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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Quoting 1601. Camille33:

Go ahead and ignore,I try to be objective,people don't know what that means.I know quite a bit more meteorological stuff than everyone on here,perhaps except Dr.Masters and a few others.


You know more than most anyone on here? Like Chantal becoming a cat4 and hitting Haiti. And Dorian roaring to major status yesterday as you predicted. Perhaps we're not the one's who don't know what objective means.
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The dry air became a factor,like right now,when the anticyclone was moved away from the system by storm motion...but it was not the cause of the system dieing.I know it's hard to imagine this,but I strongly diss-approve the assertion of people blaming the sinking motion of the mjo/dry air for this,or the tutt.That is not the case.
Member Since: July 2, 2013 Posts: 0 Comments: 1440
Quoting 1619. KoritheMan:


The dry air increased the farther west the storm got, which explains it's disheveled appearance now. Don't forget that it was also feeding on low-level inflow from the ITCZ during its early stages, which effectively offset the otherwise marginal sea surface temperatures.

The anticyclone protected it from dry air,and it would have kept protecting it. Imagine a center co-located with an upper high,increasing the latent heat release,and further strengthening it for 2 days. Then boom the llc finally is moved away from it,the upper higher that build up,imparted a heavy dose of sw shear that was not detected well by models. This is a classic dose of storm scale shear. I strongly believe dry air is a factor for developing systems which don't have an upper high,like this one. This one is not a cyclone anymore, and for shear translated dry air into the system, where the anticyclone can't do anything.
Member Since: July 2, 2013 Posts: 0 Comments: 1440
Quoting 1620. scott39:
I couldnt wait on there next management school, so I came in as an associate, with years of fast food management. I needed a job and had some money saved up, so i kissed fast food goodbye. I was told that when the next class came up, that I was a shoe in. I busted my butt for 6 months, and then I was told that I would have to move. Im not single and have kids, and they wouldnt come off any more money. Most of the management in Walmart is below average, which makes life hell for its employees.
My management is better than most Walmart from what I've heard. They actually want me to come back in December, but we'll see. Idea of not job hunting half of my college break sounds good.
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1624. scott39
Good night all.
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Quoting 1615. Camille33:

No,in this case and in every case you want an anticyclone directly over top the low level circulation. The flow is zonal and modest,so it is not very intense shear,but it's there. Anticylonic flow means the flow is turning clockwise...I don't see anything clockwise turning over the storm.

Well...here is an interesting map I captured when Dorian was approaching peak intensity on July 25....I've marked Dorian as he came into view on the lower-right of that map

Link

As seen by the upper wind barbs in that map...their was upper anticyclonicity (for lack of a better word) due north rather than completely over the storm. Dorian was strengthening at that time. And if you look at this link...and click on HDW-H...similar kind of situation tonight...or am I missing something?
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Quoting 1619. KoritheMan:


The dry air increased the farther west the storm got, which explains it's disheveled appearance now. Don't forget that it was also feeding on low-level inflow from the ITCZ during its early stages, which effectively offset the otherwise marginal sea surface temperatures.

I couldn't have said it much better Kori as I agree with what you said.
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Quoting 1618. Camille33:

We can go further into this...why didn't it get dryed out far east...it was located directly east of the dry air all along,even though the saharan air layer got eroded,the dry air didn't. The system was co-located with an anticyclone,which diffuses moisture in a pocket in all directions and allows the storm to organize. Dry air techniclly doesn't weaken storms unless it is driven into the storm,it has not,because the tutt is moving out, not towards the system.
When Dorian got disconnected from ITCZ feeding the storm, it opened up the southern side for dry air to enter the storm. Even TWC meteorologists explained this.
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1620. scott39
Quoting 1611. KoritheMan:


What was your position? Associate, or some sort of manager?
I couldnt wait on there next management school, so I came in as an associate, with years of fast food management. I needed a job and had some money saved up, so i kissed fast food goodbye. I was told that when the next class came up, that I was a shoe in. I busted my butt for 6 months, and then I was told that I would have to move. Im not single and have kids, and they wouldnt come off any more money. Most of the management in Walmart is below average, which makes life hell for its employees.
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Quoting 1618. Camille33:

We can go further into this...why didn't it get dryed out far east...it was located directly east of the dry air all along,even though the saharan air layer got eroded,the dry air didn't. The system was co-located with an anticyclone,which diffuses moisture in a pocket in all directions and allows the storm to organize.


