Tropical weather analysis - September 19, 2012

By: KoritheMan , 6:26 AM GMT on September 19, 2012

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Nadine

Nadine has weakened a little more this evening. As of the latest NHC advisory, the following information was available on the storm:

Wind: 50 mph, with higher gusts
Location: 36.2°N 32.1°W
Movement: NNE at 7 mph
Pressure: 993 mb

Nadine continues to lose organization. While no recent microwave passes are available, the satellite signature is starting to become reminiscent of a frontal low, although cyclone phase diagrams from FSU still showed at least a weak warm core in association with the system at 12 and 18z, respectively. While the satellite signature has continued to deterioriate since then, it takes time for extratropical transition, and Nadine probably still retains at least a weak mid- to upper-level warm core.



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

Significant vertical shear and cool sea surface temperatures should continue throughout the next five days. While these factors would normally argue for significant weakening, Nadine is obviously not very tropical, and is deriving at least some of its energy from temperature differentials. This should preclude significant weakening. Indeed, the majority of the guidance continues to suggest little change in strength, but Nadine has continued to weaken over the last 24 hours. However, with a major storm system now approaching the storm from the west, it seems likely that the cyclone will garner a little bit of baroclinic energy, which should act to keep the winds up, or at least not allow them to experience much additional dwindling. The future evolution -- structurally -- of Nadine, remains highly speculative. I see three possibilities: number one is for the cyclone to continue to lose central convection and degenerate into a remnant low pressure area; this seems a bit unlikely to me given the frontal appearance of the convection on satellite imagery. The second is for Nadine to transition to an extratropical cyclone over the next day or two, which is supported by the current structure. Finally, another potential event is for the cyclone to become absorbed by the north Atlantic low pressure system. This has been inconsistently suggested by some of the global models over the last few days. I tend to favor the middle option.

The steering currents around Nadine are beginning to weaken as the large scale flow to the north of the tropical cyclone becomes more zonal. A large upper low centered between the southern tip of Greenland and Newfoundland continues moving southeastward well to the north of Nadine. This evolution will drive the aforementioned frontal boundary eastward, but the amplitude of the front and the zonal flow over that portion of the north Atlantic suggests that this front will be insufficient to fully pick up Nadine. The storm is forecast to decelerate over the next day or two until the upper low drops southward as suggested by the global models. Beyond 24 hours, the approach of the low is forecast to turn Nadine southeastward. The global models have come into much better agreement on the future evolution of Nadine over the last several cycles, although the ECMWF remains an outlier at 12z in calling for a clockwise loop around the subtropical ridge. However, I recall the GFS solidly outperforming that particular model in the case of Isaac, when the latter stubbornly insisted on a hit along the Alabama coast even as the system was approaching Louisiana.

Despite the eastward shift, there continues to be some rather large disagreement pertaining to the details of that turn, as well as its duration. The GFS, after showing a much faster northeastward motion at 18z, has come in significantly slower at 0z, and keeps Nadine meanding between the Azores and the Canary Islands.

Interests in the Azores, particularly the northwestern section, should monitor the progress of Nadine over the next couple of days. Tropical storm force winds, especially in areas of higher elevation, are possible. A tropical storm warning has been issued for the archipelago (see below).

Watches and warnings


A TROPICAL STORM WARNING IS IN EFFECT FOR...
* THE AZORES ISLANDS OF FLORES...CORVO...FAIAL...PICO...SAO JORGE...
GRACIOSA...AND TERCEIRA

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.

5-day intensity forecast

INITIAL 09/19 0600Z 45 KT 50 MPH
12 hour 09/19 1800Z 45 KT 50 MPH
24 hour 09/20 0600Z 45 KT 50 MPH
36 hour 09/20 1800Z 45 KT 50 MPH
48 hour 09/21 0600Z 45 KT 50 MPH...EXTRATROPICAL
72 hour 09/22 0600Z 45 KT 50 MPH...EXTRATROPICAL
96 hour 09/23 0600Z 45 KT 50 MPH...EXTRATROPICAL
120 hour 09/24 0600Z 45 KT 50 MPH...EXTRATROPICAL

5-day track forecast



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



Central Atlantic low

A non-tropical area of low pressure has developed about 900 miles east of Bermuda. Satellite imagery suggests a well-defined surface circulation, and upper-level winds appear conducive for some development -- tropical or subtropical -- of this disturbance over the next couple of days. While a westward motion is anticipated in the interim, the models suggest recurvature ahead of a shortwave trough moving from North America by day three.

