Tropical weather analysis - September 3, 2012

By: KoritheMan , 3:18 AM GMT on September 04, 2012

Leslie

Leslie continues to move across the western Atlantic as a tropical storm. As of the latest NHC advisory, the following was posted on the cyclone:

Wind: 65 mph, with higher gusts
Location: 24.0°N 63.1°W
Movement: Stationary
Pressure: 998 mb

Leslie remains sprawling and disorganized, with little evidence of banding within the large convective cloud shield that accompanies the tropical cyclone. Earlier microwave data suggested that the low-level center was exposed to the northwest of the deep convection due to continued northwesterly shear. This assumption is confirmed by conventional satellite images. Interestingly, the earlier referenced microwave pass showed what looked like an eye feature in the 37 GhZ channel. However, this central feature was much farther into the deep convection, and it would be unusual to have a low-level eye at the bottom half of the troposphere, while having no such signature or hint at the upper half of the troposphere. I have never seen an eye evident at the 37 GhZ channel in those images before the 85 GhZ one. The former is sensitive to the lower troposphere, while the latter is more sensitive to the middle and upper troposphere. Still rather interesting to note, though.



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

The SHIPS and the global models slowly relax the shear over the next few days, especially beyond 72 hours, when a large anticyclone is forecast to coexist with Leslie. This synoptic evolution favors intensification, and despite its struggles, Leslie is still expected to become a hurricane. There is a chance that it could become a major hurricane at longer ranges, but this is not being explicitly indicated at this time.

Water vapor images show that the initial trough has bypassed Leslie, leaving the tropical storm in an area of weak steering between two ridges -- one over the central Atlantic rebuilding in the wake of the trough, and another off the east coast of the United States. There is a small upper low centered across the northwestern Bahamas. For now, this feature is not expected to have much influence on steering. In about three days, however, the models show it moving much closer to Leslie's vortex. At that point the storm is forecast to be deep enough and in close enough proximity to the low to begin moving again.

It should be noted that the UKMET and ECMWF models show a more prolonged north-northwest to northwestward motion through the weekend. This is in contrast to the GFS, which barely shows the cyclone moving near Bermuda before being picked up by a trough. There is some hinting from the CMC and NOGAPS that Leslie could pose a long-range threat to the mid-Atlantic or New England. However, none of the more reliable global models -- namely the GFS and ECMWF -- show this. Nonetheless, there is enough fragility in the pattern and disagreement amongst the models so that interests in these areas should monitor the progress of Leslie.

Regardless, the cyclone is forecast to grow considerably. The prolonged fetch of strong winds associated with the circulation of Leslie is expected to generate large swells and rip currents along much of the east coast of the United States over the next week. This wave action could eventually find its way to sections of Atlantic Canada as the cyclone begins a more definitive movement. Bermuda will also be impacted by high swells, and the cyclone could pass very close to that island over the next 5 - 7 days. Interests there should be preparing for an intensifying hurricane.

Given the split in the model guidance, the best I can do is average the solutions until Leslie responds to the upper low. This leads to a track forecast that is similar to but slower than the National Hurricane Center. There is not a lot of confidence to this forecast.

5-day intensity forecast

INITIAL 09/04 0000Z 50 KT 60 MPH
12 hour 09/05 1200Z 50 KT 60 MPH
24 hour 09/06 0000Z 55 KT 65 MPH
36 hour 09/06 1200Z 60 KT 70 MPH
48 hour 09/07 0000Z 65 KT 75 MPH
72 hour 09/08 0000Z 70 KT 80 MPH
96 hour 09/09 0000Z 80 KT 90 MPH
120 hour 09/10 0000Z 85 KT 100 MPH

5-day track forecast



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



Tropical Depression Thirteen

A small area of low pressure that originated from an upper-level trough spawned a small tropical depression over the central Atlantic today. As of the latest NHC advisory, the following was availble:

Wind: 35 mph, with higher gusts
Location: 25.9°N 42.8°W
Movement: NW at 5 mph
Pressure: 1012 mb

Convection has developed closer to the low-level center over the past couple of hours, but it remains fairly shallow.



