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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1128. will40
1124. Civicane49


i imagine you get very few of those right?
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I find it interesting that cloud tops coming off the limited convection of Dorian look to be blown to the South now instead of to the East. Perhaps the shear is changing (maybe the direction anyway)?
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Quoting 1120. washingtonian115:
I would like to experience a cat 1 at least but on a abandon island where no one lives and where no one else can get hurt...I expressed that one time on the blog and got shot down for it.I however don't like seeing hurricanes near land where people and lives are at stake.I know for sure..

And if anyone brings up that "My taxes got to pay for.."

Just know that your taxes also help pay for jail where bad people have cable medical help and 3 meals a day with a bed to sleep on..


There's a fascination about them, sure. Last year, Isaac came through here, not even a Cat 1, just a TS and it was miserable. Lingering rain and wind, flooding. Work stops, traffic stops, etc. Twas not fun at all.
Member Since: July 9, 2006 Posts: 184 Comments: 29610
1125. Patrap
..."like a Rainbow in the Dark"...

Member Since: July 3, 2005 Posts: 426 Comments: 129090
For my area:

HYDROLOGIC OUTLOOK
NATIONAL WEATHER SERVICE HONOLULU HI
308 PM HST FRI JUL 26 2013


...THE NATIONAL WEATHER SERVICE IN HONOLULU HAS ISSUED A FLOOD
POTENTIAL OUTLOOK FOR THE STATE OF HAWAII...

...HEAVY RAIN AND FLASH FLOODING POSSIBLE MONDAY AND TUESDAY...

THE LATEST FORECAST FOR TROPICAL STORM FLOSSIE TAKES THE SYSTEM
OVER THE ISLAND CHAIN EARLY NEXT WEEK AS A WEAKENING TROPICAL
CYCLONE. WHILE FLOSSIE IS EXPECTED TO REMAIN MOVING STEADILY WEST
AS IT PASSES THROUGH THE ISLAND CHAIN...ABUNDANT TROPICAL MOISTURE
DRAWN INTO THE CIRCULATION WILL BRING THE POTENTIAL FOR VERY HEAVY
RAINFALL AND FLASH FLOODING FOR PARTS OF THE STATE ON MONDAY AND
TUESDAY.

THE DETAILS ON THE TIMING AND LOCATION OF THE HEAVY RAIN REMAIN
UNCERTAIN...AND ARE HIGHLY DEPENDENT ON THE EVENTUAL TRACK OF
FLOSSIE. PLEASE STAY TUNED TO NOAA WEATHER RADIO OR LOCAL MEDIA
FOR FURTHER INFORMATION.

AN UPDATED OUTLOOK WILL BE ISSUED BY 4 AM HST SATURDAY...OR SOONER
IF NECESSARY.

FOLLOW US ON FACEBOOK AND TWITTER AT:
WWW.FACEBOOK.COM/US.NATIONALWEATHERSERVICE.HONOLU LU.GOV
WWW.TWITTER.COM/NWSHONOLULU

$$

BIRCHARD
Member Since: July 21, 2011 Posts: 83 Comments: 7167
As been mentioned earlier on the blog dry air can be overrated sometimes. There will always be dry air in the Atlantic as when there is rising air the will be dry sinking air to follow. Dry air can range on the different levels as well; there might be mid level dry air but upper to lower levels is not so dry as in this case and vice-versa. As long as a storm can output good enough inflow that lead will lead to a more anticyclonic flow and good outflow will follow thus allowing it to be able to moisten up its environment. If that is not available then system can wrap themselves in a moisture bubble like a force field to protect it from hostile atmospheric conditions. In the case of Dorian it is a small convective mass. So it was not able to support and push its moisture force field around it with continuous convective activity. The only thing keeping it alive is the vigorous circulation drawing up a little moisture from the ITCZ still it was never able to build a genuine CDO as there was not enough heat to support the firing of deep convection which is needed. In anycase as it transverse warmer water which it never really had since it started moving WNW. It may have an attempt to do something about its dry air intrusion that resulted from a lack of deep convection and south-westerly wind shear. Which has cease now and is only on the extreme northern edge of Dorian that would have probably acted as a ventilating mechanism if it had a CDO and trying to start outflow to the north. As it stand now its currently moving into warmer waters and the cooling nighttime atmosphere that starts sooner in the Central Atlantic than if it was in the Caribbean/Gulf. With a decent spin it may last at least a little longer. As it stands this is either a TD or a 40mph TS. For it to last as long as the NHC has suggest it will have to burst some deeper colder cloud tops. to conclude, if this can take advantage of warmer water it may survive. Mother nature have a way of teaching people whether or not through storms or sunny drought like weather that it is not over till she said it is, so still watch the circulation till its dissipate. 13