Probability of development in 48 hours: 20%



Lane

Lane has weakened to a tropical storm as of the latest NHC advisory, which is as follows:

Wind: 40 mph, with higher gusts
Location: 20.8°N 130.2°W
Movement: WNW at 9 mph
Pressure: 1006 mb

There is no deep convection associated with the circulation of Lane, which remains quite vigorous. There is only a small shower located about 150 miles northeast of the center. This suggests that Lane is quickly becoming a remnant low as it succumbs to southwesterly shear and cold waters. In fact, the cyclone may actually be a remnant low as of this writing.



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

Regardless of the technical status of the system, the low should dissipate in a few days and move westward in the low-level flow. A more definitive turn to the west should begin shortly as Lane has lost convection.

The track and intensity forecast philosophies remain unchanged. My track is similar to the National Hurricane Center 11:00 PM one, albeit faster and farther south based on the current motion relative to the 1200Z forecast point.

5-day intensity forecast

INITIAL 09/19 0600Z 35 KT 40 MPH
12 hour 09/19 1800Z 30 KT 35 MPH...POST-TROPICAL/REMNANT LOW
24 hour 09/20 0600Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
36 hour 09/20 1800Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
48 hour 09/21 0600Z 15 KT 15 MPH...POST-TROPICAL/REMNANT LOW
72 hour 09/22 0600Z...DISSIPATED

5-day track forecast



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

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9. KoritheMan
12:49 AM GMT on September 20, 2012
Quoting Slamguitar:
That was a good read. Do you have any opinion on if the central ATL low will make it to fully tropical or do you think this will be a strictly sub-tropical system if it develops?


It depends on how much it interacts with the upper low. Right now they are more or less collocated, which would not allow for much tropical development in the near-term. However, as we saw with Beryl, the temperature difference between the low-level warm core and the upper tropospheric cold core can, after awhile, generate instability, effectively abolishing the upper low and creating a warm core.
Member Since: March 7, 2007 Posts: 585 Comments: 20854
8. Slamguitar
9:06 PM GMT on September 19, 2012
Weird double post FTL...
Member Since: July 2, 2011 Posts: 0 Comments: 1183
7. Slamguitar
9:06 PM GMT on September 19, 2012
I'm leaning towards sub-tropical myself, mostly because the broad circulation which might make it harder to achieve full tropical characteristics. If it slows down its poleward motion, then it would have a better shot at tropical though, so who knows, lol.
Member Since: July 2, 2011 Posts: 0 Comments: 1183
6. wxchaser97
7:46 PM GMT on September 19, 2012
Quoting Slamguitar:


Well, I guess I better way to state my question is: Do you think this system will have enough time in a conductive environment to make it to fully tropical? I see the models don't think so.

I am right on the edge, as right now I don't see it happening but Beryl did it so I'm not entirely sure just yet.
Member Since: March 16, 2012 Posts: 127 Comments: 7950
5. Slamguitar
4:44 PM GMT on September 19, 2012
Quoting wxchaser97:
Thanks Kori, nice blog.

Slam, the low would probably by subtropical at least at one part of its life. Whether it becomes fully tropical hinges on how long it is over warm waters and in a good environment.


Well, I guess I better way to state my question is: Do you think this system will have enough time in a conductive environment to make it to fully tropical? I see the models don't think so.
Member Since: July 2, 2011 Posts: 0 Comments: 1183
4. wxchaser97
11:56 AM GMT on September 19, 2012
Quoting GeorgiaStormz:
I disagree with disagreeing with this blog.
I dont really have much to say though, my mind has been over the mainland US and is nearly fully out the tropics.

Severe weather is your thing, I like all weather. Does that mean you agree or disagree or what?
Member Since: March 16, 2012 Posts: 127 Comments: 7950
3. GeorgiaStormz
11:51 AM GMT on September 19, 2012
I disagree with disagreeing with this blog.
I dont really have much to say though, my mind has been over the mainland US and is nearly fully out the tropics.
Member Since: February 11, 2012 Posts: 0 Comments: 9731
2. wxchaser97
10:47 AM GMT on September 19, 2012
Thanks Kori, nice blog.

Slam, the low would probably by subtropical at least at one part of its life. Whether it becomes fully tropical hinges on how long it is over warm waters and in a good environment.
Member Since: March 16, 2012 Posts: 127 Comments: 7950
1. Slamguitar
6:51 AM GMT on September 19, 2012
That was a good read. Do you have any opinion on if the central ATL low will make it to fully tropical or do you think this will be a strictly sub-tropical system if it develops?
Member Since: July 2, 2011 Posts: 0 Comments: 1183

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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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