Invest 96L Near Cape Verdes Worth Monitoring

By: Steve Gregory , 10:01PM,GMT on August 16,2015

Invest 96L Near Cape Verdes Worth Monitoring


SUNDAY - -AUG-16
Next Regular WX Update MONDAY – AUG 17
Special Update: El NinO Part II – Scheduled for TUESDAY, AUG 18


INVEST 96L EASTERN ATLANTIC HAS POTENTIAL FOR DEVELOPMENT

The comment & image I Tweeted a few days (AUG 13) ago on an especially strong System over Africa



reached the African coast late yesterday and is now located a few hundred miles S/SW of the Cape Verdes – moving westward at ~15kts.

The system is still embedded in the weak ITCZ, but does exhibit some low to mid-level rotation, along with some deep convection thanks to a warming SST environment. The SAL flow has pushed offshore along with the wave, but most of the dust and drier air associated with it remains just north of the convective ‘ball’ associated with 96L. And with a due west track forecast by most of the specialized Tropical Cyclone models is indicated, and dust transport forecasts show most of the SAL’s most hostile components remaining to the north of the system – Invest 96L is the first system of the season to have ANY chance of development during the next 5 days. Additionally, wind shear as analyzed by CIMSS is under 10Kts and is expected to remain under 10-15Kts for the next 5 days; a level that should not be a significant hindrance to development during the next few days at least.

Explicit forecasts from the SHIPS Forecast model call for 96L to continue moving due west along 10˚N, eventually reaching 11˚N latitude in 5 days along with shear remaining under 10 Kts as it approaches 11˚N/49˚W – or about 600NM east of the Windward Islands in 5 days at Tropical Storm intensity. SST’s along the projected track will be fairly steady ~ 28˚C, warm enough to support a tropical cyclone. OTH – the specialized HWRF model moves the system somewhat slower and has it gaining more latitude, with the circulation center reaching 12˚N / 31W in 5 days – and not stronger than Depression intensity.

Note: I’ll be posting my regular, ‘Full Update’ Tomorrow – and if INVEST 96L develops more rapidly than anticipated, another Special Update on it will be posted Tuesday. Otherwise, I’ll post the Special Update on El Niño Part II on Tuesday. (I also intend to expand the text explanations for many of the images in yesterday’s El Niño Update - Part I, later this week as time permits.

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Fig 1: Overview of Tropical Atlantic Basin. With the exception of INVEST 96L located to the S-SW of the Cape Verdes – conditions across the rest of the tropics have not changed much as dry, stable air dominates much of central and western tropical Atlantic, with strong wind shear still plaguing the CARIB. But over the EATL, the SAL has weakened substantially, with a general moistening up of the low and mid-levels during the past week. The easterly waves coming off the African coast (still further south than ‘normal’ by this point in the season) are somewhat more robust than we’ve seen to date – and SST’s have risen substantially over the past 10 days, enabling sustained convection as these TW’s move off the west African coast. The strongest of these disturbances came off the coast late yesterday (96L) and is showing signs of a low to mid-level circulation – especially evident over the last few hours (See below TPWS Loop image - Fig 3).


Fig 2: MODIS TRUE-COLOR IMAGE centered on the fairly small, INVEST 96L taken early this morning. There is SOME curvature to the 2 convective bands around the centroid of circulation. The dust filled SAL (Saharan Air Layer) is clearly seen JUST to the north of 96L; but with the system so far south and still embedded within the ITCZ, the SAL is not currently seen as a major hindrance to this particular disturbance.



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Fig 3: TPW (Total Precipitable Water) image Loop (Top) / TPW Image with the ‘V’ wind component magnitude analysis over land (bottom). In the Loop, you can see counterclockwise motion surrounding the area where INVEST 96L is located – along with an expanding area of deep moisture spreading westward over the eastern Atlantic. While the SAL is still impacting areas north of 20˚N latitude, it’s a relatively ‘thin’ layer now. The top image’s wind vector component analysis shows the broad circulation surrounding 96L and the second TW right along the African coast. (Negative values to the west and north, with positive vector values to to south and east of the disturbance implies a counter-clockwise wind flow).


Fig 4: ‘Close-up’ Experimental color Enhanced IR and Water Vapor imagery of INVBEST 96L with wind shear analysis from CiMSS. Locally deep convection surrounds 96L, with surface reports showing a general circulation around 96L. The system appears likely to become a tropical depression during the next 24-48 hrs as it moves over sufficiently warm waters and within an area of relatively low wind shear (~10 Kts).


Fig 5: Shear analysis for the entire Atlantic Basin. Deep level wind shear continues across the CARIB with little change seen for the week ahead. Much lower shear values now dominate the central and eastern ATL - especially below 15˚N latitude – where 96L is expected to track during the next 5 days.



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Fig 6: Upper Air wind analysis (above 30,000 ft) Winds across the central and eastern ATL have become more conducive to cyclone formation, with hints of an upper air anti-cyclonic flow (‘H’) located about 250NM to the NW of INVEST 96L. This type of high level, upper air High Pressure system is critical to the intensification of a tropical cyclone as it serves as a way for the storm to ‘vent’ the air being pulled rapidly into the storm near the surface. Ina ‘perfect’, intense Hurricane – the upper level High sits atop the hurricane’s eye, with 2 and sometimes 3, outflow ‘channels’ of jet stream wind speeds radiating out from the center and away from the surface storm.


Fig 7: Early 12Z Cycle run of specialized Hurricane forecast models for INVEST 96L. Most of the models run earlier today are in very good agreement on the track for the system, with varying rates of intensification. The GFS related model (AVN) does NOT do much with the system, but the other models do show the system eventually becoming a tropical storm in 3 to 5 days – and these seem reasonable considering the systems current environment and likely track.

✭ The Next REGULAR, Full Weather Update will be posted on MONDAY, AUGUST 17 ✭

NOTE: SPECIAL UPDATE ON EL NINO Part II is Scheduled for this TUESDAY, AUG 18

✭ Final Note: I’ll be 'updating' the previous blog (El Niño Part I) later this week with expanded 'discussions' for many of the images. Stay Tuned! ✭



Steve