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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1728. GatorWX
Never forget td 10 of '05 lol.
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So u know Dorian looked even worse than this. HH Plan of the day will probably be cancelled later today.
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1726. barbamz

Good morning, and - surprise! - is this our RIP-ed Dorian? Risen from death.

And there is a lot of action in Europe this weekend too. Estofex with a rare level 3 in its alerts!


Storm Forecast
Valid: Sat 27 Jul 2013 06:00 to Sun 28 Jul 2013 06:00 UTC
Issued: Fri 26 Jul 2013 23:25
Forecaster: GROENEMEIJER/GATZEN/PISTOTNIK

A major outbreak of severe storms is expected across regions of France, the Benelux countries and northwestern Germany. These storms may produce tornadoes, very large hail and widespread damaging wind gusts

A level 3 was issued across northern France, the Benelux and a part of northwest Germany for tornadoes, severe wind gusts, large or very large hail and excessive precipitation.

Whole discussion here.


Severe storms happening in the Benelux countries right now.

Have a nice morning everybody. I'll update European and German developments on my blog this weekend, for all who may be interested in it besides Dorian.
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Ugh. Just got done with 48 hours of flying. Not being able to check on the tropics for the last month was killing me. I have missed two storms so it seems. Luckily I did not seem to miss much out of the 2! Dorian is not looking very well either and should be killed off soon by dry air and shear. Oh well. Can't wait to track the first hurricane of the season with you guys!
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I beg to differ:
BEGIN 6-HRLY FIXES FOR TROPICAL
STORM DORIAN AT 28/1730Z NEAR 19.5N 60.0W.

EDIT: Never Mind, they re-scheduled it for tomorrow.
Quoting 1718. KoritheMan:


No they're not.

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I know it won't last. That's why I said will see. Least a few more extra hours with him
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1722. GatorWX
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Quoting 1718. KoritheMan:


No they're not.
Would this mean they're going tomorrow?
"
I. ATLANTIC REQUIREMENTS
1. NEGATIVE RECONNAISSANCE REQUIREMENTS.
3. SUCCEEDING DAY OUTLOOK: BEGIN 6-HRLY FIXES FOR TROPICAL
STORM DORIAN AT 28/1730Z NEAR 19.5N 60.0W."

http://www.nhc.noaa.gov/text/MIAREPRPD_last.shtml

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1720. VR46L
Quoting 1719. Tribucanes:
Shear is forecast to weaken by some estimates in Dorian's path from strong to only moderate. So if Dorian can get this convection organized, which I believe she's slowly doing, can continue to push the dry air, and if the shear lessens from where it's at now West of the Windward Islands; then Dorian might really have the possibility to pull off an amazing comeback. I know that's three if's too many, but I think it could really happen.


It is decreasing but I think its too late for Dorian but maybe timing well for the wave off Africa ...

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Shear is forecast to weaken by some estimates in Dorian's path from strong to only moderate. So if Dorian can get this convection organized, which I believe she's slowly doing, can continue to push the dry air, and if the shear lessens from where it's at now West of the Windward Islands; then Dorian might really have the possibility to pull off an amazing comeback. I know that's three if's too many, but I think it could really happen.
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Quoting 1716. dfwstormwatch:
 Hurricane Hunters are flying into Dorian this afternoon.


No they're not.
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Me and StormW have used this map in the past. It denotes the strength of the tropospheric steering from from the lower half of the troposphere to the upper half of the troposphere (in knots), and also indicates the forecast model steering for those layers (the arrows). It updates at 0z and 12z every day. Here's the 0z run from Friday evening:



96 hours out, the eastern Atlantic ridge starts to rebuild westward as a weak upper trough moves away from the Azores islands and the flow begins to get more zonal to the north of the ridge.

120 hours:



The CMC is even stronger with the trades (right).

Yes ladies and gentlemen, I do believe the new tropical wave will encounter the same hostile conditions Chantal and Dorian did.

Link
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 Hurricane Hunters are flying into Dorian this afternoon.
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Quoting 1714. GatorWX:


Thought I saw that. Seems like its inevitably doomed at this point. Maybe the GOM, still watch, but...


