Atlantic Disturbance 96L Still 96L But Threat to US Increases

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

There’s been little change overnight in 96L, with RECON reports indicating a broad, disorganized circulation center with a surface pressure near 1007mb along the north coast of Hispaniola near 20.8N/71.7W. Mid-upper level winds also indicate that the larger scale circulation field was not significantly impacted by the mountainous terrain of Hispaniola, and has been moving on a generally West/West-Northwest course at around 20Kts, though a somewhat more Northwesterly (310°) heading seems to have developed over the last few hours. Although there continues to be large scale outflow associated with the high level anti-cyclone sitting atop the developing system,, there are no distinct outflow channel jets; so any initial intensification of the system later today or on Sunday will likely start out on the slow side. While wind shear of 15-20Kts continues to be of moderate intensity across large areas of the system’s structure, an area of lighter shear (<10Kts) close to the circulation center continues to move in tandem with the systems primary circulatory vortex.

The overall satellite signature continues to slowly improve, with a somewhat more symmetrical appearance, and large scale curvature especially noteworthy in the E-SE quadrant where hints of a ‘feeder’ band type structure appears to be developing.

With the system embedded in a moist environment and no significantly drier air noted on the periphery of the system, dry air should not be a hindrance to intensification during the next 48 hours. In addition, the developing system is, or soon will be, moving over very warm waters, with SST’s over 29°C (~85°F) – well above that needed to support hurricane intensities. However, if the system should slow to a crawl or even go stationary later Sunday as forecast by many models, upwelling of cooler sub-surface waters could prove significant, slowing the rate of intensification appreciably.

Since yesterday, there have been major shifts to the forecast tracks – especially after Sunday – raising the risk level for the entire eastern US from Florida to New England – with about equal odds that the potential storm will impact the coast or turn out to sea. This is not a surprise since track and intensity forecasts are notorious for major shifts for systems that are still in the formative stages. Ironically, the CMC forecast, which was consistently forecasting the system to track across south Florida, has now shifted dramatically to that shown by many of the major model suites, while the more reliable track forecast models have now shifted westward, much closer to the coast. The spread of forecast tracks is now quite large – again, a typical feature of forecasts for systems still forming. The forecast challenge has been unusually high for this system, not just because it has yet to really develop and has tracked across Hispaniola (a landmass notorious for ‘destroying’ even the most well developed and intense hurricanes) but because of the unusually high track sensitivity to a ‘weakness’ in the east-west sub-tropical ridgeline that extends from the central Atlantic to the Gulf coast. While the models continue to show a break between the high pressure center in the central Atlantic and the one near the Gulf coast during the next 36 hours, the resulting TROF within this break will be relatively weak, and will begin to dissipate staring late Monday. The exact timing of this ‘break’ – and the exact location and strength of 96L during the next 72 hours will determine exactly where and when (if ?) the system turns northward. Prior model runs were in generally good agreement that the system would turn northward when it was still about 300NM east of Florida, while the most recent model runs show a far more gradual recurvature – with some of the more reliable models showing the storm getting very close to the coast before beginning to turn northward, and are all calling for the storm to be moving much slower, with the system not expected to be near the US coast for another 4 or even 5 days.

The intensity forecasts are equally difficult, although they have continued to be quite consistent with each other and between succeeding model runs. The odds are relatively high (70%-80%) the system will become a tropical storm either late today or on Sunday, with a fairly slow rate of intensification to near CAT 1 intensity on Monday or Tuesday. This continues to be a reasonably good forecast scenario all things considered, and a CAT 1 intensity threat for the east coast of Florida northward to the Carolinas is quite real – albeit a still very uncertain one.

All things considered, I must admit this is one of the more ‘challenging’ forecasts I’ve come across in my many decades of forecasting.



Fig 1: Early morning VIS imagery shows a system still trying to get ‘its act together’ with a better organized signature than yesterday, but a still poorly organized low level circulation. Some hints of ‘feeder’ type bands are seen extending into the southern CARIB south and east of the ‘center’ witch should provide an ample flow of moisture into the system assuming it finally intensifies into a strong cyclone.



Fig 2: The Total Precipitable water (TPW) analysis show 96L surrounded by moist air, with a deep moisture plume extending from the deep tropics over northern SOAMER across the CARIB into the mid-level vortex circulation that is 96L. This should continue to supply high atmospheric moisture content to the system as it develops during the next few days.



Fig 3: 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 mid level circulation field. The shear analysis is automated, so the lower values shown may not be very precise in the delineation of its aerial extent. However, this wind shear ‘couplet’ has been moving in tandem with the disturbance, and since none of the models are forecasting adverse shear conditions ahead of the systems’ projected track – shear should not be a major hindrance to intensification for the next 72 hours.



Fig 4: There’s been no significant change to yesterday’s development of a high level (~200mb) anti-cyclone which is providing decent outflow for the developing system. Though no distinct outflow jet channels have developed, the current level of outflow does support a CAT 1 intensity storm.



Fig 5: The lower level steering winds clearly show the east-west orientated sub-tropical ridge from the central Atlantic westward to the Gulf coast region, along with a break (TROF) between the two separate High pressure centers. This TROF / Ridge-line is the primary feature set that will ultimately determine the track taken by the developing cyclone. Both the intensity of the developing system and the exact location and impact of the TROF on the sub-tropical ridge will determine how the steering currents evolve and ultimately guide the the cyclone over the next several days. If the TROF weakens quickly enough, allowing the ridge to rebuild westward into the SE US – the cyclone will track into the SE coast of the US. If the TROF/ridge weakness remains in place – the cyclone will recurve and turn northward before reaching the coast.



Fig 6: Early cycle model runs are in general agreement on the track of the developing cyclone for the next 36-48 hours, but the spread between solutions beyond then has grown quite large since yesterday; indicative of the difficulty the models are having in determining the subtle-changes and inter-play between the sub-tropical ridge, the TROF off the east coast, and the cyclone itself.



Fig 7: Intensity forecasts have been fairly consistent over the past few days - seemingly unaffected by the track changes - with slow intensification expected during the next 72 hrs to CAT 1 Hurricane force by early next week.

I’ll have another brief update late this afternoon after additional RECON and SAT imagery can be reviewed, along with all the 12Z global model run output.

Steve