ATL: ARTHUR - Post-Tropical - Discussion

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Re:

#2841 Postby ozonepete » Fri Jul 04, 2014 5:45 pm

Time_Zone wrote:How is the pressure falling? It's over cold water and high shear.

This storm confuses me...


These tropical cyclones transitioning to non-tropical are very confusing until you learn the dynamics. I know them a little better because I live up here where this happens all the time and because my friend Stacy Stewart taught me a lot of this. :) Because the thunderstorms are maintaining (for the reason I just posted) and the circulation is still very symmetric, the winds are maintaining so the pressure maintains or even drops a little bit. Also, as you go northward towards the poles the Coriolis force gets stronger, so a cyclone with a pressure of 970 mb near Cape Cod is needed to support similar winds as a 980 cyclone would further south. (These are rough numbers just for an example). Recall that Hurricane Sandy (becoming extra-tropical) had a very low pressure around 940 mb at New Jersey but top sustained winds were 70-80 mph. Hurricane Hugo at South Carolina in 1989 had a similar pressure and had sustained winds near 140 mph! There are other reasons that are pretty complex to go into here, especially the dynamic differences between tropical and extra-tropical cyclones, but the important point is that if a hurricane racing northward off the mid-Atlantic has the necessary dynamics to maintain its winds, the pressure will maintain or even drop.

This is a simplification just to give you an overall idea.
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Re: Re:

#2842 Postby Time_Zone » Fri Jul 04, 2014 6:00 pm

ozonepete wrote:
Time_Zone wrote:How is the pressure falling? It's over cold water and high shear.

This storm confuses me...


These tropical cyclones transitioning to non-tropical are very confusing until you learn the dynamics. I know them a little better because I live up here where this happens all the time and because my friend Stacy Stewart taught me a lot of this. :) Because the thunderstorms are maintaining (for the reason I just posted) and the circulation is still very symmetric, the winds are maintaining so the pressure maintains or even drops a little bit. Also, as you go northward towards the poles the Coriolis force gets stronger, so a cyclone with a pressure of 970 mb near Cape Cod is needed to support similar winds as a 980 cyclone would further south. (These are rough numbers just for an example). Recall that Hurricane Sandy (becoming extra-tropical) had a very low pressure around 940 mb at New Jersey but top sustained winds were 70-80 mph. Hurricane Hugo at South Carolina in 1989 had a similar pressure and had sustained winds near 140 mph! There are other reasons that are pretty complex to go into here, especially the dynamic differences between tropical and extra-tropical cyclones, but the important point is that if a hurricane racing northward off the mid-Atlantic has the necessary dynamics to maintain its winds, the pressure will maintain or even drop.

This is a simplification just to give you an overall idea.


Thanks for the explanation! I must say Arthur isn't looking too shabby right now despite the decrease in winds.
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Re: ATL: ARTHUR - Hurricane - Discussion

#2843 Postby somethingfunny » Fri Jul 04, 2014 6:44 pm

Also, any expansion of the windfield is symptomatic of strengthening just as much as any intensification of the maximum windspeed would be... So, though the windspeeds are dropping a bit, they're covering a much larger area. See any number of large intense storms without particularly high windspeeds (all of the ones Andrew92 mentioned are some) for examples of why windspeed isn't the only way intensity manifests.

I guess your expectation was that Arthur would surprise the NHC intensify or at least maintain its' intensity while flying north toward Canada, and frankly it has! Its' central pressure is still around the same level it had down by North Carolina, it's just that the windfield is evolving to spread hurricane-force winds over a larger area than ever before. A tight storm is Luke Skywalker and a wide storm is 100 Stormtroopers - they're equally powerful.
Last edited by somethingfunny on Fri Jul 04, 2014 6:48 pm, edited 1 time in total.
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#2844 Postby Time_Zone » Fri Jul 04, 2014 6:45 pm

Is Arthur still tropical?
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Re: ATL: ARTHUR - Hurricane - Discussion