The dry air increased the farther west the storm got, which explains it's disheveled appearance now. Don't forget that it was also feeding on low-level inflow from the ITCZ during its early stages, which effectively offset the otherwise marginal sea surface temperatures.
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Quoting 1616. KoritheMan:


Then what about the arc clouds, dear boy?

Oh ho ho it's magic
you knooooow

We can go further into this...why didn't it get dryed out far east...it was located directly east of the dry air all along,even though the saharan air layer got eroded,the dry air didn't. The system was co-located with an anticyclone,which diffuses moisture in a pocket in all directions and allows the storm to organize. Dry air techniclly doesn't weaken storms unless it is driven into the storm,it has not,because the tutt is moving out, not towards the system.
Member Since: July 2, 2013 Posts: 0 Comments: 1440
anyway guys I out for a few hours
Member Since: June 13, 2009 Posts: 2 Comments: 11260
Quoting 1606. Camille33:
A lot of people think the storm weakening cause of 1.dry air 2.the tutt 3.the mjo. That is complete and utter nonsense.The storm weakened due to the easterly winds which accelerated the low level circulation ahead of the anticyclone...which in turn having had an intense anticyclone,it got sheared by it-self.That is a fact.So please keep that in mind.


Then what about the arc clouds, dear boy?

Oh ho ho it's magic
you knooooow
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Quoting 1612. NCHurricane2009:


But what about the stretch of winds blowing from E to W in the area between 40W and 60W in between 10N and 20N? Isn't HDW-H mean upper winds? Am I reading the wind barbs wrong or something?

No,in this case and in every case you want an anticyclone directly over top the low level circulation. The flow is zonal and modest,so it is not very intense shear,but it's there. Anticylonic flow means the flow is turning clockwise...I don't see anything clockwise turning over the storm.
Member Since: July 2, 2013 Posts: 0 Comments: 1440
Quoting 1601. Camille33:

Go ahead and ignore,I try to be objective,people don't know what that means.I know quite a bit more meteorological stuff than everyone on here,perhaps except Dr.Masters and a few others.


Careful what you say, in my mind, perhaps the reasoning behind some ignoring you is not entirely in your facet of knowledge, but how you apply it and communicate to the blog that knowledge. Can't be a good meteorologist if you can't communicate well to others. No offense, I hope.
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Quoting 1608. scott39:
Thats like being let out of prison...worked there 10 years ago, and I know its gotten worse. Good for you.
It's not really that difficult short term, but it's nice to not work hours straight anymore for awhile. College is around the corner. I leave for Asheville August 10th.
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Quoting 1605. AussieStorm:


I wouldn't call that and anti-cyclone.

All I can see is winds at all levels blowing from W -> E


But what about the stretch of winds blowing from E to W in the area between 40W and 60W in between 10N and 20N? Isn't HDW-H mean upper winds? Am I reading the wind barbs wrong or something?
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Quoting 1608. scott39:
Thats like being let out of prison...worked there 10 years ago, and I know its gotten worse. Good for you.


What was your position? Associate, or some sort of manager?
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Quoting 1582. Bluestorm5:
You're one of the last one to not have him on iggy.


I will never put anyone on my ignore list. Didn't even know what "ignore" was until 7 months ago. I'll read anyone's statements and comment on them if I can, sucks for you guys if I quote those who are on yall's lists.
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Quoting 1602. NCHurricane2009:


Hey...its the other Michigan blogger! Long time...no see...

