Tropical weather analysis - August 3, 2012

By: KoritheMan , 3:46 AM GMT on August 04, 2012

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Tropical Storm Ernesto appears to be intensifying. The latest NHC advisory had this to say about the storm:

Wind: 50 mph, with higher gusts
Location: 13.9°N 65.6°W
Movement: W at 18 mph
Pressure: 1002 mb
Category: Tropical storm

Ernesto looks the best it ever did on satellite, and it is likely recon will find stronger winds during their next scheduled penetration. Unfortunately, the storm is out of range of both the San Juan and Curacao radars, making a decent look at the center difficult from that vantage point. As I write this, Dvorak numbers are starting to come up, and it is likely that satellite estimates will correspond to the improving satellite signature soon enough.

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

Although not yet perfect, the upper flow pattern over Ernesto is becoming more diffluent. The GFS/SHIPS forecasts the environment to gradually become anticyclonic over the next few days. This is particularly true when the system enters the western Caribbean on Monday. At that point, an upper low is forecast to be over the western Gulf of Mexico, which could provide some sort of upper air ventilation to the tropical cyclone at that time. This low is already apparent on water vapor imagery developing off the southwest Florida coast. Given the very high oceanic heat content currently lying dormant in the western Caribbean Sea, such a pattern could herald rapid intensification at that stage. One negative is the relatively quick forward motion of the cyclone -- it is moving at 18 mph, which makes surface convergence (and thus thunderstorms) difficult. However, Ernesto is forecast to gradually slow, and all indications are that it will be a dangerous hurricane in the western Caribbean and Gulf of Mexico. I admit that my intensity forecast is a conservative one, but I still don't know how Ernesto is going to interact with land. If it moves over the Yucatan Peninsula like the hurricane center predicts, that could temporarily halt strengthening. On the other hand, if it avoids the peninsula, it could become stronger than anticipated. My forecast is a compromise of these possibilities, and one is reminded that 5-day intensity forecasts are subject to large errors, up or down.

I am still having difficulty in pinpointing exactly where Ernesto will go. Now that it's safely tucked in the Caribbean, we know it's not going to recurve. But the million dollar question is: how strong will the central plains ridge be? I note that it has been a very persistent feature this summer, what with the extreme heat waves over that portion of the country. While I doubt this pattern will break away completely, as they often don't, the global models all agree on some sort of trough amplifying over the central and southern United States by Tuesday. Although the trough is forecast to slowly lift out by Thursday, a secondary trough, this one a bit more potent, is forecast to begin amplifying over the western United States at around roughly the same time. This could theoretically reinforce any northward motion, but this is contingent on how strong Ernesto becomes over the Caribbean. As a general rule, stronger systems tend to move in a more poleward fashion.

The first trough is already in the process of amplifying over the Great Basin. Upper air data and water vapor images show that this feature is quite strong, but I would rather be conservative before I deviate too far to the right of my previous forecasts. For one thing, there's the GFS and ECMWF solution of a Mexico landfall that remains a possibility. When these two models are in harmonious agreement, their consensus is usually hard to beat. However, these models do not seem to be handling the synoptic environment ahead of Ernesto particularly well, and the assumption of a more westward motion at longer ranges seem to hinge almost entirely on a weaker cyclone vortex missing the trough. However, it's also possible the trough won't be strong enough to fully pick up Ernesto, especially given the strength of the central United States ridge this summer. Keeping this in the back of my mind, I will not yet fully commit to a definitive northward motion when the system approaches the Gulf of Mexico. But given the uncertainties, interests along the Gulf Coast from Mexico to Alabama should monitor the progress of Ernesto, as it has the potential to be a major hurricane.

It should be noted that 91L could play an important role in the future track of Ernesto, especially if intensifies into a tropical cyclone. This option would tend to reinforce the weakness in the ridge at longer ranges, allowing Ernesto to move more northward. For now, I am leaving this possibility out of the forecast due to uncertainties in how 91L is going to evolve.

5-day intensity forecast

INITIAL 08/04 0000Z 45 KT 50 MPH
12 hour 08/04 1200Z 50 KT 60 MPH
24 hour 08/05 0000Z 55 KT 65 MPH
36 hour 08/05 1200Z 55 KT 65 MPH
48 hour 08/06 0000Z 60 KT 70 MPH
72 hour 08/07 0000Z 65 KT 75 MPH
96 hour 08/08 0000Z 75 KT 85 MPH
120 hour 08/09 0000Z 85 KT 100 MPH

5-day forecast track

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

Tropical Depression Six

A well-defined tropical wave in the eastern Atlantic located about 230 miles southwest of the Cape Verde Islands ("90L") has become a tropical depression. As of the initial NHC advisory, the following information was posted on the storm:

Wind: 35 mph, with higher gusts
Location: 13.8°N 27.8°W
Movement: WNW at 16 mph
Pressure: 1009 mb
Category: Tropical depression

The low-level center appears to be the on eastern edge of the convection, which is exhibiting a curved band pattern on satellite imagery.

