp...so it fires storms.But the air is to dry and it can't form an anticyclone without at least 12 - 18 hours of storms over a low level center,it is not organizing at all.


...What? Didn't... didn't you just say, literally, that StormJunkie didn't know what he was talking about? That the shear in front of Dorian was lessening?

You may not have said it directly, but you definitely gave an implicit tone that you possessed some degree of confidence that it would survive the mid-oceanic trough.

Holy inconsistency, Batman.
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Quoting 1568. AllStar17:


StormWalsh?

Stormw is banned,he tried to promote himself.
Member Since: July 2, 2013 Posts: 0 Comments: 1523
Doppler, in the month of July we have had here in Fort Lauderdale 2 recorded tornadoes. One hit Pompano Beach during lifeguard training and injured 4 and one destroyed a boat dealership on Wed. Talk about strange.
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Quoting 1565. scott39:
Im working on trying to learn how to read maps better. I thought some of you would be open to a lesson on them.


Well, let me know what you want to understand, and I'll do my best to elucidate it for you.
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Quoting 1564. wunderkidcayman:
come on guys you got to give it to me I did say that Dorian will make a comeback tonight most of you were RIPing and Downcasting but I had called it

Now explain to me how it is coming back? This is diurnal max in where the ocean is warmer than the air temp...so it fires storms.But the air is to dry and it can't form an anticyclone without at least 12 - 18 hours of storms over a low level center,it is not organizing at all.
Member Since: July 2, 2013 Posts: 0 Comments: 1523
1570. sar2401
Quoting AussieStorm:


I don't know where the anti-cyclone you are talking about is.....


Yeah, I'd be interested in the answer to that one also.
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Kori, shows how long I have been gone, and how inactive the seasons have been.
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Quoting 1558. plywoodstatenative:
There are bloggers here that have been around here for a while. People like Patrap, StormJunkie would be good people to talk to. They have been in here since Katrina. I come in whenever storms pop up, but really do not hang around. If anyone has questions, ask those of us vets in here, we are more than happy to help. Every so often you get weatherguy03 who pops in or StormWalsh, but thats as I said every so often


StormWalsh?
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Not to take away from Dorian but a small rotation couplet passed over the small town of Hall Summit, LA. I hope there was no tornado
Member Since: February 13, 2012 Posts: 11 Comments: 3882
Quoting 1552. scott39:
Hey you experienced bloggers....Would love your thoughts on the Anticyclone that is currently starting to shield Dorian?

My most thorough thoughts on your question here are in post 1526. I don't see Dorian making a complete home-run with this upper anticyclone though....that post best explained why with a link to help out...
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1565. scott39
Quoting 1557. KoritheMan:


I don't think there is one.
Im working on trying to learn how to read maps better. I thought some of you would be open to a lesson on them.
come on guys you got to give it to me I did say that Dorian will make a comeback tonight most of you were RIPing and Downcasting but I had called it
Member Since: June 13, 2009 Posts: 2 Comments: 12719
1563. sar2401
Quoting FlaStGuy:


His emoticon made me think he was joking. I just know how the blog typically reacts to new posters showing up in July/August as most are typically trolls. Feeding frenzy was a bad analogy. I just felt that this was a blog dominated by more experienced and adult aged individuals for the most part (though I guess I am well into that stage of life lol)

No hard feelings, I am a sarcastic person and have a semi thick skin already so I'll take comments in stride.

I don't know if Kori was joking. Might have been, but I try not to do that with new people. There are many young people on this blog, so nothing to fear there, I can almost guarantee you that, at 67, I'm the oldest person on here now. If your join date was a couple of days ago, I would immediately expect you were a troll, since we've had a bunch lately. Your join date of last year puts me more at ease, as does the maturity of your posts. I have actually been quite surprised when I found out how young some of the regulars are here, since their postings don't give it away, OTOH, we have a few adults here you might expect just graduated sixth grade at the bottom of their class. :-)
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Quoting 1559. KoritheMan:


StormW left a long time ago.


Footed it right on out of here...
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Quoting scott39:
Hey you experienced bloggers....Would love your thoughts on the Anticyclone that is currently starting to shield Dorian?


I don't know where the anti-cyclone you are talking about is.....




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Quoting 1419. AllStar17:


Who knows.


