Tropical weather analysis - September 4, 2012

By: KoritheMan , 10:32 PM GMT on September 04, 2012

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Leslie

Tenacious Leslie continues to hang on amidst strong vertical shear. As of the latest NHC advisory, the following was posted on the cyclone:

Wind: 65 mph, with higher gusts
Location: 25.3°N 62.6°W
Movement: N at 3 mph
Pressure: 994 mb

After looking rather sickly this morning, convection has returned to the center, which based on visible satellite images appears to be located under the convection.



Figure 1. Latest infrared satellite image of Tropical Storm Leslie. Image credit: NOAA

While there is still shear over Leslie and the center could easily become exposed again, the strongest shear is located well north of the tropical storm based on analysis of water vapor imagery. Leslie is moving slow enough that it should not feel this shear. The global models still insist on a more favorable upper air pattern developing over the tropical cyclone during the next couple of days. One inhibiting factor that counteract the anticyclonic wind pattern is the continued slow motion of the cyclone; most of the global models show Leslie moving little over the next three days, which could create cold water upwelling underneath the cyclone vortex. But since the underlying ocean is fairly warm for this part of the Atlantic, I am unsure how much of a negative factor this will actually be. By the weekend, Leslie is forecast to begin moving again. With the maintenance of the very favorable upper-level pattern previously alluded to, the cyclone is forecast to strengthen, and it could do so more quickly than I am indicating. Leslie easily has a chance to become a major hurricane. This is could occur very near Bermuda, and interests there should carefully monitor Leslie's progress.

Leslie is caught within a very complex steering pattern. Water vapor and 12z upper air data over the United States show an amplifying shortwave trough over the Ohio Valley and mid-Atlantic. Downstream from the trough, there is evidence of a building low- to mid-level ridge in the wake of a trough that has bypassed Leslie's longitude. This type of pattern usually means slow movement, and that is what will be forecast.

Recent satellite fixes suggest that Leslie might have become quasi-stationary again, with perhaps a very slow northward drift. This general motion is forecast to continue over the next few days, with a gradual turn to the north-northwest later in the period. Beyond Sunday, the system is forecast to come under the influence of a deep-layer trough forecast to move from the mid-Atlantic states into the waters of the western Atlantic. This trough should be sufficient to recurve Leslie. However, depending on the timing and amplitude of the trough, as well as Leslie's position in relation to it, the storm could pose a threat to New England or Atlantic Canada. None of the models are indicating the former, but they do suggest the latter. Interests in both these areas should follow Leslie's progress carefully through the early part of next week.

5-day intensity forecast

INITIAL 09/04 2100Z 55 KT 65 MPH
12 hour 09/05 0600Z 55 KT 65 MPH
24 hour 09/05 1800Z 60 KT 70 MPH
36 hour 09/06 0600Z 65 KT 75 MPH
48 hour 09/07 1800Z 70 KT 80 MPH
72 hour 09/08 1800Z 75 KT 85 MPH
96 hour 09/09 1800Z 80 KT 90 MPH
120 hour 09/10 1800Z 90 KT 105 MPH

5-day track forecast



Figure 2. My 5-day forecast track for Leslie.



Michael

Tropical Storm Michael formed from what was Tropical Depression Thirteen yesterday. As of the latest NHC advisory, the following was posted on the storm:

Wind: 50 mph, with higher gusts
Location: 27.5°N 43.7°W
Movement: NNW at 7 mph
Pressure: 1006 mb

Satellite images reveal that Michael is a rather small tropical storm, and would likely have been missed prior to the advent of the satellite era. It still doesn't beat horizontally-challenged Marco from 2008 though, which was probably no more than 75 miles across. Although cloud tops have warmed, an SSMI overpass from 1830z suggested a well-defined curved banding pattern, as well as the beginnings of a warm spot, possibly a mid-level eye. However, this feature has not apparent on conventional satellite images, and I am uncomfortable lableing this an eye for now. In any event, Michael is clearly strengthening.



