Tropical weather analysis - August 12, 2011

By: KoritheMan , 3:20 AM GMT on August 13, 2011

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Tropical Depression Six

Tropical Depression Six formed today from an area of low pressure that had been tracked for the last few days along a decaying frontal boundary in the western Atlantic. As of the latest NHC advisory on the system, here was the information:

Wind: 35 mph, with higher gusts
Location: 36.6N 61.3W
Movement: ENE at 22 mph
Pressure: 1010 mb
Category: Tropical Depression (Saffir-Simpson Hurricane Scale)

Satellite images show a not too impressive tropical cyclone, although some fairly deep convection (for a high latitude system) is over the center.

Figure 1. Latest infrared satellite image of Tropical Depression Six, courtesy of NOAA's Satellite Services Division (SSD).

The depression is not within an environment particularly conducive to strengthening, with about 30 knots of southwesterly shear impacting the system. This is acting to squash the western side of the storm, which is shown well on satellite pictures. Water vapor imagery also shows that the cyclone is embedded within an area of large scale subsidence associated with a shortwave trough emanating from southeast Canada.

These factors are forecast to persist, which should make intensification difficult, especially in light of current organizational trends. Currently, I feel that the system will not make it to tropical storm status, although I've been burned before. Although for what it's worth, the models agree with me.

This depression poses no threat to land, and should move quickly ENE until extratropical transition occurs in about 36 hours. Thereafter, a turn to the east around the northern periphery of the Bermuda High is expected.

Invest 92L

A surface trough located several hundred miles east of the northern Leeward Islands "(92L)" still poses a threat to develop into a tropical depression. Although convection is firing near the center, there are no signs of a surface circulation at this time.

Figure 2. Latest infrared satellite image of Invest 92L, courtesy of NOAA's Satellite Services Division (SSD).

There are no signs of a shearing mechanism directly over the center on satellite imagery, although southwesterly shear lies just to the north. Upper-level winds should remain at least relatively conducive for the next three or so days, before the system begins to interact with a trough currently over the Ohio Valley. At that point, southwesterly shear associated with this trough could act to induce weakening.

I expect that any development will be gradual, given the large area of dry air to the west of the system. This still has the potential to become a tropical depression, however, but the prospects of it becoming anything beyond a moderate tropical storm appear to be diminishing.

Synoptic steering takes this area out to sea, but Bermuda still needs to watch this.

Invest 93L

A tropical wave located about 575 miles southwest of the southern Cape Verde Islands ("93L") has lost nearly all of its thunderstorm activity today. On the positive side, this system has managed to establish a large amount of spin, something that was notably lacking in the preceding days. Thus, should convection ever sustain itself, we would need to be concerned about the possibility of tropical cyclogenesis.

Figure 3. Latest infrared satellite image of Invest 93L, courtesy of RAMMB imagery.

As of 0z, SHIPS analyzes 23 kt of easterly shear over 93L, but satellite imagery suggests a lighter shearing regime. Upper-level winds are forecast to become more favorable over the next couple of days, although it may run into increased shear as it approaches the Lesser Antilles late Monday and into Tuesday. Should the system remain south of the northern Leeward Islands, however, then the GFS suggests shear will be less, and the system may be able to maintain its forecast anticyclone should that happen. That would be a most favorable development in terms of this system having a shot at becoming a tropical depression.

I do feel that the 0z dynamical model consensus may be a bit too far south with 93L, as a pronounced weakness exists along 55W, associated with the same trough in the process of recurving TD6. This weakness will only be reinforced by the trough moving across the Ohio Valley. In short, the synoptic pattern does not favor a due west motion into the heart of the Caribbean like these models are indicating. Even if the system stays fairly weak over the next few days, there should still be enough of a weakness within the ridge to tug it northward a bit as it nears the Lesser Antilles.

At this stage, I think the most likely scenario is for it to enter the eastern Caribbean, and then impact portions of the Leeward Islands and Puerto Rico in a fashion similar to Emily. Thereafter, the track is uncertain, but this system could pose a long-range threat to the US east coast.

It is worth noting that if 93L eludes the poleward pull I'm forecasting, it would follow the behavior depicted in the 0z model consensus, and would likely become much stronger.

Invest 94L

A fourth area of interest ("94L") is located 675 miles north-northeast of the northern Leeward Islands. This system has become a little better organized today.

Figure 4. Latest infrared satellite image of Invest 94L, courtesy of NOAA's Satellite Services Division (SSD).

This system is very compact, and is thus prone to subtle changes in the surrounding environment. I am not anticipating significant development of this system, but it still needs to be watched, given the very favorable upper-level shear environment in which the system is embedded. The next big hurdle that could curb intensification is an area of dry air to the west of the storm, being imparted by northwesterly flow aloft associated with the trough recurving TD6.

Steering forecasts suggest that 94L should continue moving west to west-northwest over the next 12 to 24 hours, but should run into the mid-latitude westerlies thereafter. Some of the models are indicating that remnant mid-level energy associated with 94L could work its way around the entirety of the Bermuda/Azores high, and move westward next week. Even if this does occur, redevelopment is not anticipated.

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6. CybrTeddy
4:00 AM GMT on August 13, 2011
thanks for the hard work!
Member Since: July 8, 2005 Posts: 259 Comments: 24603
5. oldnewmex
3:50 AM GMT on August 13, 2011
Thank you for this summary.
Member Since: January 28, 2011 Posts: 0 Comments: 214
4. MiamiHurricanes09
3:43 AM GMT on August 13, 2011
Thanks Kori. I appreciate it.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
3. WoodyFL
3:36 AM GMT on August 13, 2011
Very good analysis. I wish others could explain it so simply as you. Very good.
Member Since: April 24, 2011 Posts: 1 Comments: 601
2. caneswatch
3:33 AM GMT on August 13, 2011
Great read Kori!
Member Since: October 8, 2008 Posts: 14 Comments: 4553
1. Grothar
3:24 AM GMT on August 13, 2011
Kori, that is a great blog. Must have taken a lot of time to do it. A lot of thought went into it. Easy to read and you have everything covered. A+
Member Since: July 17, 2009 Posts: 71 Comments: 27292

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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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