Tropical Atlantic Uneventful / Tropical Storm Rachel No Threat

By: Dr. Jeff Masters , 3:18PM,GMT on September 27,2014

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

Atlantic Basin Hostile to Tropical Cyclone Formation

The weak disturbance that was INVEST 96L is no longer of any significance as it became overwhelmed by both drier and stable air along with increased wind shear. What’s left of the disturbance has turned N/NE.

Although there are 3 other tropical Waves in the Atlantic basin – none pose any threat of development – and no cyclone formations are expected or forecast by the more reliable models for the next 7 days.

TROPICAL STORM RACHEL COULD BRING RAINS TO BAJA

In the East Pacific, Tropical Storm RACHEL is located near 19°N/116°W or some 725NM SW of the southern tip of Baja. The storm intensified some yesterday, with sustained winds now around 55Kts, with some models – along with NHC – now forecasting the system to intensify to just under Hurricane intensity within 12-24 hrs before it begins to weaken rapidly after 48 hrs. Based on my analysis of SAT imagery and trends – it appears a bit unlikely RACHEL will intensify significantly - but regardless, the longer range period will see Rachel weaken rapidly as it moves slowly N/NW or even N/NE at 7Kts into increasingly drier, stable air with increased wind shear.

The GFS has backed off a bit on tracking the remnants of RACHEL to Baja – but whether it spins down to a mere swirl near Baja, or to one just North or west of its current location – the storm will have no impact at all on land.


CLICK IMAGE to open full size image in new window

Fig 1: This mornings overview of the Tropical Atlantic (and East Pacific) shows the remnants of INVEST 96L in the mid Atlantic – actually to the northeast of yesterdays location. An active Tropical Wave from along the western shore of the Yucatan southward into the Pacific is westbound at 10Kts, while a second wave in the east central Caribbean is also westbound at about 10Kts. The only other definable wave is off the west coast of Africa and is drifting westward. None of these Atlantic waves have any chance of developing either due to a very unfavorable upper air wind environment – or dry and stable air. Unlike yesterday when no waves were discernable over Africa, today, a Wave appears to have developed in central Africa within the ITCZ / Monsoonal TROF, and is westbound at 10Kts. This wave is not forecast to develop either.




Fig 2: The color enhanced IR (Infra-Red) image of Rachel shows a small but strong area of convection near the center of the storm, but a somewhat less organized appearance compared to yesterday - with another area of deep convection to the east of the storm center. It appears that there’s been some stable air intrusion from the south, but the environment surrounding the storm remains quite moist, and a weak ‘feeder band’ appears well to the south/southeast.




Fig 3: An early morning 89Ghz microwave pass highlights both the structure of the storm and the exact center. There appears to be a partial eye wall structure and a highly symmetrical spiral band structure – though the only deep convection is found in the eastern semi-circle.



Fig 4: The satellite derived high level wind flow shows a weak, but highly symmetrical outflow atop the storm center, though based on current imagery, and the magnitude of the outflow – intensification to near hurricane force seems unlikely to this observer. A light southeasterly wind shear well under 10Kts doesl, however, support some further deepening over the next 12 hours.




Fig 5: The mean steering current analysis from UW CIMSS for the EPAC show RACHEL in a very light, and variable wind flow area. The storm is still moving NNW, but a deep layered TROF over the West Coast may soon induce a south/southwest flow which would cause RACHEL to turn Northward for awhile until the western US TROF moves east by tomorrow. Most models then show the large High pressure ridge centered northeast of Hawaii building back eastward – turning RACHEL back to the WNW and then West by Monday as the storm rapidly dissipates, and is steered within the general low level trade wind flow.



Fig 6: The official NAVY Forecast track/intensity chart for Rachel (which normally utilizes the NHC forecast in this part of the world) follows the model consensus, and disregards the implied track provided by the GFS.

I’ll have another brief update on Sunday.

Steve