Figure 3. Latest infrared satellite image of Tropical Storm Michael. Image credit: NOAA

Upper-level outflow is confined to the eastern portion of the circulation due to continued northwesterly shear. The global models show an increase in this shear over the next 24-48 hours as an upper-level trough to the north of Michael moves eastward. Since Michael is a small storm, it will be highly susceptible to any increase in shear. In fact, I would not be surprised to see it weaken a little, but a more conservative wager would be to indicate little change. Beyond 48 hours, the upper-level winds are forecast to relax, and a small anticyclone is forecast to move in tandem with the tropical cyclone. This should promote some strengthening later in the forecast period. In the mean time, a little additional intensification is forecast before the upper flow becomes prohibitive.

Water vapor and UW-CIMSS steering analysis suggests that Michael is wedged between an approaching trough to the west and a mid-level ridge to the east. This pattern should result in a north-northwestward motion in the short-term, followed by a gradual poleward bend as the trough becomes the dominant steering influence. In about three days, the trough is forecast to bypass Michael, leaving the cyclone in a region of weak steering. At that point, the primary steering mechanism will be a narrow mid-level ridge, but moreso the circulation of Leslie, which is forecast to be a large hurricane at that time. Michael should respond by turning slowly west-northwestward around the eastern side of the cyclone vortex. In general the models agree on this, but Michael has to stairstep to get there. This kind of squashed spider consensus usually indicates slow movement, as well as a big forecasting headache. My forecast track is pretty similar to that of the National Hurricane Center.

Large swells will continue affecting much of the US east coast, Bermuda, and Atlantic Canada throughout much of this week.

5-day intensity forecast

INITIAL 09/04 2100Z 45 KT 50 MPH
12 hour 09/05 0600Z 50 KT 60 MPH
24 hour 09/05 1800Z 50 KT 60 MPH
36 hour 09/06 0600Z 50 KT 60 MPH
48 hour 09/07 1800Z 50 KT 60 MPH
72 hour 09/08 1800Z 55 KT 65 MPH
96 hour 09/09 1800Z 60 KT 70 MPH
120 hour 09/10 1800Z 65 KT 75 MPH

5-day track forecast



Figure 4. My 5-day forecast track for Michael.



Remnants of Isaac

Isaac's hatred for the northern Gulf Coast continues. Even in death, he refuses to quit. A satellite archive from the last few days shows that a piece of the mid-level energy of former Hurricane Isaac detached from the main frontal zone, moved eastward across Arkansas into Tennessee, where it gradually turned southward. Now, it is located over southeastern Alabama, where it is bringing rain and severe thunderstorms. Surface observations do not indicate a well-defined circulation, and if it is going to regenerate, it will probably take a little time to do so. Isaac should be over the Gulf of Mexico in about 6-12 hours.

Upper air data, water vapor images, and real time steering data from UW-CIMSS shows that a mid-level ridge is building over the central United States as a weak shortwave moves by to the north. A break in the ridge is noted over the eastern United States from about 83W eastward. However, the flow over the central and eastern US is highly zonal, indicating a weak trough. We saw this with the same trough that brought Isaac to Louisiana. The global models have successfully identified this trough as a weak one, and it is forecast to bypass Isaac over the next day or so, and Isaac will have to wait for another trough developing off the Pacific northwest, forecast to arrive by the weekend. The result will be weak steering currents. This portion of the forecast is critical in determining which section of the Gulf Coast feels Isaac's second go round. If the storm moves significantly south and west, it could make landfall along the northern Gulf Coast as the trough picks it up. Alternatively, if the system doesn't gain any significant westward longitude before the arrival of the trough, it will move across the Florida peninsula and into the western Atlantic. This seems to be the most likely scenario for now based on the consensus of the global models and Isaac's close proximity to the east coast trough. However, they tend to perform poorly with small entities, and this is Isaac, so you never know. The synoptic setup here is similar to what we saw with Debby back in June, and it's essentially a battle between the ridge over the central US and the trough over the east.

