Atlantic Disturbance 96L Remains Disorganized - For Now…

(By Steve Gregory - Substituting for Dr. Masters who is on Vacation.)

There has generally been little change overnight in disorganized tropical disturbance 96L, though the most recent upper level analysis and VIS imagery loops suggest the environment is becoming more conducive to cyclone formation within the next 12 hours, with the potential for further development to a strong Tropical Storm force cyclone later this weekend.

While the overall appearance of the disturbance remains poorly organized (with the appearance of at least 2 separate low level vortices during the last 36 hours} the most recent visible Satellite imagery loop suggests a primary surface circulation is located to the N/NE of Puerto Rico NEAR 19N/65.5W . However, at this point, limited surface/buoy/ship reports do not yet seem to clearly reflect this possible development.

While the overall satellite signature shows a rather elongated system, with convection widely dispersed, a burst of deep convection has developed over the last few hours near and across the NW and SE quadrants of the low level circulation center. San Juan Radar also depicts some 'banding' features developing to the SE of the newly forming center.

In addition, an upper level anticyclone has apparently formed over the last 3-6 hours near and just SW of the developing cyclone center. That said – wind shear still appears to be of moderate intensity near the center of the system (15Kt-20Kts) – though much lower shear values are clearly seen just SW of the center near the developing upper level anti-cyclone. Though these developments show a significant environmental improvement supporting cyclone development since yesterday – the nearby mountainous terrain of Hispaniola and Puerto Rico are likely to inhibit any rapid type of development during the next 12-36 hours.

The more reliable global model runs over the past 24 hours continue to call for a general track into the central Bahamas this weekend before turning northward Sunday as a break in the east-west orientated subtropical ridge to the north of the system develops further into a TROF over the next 48-72 hours, steering the system Northwestward and then Northward. Most every model (with 2 distinct exceptions) keep the cyclone away from the US mainland. (The CMC continues to track the system across south Florida – but this seems unlikely unless the system does NOT develop significantly for at least the next 72 hours – while the last OPNL ECMWF and GFDL model suite shows the storm first turning northward as the other major models do, but then suddenly turns the storm NW towards the Mid-Atlantic coast towards the middle of next week. However, the ECMWF Ensembles - which are typically much more reliable for developing systems - do NOT show this turn towards the coast.) Since the system has yet to become a well established cyclone, and upper level wind flow forecasts at these longer ranges during the warm season can be quite unreliable - all of these ‘outlier’ track forecasts remain highly suspect.

In addition, overall confidence in the intensity forecasts remains relatively low, though most of the more reliable, specialized hurricane models do call for a slow intensification to a CAT 1 storm after 72 hours as the storm begins to track northward away from the central Bahamas. Sea Surface Temps (SST’s) are very warm across the SW Atlantic near the Bahamas, with readings near or just over 29°C (~85°F), solidly supporting hurricane intensity, though the depth of the warm water remains relatively shallow (and a very slow moving storm would tend to upwell the somewhat cooler sub-surface water). All things considered, however, the model intensity forecasts subjectively look reasonable given the current stage of development.

A RECON is still planned for this afternoon, and I’ll have another update on this developing system late today.




Fig 1: The above VIS depicts an elongated, poorly organized disturbance with widely dispersed, though locally heavy, convection. Over the last few hours, a surface circulation appears to be developing NE of Puerto Rico with a strong convective burst noted to the N-NW of this circulation.



Fig 2: Wind Shear analysis (mid levels) shows moderate shear conditions in the 15-20Kt range in the vicinity of the primary low-mid level circulation. Much lower shear conditions are seen just south-southwest of the center. Since the shear analysis is automated, the values shown may not be very precise, though the very slow development is likely due in part to the less than optimal shear environment.




Fig 3: The most significant development since last night is the apparent development of an anti-cyclone at high levels, centered fairly close to the primary mid-level circulation center. Weak but distinct outflow channels are also apparent to the N-NW and South of the developing cyclone.



Fig 4: The lower level steering winds (used for a shallow / developing cyclones) clearly shows the east-west orientated sub-tropical ridge from the central Atlantic westward to the Gulf coast region, along with a developing weakness between the two separate High pressure centers. If the system does NOT develop, it will likely tend to continue W-NW towards Florida. However, any significant development, along with the global model forecasts for a break in the ridge line near the central Bahamas, should lead to the cyclone turning northward late this weekend as it tracks around the western periphery of the central Atlantic High pressure center.



Fig 5: Early cycle model runs are in very good agreement on the track of the cyclone ultimately turning away from the US mainland, though there is that 'sudden turn towards the coast' by a couple models that cannot be totally discounted.



Fig 6: Intensity forecasts have been fairly consistent over the past couple days, with slow intensification expected during the next 24-72 hrs, with an increase in intensity to CAT 1 Hurricane force during the 3-5 day period.


Steve