Ask Alexander. He is the man who knows.
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Quoting 1558. plywoodstatenative:
There are bloggers here that have been around here for a while. People like Patrap, StormJunkie would be good people to talk to. They have been in here since Katrina. I come in whenever storms pop up, but really do not hang around. If anyone has questions, ask those of us vets in here, we are more than happy to help. Every so often you get weatherguy03 who pops in or StormWalsh, but thats as I said every so often


StormW left a long time ago.
Member Since: Posts: Comments:
There are bloggers here that have been around here for a while. People like Patrap, StormJunkie would be good people to talk to. They have been in here since Katrina. I come in whenever storms pop up, but really do not hang around. If anyone has questions, ask those of us vets in here, we are more than happy to help. Every so often you get weatherguy03 who pops in or StormWalsh, but thats as I said every so often
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Quoting 1552. scott39:
Hey you experienced bloggers....Would love your thoughts on the Anticyclone that is currently starting to shield Dorian?


I don't think there is one.
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I've been closely watching the RGB satellite imagery for Tropical Storm Dorian all night and in the most recent images, it appears the beginnings of outflow overhead may be taking shape as you can see to the northeast and southwest of the circulation center. In addition, organization may be trying to take place as the convection attempts to become more symmetric over the storm and the convective cluster to the northeast has taken on a curved appearance as if becoming more like a banding feature. Will be interesting to see if this continues.
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I just had to laugh at this... Link

I think everyone can relate to it.
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Who wants to make bets on how long before we start to see lower-level outflow boundaries? :P

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The vorticity max at the low levels will survive into the western bahamas due to weak environmental shear...and thus have a shot reform when the system heads into higher ssts/tchp and potentially forms an anticyclone given moister air.

Right now it has lost it's anticyclone and lost it's life engine,and it was injured.The system is not permanently disabled until it fractures it's vorticity.Keep in mind given the dry air...the atmosphere has to do a lot to saturate itself and reach the Level of free-convection and during dmax where the atmosphere is un-capped it is easier to do so.Now keep in mind when the atmospehre warms tomorrow and cinh increases...and the dry air still remains...the convergence of the vort can't eliminate 2 stones at once. This is a temporary cluster and will survive for 6 hours. If you want development wait 3 - 4 days.
Member Since: July 2, 2013 Posts: 0 Comments: 1523
1552. scott39
Hey you experienced bloggers....Would love your thoughts on the Anticyclone that is currently starting to shield Dorian?
Quoting 1538. theyoungmet:


To all the teens on the chat room, I'll join in sometime soon, but I'm on my phone because I'm on a road trip and I won't be able to get on for a while.

Also, welcome FlaStGuy! We are the newbies :)

Anyway, I was hoping that ULL was gonna aid the outflow channels for Dorian, but it might just tear him apart instead. Geez ULLs can be such double sided daggers. Dorian doesn't have the ITCZ to aid convection, so he literally will live and die by Dmin and Dmax as well as his ability to tap into a moisture feed closer to the islands past the dry air. I don't see an anticyclone coming back anytime soon though.

I guess I am the last one to welcome you on here...so ummm...welcome!

As far as the upper anticyclone...their is already one near 20N 52.5W (see post 1526). Its not so much that an anticyclone has to come back...this anticyclone has already been pre-existing in the vicinity of the Lesser Antilles over the last few days.
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1550. sar2401
Quoting FlaStGuy:


While sort of disappointing, isn't at least a bit impressive that in comparison to what he looked like around 3/4pm that Dorian was able to moisten his environment as much as he has?

I wish I could say yes but, really, look at all the other convection around the Basin. All those blobs typically increase their convection as we get toward dawn. TC's are not the only thing affected by DMax, it's any convection of any kind in the tropics. If Dorian hadn't fired off some convection, the NHC would have it as a remnant low at 5 am. It has bought itself a few more hours of life, but that's about it.
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Quoting 1536. FlaStGuy:


No, just felt like if I started posting with a handful of other new bloggers I wouldn't be subjected to the entire fury of the blog if I said something dumb.

The dynamics of this blog are frighteningly similar to what I anticipate my thesis defense to be. A feeding frenzy of more knowledgeable individuals looking to exploit my miscalculations or incorrect judgements


There are sharks in the water here for sure but most are all bark and no bite, and when i say bark i mean (copy and paste experts or quasi-plagiarism re-post operatives). Nothing to be afraid of either way :)
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Quoting 1545. sar2401:

Geez, I don't think that's what he said at all. I think he feels more comfortable posting because there are some other new members posting as well. Your kind of response was exactly what he was trying to avoid.