Based on the depth of the upper low over the Bahamas, should Isaac become relatively deep over the next day or two, it could move southwestward around the western periphery of said low, which has a strong signature at 500 mb and above. This would place the northern Gulf Coast at greater risk.

Strong northerly shear is forecast to affect the northeastern Gulf waters for the next few days, in association with the upper low and the central US ridge. However, if Isaac moves southwest, it will be moving in the same direction as the shear vector, which would likely dampen the blow. Vertical shear will be less the farther west Isaac goes.

Probability of development in 48 hours: 10%

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4. wxchaser97
11:58 AM GMT on September 05, 2012
Nice blog Kori, thanks!
Member Since: March 16, 2012 Posts: 127 Comments: 7981
3. LostTomorrows
11:56 PM GMT on September 04, 2012
Quoting KoritheMan:


Thank you for the kind words.

Concerning Leslie, Bermuda seems to have good building codes. In fact, only a few people were killed on the island during Fabian. I realize death isn't a statistic, but they shouldn't suffer massive casualties. Most likely just some power outages, roof, and tree damage.


I'm only concerned because Leslie will apparently be on a strengthening trend and pass more slowly, whereas Fabian was the opposite. Also, Leslie seems to be on pace to be much larger. All ingredients for a more damaging storm. I know Bermuda has very strict building codes; they're built on a volcano, so they have the resources below their feet for strong, weather-resistant architecture.
Member Since: August 26, 2008 Posts: 0 Comments: 618
2. KoritheMan
11:19 PM GMT on September 04, 2012
Quoting LostTomorrows:
I really enjoy your blogs, you're pretty much the only blogger here (asides from the main, obviously) whos blogs I keep track of. Very in-depth. And you don't simply abide by the NHC forecasts either.

Michael is impressing as per usual with storms that develop at a certain latitude above the MDR: Cris, Gordon, and Kirk all managed to become hurricanes when they weren't initially forecast to. I think it's because the MDR is largely being invaded by very dry air, so I think this trend might continue for a few years. I wouldn't be surprised if Michael achieves hurricane status.

And that Leslie is a terrifying monster, and I know people in Bermuda and am keeping in touch with them, they are only now starting to take Leslie seriously, it seems, she could very well be worse than Fabian was for them, and could swallow that country whole. She's massive. It is cause for worry.

And Isaac... he really is a nasty, nasty guy.


Thank you for the kind words.

Concerning Leslie, Bermuda seems to have good building codes. In fact, only a few people were killed on the island during Fabian. I realize death isn't a statistic, but they shouldn't suffer massive casualties. Most likely just some power outages, roof, and tree damage.
Member Since: March 7, 2007 Posts: 602 Comments: 21780
1. LostTomorrows
10:58 PM GMT on September 04, 2012
I really enjoy your blogs, you're pretty much the only blogger here (asides from the main, obviously) whos blogs I keep track of. Very in-depth. And you don't simply abide by the NHC forecasts either.

Michael is impressing as per usual with storms that develop at a certain latitude above the MDR: Cris, Gordon, and Kirk all managed to become hurricanes when they weren't initially forecast to. I think it's because the MDR is largely being invaded by very dry air, so I think this trend might continue for a few years. I wouldn't be surprised if Michael achieves hurricane status.

And that Leslie is a terrifying monster, and I know people in Bermuda and am keeping in touch with them, they are only now starting to take Leslie seriously, it seems, she could very well be worse than Fabian was for them, and could swallow that country whole. She's massive. It is cause for worry.

And Isaac... he really is a nasty, nasty guy.
Member Since: August 26, 2008 Posts: 0 Comments: 618

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About KoritheMan

I'm just a 23 year old with an ardent passion for weather. I first became aware of this interest after Tropical Storm Isidore struck my area in 2002.

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