If it goes mountain climbing, the chances of regeneration appear even more slim.
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1714. GatorWX
Quoting 1712. KoritheMan:
One positive though, is that there is finally some evidence of upper-level outflow for the first time in nearly about a day. That shows that there is still some upper divergence over the area, but again, it's likely being enhanced at least partly by the mid-oceanic trough.


Thought I saw that. Seems like its inevitably doomed at this point. Maybe the GOM, still watch, but...
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1713. GatorWX
Quoting 1706. OracleDeAtlantis:
Look how rapidly Dorian is moistening his environment.



Hehe, yea
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One positive though, is that there is finally some evidence of upper-level outflow for the first time in nearly about a day. That shows that there is still some upper divergence over the area, but again, it's likely being enhanced at least partly by the mid-oceanic trough.
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I'm not buying the NHC's forecast for Dorian. Moisture shield is pushing the dry air very nicely and banding formation may not be far off. Not surprised NHC took the conservative rout with the dry air and increasing shear predicted, but Dorian is night and day from eight hours ago and still improving steadily.
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Quoting 1708. RGVtropicalWx13:
Morning.. looks like Dorian fired up nicely during dmax hours!!! Just shows he's not done yet!!! Well see how this convection does for a few more hours!


Won't last.

The convection you see is being generated by dmax and upper diffluence associated with the mid-oceanic trough. Water vapor imagery shows fast southwesterly to westerly winds ahead of the trough; right now those winds are assisting him, but since the dude's not in a hurry to slow down anytime soon, it will run into the TUTT and its associated shear maximum later today. That will signal the end of Dorian once and for all.
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Quoting 1707. Civicane49:
Hurricane.
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Morning.. looks like Dorian fired up nicely during dmax hours!!! Just shows he's not done yet!!! Well see how this convection does for a few more hours!
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Member Since: July 21, 2011 Posts: 83 Comments: 7167
Look how rapidly Dorian is moistening his environment.

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Member Since: September 2, 2011 Posts: 0 Comments: 1149
1704. VR46L
Quoting 1702. Vlad959810:

No models support and develop it


Umm the euro is showing it and the Gfs yesterday but both kill if off shortly after ..

I think it looks great ... but will probably go the same way as Chantal and Dorian if it develops ...



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Its interesting that the Epac busiest month is August but July and September tend to produce one more name storm on average.http://www.nhc.noaa.gov/climo/
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Quoting 1690. Tribucanes:
Kori, what do you think of the low off of Africa, starting to show real signs this morning.
Quoting 1685. stoormfury:
very impressive wave off the African coast 90L COMING SOON ?? IT LOOKS LIKE THE SHOW IS ABOUT TO BEGIN

No models support and develop it
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Quoting 1699. Tribucanes:
Well Kori, looks like your right, NHC planning the funeral.


Planning is all well and good, but maybe all Dorian needs is for a few friends by his bedside. That could be the motivational push. ;)
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Have a good early morning nightowls, until tomorrow.....
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Well Kori, looks like your right, NHC planning the funeral.
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TROPICAL STORM FLOSSIE DISCUSSION NUMBER 10
NWS NATIONAL HURRICANE CENTER MIAMI FL EP062013
200 AM PDT SAT JUL 27 2013

FLOSSIE HAS STRENGTHENED DURING THE LAST SEVERAL HOURS. MICROWAVE
IMAGES SHOWED THE DEVELOPMENT OF A MID-LEVEL EYE A LITTLE BEFORE
0600 UTC...AND GLIMPSES OF THAT FEATURE HAVE BECOME APPARENT IN
RECENT GEOSTATIONARY SATELLITE IMAGES. DVORAK CLASSIFICATIONS HAVE
INCREASED TO 3.5/55 KT...BUT GIVEN THE IMPROVEMENT IN ORGANIZATION
SINCE THEN...THE INITIAL WIND SPEED IS SET AT 60 KT. FLOSSIE HAS
CROSSED THE 26C ISOTHERM AND IS HEADED FOR SLIGHTLY COOLER WATERS
DURING THE NEXT DAY OR TWO. THESE COOLER WATERS AND DRIER AIR
SHOULD CAUSE THE CYCLONE TO HOLD STEADY IN STRENGTH OR BEGIN TO
WEAKEN. EVEN THOUGH THE WATER TEMPERATURES INCREASE ALONG THE
EXPECTED TRACK OF FLOSSIE BEGINNING IN ABOUT 48 HOURS...WESTERLY
SHEAR AND A DRIER AIRMASS SHOULD RESULT IN ADDITIONAL WEAKENING.
THE NHC WIND SPEED FORECAST IS IN GOOD AGREEMENT WITH THE INTENSITY
MODEL CONSENSUS AND IS SLIGHTLY HIGHER THAN THE PREVIOUS FORECAST
IN THE SHORT TERM.

