Hurricane CRISTOBAL Northbound – ‘97L’ Needs Monitoring – Strong System over Africa
By: SteveGregory , 4:01 PM GMT on August 26, 2014
CRISTOBAL intensified into a hurricane early last night as wind shear eased a bit, and outflow improved somewhat to the North and south. Shear remains relatively high near 20Kts, and with no improvement in the outflow pattern above the storm expected and the proximity to drier to its west – significant intensification appears unlikely as the 988mb storm moves Northward during the next 2 days along the western periphery of the sub-tropical (Bermuda) High centered in the central Atlantic. All the models are now in excellent agreement on both the track and intensity forecast for CRISTOBAL, and except for large waves and localized rip currents along the east coast – CRISTOBAL will have no impact on the US mainland
INVEST 97L POSES A POTENTIAL THREAT OF DEVELOPMENT THIS WEEKEND
Although NHC dropped 97L overnight, this system is now BACK on the NHC home page chart, most likely due to several global models now forecasting it to develop this weekend as it approaches the Leeward Islands – with the GFS and a few other models tracking it as a cyclone into the southeast coast of the US mainland late next week. Because NHC ‘dropped’ the system over night, none of the specialized hurricane models we executed during the 12Z cycle run. However, the models will almost certainly be initialized for the 18Z cycle run later today. That said – the system is currently surrounded by dry and somewhat stable air with no significant outflow signature seen on SAT imagery at this time; so development, if any, will be quite slow for the next 72 hours.
STRONG DISTURBANCE STILL WESTBOUND OVER AFRICA
Finally, the strongest African tropical disturbance of the season is now located over west-central Africa with a long history of deep convection and a well established rotation in low to mid levels. The disturbance is now moving slowly westward, and is expected to emerge off the west African coast this weekend. Some model projections forecast this system to gradually intensify next week – but also put it on a more Northwesterly track next week – implying this system will start heading out to sea by later next week.
Fig 1: Early morning VIS imagery shows the center of CRISTOBAL on the NW side of deep convection moving Northward at about 5Kts (based on the last 6 hours of RECON/SAT image tracking). The overall satellite signature is highly unusual for a tropical cyclone, especially of hurricane intensity – with the system appearing to be along the southern edge of a mid-latitude frontal boundary. In some respects, that is exactly what has occurred over the last 24 hours, with the southward plunge of well defined dry air surge to the immediate west of the cyclone, leading to a ‘linear’ type orientation of convection extending from well north of CRISTOBAL southward to the storm itself. There is a small possibility that this ‘dynamic’ boundary actually helped intensify the cyclone, despite the moderate shear and dry air that is just west of the storms core circulation. Normally, dry air this close to a relatively weak and sheared system like we had would weaken or totally halt intensification – but in this case (the first I’ve ever seen) – the opposite occurred. It’s worth noting that the dry air surge extended into the northern GOM as well – triggering a line of strong convection there.
Fig 2: The above overview of the tropical Atlantic shows a significant tropical wave/disturbance that was (and now again, is) 97L approaching 50W, and is still westbound at ~16Kts. Dry air is to the north and northwest of the system does not appear to be infiltrating the central area of what isolated convection there is. However, the system still remains in a less than favorable area of somewhat stable air – with no upper level wind support. As the system approaches the far eastern CARIB late this week, the environment should become somewhat more favorable for development.
Fig 3: Enhanced IR imagery over Africa earlier this morning highlights the very strong system in west-central Africa. This system had been moving W/SW for the last 36 hours – but has slowed its forward motion somewhat, and is expected to move on a West/Northwest (290°) during the next few days.
Fig 4: The above image over Africa includes satellite derived winds – and show a well established cyclonic flow around the major system of interest – just as it had for the last 2 days
Fig 5: The global models like the NAVY GEM above – and the GFS & CMC forecasts (not shown) all forecast the system to slowly intensify next week, with the GFS then carrying the system into the SE U.S. coast as a cyclone. Clearly, this system needs close monitoring.