Edit: fix errors and added a little more!
Member Since: May 25, 2012 Posts: 0 Comments: 2043
The fat lady hasn't sung yet !
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Quoting 1110. Patrap:
The outflow boundaries or Mini Gust Fronts can "Plow the Road" per se until the System can build a good Barotropic Warm Column.

Dorian is a sneaker...don't discount it.





Yeah, that's what I was saying. ;)
Member Since: July 9, 2006 Posts: 184 Comments: 29610
Quoting 1102. mitthbevnuruodo:


I love to see a major hurricane, for the beauty of nature that one is...with it's well formed eye and perfect banding,it's amazing to view. But want it to be a fish storm...in any basin. I have no desire to see it hit anyone.

I think some DO like to see destruction. Some people are just messed up. But I just like to see the beauty of nature..albeit from afar as possible, and not ruining anyone elses lives either. I'm just interested in weather, mother nature and geological events...and do wish there wasn't a human impact with them. It's just unavoidable sometimes sadly though
I would like to experience a cat 1 at least but on a abandon island where no one lives and where no one else can get hurt...I expressed that one time on the blog and got shot down for it.I however don't like seeing hurricanes near land where people and lives are at stake.I know for sure..

And if anyone brings up that "My taxes got to pay for.."

Just know that your taxes also help pay for jail where bad people have cable medical help and 3 meals a day with a bed to sleep on..
Member Since: August 14, 2010 Posts: 10 Comments: 17482
1119. ncstorm
convection starting to fire again..oh downcasters??
Member Since: August 19, 2006 Posts: 13 Comments: 16039
Quoting 1108. sar2401:

Yes, anything is possible. Other storms have come back from the dead. Some have become TD's others have become major hurricanes. Some people also win the lottery. That's also possible, it's just not probable. From what I see of Dorian now, it's not probable that it will survive the night. If it does, I'm going out and buying a lottery ticket. :-)
I don't know about tonite, but I personally wouldn't put the probability of a comeback out there in lottery territory over the next 5 days. 4/1-5/1(20-25%) or so probability of TS or higher on the NHC prob chart....
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1117. KEEPEROFTHEGATE (Mod)
Quoting 1089. EyEtoEyE:
If I remember right Andrew looked almost like Dorian does now , and then BAM , same type of environment , I know everybody , will quote call me crazy , go right ahead , I've been called alot worse , when you know the facts , because the NHC called Andrew a depression , or a very weak TS , In the same area , give or take a few hundred miles , then Andrew got into much warmer waters , and we all know the rest of that story ! Dorian gets in the same warm waters , watch out , I am saying don't be hasty , on writing HIM off, I really don't care what time of season it happens in , July or August , like Andrew was in , you must not count any storm out !



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1116. will40
like i say until you can only see blue seas beneath it it is still alive
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1115. Patrap
Member Since: July 3, 2005 Posts: 426 Comments: 129090
1114. Patrap
Tropical Storm 04L

UW-CIMSS Automated Satellite-Based
Advanced Dvorak Technique (ADT)
Version 8.1.4
Tropical Cyclone Intensity Estimation Algorithm

Current Intensity Analysis



UW - CIMSS
ADVANCED DVORAK TECHNIQUE
ADT-Version 8.1.4
Tropical Cyclone Intensity Algorithm

----- Current Analysis -----
Date : 27 JUL 2013 Time : 011500 UTC
Lat : 18:00:55 N Lon : 47:12:57 W


CI# /Pressure/ Vmax
3.0 /1006.6mb/ 45.0kt


Final T# Adj T# Raw T#
2.0 2.0 1.5

Center Temp : +15.3C Cloud Region Temp : -6.9C

Scene Type : SHEAR (>1.25^ TO DG)

