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 Post subject: 91C and the NE Pacific Analysis
PostPosted: Fri Nov 03, 2006 4:28 am 
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Unfortunately, this cyclone gives no reason to be excited. After looking at 91C last night and then again tonight, I believe this is not subtropical or tropical and never was, just a case of warm seclusion at its finest.

Too many people tend to focus on what is happening now. They look at a current satellite image and then get silly over what might happen in the future without investigating why the current object exists in the first place.

I have included satellite imagery of the northeastern Pacific Ocean from 22 Oct 00Z through 03 Nov 03Z.

Animated GIF Link

Now, using quickly thrown together satellite analyzes, I will give a day-by-day breakdown of the synoptic situation. I have labeled every low (after the phonic alphabet) and every high (after Spanish numbers) to ease the description.
Image
October 22nd:
Meridianal flow exists over the NEPAC, with Alpha making landfall in southern Alaska. Moisture associated with the warm front is affecting British Columbia. Behind Alpha is Bravo, which is in Bristol Bay. The associated occlusion is crossing the Alaska Peninsula. Between the two systems is an area of ridging, which is helping to kick off some stratocumulus. The associated area of high pressure--Uno, which is northwest of Hawai’i, plays a role with 91C.

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October 23rd:
Alpha has pushed into the Northwest Territories and Nunavut, with a frontolysizing cold front stretching into the Pacific Ocean. Helping to weaken this front is Uno north of Hawai’i. This feature is helping to orient Bravo and Charlie more zonal. Charlie is a low that developed two days prior on the cold front behind Bravo.

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October 24th:
Alpha’s cold front still is hanging around across the US Northwest, but there is no temperature advection occurring. Bravo was absorbed by Charlie, which caused significant deepening. Charlie’s warm and cold front have made landfall to the American West Coast and is dragging behind it an elongated stationary boundary. Delta has appeared on the map in the Bering Sea—the parent low of the cold front behind Echo. Echo, however, developed near the triple-point thanks to better jet support. The front behind Delta and Echo will eventually help form 91C.

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October 25th:
Charlie rapidly weakened as it crossed the Rocky Mountains of Canada. Meanwhile, the long frontal train that followed Charlie spawned a new low, Gulf, that is crossing Montana. Delta and Echo lost jet support while another low developed on that particular frontal chain, forming Foxtrot. This low is affecting Alaska and coastal British Columbia. Uno, north of Hawai’i, is continuing to help keep the pattern zonal.

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October 26th:
Charlie is entering Nunavut, heading toward the Hudson Bay. Delta and Echo have dissipated, while Foxtrot weakens in the Gulf of Alaska, thanks to lost jet support. Meanwhile the occlusion that was associated with Foxtrot has frontolysized. However, another low has formed on the chain, creating Hotel in northern British Columbia. Hotel, the next low (India), and the front between them are being stretched out thanks to Uno (that is now just west of California) and another high, Dos, that is southwest of Alaska (off the map). Essentially, this has created a large “col” area over the NEPAC.

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October 27th:
The remnants of Foxtrot remain in the Gulf of Alaska. Meanwhile, Hotel is marching off toward Hudson Bay. India is now getting closer to British Columbia, while a new low on the frontal boundary, Juliet (or in the imagery—labeled as 91C), forms under the polar front jet. Still, Uno and Dos in the central Pacific are keeping this pseudo-col pattern in place.

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October 28th:
Hotel is exiting the map while the warm front of Kilo is entering. The front behind Kilo will help push Dos eastward. Meanwhile, Uno is keeping still. With Uno and Dos getting closer, India is exiting northeastward. However… Kilo is starting to force Foxtrot southward, causing a dip to form between India and 91C.

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October 29th:
India is passing north of Uno’s influence, crossing British Columbia. Meanwhile, 91C is gaining more jet support and is undergoing cyclogenesis. Dos is now being squeezed between Kilo’s cold front and 91C’s warm front. To make matter more complicated, the remnants of Foxtrot is forcing ridging to cut 91C and India apart.

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October 30th:
Kilo is now dying over Alaska, while another wave on its front forms Lima. Meanwhile, India and 91C are separating. That is because 91C is developing more and Kilo/Lima’s front is forcing Dos north of 91C. Uno is prohibiting 91C from eastward progression, causing 91C to go quasi-stationary.

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Halloween:
While Kilo dissipates, Lima and a new low, Mike, slow their progression, thanks to Dos. 91C is wrapping itself up and has became a cut-off low because of Uno and Dos’s blocking. 91C, meanwhile, has undergone what is called warm seclusion. This is a mature phase in an extratropical cyclone’s life that is found in intense marine cyclones. In 91C’s case, warm low-level temperatures are trapped (or secluded) by a bent-back warm front. This low becomes somewhat warm-core because of the trapped warm air. This also helps to form a cloud-free area in the vorticity maximum, appearing like an eye. Also, the trapped warm air, coupled with relatively cool waters (in this case about 15°C) help fire up convection.

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November 1st:
Mike crosses British Columbia, north of Dos. Meanwhile, Lima is getting wrapped up in 91C’s circulation as that circulation expands and falls apart. I drew frontolysizing fronts only for continuity as there really is not much temperature advection occurring. Most of the air has been modified, except for a pocket of warm air, associated with the occlusion, that is near the center of 91C.

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November 2nd:
The pattern has practically fell apart at this point. Dos, Mike, and Lima have dissipated. The remnants of 91C’s warm front have merged with the cold front of Lima west of 91C. I have placed a trough here, although one could actually analyze something with better surface observations. Basically, this region is a meteorological mess. The occlusion of 91C has split, with the one section (that is somewhat attached to 91C) weakening and another section forming a new low, November. 91C, meanwhile, continues to spin up and reach its peak as warm air and vorticity keep going.

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November 3rd:
November has made landfall near Seattle, while 91C follows behind it. The warm air trapped in 91C has modified and is now closer to the surface temperature. This has caused 91C to experience cyclolysis. Meanwhile, the mess that was formerly known as 91C’s warm front and Lima’s cold front has developed its own low—Oscar.

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Image
Near-term forecast:
Expect November to fizzle while 91C makes landfall in Washington. 91C will quickly die off. Meanwhile, Oscar will strengthen a little bit before being pushed eastward with the flow.




-Mike


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 Post subject:
PostPosted: Fri Nov 03, 2006 7:04 pm 
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Nice analysis Mike. This event could make a great case study in the future.


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 Post subject:
PostPosted: Tue Nov 07, 2006 2:12 pm 
I am going to have to disagree.

IMO, this was a TC embedded within a larger non-tropical low, much like the unnamed hurricane of 1991 and Olga in 2001 (Now, there is some debate within the community as to whether or not those were actually hurricanes) This type of development can occur over colder water due to the very cold tropopause temperatures. The system also had a tight RMW.

The consensus of those who I have spoken to is that this probably was tropical (though I agree, that may be pushing it a little)


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