THE TROPICAL STORM REMAINS ON A STEADY WEST-NORTHWESTWARD PATH AT
ABOUT 17 KT. THIS MOTION IS EXPECTED TO CONTINUE FOR ABOUT ANOTHER
DAY...FOLLOWED BY A SLIGHT DECREASE IN FORWARD SPEED AND A TURN
TOWARD THE WEST. THE MODELS ARE IN FAIRLY GOOD AGREEMENT...AND
LITTLE CHANGE WAS MADE TO THE PREVIOUS NHC TRACK FORECAST. FLOSSIE
IS EXPECTED TO BE NEAR OR OVER THE HAWAIIAN ISLANDS IN ABOUT 3
DAYS.

BASED ON THE CURRENT FORECAST...FLOSSIE WILL ENTER THE CENTRAL
PACIFIC HURRICANE CENTER AREA OF RESPONSIBILITY AROUND 1800 UTC.

FORECAST POSITIONS AND MAX WINDS

INIT 27/0900Z 17.5N 137.6W 60 KT 70 MPH
12H 27/1800Z 18.1N 140.2W 60 KT 70 MPH
24H 28/0600Z 18.9N 143.5W 55 KT 65 MPH
36H 28/1800Z 19.4N 146.6W 50 KT 60 MPH
48H 29/0600Z 19.7N 149.6W 40 KT 45 MPH
72H 30/0600Z 20.0N 155.5W 35 KT 40 MPH...INLAND
96H 31/0600Z 20.5N 162.0W 30 KT 35 MPH
120H 01/0600Z...DISSIPATED

$$
FORECASTER CANGIALOSI
Member Since: July 21, 2011 Posts: 83 Comments: 7167
BULLETIN
TROPICAL STORM FLOSSIE ADVISORY NUMBER 10
NWS NATIONAL HURRICANE CENTER MIAMI FL EP062013
200 AM PDT SAT JUL 27 2013

...FLOSSIE STRENGTHENS...
...ALMOST A HURRICANE...


SUMMARY OF 200 AM PDT...0900 UTC...INFORMATION
----------------------------------------------
LOCATION...17.5N 137.6W
ABOUT 1155 MI...1860 KM E OF HILO HAWAII
MAXIMUM SUSTAINED WINDS...70 MPH...110 KM/H
PRESENT MOVEMENT...WNW OR 285 DEGREES AT 20 MPH...32 KM/H
MINIMUM CENTRAL PRESSURE...995 MB...29.38 INCHES
Member Since: July 21, 2011 Posts: 83 Comments: 7167
TROPICAL STORM DORIAN DISCUSSION NUMBER 13
NWS NATIONAL HURRICANE CENTER MIAMI FL AL042013
500 AM AST SAT JUL 27 2013

DISORGANIZED CONVECTION HAS DEVELOPED NEAR THE CENTER OF DORIAN
DURING THE PAST SEVERAL HOURS. HOWEVER...WATER VAPOR IMAGERY SHOWS
WIDESPREAD MID-/UPPER-LEVEL DRY AIR NEAR THE CYCLONE...WHICH
SUGGESTS THIS CONVECTION COULD BE SHORT-LIVED. THE INITIAL
INTENSITY IS KEPT AT 35 KT BASED ON A SATELLITE INTENSITY ESTIMATE
FROM TAFB AND EARLIER ASCAT DATA. IT SHOULD BE NOTED THAT RECENT
OSCAT DATA SUGGESTS THE POSSIBILITY THAT DORIAN NO LONGER HAS A
CLOSED CIRCULATION.