Positioning Method : FORECAST INTERPOLATION

Ocean Basin : ATLANTIC
Dvorak CI > MSLP Conversion Used : ATLANTIC

Tno/CI Rules : Constraint Limits : 1.0T/6hr
Weakening Flag : ON
Rapid Dissipation Flag : FLAG

C/K/Z MSLP Estimate Inputs :
- Average 34 knot radii : 50km
- Environmental MSLP : 1018mb

Satellite Name : MSG3
Satellite Viewing Angle : 57.0 degrees


Member Since: July 3, 2005 Posts: 426 Comments: 129090
Looking for a downgrade all the way to remnant low/invest.

Dorian is toasted by the dry air.
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Quoting 1105. KoritheMan:

Agreed. I'm not one of those people, though. :)

Despite what a lot of people think, the desire for a major hurricane and compassion are not mutually exclusive.


Totally agree man

And have never thought you one for wanting one to see destruction, so no worries from here
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Quoting 1093. CosmicEvents:
Maybe the fact that Dorian has always been such a diminutive storm, as KMan said last night....might help to get it to tighten up and regenerate quicker than a big or even normal sized storm.


Never know with these tropical storms obviously...and yes, Andrew looked well dodgy for a while/

I've not much faith in Dorian, but I discount nothing. As so much can happen, and storms of course sometimes defy the odds. Maybe I'm just paranoid LOL But I would always keep an eye on ANYthing!
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1110. Patrap
The outflow boundaries or Mini Gust Fronts can "Plow the Road" per se until the System can build a good Barotropic Warm Column.

Dorian is a sneaker...don't discount it.



Member Since: July 3, 2005 Posts: 426 Comments: 129090
Quoting 1093. CosmicEvents:
Maybe the fact that Dorian has always been such a diminutive storm, as KMan said last night....might help to get it to tighten up and regenerate quicker than a big or even normal sized storm.


;) I saw that. That, kman, he went and pulled out a fancy word! Had to get Webster's out! ;P I like kman, and certainly respect his tropical wisdom!
Member Since: July 9, 2006 Posts: 184 Comments: 29610
1108. sar2401
Quoting CosmicEvents:
Maybe the fact that Dorian has always been such a diminutive storm, as KMan said last night....might help to get it to tighten up and regenerate quicker than a big or even normal sized storm.

Yes, anything is possible. Other storms have come back from the dead. Some have become TD's others have become major hurricanes. Some people also win the lottery. That's also possible, it's just not probable. From what I see of Dorian now, it's not probable that it will survive the night. If it does, I'm going out and buying a lottery ticket. :-)
Member Since: October 2, 2004 Posts: 0 Comments: 16289
Dorian it ain't over til you're over!
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Quoting 1101. wunderkidcayman:

yeah I agree


yeah there is a bit


but more like on the N side of the LLCOC
Member Since: June 13, 2009 Posts: 2 Comments: 12430

Quoting 1102. mitthbevnuruodo:




I think some DO like to see destruction. Some people are just messed up.
Agreed. I'm not one of those people, though. :)

Despite what a lot of people think, the desire for a major hurricane and compassion are not mutually exclusive.
Member Since: March 7, 2007 Posts: 597 Comments: 21094
One thing that helped Chantal, imo (others may disagree), was that the outflow boundaries created by the collapsing storms in front of the system served to help moisten the environment. Remember, this is a small, very shallow system trying to generate sfc lift amidst the dry air environment. If it can generate a little lift here and there in front of it, it just may buy it enough time to get to the warmer waters of 50w.

A couple of days ago I said everything would be gradual (except its forward speed) about Dorian. If it got bigger it'd be gradual. If it died it'd be gradual. This is a small system, so small elements matter, even small collapsing thunderstorms out in front of it or on the side of the dry air especially. Increments. Small increments, may make all the difference here.

The small LLC looks a tad oblong, and indeed the west or southwest side of the system may be open and I'd credit that more to the fast, low-level easterly flow more so than the dry air. Of course, the dry air is not helping it either. Still, there appears to be a vigorous shallow circulation. It may be a cooked system, but it's certainly still fighting.
Member Since: July 9, 2006 Posts: 184 Comments: 29610
1103. Patrap
18.0 N

44.5 W

Member Since: July 3, 2005 Posts: 426 Comments: 129090
Quoting 1087. KoritheMan:

Yes. All of my friends know that I want to see a major hurricane. If admitting that to people makes them hate me, I really don't care.