#2845 Postby ozonepete » Fri Jul 04, 2014 6:49 pm

:uarrow: You're welcome! And yes right now it still is. Now while it's maintained really well so far it is just now about to go from 25C to 18C water in a really short time. Look at one of the satellite floaters and turn on SSTs at the top selection boxes. See that incredibly steep isotherm for SSTs that it's about to cross where the lines are so tightly packed? Wow! That's a really fast drop. Now you would expect the strength to drop off noticeably, but there's one catch. As it ingests that much lower octane fuel, it will now start undergoing extra-tropical transition. Look at a current surface chart and you can see that the cold front just west of it is getting close to the COC of Arthur. So it will now start ingesting the cooler drier air behind the front into its circulation. The cooler air will get pulled into its southern and western half while the warm tropical air on the east will remain on the eastern side and spread across the northern side and it will start becoming like a mid-latitude cyclone which are driven by difference in temp horizontally rather than vertically. Also, cold air at 500 mb (near 20,000 feet roughly) will move in over the cyclone and this will invigorate it by keeping the temperature difference between the low levels and upper levels fairly significant. So this extra-tropical transition from tropical to mid-latitude type will prevent it from weakening as rapidly. The transition is offsetting the SST drop! Pretty neat, huh? In cases where the 500mb cold pool (trough) moving over it is really strong you can actually get strengthening. That's what happened with Sandy.
Last edited by ozonepete on Fri Jul 04, 2014 6:54 pm, edited 1 time in total.
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Re: ATL: ARTHUR - Hurricane - Discussion

#2846 Postby somethingfunny » Fri Jul 04, 2014 6:53 pm

ozonepete wrote::uarrow: You're welcome! And yes right now it still is. Now while it's maintained really well so far it is just now about to go from 25C to 18C water in a really short time. Look at one of the satellite floaters and turn on SSTs at the top selection boxes. See that incredibly steep isotherm for SSTs that it's about to cross where the lines are so tightly packed? Wow! That's a really fast drop. Now you would expect the strength to drop off noticeably, but there's one catch. As it ingests that much lower octane fuel, it will now start undergoing extra-tropical transition. Look at a current surface chart and you can see that the cold front just west of it is getting close to the COC of Arthur. So it will now start ingesting the cooler drier air behind the front into its circulation. The cooler air will get pulled into its southern and western half while the warm tropical air on the east will remain on the eastern side and spread across the northern side and it will start becoming like a mid-latitude cyclone which are driven by difference in temp horizontally rather than vertically. Also, cold air at 500 mb (near 120,000 feet roughly) will move in over the cyclone and this will invigorate it by keeping the temperature difference between the low levels and upper levels fairly significant. So this extra-tropical transition from tropical to mid-latitude type will prevent it from weakening as rapidly. The transition is offsetting the SST drop! Pretty neat, huh? In cases where the 500mb cold pool (trough) moving over it is really strong you can actually get strengthening. That's what happened with Sandy.


So what is the forecast for Extratropical Storm Arthur once that happens? Is this one that Europe needs to be watching out for?
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Re: ATL: ARTHUR - Hurricane - Discussion

#2847 Postby ozonepete » Fri Jul 04, 2014 7:04 pm

somethingfunny wrote:
ozonepete wrote::uarrow: You're welcome! And yes right now it still is. Now while it's maintained really well so far it is just now about to go from 25C to 18C water in a really short time. Look at one of the satellite floaters and turn on SSTs at the top selection boxes. See that incredibly steep isotherm for SSTs that it's about to cross where the lines are so tightly packed? Wow! That's a really fast drop. Now you would expect the strength to drop off noticeably, but there's one catch. As it ingests that much lower octane fuel, it will now start undergoing extra-tropical transition. Look at a current surface chart and you can see that the cold front just west of it is getting close to the COC of Arthur. So it will now start ingesting the cooler drier air behind the front into its circulation. The cooler air will get pulled into its southern and western half while the warm tropical air on the east will remain on the eastern side and spread across the northern side and it will start becoming like a mid-latitude cyclone which are driven by difference in temp horizontally rather than vertically. Also, cold air at 500 mb (near 20,000 feet roughly) will move in over the cyclone and this will invigorate it by keeping the temperature difference between the low levels and upper levels fairly significant. So this extra-tropical transition from tropical to mid-latitude type will prevent it from weakening as rapidly. The transition is offsetting the SST drop! Pretty neat, huh? In cases where the 500mb cold pool (trough) moving over it is really strong you can actually get strengthening. That's what happened with Sandy.


So what is the forecast for Extratropical Storm Arthur once that happens? Is this one that Europe needs to be watching out for?