His emoticon made me think he was joking. I just know how the blog typically reacts to new posters showing up in July/August as most are typically trolls. Feeding frenzy was a bad analogy. I just felt that this was a blog dominated by more experienced and adult aged individuals for the most part (though I guess I am well into that stage of life lol)

No hard feelings, I am a sarcastic person and have a semi thick skin already so I'll take comments in stride.
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1547. vis0
Quoting 277. barbamz:


Doesn't Dorian resemble the horsehead nebula in some way? I remember an article some days ago dealing with this resemblance of very different systems in our world, using the idea of fractals. I've tried to find it again, but I've failed. :(

If people only knew, BTW GTcc's blob AREA off the SE has a chance to help Dorian (as stated 3 days ago)
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Quoting 1545. sar2401:

Geez, I don't think that's what he said at all. I think he feels more comfortable posting because there are some other new members posting as well. Your kind of response was exactly what he was trying to avoid.


*sigh*

I really wish I was better at expressing my sense of humor. I thought that emoticon at the end would have clarified that.

Sorry if I offended anyone.
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1545. sar2401
Quoting KoritheMan:


Oh, so you're here to set the "newbies" straight, is that it? Charming. ;)

Geez, I don't think that's what he said at all. I think he feels more comfortable posting because there are some other new members posting as well. Your kind of response was exactly what he was trying to avoid.
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1544. Kumo
Quoting 1536. FlaStGuy:


No, just felt like if I started posting with a handful of other new bloggers I wouldn't be subjected to the entire fury of the blog if I said something dumb.

The dynamics of this blog are frighteningly similar to what I anticipate my thesis defense to be. A feeding frenzy of more knowledgeable individuals looking to exploit my miscalculations or incorrect judgements


You'll be fine. You've just got to take all criticism in stride and learn to roll with the punches. It will help you to grow a thick skin.
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Quoting 1537. sar2401:

I see the center more at 18N 47W, with the heaviest thunderstorms just to the southwest. Frankly, even the thunderstorms that are occurring aren't very impressive, especially since it's right at DMax. It should be pulling those thunderstorms in and forming a distinct LLC. I thought Dorian would do a better job of getting through the dry air and strong Easterly's but it's very apparent it's down to a TD at best right now. I expect ti to be a remnant low by 11 am.


While sort of disappointing, isn't at least a bit impressive that in comparison to what he looked like around 3/4pm that Dorian was able to moisten his environment as much as he has?
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Well you know the storms tend to bring me back in here. Btw how is Portlight doing and is there anything that is needed right now?

My view on dorian is this: There is always that one storm that acts like a steam roller at the true beginning of the season, meaning this time of year, sucking up all the dry air while not doing much of anything. Yes its going to be interesting to see if he can regain some organization and be something. But if you look at a water vapor to the east of Dorian, you will notice that the dry air patch that was there is gone. So I think we will start seeing a pick up on action soon.
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Quoting 1499. scott39:
i dont see anyone else(besides NCHurricane) chatting about the upper level anticyclone, that Dorian is about to have. This is going to be key for his survival and further developement. If it lines up like its forecasted too....all systems go.
It's easier to see on the broader water vapor animation. It's also why we suddenly see the shear dropping out ahead of Dorian in the shape of a big doughnut.

That broad ULL to the NW has become Dorian's friend, at least for now.



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You know me mlc, I'll still be keeping an eye on it to see what it does. I know as well as anyone that you can never say never in the tropics...

That said, the last time the models (and I mean a several day consensus of most of them) made a colossal mistake was Katrina. Think a lot of effort was put in to improving them since that incident.

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Quoting 1536. FlaStGuy:


No, just felt like if I started posting with a handful of other new bloggers I wouldn't be subjected to the entire fury of the blog if I said something dumb.

The dynamics of this blog are frighteningly similar to what I anticipate my thesis defense to be. A feeding frenzy of more knowledgeable individuals looking to exploit my miscalculations or incorrect judgements


I personally think that's the completely wrong attitude to take, but that's just me.
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Quoting 1520. Bluestorm5:
Can't see Dorian surviving shear unless it have anticyclone or something.



To all the teens on the chat room, I'll join in sometime soon, but I'm on my phone because I'm on a road trip and I won't be able to get on for a while.

Also, welcome FlaStGuy! We are the newbies :)

Anyway, I was hoping that ULL was gonna aid the outflow channels for Dorian, but it might just tear him apart instead. Geez ULLs can be such double sided daggers. Dorian doesn't have the ITCZ to aid convection, so he literally will live and die by Dmin and Dmax as well as his ability to tap into a moisture feed closer to the islands past the dry air. I don't see an anticyclone coming back anytime soon though.
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1537. sar2401
Quoting ecflweatherfan:
I know it's kinda odd to say, with Dorian especially, but the CoC is completely covered by thunderstorms at this time. It's really hard to make it out, but my best guess using extrapolation from movement prior to thunderstorm explosions, would put it around 18N 49W, right under the middle of the deepest thunderstorm activity.