Figure 3. Latest infrared satellite image of Tropical Depression Six. Image credit: RAMMB

Upper-level winds in the path of the system are fairly light, which could favor some additional intensification over the next day or so. After that, the cyclone is forecast to enter a drier airmass and move across slightly cooler sea surface temperatures, the combination of which should halt further intensification. None of the global models intensify this system much, but neither do they with Ernesto. However, in this case, they appear to have a more realistic presentation of the surrounding environment so that their forecasts are more believable. Beyond 72 hours, some westerly shear could begin to affect the system. However, this shear isn't too strong in the GFS fields relative to the vortex location, so it is uncertain how much of this shear will actually penetrate the core. My forecast will call for slight strengthening over the next 24 hours, followed by slowly weakening storm. However, the cyclone could just as easily dissipate.

5-day intensity forecast

INITIAL 08/04 0300Z 30 KT 35 MPH
12 hour 08/04 1200Z 35 KT 40 MPH
24 hour 08/05 0000Z 40 KT 45 MPH
36 hour 08/05 1200Z 40 KT 45 MPH
48 hour 08/06 0000Z 40 KT 45 MPH
72 hour 08/07 0000Z 35 KT 40 MPH
96 hour 08/08 0000Z 30 KT 35 MPH
120 hour 08/09 0000Z 25 KT 30 MPH

The depression is south of a mid-level ridge. An upstream shortwave trough over France carries lingering southwesterly flow all the way to north Africa, which is creating a slight weakness in this ridge. Therefore, a more climatological due westward motion is not anticipated in the near-term. By Monday, the tropical cyclone is forecast to be moving west. However, concurrent with this, the storm will be approaching a break in the subtropical ridge associated with a large upper-level low pressure system; water vapor imagery indicates a general weakness west of 35W. Assuming the cyclone maintains some semblance of convection at that time, it could follow this weakness out to sea, or it could move more westward with the low-level flow as a weaker entity. I favor the latter option.

5-day track forecast

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

Invest 91L

A large area of showers extending across the Bahamas southward toward Cuba is associated with a surface trough. The associated shower activity is poorly-organized, surface pressure are not falling, there are no signs of a circulation, and upper-level winds are not currently favorable for development. Based on a couple of surface observations, satellite interpretation, and doppler radar data out of Miami, any center is probably located to the south of Nassau, but there is certainly margin for error. However, since the system is disorganized, a center reformation could occur underneath the deepest convection to the south, closer to the north coast of Cuba. If this happens, 91L will have more time over water, and would likely be a stronger entity in the Gulf of Mexico.

Figure 5. Latest infrared satellite image of Invest 91L. Image credit: NOAA

The upper low west of Naples is forecast to move westward away from 91L over the next few days, which would place the system in a more diffluent southeasterly shearing regime when it enters the Gulf of Mexico. I am not particularly enthusiastic about a strong system, but the environment over the Gulf appears favorable enough for the formation of a tropical depression or tropical storm should 91L hold together. I am not yet going to call for genesis, however.

Based on 0z upper air data from the Bahamas and Florida, the system is not in a hurry to move. I expect a very slow northwestward motion for the next 24 hours, with the system picking up speed after that. Depending on where the center is, the system should be inland by tomorrow night. However, if no significant motion occurs, this may be somewhat delayed. Once in the Gulf of Mexico, the models have differing solutions regarding landfall, and areas anywhere from Louisiana to the Florida panhandle should monitor the progress of 91L. Interestingly, the Euro does not bring the system into the Gulf, instead calling for a more northward motion toward Georgia. The latter scenario seems unlikely since the weak trough over the eastern US appears to be lifting out.

Regardless of development, locally heavy rainfall will affect portions of the Bahamas and south Florida over the next day or so.

Probability of development in 48 hours: 30%

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7. stormwatcherCI
10:47 AM GMT on August 04, 2012
Excellent analysis Kori. Thank you.
Member Since: October 9, 2008 Posts: 0 Comments: 8449
6. fmhurricane2009
4:57 AM GMT on August 04, 2012
Impressive Post Kori! One of the best that you have done. (And that's no small feat because your overall quality is so high) I concur with everything in your forecasts, especially with Ernesto, but I just don't have the stamina that is needed to create such a informative discussion. Thank you for all you do!
Member Since: August 15, 2009 Posts: 3 Comments: 228
5. Tribucanes
4:07 AM GMT on August 04, 2012
Very well thought out Kori. 91L at a 30% chance of forming is not far off. Really hoping you get into a meteorology program at FSU or in the Midwest. Your one of the best here and I, like so many others, appreciate your time and efforts. PM you soon.
Member Since: April 18, 2012 Posts: 0 Comments: 2437
4. allancalderini
3:59 AM GMT on August 04, 2012
Amazing blog ;) I really like it.
Member Since: October 15, 2011 Posts: 0 Comments: 4487
3. wxchaser97
3:57 AM GMT on August 04, 2012
Thanks Kori, looks good, same forecast as mine for Ernesto.
Member Since: March 16, 2012 Posts: 127 Comments: 7981
2. TropicalAnalystwx13
3:53 AM GMT on August 04, 2012
Thanks Kori!
Member Since: July 6, 2010 Posts: 113 Comments: 34093
1. Ameister12
3:52 AM GMT on August 04, 2012
Thanks, Kori!
Member Since: August 9, 2009 Posts: 10 Comments: 5129

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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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