Atlantic Disturbance 96L and Tropical Storm Rachel

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

INVEST 96L NOT A THREAT

A small area of disturbed weather with periodic bursts of convection near 13°N/42°W remains disorganized as it continues to drift WNW at around 5Kts. Although wind shear remain moderate near 15Kts, there are indications of some outflow at high levels – the system remains surrounded by relatively dry and somewhat stable air which is inhibiting development. None of the global or dynamical specialized hurricane models are forecasting development, while the typically overdone statistically based intensity models are forecasting a slight intensification over the next 5 days. Assuming the system even survives at all, it’s likely to move WNW and then NW before getting absorbed by an approaching upper level TROF.

There are only a couple other Tropical waves across the Tropical Atlantic basin and they’re embedded in generally hostile environments. Meanwhile, I can no longer identify any upstream tropical waves over Africa – a sure sign that the ‘Cape Verdes’ Hurricane Season is rapidly winding down – with the Caribbean region likely to be only area where a cyclone that could threaten the US may yet develop during the remainder of the Fall season.

TROPICAL STORM RACHEL COULD BRING RAINS TO BAJA

Tropical Storm Rachel is located about 450NM SW of the southern tip of Baja, moving WNW at 14Kts. Though NHC is carrying this system as a relatively weak storm with sustained winds near 45Kts, there has been a strong increase in deep convection closer to the low level storm center over the past few hours, and it’s possible winds have increased to near 55Kts – though there has not been a recent Microwave or ASCAT satellite pass to confirm this. Regardless of any intensity change in the short term, the storm is heading towards cooler SST’s and will begin to slowly weaken during the next 72 hrs.

The longer term track of RACHEL is somewhat problematic beyond 72 hours. The western end of a mid-level high pressure ridge over Mexico is expected to weaken during the next few days as a major, deep-layered TROF in the Northeast Pacific digs southeastward towards the western US. The GFS, which is often quite good at track forecasts, calls for Rachel to turn northward this weekend and then head Northeast towards central Baja around the periphery of the high pressure ridge to its east. IF this occurs, the only significant threat would be for locally heavy rainfall, with a low but real chance that high level moisture would eventually stream northeastward across northern Mexico towards the Rockies ahead of the mid-latitude TROF now heading for the western US that will, by next week, be moving eastward across the Rockies. However, depending on how quickly RACHEL spins down to depression or even remnant Low, the much shallower system would most likely never take the turn to the Northeast, but would simply continue on a WNW or NW track as a dissipating remnant swirl.



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Fig 1: This mornings overview of the Tropical Atlantic (and East Pacific) shows INVEST 96L essentially in the same area as yesterday about 1,100NM east of the Caribbean, along with Tropical Storm RACHEL well to the southwest of Baja. A Tropical Wave from near western Cuba extends S/SW across the Yucatan and out over the Pacific. This wave is westbound at 18Kts, but lies within an unfavorable area for development. The tropical wave in the far eastern Caribbean is westbound at 14Kts, within an area of deep tropical moisture, but an extremely hostile wind environment. The only other significant tropical wave remains nearly stationary along the west coast of Africa – and is showing signs of being absorbed into the Intertropical Convergence Zone (ITCZ). For the first time since the start of Hurricane Season – no tropical waves are observed over Africa as the ITCZ / African Monsoonal TROF has shifted southward towards the equatorial region, and is also weakening. It would be somewhat surprising to see any strong tropical waves come off the African coast during the remainder of the season.




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Fig 2: The (Bottom) image frame of the 72 hour Total Precipitable Water (TPW) ‘Loop’ (top) helps to identify Tropical Waves across the Atlantic. The darker the orange shading – the greater the depth and total volume of moisture derived from satellite imagery. The Loop imagery itself also helps to determine any ‘rotation’ that may be associated with the tropical waves. The more ‘specialized’ last image frame from the loop (Bottom) also includes component wind vector information to show actual areas and magnitudes of ‘rotation. INVEST 96L has become a somewhat ‘lone’ area of deep moisture in mid-Atlantic – and while watching the 72-hour loop, you can see the original large area of deep moisture that had been in this region a few days ago shifted westward to where the TW in the eastern CARIB is now. At the same time, the deep moisture that was orientated east-west as part of the ITCZ ‘bubbled’ northward, and then ‘pinched off’ into the small disturbance 96L. Over the past 24 hours, dry air appears to be infiltrating the deep moisture field associated with 96L.




Fig 3: A closer look at the mostly satellite derived low level winds (with a few surface OBS as well) surrounding 96L show a sharp inverted TROF structure, but little to no clear evidence of an actual low level circulation.




Fig 4: Upper air winds (above 25,000 Ft) is overlaid with the wind shear analysis from the UW CIMSS group, and shows moderate SW wind shear averaging about 15Kts across 96L. There is, however, some outflow wind vectors near some of the heavier convection – but overall, as the system moves WNW over the next few days – it will eventually encounter strong wind shear – and development of this system is extremely unlikely.




Fig 5: The larger overview of the western Atlantic region clearly hints at a rapid shift towards he cool season with a very deep and long wave upper air TROF from the northern Atlantic southwestward to a strong upper Low now over Puerto Rico and continuing into the southern CARIB. A high amplitude ridge extends from near the Yucatan northeastward to just off the east coast. Overall, we find a chaotic and highly energetic but hostile wind pattern across the GOM eastward to the central Atlantic.




Fig 6: The early morning color enhanced IR (Infra-Red) image of Tropical Storm Rachel continues to show very deep convection, with cloud top temperatures near -60°C (indicative of tops near 45,000’).




Fig 7: A closer look at the high level winds near RACHEL shows a very well developed, though not especially strong, anti-cyclonic (clockwise) outflow wind pattern atop the storm. But as the system moves further to the WNW, cooler SST’s and an increasing drier environment surrounding the storm will lead to a slow but steady weakening.




Fig 8: Steering level winds show Rachel still in a E/SE wind flow, but approaching a break in the ridge centered over Mexico and another strong high pressure center well north of Hawaii. The ‘break’ in the ridge is due a relatively deep-layered TROF approaching the west coast. If the storm maintains it strength long enough (as the GFS is calling for) it will eventually be caught up in the southwesterly wind flow around the western periphery of the High over Mexico, and will head towards Baja – weakening considerably by the time it reaches that area. If the storm weakens sooner than the GFS is forecasting, the shallow system will never turn Northeast – instead simply drifting WNW within the general trade wind environment.

The next Tropical Update will be issued tomorrow.

Steve