Nobody wants destruction, even those who want a storm. And anyone who suggests they do is not thinking rationally, as is typically the case during controversial topics like this.

But "I don't understand your mentality, therefore you're sick and twisted" is not justification for saying what you said, nor will it ever be.

I'm not really going to try and debate the point any further, because it'll just be an emotional pissing contest between both of us.


I love to see a major hurricane, for the beauty of nature that one is...with it's well formed eye and perfect banding,it's amazing to view. But want it to be a fish storm...in any basin. I have no desire to see it hit anyone.

I think some DO like to see destruction. Some people are just messed up. But I just like to see the beauty of nature..albeit from afar as possible, and not ruining anyone elses lives either. I'm just interested in weather, mother nature and geological events...and do wish there wasn't a human impact with them. It's just unavoidable sometimes sadly though
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Quoting 1086. moonlightcowboy:
I'm still seeing low-level center at 18n,47.5w.


yeah I agree

Quoting 1092. weatherlover94:
Is that convection any where near the center ?

yeah there is a bit

Member Since: June 13, 2009 Posts: 2 Comments: 12430
1100. sar2401
Quoting weatherlover94:
Is that convection any where near the center ?

No, it's way off to the edges.
Member Since: October 2, 2004 Posts: 0 Comments: 16289
1099. pottery
Quoting sar2401:

Well, it is only the end of July. Hopefully, we're not still saying that at the end of August. I had remarked a few days ago how remarkably free both the Gulf and Caribbean were from any convection, something I've never seen before in late July. This has to mean a lot of stable, dry air covering essentially the entire basin. It's going to take a storm a lot bigger than Dorian, or a major retreat of the A-B high, to change that. At least up until the end of the first week of August, I sure don't see any major change.

I agree with that.
I'm looking at Aug 10th-15th before that can change enough to make a big difference.
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1098. Patrap
The Low Level Swirl has good vorticity,and I for one know better than to discount a CV seed SE of CONUS.



Respect to da Basin yo,'

Member Since: July 3, 2005 Posts: 426 Comments: 129090
1097. KEEPEROFTHEGATE (Mod)
Quoting 1086. moonlightcowboy:
I'm still seeing low-level center at 18n,47.5w.

ya that's about right
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1096. sar2401
Quoting GrandCaymanMed:


Look at post 1064 above. There are a few thunderstorms popping. It doesn't look like your
classic cyclone, but thunderstorms are still
popping.

Umm, yeah, I posted that to show how pathetic Dorian has become. I sailed the Gulf and Caribbean for almost 30 years. Those measly little showers and thunderstorms can pop up from a flock of albatross flying a little fast on a nice war, humid night. :-) I'm not even sure they are part of Dorian. It needs to put on a big time show between now and DMax to have any chance. It's really simple at this point. DMax is about five hours from now. If Dorian doesn't start turning red all over by about 1:00 am EDT, it's toast.
Member Since: October 2, 2004 Posts: 0 Comments: 16289
1095. SRQfl
Is Dorian shedding his skin? Or withering away?



Member Since: October 1, 2007 Posts: 0 Comments: 155
Quoting 1055. KEEPEROFTHEGATE:




what you can't see that sparsely sporadic convective blow up starting


Sir you have a very good eye , on the storm
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Maybe the fact that Dorian has always been such a diminutive storm, as KMan said last night....might help to get it to tighten up and regenerate quicker than a big or even normal sized storm.
Member Since: Posts: Comments:
Is that convection any where near the center ?
Member Since: September 8, 2010 Posts: 0 Comments: 2218
Quoting 977. Bluestorm5:
Igor was under a lot of dry air as well, but managed to miss out on Category 5 by 1 mph. Dry air will always be there most of the times, but it depends on how storms protect itself from them how strong they get. Dorian was feeding on ITCZ and didn't have lot of protection build up.