THE INITIAL MOTION IS 275/19. A STRONG LOW-/MID-LEVEL RIDGE NORTH
OF DORIAN IS EXPECTED TO STEER THE CYCLONE QUICKLY WESTWARD FOR THE
NEXT 72 HR. THE TRACK GUIDANCE IS IN GOOD AGREEMENT WITH THIS
SCENARIO...AND THE NEW TRACK FORECAST IS AN UPDATE OF THE PREVIOUS
FORECAST.

DORIAN IS FORECAST TO EXPERIENCE 15-20 KT OF VERTICAL WIND SHEAR
DURING THE FORECAST PERIOD...AS WELL AS MOVE THROUGH A DRY
MID-/UPPER-LEVEL AIR MASS. THIS COMBINATION SHOULD CAUSE
ADDITIONAL WEAKENING...WITH THE CYCLONE DEGENERATING TO A REMNANT
LOW BY 36 HR. AN ALTERNATIVE SCENARIO IS THAT THE SYSTEM COULD
DEGENERATE TO A TROPICAL WAVE AT ANY TIME DURING THE NEXT 72 HR...
AND THUS DISSIPATE EARLIER THAN CURRENTLY FORECAST. UPPER-LEVEL
WINDS IN THE AREA WEST OF THE FORECAST POINT OF DISSIPATION ARE NOT
EXPECTED TO BE CONDUCIVE FOR RE-GENERATION AT THIS TIME.

FORECAST POSITIONS AND MAX WINDS

INIT 27/0900Z 18.2N 50.0W 35 KT 40 MPH
12H 27/1800Z 18.6N 53.1W 30 KT 35 MPH
24H 28/0600Z 19.2N 57.2W 30 KT 35 MPH
36H 28/1800Z 19.8N 61.1W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 29/0600Z 20.4N 64.7W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 30/0600Z 21.0N 71.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 31/0600Z...DISSIPATED

$$
FORECASTER BEVEN
Member Since: July 21, 2011 Posts: 83 Comments: 7167
Quoting 1692. Tribucanes:
I don't know Kori, Dorian's out of ICU and getting checked out of the hospital currently. Key words, "I don't know."


Yes, but the convection still noticeably lacks banding. It's pretty shapeless right now.
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BULLETIN
TROPICAL STORM DORIAN ADVISORY NUMBER 13
NWS NATIONAL HURRICANE CENTER MIAMI FL AL042013
500 AM AST SAT JUL 27 2013

...DORIAN CONTINUES MOVING RAPIDLY WESTWARD AS A TROPICAL STORM...


SUMMARY OF 500 AM AST...0900 UTC...INFORMATION
----------------------------------------------
LOCATION...18.2N 50.0W
ABOUT 860 MI...1385 KM E OF THE NORTHERN LEEWARD ISLANDS
MAXIMUM SUSTAINED WINDS...40 MPH...65 KM/H
PRESENT MOVEMENT...W OR 275 DEGREES AT 22 MPH...35 KM/H
MINIMUM CENTRAL PRESSURE...1011 MB...29.85 INCHES
Member Since: July 21, 2011 Posts: 83 Comments: 7167
Quoting 1690. Tribucanes:
Kori, what do you think of the low off of Africa, starting to show real signs this morning.


I don't see anything out of the ordinary for now, but the GFS projects very favorable upper-level winds for the next several days. At the very least, it has that going for it.
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I don't know Kori, Dorian's out of ICU and getting checked out of the hospital currently. Key words, "I don't know."
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Quoting 1640. Civicane49:
Still a tropical storm in the 06z update.

AL, 04, 2013072706, , BEST, 0, 180N, 490W, 35, 1011, TS,

Flossie up to 55 knots.

EP, 06, 2013072706, , BEST, 0, 172N, 1367W, 55, 998, TS,
Quoting 1640. Civicane49:
Still a tropical storm in the 06z update.

