Two Areas Both Near the US that May Need Watching

FWIW, Kevin and I just finished the discussion below. Looks like a busy weekend ahead for us.

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IWIC Atlantic Tropical Weather Discussion - 29 July 2004 - 9:20 PM EDT

The Atlantic Basin has become a bit more active today. There remains two prominent areas of interest, one being a surface trough just northeast of the Bahamas, and the other a broad surface low pressure just east of the Florida Straits.

The surface trough northeast of the Bahamas has been gradually organizing over the past 12 to 24 hours. Visible satellite imagery hints at a possible weak low level circulation developing near 26ºN, 72ºW, though a few more hours of microwave shortwave satellite imagery will be needed to confirm this. Regardless, given these recent satellite imagery trends, we are confident that a definate low level circulation will develop around the coordinates given above within the next 12 hours. Weak to moderate low level convergence is noted around the possible low, particularly from the south. Convection as a whole associated with the system is on a diurnal wane, though an upper level low to the south is helping to enhance convection southeast of the potential center by creating strong lift. At the same time, it is currently restricting convective development in the southwest quadrant. Meanwhile, an upper level ridge is noted over the northern half of the system, enabling a decent anticyclonic outflow pattern to establish in the northern and eastern quadrants. This overall situation is more baroclinic than barotropic, though changes appear to be in store. Model guidance indicates that the upper level low will continue to slowly slide southwestward. In response, this will allow for the upper level ridge to expand further south so that it encompasses nearly the entire system. This will permit ventilation aloft in all quadrants, a necessity for a true tropical cyclone. The progged southwestward or southward position of the upper level low will aid convection in the center of the disturbance as well. With a large upper level ridge progged to be located directly over the surface center, wind shear should not be a problem. Global models are not too agressive on intensity despite this, however. Most only show development into a tropical depression or tropical storm at the most. Some, such as the , do not show anything more than an inverted trough. The dynamical SHIPS model is more bullish than the global model suite, and strengthens it to a moderate tropical storm in the next 48 hours. Considering the recent organization trend and the already building favorable upper tropospheric pattern, there is good potential for tropical depression or tropical storm development within the next 24 hours. However, taking model guidance into account and the lack of a lot of time spent over water, nothing more is anticipated.

The future track of the system depends solely on the position of the subtropical ridge and the timing arrival of a trough currently located over the central United States plains. Motion over the past 12 to 24 hours has been very slowly towards the west-northwest, which is expected to continue in the near future along the prehiphery of the subtropical ridge. Thereafter, some uncertainty arises. Once the trough approaches the United States east coast, the westward flank of the subtropical ridge will be pushed further east, thus curving the system north or northeastward. When this will happen, and whether it will happen before landfall, is the main question. The majority of the global models take the trough far enough east to influence the system in 60 to 72 hours. Given the current slow motion, this supports a right curve while still over water, thus taking it into eastern North Carolina rather than Georgia or South Carolina. The most recent run of the tropical models suggests otherwise, though little trust should be put into these models as most of them are primarily designed to handle purely tropcial situations south of 20ºN. Based on all of this, landfall in northern South Carolina or North Carolina is expected from the south-southwest late Sunday or early Monday. There remains some uncertainty in the exact time and location of landfall, as well as the intensity of the system. Thus, residents living along the coast from northern Florida to Virginia should monitor the progress of this disturbance. An Air Force Reserve Reconnaissance plane will investigate the area tomorrow if needed.

The second area of interest lies in the Gulf of Mexico. A broad area of low pressure is located just to the west of Dry Tortugas in the Florida Straits. Convection is increasing around its fringes this evening, mostly due to diurnal enhancement. Nonetheless, the organization as a whole is better now than it was 12 to 24 hours ago, with moderate low level convergence and anticyclonic flow aloft. The synoptic environment is gradually becoming more conducive for development. Model guidance still depicts a large upper level ridge building over the Gulf of Mexico in the near future. This will allow for greatly reduced shear values and strong evacuation aloft. Additionally, water temperatures and hurricane heat potential are more than sufficient to support a tropical cyclone in the Gulf of Mexico. If convection persists overnight and the circulation becomes better defined, this system will become a candidate for classification tomorrow. As mentioned above, conditions will allow for at least gradual strengthening over the next few days. It is also important to note that this system could be a slow mover even in the longer term, allowing more time over water. All factors subsisting, the prospect of strengthening looks good with this system. The Dynamical SHIPS model brings the low to a minimal hurricane in 120 hours. Global models are much less aggressive, but nearly all of them still show development. Based on the progged upper level pattern, recent organization trends, and model guidance, intensification to a tropical depression or tropical storm is a decent possibility over the next 36 hours. It is uncertain as to how strong this will become, but any low that stays over the warm Gulf of Mexico waters for several days under a large ridge aloft is always subject to concern.

The system is moving slowly towards the southwest at this time due to a temporary fluctuation in the low-level steering currents in the eastern Gulf of Mexico. Earlier today, east-southeasterly steering currents were observed. The recent fluctuation in steering can be attributed to the flattening of ridging to the north and east of the system. As the trough along the United States east coast moves further west, the western flank of the ridge in the subtropical Atlantic should weaken, allowing steering currents in the Gulf to switch to an easterly or east-southeasterly direction. Model guidance is extremely diverged on the future track. The BAMM and BAMD tropical models take it westward towards the Mexican or Texan coastline, as does the UKMET. On the other hand, a lot of the other global models, such as the ETA, , and curve it northward towards the northern Gulf coast. It is too soon to say with certainty which track the system will take. However, given the expected passage of a trough across the eastern half of the United States in the forecast period, a more southerly turn in the steering flow in the Gulf of Mexico would seem reasonable. Provided the system is more than just a tropical wave or a low with minimum central pressure lower than 1010 millibars, a turn more towards the north is the more likely scenario at this time. Furthermore, the tropical models are not very reliable in this type of situation. As with the western Atlantic disturbance, the environment is not completely barotropic, which is the pattern the tropical models perform best. The most likely target for landfall is from central Louisiana to extreme western Florida Panhandle. A more westward motion cannot be discounted yet, so interests in both the northern and western Gulf coasts should pay attention to this system over the upcoming weekend. An Air Force Reserve Reconnaissance plane will investigate this area tomorrow as well, if necessary.

On another note, the continues to show tropical cyclone development from a tropical wave in the eastern Atlantic within the next 72 hours. However, no other model is in agreement with this solution, and the air ahead still looks a bit seasonably dry. We will continue to monitor this region of the Atlantic basin, but significant development is not expected at this time.

http://independentwx.com/atlanticdiscussion.html

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Rob Mann
IndependentWx.Com

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