Here's the NHC forecast which seems pretty reasonable (as they always are). After crossing the Canadian maritimes as an extra-tropical storm with TS winds it will actually reach Greenland as a weak extra-tropical storm with depression winds. They usually either peter out or get caught in the westerlies and go across the Atlantic after completely transitioning into a run-of-the-mill mid-latitude low pressure. A few have remained as strong extra-tropical storms and made it all the way over to Europe where they caused some damage. Arthur is a little more rare here because they usually go eastward and rarely ever get all the way north to Greenland.

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Re: ATL: ARTHUR - Hurricane - Discussion

#2848 Postby supercane4867 » Fri Jul 04, 2014 8:38 pm

54kt wind gust recorded in Nantucket with sustained TS condition

KACK: Nantucket, Nantucket Memorial Airport, MA, United States [54kt, 28m/s]
KACK 050040Z 35037G52KT 3/4SM R24/2600VP6000FT -RA BR VV005 18/18 A2951
KACK 050040Z 35037G52KT 3/4SM R24/2600VP6000FT -RA BR VV005 18/18 A2951 RMK AO2 PK WND 36054/0017 RAE09B29 PRESFR P0036
KACK 050053Z 35038G52KT 1SM R24/4000VP6000FT BR VV005 18/18 A2950 RMK AO2 PK WND 36054/0017 RAE09B29E42 SLP988 P0040 T01780178
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Re: ATL: ARTHUR - Hurricane - Discussion

#2849 Postby ozonepete » Fri Jul 04, 2014 9:22 pm

Thanks, supercane! Here's the translated data for those who can't read METARS. Clearly full tropical storm conditions going on there. Here's the link if anyone wants to follow the conditions there: http://w1.weather.gov/obhistory/KACK.html

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#2850 Postby CrazyC83 » Fri Jul 04, 2014 9:28 pm

Seeing pictures of damage in the Cape and Islands from Arthur as well.
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#2851 Postby dexterlabio » Fri Jul 04, 2014 9:33 pm

Arthur looks pretty good for a hurricane in such high latitude.
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Re: ATL: ARTHUR - Hurricane - Discussion

#2852 Postby ozonepete » Fri Jul 04, 2014 9:55 pm

You can see the extra-tropical transition going on now. Look at the difference between the first (about 7PM) and second (about 11PM) satellite images. The cyclone is getting stretched from north to south to a northwest to southeast orientation as the 500 mb trough interacts with the tropical cyclone. You can see that northwest to southeast lobe of the 500 mb trough pushing out over Arthur on the last diagram of 500 mb heights (follow the v shaped trough out there over the northeast and the lobe that sticks out over the southern New England coast.) The very cold air at the bottom of that trough is slowly getting pulled over the top of the cyclone and will enable the thunderstorms to maintain even though the SSTs are dropping. it will also help transition Arthur from tropical to extra-tropical.

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#2853 Postby CrazyC83 » Fri Jul 04, 2014 10:05 pm

Should be ET in a few hours at this rate.
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Re:

#2854 Postby ozonepete » Fri Jul 04, 2014 10:18 pm

CrazyC83 wrote:Should be ET in a few hours at this rate.


Yup, Craze. Really fast now. Here's excerpts from the 11PM discussion:

HURRICANE ARTHUR DISCUSSION NUMBER 17
NWS NATIONAL HURRICANE CENTER MIAMI FL AL012014
1100 PM EDT FRI JUL 04 2014

Satellite and radar data indicates that Arthur has begun extratropical transition. The eye feature seen previously has dissipated, and the remaining convection is now displaced somewhat to the northeast of the low-level center. In addition, microwave total precipitable water data from a recent SSM/IS overpass shows very dry air entraining into the west and south sides of the cyclone. SFMR data from the previous Air Force Reserve Hurricane Hunter flight showed maximum winds of 60-65 kt, and based on this the initial intensity is reduced to a possibly generous 65 kt.
.....
Arthur is expected to complete extratropical transition in about 12 hours. After transition, the former tropical cyclone should steadily weaken, with winds dropping below gale force by 96 hours. The intensity forecast has been revised downward based on input from the OPC, as well as from the GFS and ECMWF models.
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Re: ATL: ARTHUR - Hurricane - Discussion

#2855 Postby HurricaneTracker2031 » Sat Jul 05, 2014 1:33 am

Arthur is moving closer to Nova Scotia and is weakening. It appears it is being absorbed by the cold front already...