AL, 04, 2013072706, , BEST, 0, 180N, 490W, 35, 1011, TS,

Flossie up to 55 knots.

EP, 06, 2013072706, , BEST, 0, 172N, 1367W, 55, 998, TS,
Flossie might reach hurricane status after all.
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Kori, what do you think of the low off of Africa, starting to show real signs this morning.
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Quoting 1688. Tribucanes:
Dorian has been tightening up and the beginning of forming up of a new moisture shield is happening too. Spin has become good enough to pull in some of the moisture of Dorian's old moisture shield that was left behind. With shear expected to be high, it makes me wonder why some intensity models have Dorian blowing up again. Would have to be within the next 24-36hrs for that to happen. Would love to be privy to the NHC discussion going on right now.


It's going to die today. Mark my words. :)
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Dorian has been tightening up and the beginning of forming up of a new moisture shield is happening too. Spin has become good enough to pull in some of the moisture of Dorian's old moisture shield that was left behind. With shear expected to be high, it makes me wonder why some intensity models have Dorian blowing up again. Would have to be within the next 24-36hrs for that to happen. Would love to be privy to the NHC discussion going on right now.
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Quoting 1680. VR46L:


The Ships Usually is , I don't know I must be missing something I can't see him coming back


The SHIPS' intensity algorithm is based on historical tropical cyclones. It's a statistical model, hence the expedient acronym "Statistical Hurricane Intensity Prediction Scheme".

As far as I'm aware it doesn't get more sophisticated than that, which is why there is typically so much run to run variability.
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1686. VR46L
Quoting 1685. stoormfury:
very impressive wave off the African coast 90L COMING SOON ?? IT LOOKS LIKE THE SHOW IS ABOUT TO BEGIN


I dont see why not,It looks good been watching it 2 days now !
but it will probably go the same way as Chantal and Dorian
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very impressive wave off the African coast 90L COMING SOON ?? IT LOOKS LIKE THE SHOW IS ABOUT TO BEGIN
Member Since: August 22, 2006 Posts: 0 Comments: 2722
1684. VR46L
Is anyone elses filter playing up .. I have it set to show all and its seems to be showing Average instead !
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Flossie has been taking advantage of warm sea surface temperatures over the past few days. Since it is crossing the 26C isotherm, Flossie appears to have reached its peak intensity with 70 mph winds. Steady weakening is anticipated as the system will traverse into cool waters at 25C during the next couple of days. Although water temperatures will increase at about 150W, increasing westerly shear and drier air should result in further weakening. Flossie will probably hit the Big Island of Hawaii as a weakening tropical storm/depression by Monday evening. Tropical storm watches and warnings may be required for the Big Island and the Maui County by late tomorrow.

Member Since: July 21, 2011 Posts: 83 Comments: 7167
1682. VR46L
Quoting 1681. dfwstormwatch:
you guys know that the shear your pointing out was at around 50 knots 24 hours ago right?


Was it ! I thought It was 40kts then either way its pretty destructive shear levels to the NW of Dorian ..
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you guys know that the shear your pointing out was at around 50 knots 24 hours ago right?
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1680. VR46L
Quoting 1678. yankees440:


Model intensity still split...Ships much more aggressive


The Ships Usually is , I don't know I must be missing something I can't see him coming back
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Quoting 1793. OracleDeAtlantis:
Tonight as I was watching Jim Cantore on the Weather Channel, he talked about how forecasting a hurricane was like a triangle and then he drew one with his hands in the air.

By accident, a couple of nights ago, I merged one of two components from my initial, intuitive hurricane forecast this year(Post 543,) with my recent update(Post 2553,) by swapping the animation file underneath the first graphic.

What you're about to see was not preplanned or choreographed, the pieces fell into place, visually congruent.

With intuitive forecasting, this is what I call an underline, it reaffirms the first forecast.

The visually congruent scene at the very end of this curious movie, is arguably what the American Indians used to say to those who believed the earth had no voice, or medicine.

Note: It helps to change your browser's View setting to allow "full screen."





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Model intensity still split...Ships much more aggressive
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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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