IGOR was a mature and slow moving storm, unlike DORIAN
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Quoting 1066. wunderkidcayman:
Advisory in less than 1 hour 30 mins from now
All gone. Next wave, please.
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If I remember right Andrew looked almost like Dorian does now , and then BAM , same type of environment , I know everybody , will quote call me crazy , go right ahead , I've been called alot worse , when you know the facts , because the NHC called Andrew a depression , or a very weak TS , In the same area , give or take a few hundred miles , then Andrew got into much warmer waters , and we all know the rest of that story ! Dorian gets in the same warm waters , watch out , I am saying don't be hasty , on writing HIM off, I really don't care what time of season it happens in , July or August , like Andrew was in , you must not count any storm out !
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WOW, just in from work and last I saw was around noon GMT. And though, haven't thought Dorian looked like it would flourish cause of the conditions, not sure I was expecting the mess am seeing now! I know it could regenerate later, but geesh, how ugly at the mo!

As for N Wales, I REALLY hope the forecast holds (they often can't forecast accurately the day before for this area haha), but so far, come Tues, 65'! Oh, how I would LOVE that! LOL

Cheers all!
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Quoting 1028. sar2401:

People need to stop hiding that they want to see a major hurricane cause untold death and destruction for their amusement, so they won't be bored? Maybe you can clarify that.

Yes. All of my friends know that I want to see a major hurricane. If admitting that to people makes them hate me, I really don't care.

Nobody wants destruction, even those who want a storm. And anyone who suggests they do is not thinking rationally, as is typically the case during controversial topics like this.

But "I don't understand your mentality, therefore you're sick and twisted" is not justification for saying what you said, nor will it ever be.

I'm not really going to try and debate the point any further, because it'll just be an emotional pissing contest between both of us.

You, me, or anybody else, have no right to label people with differing opinions as insane, sick, twisted, or any other random derogatory label. People need to get off their soapboxes and realize that; the sooner the better.

EDIT: I will say though, that I think there's a time and place to admit you want to see a hurricane. It's all in the presentation. That's why JFV has garnered such a bad reputation around these parts, because he lacks compassion. For example, I would never admit to wanting a hurricane while one is bearing down somewhere. That's disrespectful. I would admit to wanting one during quiet and solitude like now though, and if that doesn't sit well with some people, so be it.
Member Since: March 7, 2007 Posts: 597 Comments: 21094
I'm still seeing low-level center at 18n,47.5w.

Member Since: July 9, 2006 Posts: 184 Comments: 29610

Quoting 1076. pottery:

Yeah, but it's losing convection.
It's surrounding environment is too dry.
It already lost convection but its trying one last time to do something as it now is entering 27-28C its not over just yet...
Member Since: May 25, 2012 Posts: 0 Comments: 2043
1084. sar2401
Quoting CosmicEvents:
Approaching warmer waters after 50w.

I think it's going to be about eight hours too late to matter.
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1083. Patrap
TS Dorian

"Tenacious "D"


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1081. Relix
Miracle 50W plus DMAX coming up. If Dorian doesn't make it tonight NHC will call it quits by 5AM. Go Dorian, I support you :P
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Quoting 1067. sar2401:

It's not popping up thunderstorms. What thunderstorms it had have dissipated. Can you show me a picture where it's "popping" up anything? I sure don't see it in this loop.


Look at post 1064 above. There are a few thunderstorms popping. It doesn't look like your
classic cyclone, but thunderstorms are still
popping.
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1079. Patrap
TS Dorian obviously has shed the Upper and Mid Level old vortex, and has begun anew,...with the CV Low Level seeder.



Member Since: July 3, 2005 Posts: 426 Comments: 129090
1078. sar2401
Quoting pottery:

Still a lot of dry air where it matters.
The convection with wave coming off now will most likely not survive for long, as there is plenty dry ahead of it.

What a Most Peculiar season !

Here in Trinidad it's still not raining
Getting a little scary....

Well, it is only the end of July. Hopefully, we're not still saying that at the end of August. I had remarked a few days ago how remarkably free both the Gulf and Caribbean were from any convection, something I've never seen before in late July. This has to mean a lot of stable, dry air covering essentially the entire basin. It's going to take a storm a lot bigger than Dorian, or a major retreat of the A-B high, to change that. At least up until the end of the first week of August, I sure don't see any major change.
Member Since: October 2, 2004 Posts: 0 Comments: 16289

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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.