Figure 3. Latest infrared satellite image of Tropical Depression Thirteen. Image credit: NOAA

The large scale environment does not appear especially conducive for intensification. Northwesterly shear is forecast to persist, and there is some dry air in the cyclone's surroundings. Also, there is little evidence of anticyclonic outflow at this time. However, only a small increase in organization could result in the system becoming a tropical storm. This system is forecast to dissipate in a few days. Small systems like this one are highly susceptible to shear, so even a subtle increse in the vertical shear could result in the system being incapacitated quicker than I am indicating.

Given the short history of the tropical cyclone, initial movement is difficult to determine. Based on shortwave infrared fixes, it looks to me as if the system is beginning to turn to the west-northwest. This is consistent with the overall pattern, with the cyclone located between an upper trough to the northwest and a mid-level ridge to the east. As the trough amplifies, the cyclone is expected to turn north, then northeast with a slight increase in forward speed. My track is to the right of the one from the National Hurricane Center, predicated on the assumption that the depression will feel greater influence from the trough. Alternatively, the cyclone is small enough that it may not fully respond to the trough, and instead simply meander.

5-day intensity forecast

INITIAL 09/04 0000Z 30 KT 35 MPH
12 hour 09/04 1200Z 35 KT 40 MPH
24 hour 09/05 0000Z 35 KT 40 MPH
36 hour 09/05 1200Z 30 KT 35 MPH
48 hour 09/06 0000Z 30 KT 35 MPH
72 hour 09/07 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
96 hour 09/08 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
120 hour 09/09 0000Z...DISSIPATED

5-day track forecast



Figure 4. My 5-day forecast track for Tropical Depression Thirteen.



John

John continues to move across the open Pacific. As of the latest NHC advisory, the following was posted on the storm:

Wind: 35 mph, with higher gusts
Location: 21.7°N 114.6°W
Movement: NW at 13 mph
Pressure: 1003 mb

John is experiencing northeasterly shear. Satellite images show the low-level center to be exposed well to the northeast of the shapeless and pulsating convection.



Figure 5. Latest infrared satellite image of Tropical Storm John. Image credit: NOAA

John does not have long to live now that the low-level center is racing northwestward away from the convection. While the shear is forecast to decrease as John moves away from the axis of the upper-level ridge, water vapor imagery shows dry air in the immediate path of the tropical cyclone. Naturally, cool sea surface temperatures will coexist with the subsident airmass. The combination of these features should quickly obliterate John, and the cyclone is forecast to degenerate into a remnant low later on Tuesday.

John remains south of a weakness in the ridge over the southwestern United States as an upper trough amplifies off the California coast. This evolution should cause a general northwest motion until the system becomes a remnant low. Thereafter, the cyclone is forecast to turn west-northwest as it becomes a shallower vortex.

5-day intensity forecast

INITIAL 09/04 0000Z 30 KT 35 MPH
12 hour 09/04 1200Z 30 KT 35 MPH
24 hour 09/05 0000Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
36 hour 09/05 1200Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
48 hour 09/06 0000Z 20 KT 35 MPH...POST-TROPICAL/REMNANT LOW
72 hour 09/07 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
96 hour 09/08 0000Z...DISSIPATED

5-day track forecast



Figure 6. My 5-day forecast track for John.


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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3. Civicane49
3:47 AM GMT on September 04, 2012
Great to see you, Kori. Thanks for the blog update.
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2. wxchaser97
3:45 AM GMT on September 04, 2012
Good to see you Kori, thanks for the blog!
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1. KEEPEROFTHEGATE (Mod)
3:37 AM GMT on September 04, 2012
well where have you been

last i saw ya
was at the start of I
storms approach on LA
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