NC! I haven't been able to see/talk to you in a while. There are a couple other bloggers that are in southern Michigan, and of course Dr. Masters.
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1608. scott39
Quoting 1604. Bluestorm5:
I dunno why I'm up this late... work at 10:30. At least only one more week of Walmart.
Thats like being let out of prison...worked there 10 years ago, and I know its gotten worse. Good for you.
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Quoting 1606. Camille33:
A lot of people think the storm weakening cause of 1.dry air 2.the tutt 3.the mjo. That is complete and utter nonsense.The storm weakened due to the easterly winds which accelerated the low level circulation ahead of the anticyclone...which in turn having had an intense anticyclone,it got sheared by it-self.That is a fact.So please keep that in mind.
Also the natural vorticity doesn't weaken because of storm-scale occuranses...it can only weaken/dissipate with a strong headwind shear at the level it is located.
Member Since: July 2, 2013 Posts: 0 Comments: 1440
A lot of people think the storm weakening cause of 1.dry air 2.the tutt 3.the mjo. That is complete and utter nonsense.The storm weakened due to the easterly winds which accelerated the low level circulation ahead of the anticyclone...which in turn having had an intense anticyclone,it got sheared by it-self.That is a fact.So please keep that in mind.
Member Since: July 2, 2013 Posts: 0 Comments: 1440
Quoting NCHurricane2009:

Their is an upper anticyclone near 52.5W 20N...using this tool:

Link

Click on HDW-H checkbox...and you can use the upper-level wind barbs to trace out said upper anticyclone...


I wouldn't call that and anti-cyclone.

All I can see is winds at all levels blowing from W -> E
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I dunno why I'm up this late... work at 10:30. At least only one more week of Walmart.
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1603. scott39
Quoting 1600. NCHurricane2009:

Another trick people like to pull is giving the same thing different names. Anticyclone also means ridge and also means high pressure. Vortex also means low (or low pressure). A trough, low, or vortex can be referred to as vorticity....

Anticyclone though is usually reserved for ridges that are fully closed....vortex or lows is usually reserved for troughs that have fully closed into a spin.

Ridges are usually symbolized with zig-zag lines or Hs...troughs are usually symbolized with dashed lines or Ls. And did you know that fronts are basically troughs with air temp mass contrasts across them? (but I think from watching TV you know the symbols for fronts)
Thanks for your help. Have a good night
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Quoting 1592. wxchaser97:

Convection that formed from D-MAX that isn't that organized isn't much of a comeback imo. I'd wait to see if the convection persists through all the dry, stable air and D-MIN tomorrow.


Hey...its the other Michigan blogger! Long time...no see...
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Quoting 1598. sar2401:

***cough***ignore***cough***

Go ahead and ignore,I try to be objective,people don't know what that means.I know quite a bit more meteorological stuff than everyone on here,perhaps except Dr.Masters and a few others.
Member Since: July 2, 2013 Posts: 0 Comments: 1440
Quoting 1586. scott39:
You got my intrest up on your earlier posts about it. Thanks for the map illustrations. It helped. 2 days ago I caught the tail end of a met saying something about a anticyclone being in place to help Dorian. we shall see.

Another trick people like to pull is giving the same thing different names. Anticyclone also means ridge and also means high pressure. Vortex also means low (or low pressure). A trough, low, or vortex can be referred to as vorticity....

Anticyclone though is usually reserved for ridges that are fully closed....vortex or lows is usually reserved for troughs that have fully closed into a spin.