I see the center more at 18N 47W, with the heaviest thunderstorms just to the southwest. Frankly, even the thunderstorms that are occurring aren't very impressive, especially since it's right at DMax. It should be pulling those thunderstorms in and forming a distinct LLC. I thought Dorian would do a better job of getting through the dry air and strong Easterly's but it's very apparent it's down to a TD at best right now. I expect it to be a remnant low by 11 am.
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Quoting 1534. KoritheMan:


Oh, so you're here to set the "newbies" straight, is that it? Charming. ;)


No, just felt like if I started posting with a handful of other new bloggers I wouldn't be subjected to the entire fury of the blog if I said something dumb.

The dynamics of this blog are frighteningly similar to what I anticipate my thesis defense to be. A feeding frenzy of more knowledgeable individuals looking to exploit my miscalculations or incorrect judgements
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You too PWS.

@ 1529. I know enough to know that NO model has brought Dorian to hurricane strength over the past 5 days, and almost all the models have shown it dissipating for the past couple days. I also know that when I take what the models have shown and look at the current observations, like wind shear, that Dorian has little to no chance to survive what it is headed towards. IF it misses the mountains and is still a wave in a couple days, then there may be a 5-10% chance that it redevelops, but that is highly unlikely. So calm down. It is July and we have had four named systems in the Atl. Pretty impressive if you ask me...As for Dorian though, she has most likely seen her best days already.
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Quoting 1514. FlaStGuy:


Well to be fair, I've been tempted to post earlier, but have always felt I lacked anything intellectual enough to add to the conversation. AND I've notice more newbies this seasona nd felt the time was right lol


Oh, so you're here to set the "newbies" straight, is that it? Charming. ;)
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Quoting 1519. StormJunkie:
Yes, a little convection is starting to build near the center.

BUT, it is all for naught imho...While the shear is dropping ever so slightly, this is not a system that can ward off the 40kts of shear it is headed towards...

His core is too small and feeble to handle what he's about to head in to. Again, there is a reason you can't find a model that develops him.





SJ, I can't disagree with you, but I think we should remember this is quite the shallow system. Upper-level shearing should not be as adverse to a shallow system. Sure, a shallow system can't grow, mature in 40 kts upper-level shear, but shouldn't necessarily kill it either. Any mid-level shearing would be more likely to affect it. Right?

It does look like dry air is about to be less of an issue, but clearly this sat pic helps put its "diminutive" size in perspective. Because it is so small, the slightest vulnerabilities can affect it, wound it, even kill it, or not if it can manage to remain intact and stable as a shallow system until conditions become more favorable. Lots of time yet, even ahead of the upper level shearing. Sorry, just can't write it off yet, but statistics likely support all that you've mentioned. :)
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Quoting 1527. sar2401:

Those easterly trade winds have been a big deal for Dorian, the same as they were fro Chantal. You can see the Easterly's tearing p the outflow from Dorian on the NW side. I think the convect we see tonight on the west side of Dorian, even though it's well displaced from the center, is Dorian's last ditch attempt to change direction more to the WSW and escape the Easterly trades.


huh? Convection is over the LLC, Dorian won't go wsw until maybe past 65W when the ridge to its N becomes more NE-SW, easterly trades were with Chantal but not with Dorian...she weakend mostly from dry air and building high pressure to his north
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Quoting 1529. Camille33:

Do you even know what your talking about???The tutt is collapsing via heat release being transported,it's losing it's cold core and diminishing,thus shear will continue to drop.Also the system is sustaining tstms so far near the llc,and sustained heat release can form an anticyclone.


Ahem.
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Well I'l try my best Storm lol.

As for Dorian, he has definitely be an interesting and frustrating storm to watch. The high pressure set up for the Atlantic just seems quite scary to me this season, especially with the types of systems that have been rolling out of W. Africa the past month and a half or so.
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Quoting 1519. StormJunkie:
Yes, a little convection is starting to build near the center.



BUT, it is all for naught imho...While the shear is dropping ever so slightly, this is not a system that can ward off the 40kts of shear it is headed towards...



His core is too small and feeble to handle what he's about to head in to. Again, there is a reason you can't find a model that develops him.

Do you even know what your talking about???The tutt is collapsing via heat release being transported,it's losing it's cold core and diminishing,thus shear will continue to drop.Also the system is sustaining tstms so far near the llc,and sustained heat release can form an anticyclone.
Member Since: July 2, 2013 Posts: 0 Comments: 1523
1528. 7544
hmmm looks like dorian has found a sweet spot just in time for dmax ill stick with the eruo on this one and dejevue lol
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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