Analysis on Arthur and other tropical systems: http://goo.gl/FpWqpA
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#2856 Postby Andrew92 » Sat Jul 05, 2014 1:52 am

AL, 01, 2014070506, , BEST, 0, 423N, 676W, 60, 981, TS, 34, NEQ, 100, 140, 100, 70, 1014, 180, 45, 75, 0, L, 0, , 0, 0, ARTHUR, M,

Expect a downgrade at the 5 AM advisory to a tropical storm, if not declared extratropical.

-Andrew92
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#2857 Postby supercane4867 » Sat Jul 05, 2014 3:50 am

...ARTHUR LOSING TROPICAL CHARACTERISTICS...
...WEATHER RAPIDLY DETERIORATING OVER WESTERN NOVA SCOTIA...
5:00 AM EDT Sat Jul 5
Location: 43.1°N 66.9°W
Moving: NE at 22 mph
Min pressure: 981 mb
Max sustained: 70 mph
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Re: ATL: ARTHUR - Post-Tropical - Discussion

#2858 Postby cycloneye » Sat Jul 05, 2014 7:01 am

Is Post-Tropical.

...ARTHUR BECOMES A POST-TROPICAL CYCLONE...
...STRONG WINDS AND HEAVY RAINS CONTINUE OVER PORTIONS
OF SOUTHEASTERN CANADA AND DOWN EAST MAINE...


SUMMARY OF 800 AM EDT...1200 UTC...INFORMATION
----------------------------------------------
LOCATION...44.5N 66.5W
ABOUT 50 MI...80 KM NNW OF YARMOUTH NOVA SCOTIA
MAXIMUM SUSTAINED WINDS...65 MPH...100 KM/H
PRESENT MOVEMENT...NNE OR 30 DEGREES AT 23 MPH...37 KM/H
MINIMUM CENTRAL PRESSURE...983 MB...29.03 INCHES
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#2859 Postby Hybridstorm_November2001 » Sat Jul 05, 2014 7:37 am

Winds now 30 mph sustained gusting to over 50 mph with heavy rain, reminds me of Irene in 2011 only in that case the winds and rain distinctly were separated. Lost power twice already this morning, so using generator now. Wind wise this is as strong as all but the absolute worse nor'easters we get in fall & winter. It makes me happy it wasn't any stronger, or hit more directly.
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Re: ATL: ARTHUR - Post-Tropical - Discussion

#2860 Postby tropicana » Sat Jul 05, 2014 8:26 am

POST TS ARTHUR LEAVES MORE THAN 120,000 WITHOUT POWER IN MARITIMES
Sat Jul 5 2014

More than 120,000 homes and businesses in the Maritimes are without electricity Saturday morning as post-tropical storm Arthur makes its way into the region. The storm was downgraded from hurricane status with sustained winds of about about 110 km/h as it approached landfall in the Yarmouth area of Nova Scotia.

The number of power outages in Nova Scotia and New Brunswick has climbed rapidly since the storm made landfall in Digby County around 7:30 AT. In Nova Scotia, more than 73,000 were affected by power outages as of 9:30 AT, stretching from Yarmouth in the southwest to Halifax and the Annapolis Valley.

In New Brunswick, the number of people without NB Power service more than doubled to 51,000 customers between 8:30 a.m. and 9:30 a.m.
The Fredericton region is the hardest hit in New Brunswick with 30,000 outages. More than 9,000 customers in the Woodstock area also lost power.

Outages in the range of 2,300 to 3,200 customers were reported in St. Stephen, Moncton and Rothesay.
The city of Fredericton was urging people to stay off ctiy streets and roads Saturday because of the number of trees down.
"If at all possible, residents are being asked to stay off the roads. If you have to be out on the roads, please use extreme caution," said a notice from city officials. "Trees are blocking roads; traffic lights are out in many areas of the city."

CBC meteorologist Peter Coade said the centre of the storm was over land by 7:30 a.m. AT. Coade said Arthur appears to have made landfall in the Digby County area north of Yarmouth, Winds ahead of the storm were gusting to 95 km/h at Baccaro Point in southwest Nova Scotia.
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