Ridges are usually symbolized with zig-zag lines or Hs...troughs are usually symbolized with dashed lines or Ls. And did you know that fronts are basically troughs with air temp mass contrasts across them? (but I think from watching TV you know the symbols for fronts)
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1599. scott39
Quoting 1595. scottsvb:
Dorian is just pulsating..what storms do at night over the warm waters.....As I said 2 days ago, he will weaken and be just a open wave by Saturday... he is right on schedule. Dorian is a easy forecast. Hardest time of the year is Oct.
I hope your right. I cant remember if your a met or not.
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1598. sar2401
Quoting KoritheMan:


...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.

***cough***ignore***cough***
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Quoting 1589. KoritheMan:


I don't ignore people that I consider entertaining. Rule of thumb in the life of Kori.

I don't ignore people. I just either use the self moderation tools that I have or just not quote them.
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Quoting 1594. KoritheMan:


*cough*

Whatever you say, dear.

*cough*

I said the storm would become a wave yesterday,it has,so what exactly are you trying to allude to?
Member Since: July 2, 2013 Posts: 0 Comments: 1440
Dorian is just pulsating..what storms do at night over the warm waters.....As I said 2 days ago, he will weaken and be just a open wave by Saturday... he is right on schedule. Dorian is a easy forecast. Hardest time of the year is Oct.
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Quoting 1590. Camille33:
There is a lot of wish-casting tonight,lets get back on track.


*cough*

Whatever you say, dear.

*cough*
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Quoting 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


Link
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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

Convection that formed from D-MAX that isn't that organized isn't much of a comeback imo. I'd wait to see if the convection persists through all the dry, stable air and D-MIN tomorrow.
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Quoting 1555. AussieStorm:
I just had to laugh at this... Link

I think everyone can relate to it.

True except I like Spring :)
Member Since: February 13, 2012 Posts: 11 Comments: 3669
There is a lot of wish-casting tonight,lets get back on track.
Member Since: July 2, 2013 Posts: 0 Comments: 1440
Quoting 1582. Bluestorm5:
You're one of the last one to not have him on iggy.


I don't ignore people that I consider entertaining. Rule of thumb in the life of Kori.
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Quoting 1555. AussieStorm:
I just had to laugh at this... Link

I think everyone can relate to it.


LOL
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1587. sar2401
Quoting 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

It has done what any storm in the tropics does at night, increase convection. The center has not consolidated, there's no banding, the outflow is getting ripped to shreds, and the warmer SST's don't seem to have helped. Really, a few thunderstorms over what is now probably a TD is not at all impressive.
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1586. scott39
Quoting 1566. NCHurricane2009:

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...
Quoting 1561. AussieStorm:


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

You got my intrest up on your earlier posts about it. Thanks for the map illustrations. It helped. 2 days ago I caught the tail end of a met saying something about a anticyclone being in place to help Dorian. we shall see.
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Quoting 1575. KoritheMan:


...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.

Wake up,I said in 3 - 4 days it will re-form.The low level vort will not get killed due to modest environmental shear.It is not forming right now.
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Quoting 1574. Camille33:

Stormw is banned,he tried to promote himself.


I don't know if it was about promoting himself...I think he tried to tip toe around some stuff...Or maybe it was about promoting himself...lol I guess TrapicalAmanda went out in that ban too...

Dorian is quite the little system for sure...

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Quoting 1561. AussieStorm:


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





Their is an upper anticyclone near 52.5W 20N...using this tool:

Link

Click on HDW-H checkbox...and you can use the upper-level wind barbs to trace out said upper anticyclone...
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Quoting 1575. KoritheMan:


...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.
You're one of the last one to not have him on iggy.
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Quoting 1577. AllStar17:


Last year was very active.


I understand it was active, that a miswording on my part. I did was busy most of last year, did not truly have a chance to pop in here
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Quoting 1575. KoritheMan:


...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.


Give it a minute and he'll change again. LOL. I'm still trying to comprehend the first post. Boy.
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Quoting 1573. plywoodstatenative:
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.

Yes, I remember hearing about those
Member Since: February 13, 2012 Posts: 11 Comments: 3669
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

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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.